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{{Short description|NASA mission to explore Mars via two rovers}}
{{Use mdy dates|date=February 2019}} 
[[File:NASA Mars Rover.jpg|thumb|upright=1.4|Artist's conception of MER rovers on Mars]]
[[File:MER twins white cyc.png|thumb|upright=1.4|MER imaged from different angles]]

[[NASA]]'s '''Mars Exploration Rover''' ('''MER''') mission was a [[robotic space mission]] involving two [[Mars rover]]s, ''[[Spirit (rover)|Spirit]]'' and ''[[Opportunity (rover)|Opportunity]]'', exploring the planet [[Mars]]. It began in 2003 with the launch of the two [[rover (space exploration)|rover]]s to explore the Martian surface and [[Geology of Mars|geology]]; both landed on Mars at separate locations in January 2004.  Both rovers far outlived their planned missions of 90 [[Timekeeping on Mars#Sols|Martian solar days]]: MER-A ''Spirit'' was active until March 22, 2010,<ref>{{Cite web|url=https://mars.nasa.gov/mer/mission/rover-status/spirit/2010/all/|title=Rover Update: 2010: All|last=mars.nasa.gov|website=mars.nasa.gov|access-date=February 14, 2019}}</ref> while MER-B ''Opportunity'' was active until June 10, 2018.<ref>{{Cite web|url=https://www.cnn.com/2019/02/13/world/nasa-mars-opportunity-rover-trnd/index.html|title=After 15 years, the Mars Opportunity rover's mission has ended|first=Ashley|last=Strickland|website=CNN|date=February 13, 2019|access-date=February 14, 2019}}</ref>

==Objectives==
The mission's scientific objective was to search for and characterize a wide range of [[rock (geology)|rocks]] and [[soil]]s that hold clues to past [[Water on Mars|water activity]] on [[Mars]]. The mission is part of [[NASA]]'s [[Mars Exploration Program]], which includes three previous successful landers: the two [[Viking program]] landers in 1976 and [[Mars Pathfinder]] probe in 1997.<ref name="Mission overview">{{cite web |url=http://marsrovers.nasa.gov/overview |title=Mars Exploration Rover Mission Overview |publisher=NASA |access-date=November 25, 2009 |archive-url=https://web.archive.org/web/20090603184238/http://marsrovers.nasa.gov/overview/ |archive-date=June 3, 2009 }}</ref>

The scientific objectives of the Mars Exploration Rover mission were to:<ref>{{cite web |title=Objectives - NASA |url=https://mars.nasa.gov/mer/mission/science/objectives/ |website=mars.nasa.gov |access-date=23 April 2023}}</ref>

*Search for and characterize a variety of rocks and soils that hold clues to past [[Water on Mars|water]] activity. In particular, samples sought include those that have minerals deposited by water-related processes such as [[Precipitation (meteorology)|precipitation]], [[Water vapor|evaporation]], [[Sedimentary rock|sedimentary cementation]], or [[hydrothermal activity]].
*Determine the distribution and composition of minerals, rocks, and soils surrounding the landing sites.
*Determine what [[Geology of Mars|geologic processes]] have shaped the local terrain and influenced the chemistry. Such processes could include water or wind erosion, sedimentation, hydrothermal mechanisms, volcanism, and cratering.
*Perform calibration and validation of surface observations made by [[Mars Reconnaissance Orbiter]] (MRO) instruments. This will help determine the accuracy and effectiveness of various instruments that survey Martian geology from orbit.
*Search for iron-containing minerals, and to identify and quantify relative amounts of specific mineral types that contain water or were formed in water, such as iron-bearing carbonates.
*Characterize the [[mineralogy]] and textures of rocks and soils to determine the processes that created them.
*Search for geological clues to the environmental conditions that existed when liquid water was present.
*Assess whether those environments were conducive to life.

The total cost of building, launching, landing and operating the [[Mars rover|rovers]] on the surface for the initial 90-[[Martian day#Sols|sol]] primary mission was US$820&nbsp;million.<ref name="MissionExtensions">{{cite web|url=https://www.nbcnews.com/id/wbna21327647 |title=NASA extends Mars rovers' mission |publisher=NBC News |date=October 16, 2007 |access-date=April 5, 2009}}</ref> Each rover received five mission extensions, as they continued to function past their initially planned duration. The fifth mission extension was granted in October 2007, and ran to the end of 2009.<ref name="MissionExtensions" /><ref name="nasa">{{cite web|url=http://marsrovers.jpl.nasa.gov/newsroom/pressreleases/20071015a.html |title=Mars Exploration Rover Mission: Press Releases|publisher=marsrovers.jpl.nasa.gov|access-date=May 25, 2015}}</ref> The total cost of the first four mission extensions was $104&nbsp;million, and the fifth mission extension cost at least $20&nbsp;million.<ref name="MissionExtensions" /> The mission was managed by the [[Jet Propulsion Laboratory]] (JPL), which designed, built, and operated the rovers.

In recognition of the vast amount of [[Scientific information from the Mars Exploration Rover mission|scientific information]] amassed by both rovers, two [[asteroid]]s have been named in their honor: [[37452 Spirit]] and [[39382 Opportunity]].

==Mission timeline==
=== Launch and landing ===
{{Multiple image
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| image1 = Lancement Spirit fusee Delta IIs 10062003.jpg
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| caption1 = Launch of MER-A ''Spirit''
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| caption2 = Launch of MER-B ''Opportunity''
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The MER-A (''Spirit'') and MER-B (''Opportunity'') were launched on June 10, 2003 and July 7, 2003, respectively. Though both probes launched on [[Boeing]] [[Delta II]] 7925-9.5 rockets from [[Cape Canaveral Space Launch Complex 17]] (CCAFS SLC-17), MER-B was on the heavy version of that launch vehicle, needing the extra energy for [[Trans-Mars injection]]. The launch vehicles were integrated onto pads right next to each other, with MER-A on CCAFS SLC-17A and MER-B on CCAFS SLC-17B. The dual pads allowed for working the 15- and 21-day planetary launch periods close together; the last possible launch day for MER-A was June 19, 2003 and the first day for MER-B was June 25, 2003. NASA's [[Launch Services Program]] managed the launch of both spacecraft.

The probes landed in January 2004 in widely separated equatorial locations on Mars. On January 21, 2004, the [[Deep Space Network]] lost contact with ''Spirit'', for reasons originally thought to be related to a flare shower over [[Australia]]. The rover transmitted a message with no data, but later that day missed another communications session with the [[Mars Global Surveyor]]. The next day, JPL received a beep from the rover, indicating that it was in fault mode. On January 23, the flight team succeeded in making the rover send. The fault was believed to have been caused by an error in the rover's [[flash memory]] subsystem. The rover did not perform any scientific activities for ten days, while engineers updated its software and ran tests. The problem was corrected by reformatting ''Spirit''{{'s}} flash memory and using a [[software patch]] to avoid memory overload; ''Opportunity'' was also upgraded with the patch as a precaution. ''Spirit'' returned to full scientific operations by February 5. Both rovers' missions were initially just 90-[[Martian day#Sols|sol]]-long.

On March 23, 2004, a news conference was held announcing "major discoveries" of evidence of past liquid water on the Martian surface. A delegation of scientists showed pictures and data revealing a stratified pattern and [[cross bedding]] in the rocks of the outcrop inside a crater in [[Meridiani Planum]], landing site of MER-B, ''Opportunity''. This suggested that water once flowed in the region. The irregular distribution of [[chlorine]] and [[bromine]] also suggests that the place was once the shoreline of a salty sea, now evaporated.

{{Multiple image
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| image1 = Animation of Spirit around Sun.gif
| image2 = Animation of Opportunity trajectory.gif
| footer = Animations of ''Spirit'' (left) and ''Opportunity'' (right) orbits. <br />{{legend2| Yellow| Sun}}{{·}}{{legend2| RoyalBlue| Earth}}{{·}}{{legend2|Lime| Mars}}{{·}}{{legend2|Magenta| Rover}}
}}

===Mission extensions===

On April 8, 2004, NASA announced that it was extending the mission life of the rovers from three to eight months. It immediately provided additional funding of US$15&nbsp;million through September, and $2.8&nbsp;million per month for continuing operations. Later that month, ''Opportunity'' arrived at [[Endurance crater]], taking about five days to drive the 200 meters. NASA announced on September 22 that it was extending the mission life of the rovers for another six months. ''Opportunity'' was to leave Endurance crater, visit its discarded heat shield, and proceed to [[Victoria crater]]. ''Spirit'' was to attempt to climb to the top of the [[Columbia Hills (Mars)|Columbia Hills]].

With the two rovers still functioning well, NASA later announced another 18-month extension of the mission to September 2006. ''Opportunity'' was to visit the "Etched Terrain" and ''Spirit'' was to climb a rocky slope toward the top of [[Husband Hill]]. On August 21, 2005, ''Spirit'' reached the summit of Husband Hill after 581 [[Timekeeping on Mars#Sols|sols]] and a journey of {{convert|4.81|km|mi|sp=us}}.

''Spirit's'' front right wheel ceased working on March 13, 2006, while the rover was moving itself to [[McCool Hill]]. Its drivers attempted to drag the dead wheel behind ''Spirit'', but this only worked until reaching an impassable sandy area on the lower slopes. Drivers directed ''Spirit'' to a smaller sloped feature, dubbed "Low Ridge Haven", where it spent the long Martian winter, waiting for spring and increased solar power levels suitable for driving. That September, ''Opportunity'' reached the rim of Victoria crater, and Spaceflight Now reported that NASA had extended mission for the two rovers through September 2007.<ref>{{cite web |url=http://www.nasa.gov/mission_pages/mer/mer-20060927.html |title=NASA Mars Rover Arrives at Dramatic Vista on Red Planet |publisher=nasa.gov |access-date=September 28, 2006}}</ref><ref>{{cite web |url=http://spaceflightnow.com/news/n0609/26rovers/ |title=Mars rover, Global Surveyor, Odyssey missions extended |access-date=September 27, 2006}}</ref> On February 6, 2007, ''Opportunity'' became the first spacecraft to traverse {{convert|10|km|mi|abbr=off|sp=us|spell=in}} on the surface of Mars.<ref>{{cite web |url=http://www.nasa.gov/mission_pages/mer/images/opp-20070207.html |title=Opportunity Passes 10-Kilometer Mark |publisher=nasa.gov |access-date=February 8, 2007}}</ref>

In July 2007, during the fourth mission extension, Martian dust storms blocked sunlight to the rovers and threatened the ability of the craft to gather energy through their [[solar panel]]s, causing engineers to fear that one or both of them might be permanently disabled. However, the dust storms lifted, allowing them to resume operations.<ref>{{cite web | url = http://www.nasa.gov/mission_pages/mer/mer-20070824.html | title = Mars Exploration Rover Status Report: Rovers Resume Driving | publisher = nasa.gov | access-date = September 3, 2007 | archive-date = September 2, 2014 | archive-url = https://web.archive.org/web/20140902071407/http://www.nasa.gov/mission_pages/mer/mer-20070824.html }}</ref>

[[File:Spirit and Opportunity by the numbers.jpg|thumb|''Spirit'' and ''Opportunity'' by the numbers]]

''Opportunity'' was poised to enter Victoria Crater from its perch on the rim of Duck Bay on June 28, 2007,<ref>{{cite web |url=http://marsrovers.jpl.nasa.gov/newsroom/pressreleases/20070628a.html |title=NASA Mars Rover Ready For Descent Into Crater |publisher=jpl.nasa.gov |access-date=July 15, 2007 |archive-date=July 6, 2007 |archive-url=https://web.archive.org/web/20070706163700/http://marsrovers.jpl.nasa.gov/newsroom/pressreleases/20070628a.html }}</ref> but due to extensive dust storms, it was delayed until the dust had cleared and power returned to safe levels.<ref>{{cite web |url=http://marsrovers.jpl.nasa.gov/mission/status_opportunityAll.html#sol1220 |title=Opportunity Waiting for Dust to Settle |publisher=jpl.nasa.gov |access-date=July 15, 2007 |archive-date=June 20, 2014 |archive-url=https://web.archive.org/web/20140620024319/http://marsrovers.jpl.nasa.gov/mission/status_opportunityAll.html#sol1220 }}</ref> Two months later, ''Spirit'' and ''Opportunity'' resumed driving after hunkering down during raging dust storms that limited solar power to a level that nearly caused the permanent failure of both rovers.<ref>{{cite web |url=http://www.nasa.gov/mission_pages/mer/mer-20070824.html |title=Mars Exploration Rover Status Report: Rovers Resume Driving |publisher=NASA |access-date=August 30, 2007 |archive-date=September 2, 2014 |archive-url=https://web.archive.org/web/20140902071407/http://www.nasa.gov/mission_pages/mer/mer-20070824.html }}</ref>

On October 1, 2007,<ref>{{cite web |url=http://marsrovers.jpl.nasa.gov/mission/status_spiritAll.html#sol1337 |title=Hardy Rover Continues to Celebrate Milestones |publisher=NASA |access-date=October 16, 2007}}</ref> both ''Spirit'' and ''Opportunity'' entered their fifth mission extension that extended operations into 2009,<ref>{{cite web |url=http://www.space.com/missionlaunches/071015-marsrover-extendedmission.html |title=NASA Extends Mars Rover Mission a Fifth Time |date=October 15, 2007 |publisher=NASA |access-date=October 16, 2007}}</ref> allowing the rovers to have spent five years exploring the Martian surface, pending their continued survival.

On August 26, 2008, ''Opportunity'' began its three-day climb out of Victoria crater amidst concerns that power spikes, similar to those seen on ''Spirit'' before the failure of its right-front wheel, might prevent it from ever being able to leave the crater if a wheel failed. Project scientist Bruce Banerdt also said, "We've done everything we entered Victoria Crater to do and more."  ''Opportunity'' will return to the plains in order to characterize Meridiani Planum's vast diversity of rocks—some of which may have been blasted out of craters such as Victoria. The rover had been exploring Victoria Crater since September 11, 2007.<ref>{{cite web |url=https://www.jpl.nasa.gov/news/nasas-mars-rover-opportunity-climbing-out-of-victoria-crater |title=NASA's Mars Rover Opportunity Climbing out of Victoria Crater |publisher=jpl.nasa.gov |access-date=March 7, 2023}}</ref><ref>{{cite web |url=http://marsrovers.jpl.nasa.gov/newsroom/pressreleases/20080829a.html |title=NASA Mars Rover Opportunity Ascends to Level Ground |publisher=jpl.nasa.gov |access-date=August 29, 2008}}</ref> As of January 2009, the two rovers had collectively sent back 250,000 images and traveled over {{convert|21|km|mi|sp=us}}.<ref name="CNNfifthyear">{{Cite news |url=http://www.cnn.com/2009/TECH/01/03/mars.rovers.five.years/index.html |title=NASA's rovers mark five years on Red Planet |publisher=CNN |date=January 3, 2009 |access-date=January 3, 2009}}</ref>

After driving about {{convert|3.2|km|mi|sp=us}} since it left Victoria crater, ''Opportunity'' first saw the rim of [[Endeavour (crater)|Endeavour crater]] on March 7, 2009.<ref name="NASA20090318PressRelease">{{Cite news |title=One Mars Rover Sees A Distant Goal; The Other Takes A New Route |publisher=NASA/JPL |url=http://marsrovers.jpl.nasa.gov/newsroom/pressreleases/20090318a.html |date=March 18, 2009 |access-date=March 20, 2009}}</ref> It passed the {{convert|16|km|abbr=on}} mark along the way on sol 1897.<ref>{{cite web |url=http://www.space.com/missionlaunches/090526-opp-10-miles.html |title=Opportunity Rover Passes 10-Mile Mark on Mars |publisher=Space.com |date=May 26, 2009 |access-date=May 27, 2009}}</ref> Meanwhile, at Gusev crater, ''Spirit'' was dug in deep into the Martian sand, much as ''Opportunity'' was at [[Opportunity rover#Trouble with sand trap|Purgatory Dune]] in 2005.<ref>{{cite web |url=http://www.space.com/missionlaunches/090521-spirit-stuck.html |title=Spirit Stuck In 'Insidious Invisible Rover Trap' on Mars |publisher=Space.com |date=May 21, 2009 |access-date=May 27, 2009}}</ref>

On January 26, 2010, after six years on Mars, NASA announced that ''Spirit'' would be used as a stationary research platform, after several months of unsuccessful attempts to free the rover from soft sand.<ref>{{cite web |url=http://www.nasa.gov/mission_pages/mer/news/mer20100126.html |title=Now a Stationary Research Platform, NASA's Mars Rover Spirit Starts a New Chapter in Red Planet Scientific Studies |publisher=NASA |date=January 26, 2010 |access-date=January 29, 2010 |archive-date=April 12, 2022 |archive-url=https://web.archive.org/web/20220412004959/http://www.nasa.gov/mission_pages/mer/news/mer20100126.html }}</ref>

NASA announced on March 24, 2010, that ''Opportunity'', which had an estimated remaining drive distance of {{convert|12|km|mi|abbr=on}} to Endeavour Crater, had traveled over {{convert|20|km|mi|1|abbr=on}} since the start of its mission.<ref name="NASA20100324">{{cite web |url=http://marsrover.nasa.gov/newsroom/pressreleases/20100324b.html |title=Opportunity Surpasses 20 Kilometers of Total Driving |publisher=NASA |date=March 24, 2010 |access-date=April 18, 2010 |archive-url=https://web.archive.org/web/20100528134748/http://marsrover.nasa.gov/newsroom/pressreleases/20100324b.html |archive-date=May 28, 2010}}</ref> Each rover was designed with a mission driving distance goal of just 600 meters.<ref name="NASA20100324"/> 

In March 2010 it was announced that ''Spirit'' may have gone into hibernation for the Martian winter and might not wake up again for months.<ref>{{cite web |url=http://marsrover.nasa.gov/newsroom/pressreleases/20100331a.html |title=Spirit May Have Begun Months-Long Hibernation |publisher=NASA |date=March 31, 2010 |access-date=April 18, 2010 |archive-url=https://web.archive.org/web/20100528135357/http://marsrover.nasa.gov/newsroom/pressreleases/20100331a.html |archive-date=May 28, 2010}}</ref> On May 22, 2011, NASA cease attempts to contact with ''Spirit'', which had been stuck in a sand trap for two years. The last successful communication with the rover was on March 22, 2010. The final transmission to ''Spirit'' was on May 25, 2011.<ref name="NASA20110524MissionNews">{{cite news |title=NASA Concludes Attempts to Contact Mars Rover Spirit |publisher=NASA/JPL |url=http://www.nasa.gov/mission_pages/mer/news/mer20110524.html |date=May 24, 2011 |access-date=May 25, 2011 |archive-date=March 16, 2012 |archive-url=https://web.archive.org/web/20120316091816/http://www.nasa.gov/mission_pages/mer/news/mer20110524.html }}</ref>

On May 16, 2013, NASA announced that ''Opportunity'' had driven further than any other NASA vehicle on a world other than Earth.<ref name="40YearRecordNASA20130516">{{cite news |title=Nine-Year-Old Mars Rover Passes 40-Year-Old Record |publisher=NASA/JPL |url=http://marsrovers.jpl.nasa.gov/newsroom/pressreleases/20130516a.html |date=May 16, 2013 |access-date=May 25, 2013}}</ref> After ''Opportunity's'' total [[odometry]] went over {{convert|35.744|km|mi|abbr=on}}, the rover surpassed the total distance driven by the [[Apollo 17]] [[Lunar Roving Vehicle]].<ref name="40YearRecordNASA20130516"/> And on July 28, 2014, ''Opportunity'' had driven further than any other vehicle on a world other than Earth.<ref name="40YearRecordNASA20130516"/><ref name="NASA-20140728a">{{cite web |last1=Webster |first1=Guy |last2=Brown |first2=Dwayne |title=NASA Long-Lived Mars Opportunity Rover Sets Off-World Driving Record |url=http://www.jpl.nasa.gov/news/news.php?release=2014-245 |date=July 28, 2014 |work=[[NASA]] |access-date=July 29, 2014}}</ref><ref name="FRB-20140729">{{cite web |last=Knapp |first=Alex |title=NASA's Opportunity Rover Sets A Record For Off-World Driving |url=https://www.forbes.com/sites/alexknapp/2014/07/29/nasas-opportunity-rover-sets-a-record-for-off-world-driving |date=July 29, 2014 |work=[[Forbes]] |access-date=July 29, 2014}}</ref> ''Opportunity'' covered over {{convert|40|km|mi|abbr=on}}, surpassing the total distance of {{convert|39|km|mi|abbr=on}} driven by the [[Lunokhod 2]] [[lunar rover]], the previous record-holder.<ref name="40YearRecordNASA20130516"/><ref name="NASA-20140728a" /> On March 23, 2015, ''Opportunity'' had driven the full {{convert|42.2|km|mi|abbr=on}} distance of a [[marathon]], with a finish time of roughly 11 years and 2 months.<ref>{{cite news |url=https://www.nasa.gov/press/2015/march/nasas-opportunity-mars-rover-finishes-marathon-clocks-in-at-just-over-11-years |title=NASA's Opportunity Mars Rover Finishes Marathon, Clocks in at Just Over 11 Years |publisher=NASA/JPL |date=March 23, 2015 |access-date=July 8, 2015}}</ref>

On January 24, 2014, NASA reported that then-current studies by the remaining rover, [[Opportunity (rover)|''Opportunity'']], as well as by the newer [[Mars Science Laboratory]] rover [[Curiosity (rover)|''Curiosity'']] would now be searching for evidence of ancient life, including a [[biosphere]] based on [[autotroph]]ic, [[chemotroph]]ic and/or [[Lithotroph#Chemolithotrophs|chemolithoautotrophic]] [[microorganism]]s, as well as ancient water, including [[Lacustrine plain|fluvio-lacustrine environments]] ([[plain]]s related to ancient rivers or lakes) that may have been [[Planetary habitability|habitable]].<ref name="SCI-20140124a">{{cite journal|last=Grotzinger |first=John P.|title=Introduction to Special Issue - Habitability, Taphonomy, and the Search for Organic Carbon on Mars|journal=[[Science (journal)|Science]] |date=January 24, 2014 |volume=343 |issue=6169 |pages=386–387 |doi=10.1126/science.1249944|pmid=24458635|bibcode=2014Sci...343..386G|doi-access=free }}</ref><ref name="SCI-20140124special">{{cite journal |author=Various|title=Special Issue - Table of Contents - Exploring Martian Habitability|url=https://www.science.org/toc/science/343/6169|date=January 24, 2014|journal=[[Science (journal)|Science]] |volume=343 |number=6169 |pages=345–452|access-date=January 24, 2014 }}</ref><ref name="SCI-20140124">{{cite journal |author=Various |title=Special Collection - Curiosity - Exploring Martian Habitability|url=https://www.science.org/action/doSearch?AllField=Curiosity+Mars|date=January 24, 2014 |journal=[[Science (journal)|Science]] |access-date=January 24, 2014 }}</ref><ref name="SCI-20140124c">{{cite journal|author=Grotzinger, J.P.|title=A Habitable Fluvio-Lacustrine Environment at Yellowknife Bay, Gale Crater, Mars |date=January 24, 2014 |journal=[[Science (journal)|Science]] |volume=343 |issue=6169 |doi=10.1126/science.1242777 |display-authors=etal |pmid=24324272 |article-number=1242777|bibcode=2014Sci...343A.386G |citeseerx=10.1.1.455.3973 |s2cid=52836398 }}</ref> The search for evidence of [[Planetary habitability|habitability]], [[taphonomy]] (related to [[fossils]]), and [[organic carbon]] on the planet Mars was then switched to a primary NASA objective.<ref name="SCI-20140124a" />

In June 2018, ''Opportunity'' was caught in a global-scale dust storm and the rover's solar panels were not able to generate enough power, with the last contact on June 10, 2018. NASA resumed sending commands after the dust storm subsided but the rover remained silent, possibly due to a catastrophic failure or a layer of dust covering its solar panels.<ref name='covered panels'>[https://www.space.com/42115-mars-rover-opportunity-still-silent-4-months-later.html Opportunity Rover Still Silent on Mars, 4 Months After Epic Dust Storm Began] {{Webarchive|url=https://web.archive.org/web/20181015192425/https://www.space.com/42115-mars-rover-opportunity-still-silent-4-months-later.html |date=October 15, 2018 }}. Mike Wall, ''Space.com''. October 12, 2018.</ref> A press conference was held on February 13, 2019, announcing that after numerous attempts to contact ''Opportunity'', with no response since June 2018, NASA declared the ''Opportunity'' mission over, which also drew the 16-year long Mars Exploration Rover mission to a close.<ref>{{Cite web|url=https://mars.nasa.gov/mer/mission/status_opportunityAll.html|title=Mars Exploration Rover Mission: All Opportunity Updates|website=mars.nasa.gov|access-date=February 10, 2019|archive-url=https://web.archive.org/web/20180325061258/https://mars.nasa.gov/mer/mission/status_opportunityAll.html|archive-date=March 25, 2018|url-status=live}}</ref><ref>[https://twitter.com/MarsRovers/status/1095454710432096257 Opportunity status on January 12, 2019]</ref><ref>{{cite web | url = https://www.nytimes.com/2019/02/13/science/mars-opportunity-rover-dead.html | title = NASA's Mars Rover Opportunity Concludes a 15-Year Mission | first = Kenneth | last = Chang | date = February 13, 2019 | access-date = February 13, 2019 | work = [[The New York Times]] }}</ref>

==Spacecraft design==
[[File:Fusee Delta II lancement rover Mars.jpg|thumb|MER launch configuration, break apart illustration]]

The Mars Exploration Rover was designed to be stowed atop a [[Delta II rocket]]. Each spacecraft consists of several components:
* Rover: {{convert|185|kg|lb|abbr=on}}
* Lander: {{convert|348|kg|lb|abbr=on}}
* Backshell / Parachute: {{convert|209|kg|lb|abbr=on}}
* Heat Shield: {{convert|78|kg|lb|abbr=on}}
* Cruise Stage: {{convert|193|kg|lb|abbr=on}}
* Propellant: {{convert|50|kg|lb|abbr=on}}
* Instruments: {{convert|5|kg|lb|abbr=on}}<ref>{{cite book|chapter=Spirit and Opportunity: Wheels on Mars|publisher=SpringerLink|url=https://link.springer.com/book/9780387489254|title=Planet Mars|pages=201–204|year=2008|isbn=978-0-387-48925-4}}</ref>
Total mass is {{convert|1,063|kg|lb|abbr=on}}.

===Cruise stage===
[[File:Opportunity Cruise Stage.jpg|thumb|Cruise stage of ''Opportunity'' rover]]
[[File:MER cruise stage diagram.jpg|thumb|MER cruise stage diagram]]
The cruise stage is the component of the spacecraft that is used for travel from Earth to Mars. It is very similar to the Mars Pathfinder in design and is approximately 2.65 [[metre|meters]] (8.7&nbsp;feet) in [[diameter]] and {{convert|1.6|m|ft|abbr=on}} tall, including the entry vehicle (see below).

The primary structure is aluminium with an outer ring of ribs covered by the solar panels, which are about {{convert|2.65|m|ft|abbr=on}} in diameter. Divided into five sections, the solar arrays can provide up to 600 [[watt]]s of power near Earth and 300&nbsp;W at Mars.

Heaters and [[multi-layer insulation]] keep the electronics "warm". A [[freon]] system removes heat from the flight computer and communications hardware inside the rover so they do not overheat. Cruise avionics systems allow the flight computer to interface with other electronics, such as the [[Sun sensor]]s, star scanner and heaters.

====Navigation====
The star scanner (without a backup system) and [[Sun sensor]] allowed the spacecraft to know its orientation in space by analyzing the position of the Sun and other stars in relation to itself. Sometimes the craft could be slightly off course; this was expected, given the 500-million-kilometer (320&nbsp;million mile) journey. Thus navigators planned up to six trajectory correction maneuvers, along with health checks.

To ensure the spacecraft arrived at Mars in the right place for its landing, two light-weight, aluminium-lined tanks carried about 31&nbsp;kg (about 68&nbsp;lb) of [[hydrazine]] [[propellant]]. Along with cruise guidance and control systems, the propellant allowed navigators to keep the spacecraft on course. Burns and pulse firings of the propellant allowed three types of maneuvers:
* An axial burn uses pairs of thrusters to change spacecraft velocity;
* A lateral burn uses two "thruster clusters" (four thrusters per cluster) to move the spacecraft "sideways" through seconds-long pulses;
* Pulse mode firing uses coupled thruster pairs for spacecraft [[precession]] maneuvers (turns).

====Communication====
The spacecraft used a high-frequency [[X band]] [[radio]] [[wavelength]] to communicate, which allowed for less power and smaller [[antenna (electronics)|antennas]] than many older craft, which used [[S band]].

Navigators sent commands through two antennas on the cruise stage: a cruise [[low-gain antenna]] mounted inside the inner ring, and a cruise medium-gain antenna in the outer ring. The low-gain antenna was used close to Earth. It is omni-directional, so the transmission power that reached Earth fell faster with increasing distance. As the craft moved closer to Mars, the Sun and Earth moved closer in the sky as viewed from the craft, so less energy reached Earth. The spacecraft then switched to the medium-gain antenna, which directed the same amount of transmission power into a tighter beam toward Earth.

During flight, the spacecraft was [[gyroscope|spin-stabilized]] with a spin rate of two [[revolutions per minute]] (rpm). Periodic updates kept antennas pointed toward Earth and solar panels toward the Sun.

===Aeroshell===
[[File:Mars Rover Aeroshell.jpg|thumb|Overview of the Mars Exploration Rover aeroshell]]
The aeroshell maintained a protective covering for the lander during the seven-month voyage to Mars. Together with the lander and the rover, it constituted the "entry vehicle". Its main purpose was to protect the lander and the rover inside it from the intense heat of entry into the thin Martian atmosphere. It was based on the Mars Pathfinder and Mars Viking designs.

;Parts
The aeroshell was made of two main parts: a [[heat shield]] and a backshell. The heat shield was flat and brownish, and protected the lander and rover during entry into the Martian atmosphere and acted as the first [[aerobrake]] for the spacecraft. The backshell was large, cone-shaped and painted white. It carried the [[parachute]] and several components used in later stages of entry, descent, and landing, including:

* A parachute (stowed at the bottom of the backshell);
* The backshell electronics and batteries that fire off pyrotechnic devices like separation nuts, rockets and the parachute mortar;
* A Litton LN-200 Inertial Measurement Unit (IMU), which monitors and reports the orientation of the backshell as it swings under the parachute;
* Three large [[solid rocket]] motors called RAD rockets (Rocket Assisted Descent), each providing about a ton of force (10 [[kilonewton]]s) for nearly 4 seconds;<ref>{{Cite web|last=mars.nasa.gov|title=What are the RAD Rockets?|url=https://mars.nasa.gov/mer/mission/spacecraft/entry-descent-and-landing-configuration/aeroshell/rad-rockets/|access-date=2021-08-26|website=mars.nasa.gov}}</ref>
* Three small solid rockets called TIRS (mounted so that they aim horizontally out the sides of the backshell) that provide a small horizontal kick to the backshell to help orient the backshell more vertically during the main RAD rocket burn.

;Composition
Built by [[Lockheed Martin]] Space in Denver, Colorado, the aeroshell is made of an aluminium honeycomb structure sandwiched between [[graphite-epoxy]] face sheets. The outside of the aeroshell is covered with a layer of [[phenol]]ic honeycomb. This honeycomb is filled with an [[ablation|ablative]] material (also called an "ablator"), that dissipates heat generated by atmospheric friction.

The ablator itself is a blend of [[Cork (material)|cork]] wood, [[binder (material)|binder]] and many tiny [[silica]] glass spheres. It was invented for the heat shields flown on the Viking Mars lander missions. A similar technology was used in the first US [[crewed space mission]]s [[Mercury program|Mercury]], [[Gemini program|Gemini]] and [[Apollo program|Apollo]]. It was specially formulated to react chemically with the Martian atmosphere during [[atmospheric entry|entry]] and carry heat away, leaving a hot wake of gas behind the vehicle. The vehicle slowed from {{convert|19000|to|1600|km/h|m/s|abbr=on|sigfig=2}} in about a minute, producing about 60&nbsp;m/s<sup>2</sup> (6 ''[[g-force|g]]'') of [[acceleration]] on the lander and rover.

The backshell and heat shield are made of the same materials, but the heat shield has a thicker, {{convert|1/2|in|mm|abbr=on|order=flip}}, layer of the ablator. Instead of being painted, the backshell was covered with a very thin aluminized [[PET film (biaxially oriented)|PET film]] blanket to protect it from the cold of deep space. The blanket vaporized during entry into the Martian atmosphere.

===Parachute===
[[File:Parachute1.jpg|thumb|Mars Exploration Rover's parachute test]]
The parachute helped slow the spacecraft during entry, descent, and landing. It is located in the backshell.<ref name="nasa_expl_rover_mission">{{cite web| url =https://mars.nasa.gov/mer/mission/spacecraft_edl_parachute.html | title = Mars Exploration Rover Mission: The Mission | publisher = nasa.gov | access-date = September 12, 2020 | archive-url = https://web.archive.org/web/20190930004254/https://mars.nasa.gov/mer/mission/spacecraft_edl_parachute.html |archive-date = September 30, 2019 |url-status=live}}</ref>

;Design
The 2003 parachute design was part of a long-term Mars parachute technology development effort and is based on the designs and experience of the Viking and Pathfinder missions. The parachute for this mission is 40% larger than Pathfinder's because the largest load for the Mars Exploration Rover is 80 to 85 [[newton (units)|kilonewton]]s (kN) or {{convert|18000|to|19000|lbf|kN|abbr=on|order=flip}} when the parachute fully inflates. By comparison, Pathfinder's inflation loads were approximately 35&nbsp;kN (about 8,000&nbsp;lbf). The parachute was designed and constructed in [[South Windsor, Connecticut]] by [[Pioneer Aerospace]], who also designed the parachute for the ''[[Stardust (spacecraft)|Stardust]]'' mission.<ref name="nasa_expl_rover_mission"/>

;Composition
The parachute is made of two durable, lightweight fabrics: [[polyester]] and [[nylon]]. A triple bridle made of [[Kevlar]] connects the parachute to the backshell.

The amount of space available on the spacecraft for the parachute was so small that the parachute had to be pressure-packed. Before launch, a team tightly folded the 48 suspension lines, three bridle lines, and the parachute. The parachute was loaded in a special structure that then applied a heavy weight to the parachute package several times. Before placing the parachute into the backshell, the parachute was heat set to [[sterilization (microbiology)|sterilize]] it.<ref name="nasa_expl_rover_mission"/>

;Connected systems
[[File:Rocket assisted descent.jpg|right|thumb|Descent is halted by [[retrorocket]]s and lander is dropped {{convert|10|m|ft|abbr=on}} to the surface in this computer generated impression.]]
After the parachute was deployed at an altitude of about {{convert|10|km|mi|abbr=on}} above the surface, the heatshield was released using 6 separation nuts and push-off springs. The lander then separated from the backshell and "rappelled" down a metal tape on a [[centrifugal brake|centrifugal braking system]] built into one of the lander petals. The slow descent down the metal tape placed the lander in position at the end of another bridle (tether), made of a nearly {{convert|20|m|ft|abbr=on}} long braided [[Zylon]].<ref name="nasa_expl_rover_mission"/> Zylon is a fiber material, similar to Kevlar, that is sewn in a webbing pattern (like shoelace material) to make it stronger. The Zylon bridle provides space for airbag deployment, distance from the solid rocket motor exhaust stream, and increased stability. The bridle incorporates an electrical harness that allows the firing of the solid rockets from the backshell as well as provides data from the backshell inertial measurement unit (which measures rate and tilt of the spacecraft) to the flight computer in the rover.<ref name="nasa_expl_rover_mission"/>

Because the atmospheric density of Mars is less than 1% of Earth's, the parachute alone could not slow down the Mars Exploration Rover enough to ensure a safe, low landing speed. The spacecraft descent was assisted by rockets that brought the spacecraft to a dead stop {{convert|10|–|15|m|ft|abbr=on}} above the Martian surface.<ref name="nasa_expl_rover_mission"/>

A [[radar altimeter]] unit was used to determine the distance to the Martian surface. The radar's antenna was mounted at one of the lower corners of the lander tetrahedron. When the radar measurement showed the lander was the correct distance above the surface, the Zylon bridle was cut, releasing the lander from the parachute and backshell so that it was free and clear for landing. The radar data also enabled the timing sequence on airbag inflation and backshell RAD rocket firing.<ref name="nasa_expl_rover_mission"/>

===Airbags===
[[File:Spirit-airbag2.jpg|thumb|right|Inflated airbag in the lab]]
[[Airbag]]s used in the Mars Exploration Rover mission are the same type that [[Mars Pathfinder]] used in 1997. They had to be strong enough to cushion the spacecraft if it landed on rocks or rough terrain and allow it to bounce across Mars's surface at highway speeds (about 100&nbsp;km/h) after landing. The airbags had to be inflated seconds before touchdown and deflated once safely on the ground.

The airbags were made of [[Vectran]], like those on Pathfinder. Vectran has almost twice the strength of other synthetic materials, such as Kevlar, and performs better in cold temperatures. Six 100 [[Denier (measure)|denier]] (10&nbsp;mg/m) layers of Vectran protected one or two inner bladders of Vectran in 200 denier (20&nbsp;mg/m). Using 100 denier (10&nbsp;mg/m) leaves more fabric in the outer layers where it is needed, because there are more threads in the weave.

Each rover used four airbags with six lobes each, all of which were connected. Connection was important, since it helped abate some of the landing forces by keeping the bag system flexible and responsive to ground pressure. The airbags were not attached directly to the rover, but were held to it by ropes crisscrossing the bag structure. The ropes gave the bags shape, making inflation easier. While in flight, the bags were stowed along with three gas generators that are used for inflation.<ref>{{cite web|title=Mars Exploration Rovers - How to Land Softly on a Hard Planet|url=https://mars.nasa.gov/mer/spotlight/airbags01.html|website=NASA
|access-date=March 9, 2023}}</ref>

===Lander===
[[File:Opportunity Lander Petals PIA04848.jpg|thumb|left|MER lander petals opening]]

The spacecraft lander is a protective shell that houses the rover, and together with the airbags, protects it from the forces of impact.

The lander is a [[tetrahedron]] shape, whose sides open like petals. It is strong and light, and made of beams and sheets. The beams consist of layers of [[graphite]] fiber woven into a fabric that is lighter than aluminium and more rigid than steel. Titanium fittings are glued and fitted onto the beams to allow it to be bolted together. The rover was held inside the lander by [[Screw|bolts]] and special nuts that were released after landing with small explosives.

====Uprighting====
After the lander stopped bouncing and rolling on the ground, it came to rest on the base of the tetrahedron or one of its sides. The sides then opened to make the base horizontal and the rover upright. The sides are connected to the base by hinges, each of which has a motor strong enough to lift the lander. The rover plus lander has a [[mass]] of about {{convert|533|kg|lbs}}. The rover alone has a mass of about {{convert|185|kg|lb|abbr=on}}. The gravity on Mars is about 38% of Earth's, so the motor does not need to be as powerful as it would on Earth.

The rover contains [[accelerometer]]s to detect which way is down (toward the surface of Mars) by measuring the pull of gravity. The rover computer then commanded the correct lander petal to open to place the rover upright. Once the base petal was down and the rover was upright, the other two petals were opened.

The petals initially opened to an equally flat position, so all sides of the lander were straight and level. The petal motors are strong enough so that if two of the petals come to rest on rocks, the base with the rover would be held in place like a bridge above the ground. The base will hold at a level even with the height of the petals resting on rocks, making a straight flat surface throughout the length of the open, flattened lander. The flight team on Earth could then send commands to the rover to adjust the petals and create a safe path for the rover to drive off the lander and onto the Martian surface without dropping off a steep rock.

====Moving the payload onto Mars====
[[File:MER Spirit Lander Pan Sol16-A18R1 br2.jpg|thumb|''Spirit'''s lander on Mars]]
The moving of the rover off the lander is called the egress phase of the mission. The rover must avoid having its wheels caught in the airbag material or falling off a sharp incline. To help this, a retraction system on the petals slowly drags the airbags toward the lander before the petals open. Small ramps on the petals fan out to fill spaces between the petals. They cover uneven terrain, rock obstacles, and airbag material, and form a circular area from which the rover can drive off in more directions. They also lower the step that the rover must climb down. They are nicknamed "batwings", and are made of Vectran cloth.

About three hours were allotted to retract the airbags and deploy the lander petals.

==Rover design==
[[File:Mars Exploration Rover.stl|thumb|Interactive 3D model of the MER]]
[[File:Mars Exploration Rover.svg|thumb|Schematic drawing of the MER]]
The rovers are six-wheeled, solar-powered robots that stand {{convert|1.5|m|ft|abbr=on}} high, {{convert|2.3|m|ft|abbr=on}} wide and {{convert|1.6|m|ft|abbr=on}} long. They weigh {{convert|180|kg|lb|abbr=on}}, {{convert|35|kg|lb|abbr=on}} of which is the wheel and suspension system.<ref>{{cite web| url = http://hobbiton.thisside.net/rovermanual/| archive-url = https://web.archive.org/web/20040716083742/http://hobbiton.thisside.net/rovermanual/| archive-date = July 16, 2004| title = MER Technical Data | access-date = July 15, 2007}}</ref>

The main box-like chassis forms the Warm Electronics Box (WEB).

===Drive system===
Each rover has six aluminium wheels mounted on a [[rocker-bogie]] suspension system, similar to that on [[Sojourner_(rover)|Sojourner]],<ref>{{Cite web|url=https://mars.nasa.gov/mer/mission/rover/wheels-and-legs/|title=The Rover's Wheels - NASA Mars|date=August 6, 2019|archive-url=https://web.archive.org/web/20190806220821/https://mars.nasa.gov/mer/mission/rover/wheels-and-legs/|archive-date=August 6, 2019}}</ref> that ensures wheels remain on the ground while driving over rough terrain. The design reduces the range of motion of the rover body by half, and allows the rover to go over obstacles or through holes (depressions) that are more than a wheel diameter ({{convert|250|mm|in|sp=us}}) in size. The rover wheels are designed with integral compliant [[flexure]]s which provide shock absorption during movement.<ref>{{cite web|url=http://mars.nasa.gov/mer/spotlight/wheels01.html |title= Wheels in the Sky |website=NASA Jet Propulsion Laboratory |access-date=February 14, 2017}}</ref> Additionally, the wheels have cleats which provide grip for climbing in soft sand and scrambling over rocks.

Each wheel has its own drive motor. The two front and two rear wheels each have individual steering motors. This allows the vehicle to turn in place, a full revolution, and to swerve and curve, making arcing turns. The motors for the rovers have been designed by the Swiss company [[Maxon Motor]].<ref>{{cite web|url=http://storkdrives.com/2017/07/nasa-relies-maxon-technology/|title=Once again, NASA relies on maxon technology|website=Maxon Motor|access-date=February 14, 2019|archive-date=February 14, 2019|archive-url=https://web.archive.org/web/20190214174512/http://storkdrives.com/2017/07/nasa-relies-maxon-technology/}}</ref> The rover is designed to withstand a tilt of 45 degrees in any direction without overturning. However, the rover is programmed through its "fault protection limits" in its hazard avoidance software to avoid exceeding tilts of 30 degrees.

Each rover can spin one of its front wheels in place to grind deep into the terrain. It is to remain motionless while the digging wheel is spinning. The rovers have a top speed on flat hard ground of 50&nbsp;mm/s (2 in/s). The average speed is 10&nbsp;mm/s, because its hazard avoidance software causes it to stop every 10 seconds for 20 seconds to observe and understand the terrain into which it has driven.

===Power and electronic systems===
{{See also|Comparison of embedded computer systems on board the Mars rovers}}
[[File:MER vs. Sojourner PIA04827.jpg|thumb|left|200px|Mars Exploration Rover (rear) and [[Mars Pathfinder|''Sojourner'']] rover]]

When fully illuminated, the rover [[Multijunction photovoltaic cell|triplejunction]]<ref>{{Cite journal| doi = 10.1016/S0094-5765(02)00287-4| title = The performance of gallium arsenide/germanium solar cells at the Martian surface  | year = 2004 | author1 = D. Crisp | author2 = A. Pathare | author3 = R. C. Ewell | journal = Progress in Photovoltaics: Research and Applications | volume = 54 | issue = 2 | pages = 83–101 | bibcode=2004AcAau..54...83C}}</ref>  solar arrays generate about 140 watts for up to four hours per Martian day ([[Timekeeping on Mars#Sols|sol]]). The rover needs about 100 watts to drive. Its power system includes two rechargeable [[lithium ion]] batteries weighing {{convert|7.15|kg|lb|abbr=on}} each, that provide energy when the sun is not shining, especially at night. Over time, the batteries will degrade and will not be able to recharge to full capacity.

It was thought that by the end of the 90-sol mission, the capability of the solar arrays to generate power would likely be reduced to about 50 watts. This was due to anticipated dust coverage on the solar arrays, and the change in season. Over three Earth years later, however, the rovers' power supplies hovered between 300 [[watt-hour]]s and 900 watt-hours per day, depending on dust coverage. Cleaning events (dust removal by wind) have occurred more often than NASA expected, keeping the arrays relatively free of dust and extending the life of the mission. During a 2007 global dust storm on Mars, both rovers experienced some of the lowest power of the mission; ''Opportunity'' dipped to 128 watt-hours. In November 2008, ''Spirit'' had overtaken this low-energy record with a production of 89 watt-hours, due to dust storms in the region of Gusev crater.<ref>{{Cite news|url=https://news.bbc.co.uk/2/hi/science/nature/6909656.stm|access-date=July 22, 2007|title=Mars dust storms threaten rovers | work=BBC News | date=July 21, 2007}}</ref>

The rovers run a [[VxWorks]] [[embedded operating system]] on a [[Radiation hardened|radiation-hardened]] 20 [[megahertz|MHz]] [[RAD6000]] [[central processing unit|CPU]] with 128 [[megabyte|MB]] of [[DRAM]] with error detection and correction and 3 MB of [[EEPROM]].<ref>{{cite web |title=The Rover's "Brains" - NASA |url=https://mars.nasa.gov/mer/mission/rover/brains/ |website=mars.nasa.gov |access-date=23 April 2023}}</ref> Each rover also has 256 MB of [[flash memory]]. To survive during the various mission phases, the rover's vital instruments must stay within a temperature of −40&nbsp;°C to +40&nbsp;°C (−40&nbsp;°F to 104&nbsp;°F). At night, the rovers are heated by eight [[radioisotope heater units]] (RHU), which each continuously generate 1 [[watt|W]] of thermal energy from the decay of [[radioisotope]]s, along with electrical heaters that operate only when necessary. A sputtered gold film and a layer of [[silica]] [[aerogel]] are used for insulation.<ref>{{cite web |title=The Rover's Temperature Controls - NASA |url=https://mars.nasa.gov/mer/mission/rover/temperature/ |website=mars.nasa.gov |access-date=23 April 2023}}</ref>

===Communication===
[[File:MER white cyc ANTENNA LABELS.jpg|thumb|MER antennas]]

The rover has an [[X band]] [[low-gain antenna|low-gain]] and an X band [[high-gain antenna]] for communications to and from the Earth, as well as an [[ultra high frequency]] monopole antenna for relay communications. The low-gain antenna is [[Omnidirectional antenna|omnidirectional]], and transmits data at a low rate to [[Deep Space Network]] antennas on Earth. The high-gain antenna is directional and steerable, and can transmit data to Earth at a higher rate. The rovers use the UHF monopole and its CE505 radio to communicate with spacecraft orbiting Mars, the [[Mars Odyssey]] and (before its failure) the [[Mars Global Surveyor]] (already more than 7.6 [[terabit]]s of data were transferred using its Mars Relay antenna and [[Mars Orbiter Camera]]'s memory buffer of 12 MB).<ref name="malin">{{cite journal|first1=Michael C. |last1=Malin |first2=Kenneth S. |last2=Edgett |first3=Bruce A. |last3=Cantor |first4=Michael A. |last4=Caplinger |author5=G. Edward Danielson |first6=Elsa H. |last6=Jensen |first7=Michael A. |last7=Ravine |first8=Jennifer L. |last8=Sandoval |first9=Kimberley D. |last9=Supulver |title=An overview of the 1985–2006 Mars Orbiter Camera science investigation|journal=Mars - the International Journal of Mars Science and Exploration|date=January 6, 2010|volume=5|pages=1–60|doi=10.1555/mars.2010.0001|bibcode = 2010IJMSE...5....1M |s2cid=128873687 }}</ref> Since MRO went into orbit around Mars, the landers have also used it as a relay asset. Most of the lander data is relayed to Earth through Odyssey and MRO. The orbiters can receive rover signals at a much higher data rate than the Deep Space Network can, due to the much shorter distances from rover to orbiter. The orbiters then quickly relay the rover data to the Earth using their large and [[Signal strength|high-powered]] antennas.

===Scientific instrumentation===
[[File:MER Pancam.JPG|thumb|right|Pancam Mast Assembly (PMA)]]

Each rover has nine cameras,<ref name="cameras">{{cite web |title=The Rover's "Eyes" and Other "Senses" - NASA |url=https://mars.nasa.gov/mer/mission/rover/eyes-and-senses/ |website=mars.nasa.gov |access-date=23 April 2023}} {{PD-notice}}</ref> which produce 1024-pixel by 1024-pixel images at 12 bits per pixel,<ref>{{Cite web|url=http://tmo.jpl.nasa.gov/progress_report/42-155/155J.pdf|title=The IECR Progressive Wavelet Image Compressor|date=2003-11-15|website=tmo.jpl.nasa.gov |archive-url=https://web.archive.org/web/20050123032851/http://tmo.jpl.nasa.gov/progress_report/42-155/155J.pdf |archive-date=2005-01-23 |first1 = A |first2=M |last1=Kiely |last2=Klimesh}}</ref> but most navigation camera images and image thumbnails are truncated to 8 bits per pixel to conserve memory and transmission time. All images are then compressed using [[ICER (file format)|ICER]] before being stored and sent to Earth. Navigation, thumbnail, and many other image types are compressed to approximately 0.8 to 1.1 bits/pixel. Lower bit rates (less than 0.5-bit/pixel) are used for certain wavelengths of multi-color panoramic images. ICER is based on [[wavelets]], and was designed specifically for deep-space applications. It produces progressive compression, both lossless and lossy, and incorporates an error-containment scheme to limit the effects of data loss on the deep-space channel. It outperforms the lossy JPEG image compressor and the lossless Rice compressor used by the [[Mars Pathfinder]] mission.

Several cameras and instruments were mounted on the Pancam Mast Assembly (PMA):
* Panoramic Cameras ([[Pancam]]), two cameras with color filter wheels for determining the texture, color, [[mineralogy]], and structure of the local terrain. Two cameras work in combination to take detailed, multi-wavelength, 3-D panoramic pictures of the Martian landscape surrounding the rover, sky and the Sun. There are 30 cm between two cameras. Each camera had filters for 400-1100 nm (near-UV to near-IR), with maximum image resolution of 1024 x 1024 pixels.<ref name="pancam">{{cite web |title=The Panoramic Camera (Pancam) - NASA |url=https://mars.nasa.gov/mer/mission/instruments/pancam/ |website=mars.nasa.gov |access-date=20 April 2023}} {{PD-notice}}</ref><ref name="cameras"/>
* Navigation Cameras ([[Navcam]]), two cameras that have larger fields of view but lower resolution and are monochromatic, for navigation and driving.<ref name="cameras"/>
* A periscope assembly for the Miniature Thermal Emission Spectrometer ([[Mini-TES]]), an [[infrared spectrometer]] used to determine the mineralogy of rocks and soils from a distance by detecting their patterns of thermal radiation. It was specially tuned to look for minerals formed in water, like carbonates and clays.<ref>{{cite web |title=Miniature Thermal Emission Spectrometer (Mini-TES) - NASA |url=https://mars.nasa.gov/mer/mission/instruments/mini-tes/ |website=mars.nasa.gov |access-date=20 April 2023}} {{PD-notice}}</ref> The periscope assembly features two beryllium fold mirrors, a shroud that closes to minimize dust contamination in the assembly, and stray-light rejection baffles that are strategically placed within the graphite epoxy tubes. The Mini-TES was built by [[Arizona State University]].

The cameras were mounted 1.5 meters high on the Pancam Mast Assembly, which allows 360-degree rotation.<ref name="pancam"/> The PMA is deployed via the Mast Deployment Drive (MDD). The Azimuth Drive, mounted directly above the MDD, turned the assembly horizontally a whole revolution with signals transmitted through a rolling tape configuration. The camera drive points the cameras in elevation, almost straight up or down. A third motor points the Mini-TES fold mirrors and protective shroud, up to 30° above the horizon and 50° below. <!--The PMA's conceptual design was done by Jason Suchman at JPL, the Cognizant Engineer who later served as Contract Technical Manager (CTM) once the assembly was built by [[Ball Aerospace & Technologies Corp.]], [[Boulder, Colorado]]. Raul Romero served as CTM once subsystem-level testing began. Satish Krishnan did the conceptual design of the High-Gain Antenna Gimbal (HGAG), whose detailed design, assembly, and test was also performed by Ball Aerospace at which point Satish acted as the CTM.-->

Four monochromatic Hazard Avoidance Cameras ([[Hazcam]]s) were mounted on the rover's body, two in front and two behind. The rover used pairs of Hazcam images to map out the shape of the terrain as far as 3 meters (10 feet) in front of it, in a "wedge" shape that is over 4 meters wide at the farthest distance.<ref name="cameras"/>

{{Multiple image
| align = right
| total_width = 400
| perrow = 2,2
| image1 = MER arm with instruments.jpg
| caption1 = Instruments on the rover's arm
| image2 = MER Rock Abrasion Tool.JPG
| caption2 =Rock Abrasion Tool (RAT)
| image3 = MER APXS PIA05113.jpg
| caption3 =[[Alpha particle X-ray spectrometer]] (APXS)
| image4 = PIA23248-MarsOpportunityRover-BeforeAndAfterProcessingImage-20190604.jpg
| caption4 = MER photo before and after processing
}}

The instrument deployment device (IDD), also called the rover arm. The arm has a turret shaped like a cross that holds several instruments. The arm can spin through a 350-degree turning range and rotate vertically through 340-degree range. The arm has three joint and a maximum reach of 90 centimeters.<ref>{{cite web |title=The Rover's "Arm" - NASA |url=https://mars.nasa.gov/mer/mission/rover/arm/ |website=mars.nasa.gov |access-date=20 April 2023}} {{PD-notice}}</ref>
* [[Mössbauer spectrometer]] (MB) [[MIMOS II]], developed by Göstar Klingelhöfer at the [[Johannes Gutenberg]] University in [[Mainz]], [[Germany]], was used for close-up investigations of the mineralogy of iron-bearing rocks and soils.<ref>{{Cite journal
|title= The miniaturized Mössbauer spectrometer MIMOS II for extraterrestrial and outdoor terrestrial applications: A status report
|author1=Klingelhöfer G. |author2=Bernhardt But. |author3=Foh J. |author4=Bonnes U. |author5=Rodionov D. |author6=De Souza P. A. |author7=Schroder C. |author8=Gellert R. |author9=Kane S. |author10=Gutlich P. |author11=Kankeleit E. |journal= Hyperfine Interactions|volume=144|issue=1 |pages= 371–379|year= 2002|doi=10.1023/A:1025444209059 |bibcode = 2002HyInt.144..371K |s2cid=94640811 }}</ref><ref>{{cite web |url=http://www.lpi.usra.edu/meetings/phobosdeimos2007/pdf/7038.pdf |title=THE MINIATURISED MOESSBAUER SPECTROMETER MIMOS II: APPLICATION FOR THE "PHOBOS-GRUNT" MISSION. |author=Klingelhoefer|year=2007 |display-authors=etal}}</ref> One Mössbauer measurement took about 12 hours.<ref>{{cite web |title=Mössbauer Spectrometer (MB) - NASA |url=https://mars.nasa.gov/mer/mission/instruments/mb/ |website=mars.nasa.gov |access-date=20 April 2023}} {{PD-notice}}</ref>
* [[Alpha particle X-ray spectrometer]] (APXS), developed by the [[Max Planck Institute for Chemistry]] in [[Mainz]], [[Germany]], was used to determines the elemental chemistry of rocks and soils using alpha particles and X-rays.<ref name="apxs">{{cite web |title=Alpha Particle X-Ray Spectrometer (APXS) - NASA |url=https://mars.nasa.gov/mer/mission/instruments/apxs/ |website=mars.nasa.gov |access-date=20 April 2023}} {{PD-notice}}</ref><ref>{{Cite journal|title=The new Athena alpha particle X-ray spectrometer for the Mars Exploration Rovers|author1=R. Rieder |author2=R. Gellert |author3=J. Brückner |author4=G. Klingelhöfer |author5=G. Dreibus |author6=A. Yen |author7=S. W. Squyres |journal= Journal of Geophysical Research|year= 2003|volume= 108|issue=E12 |page= 8066| doi=10.1029/2003JE002150|bibcode = 2003JGRE..108.8066R |doi-access=free }}</ref> Most APXS measurements were taken at night and required at least 10 hours of accumulation time.<ref name="apxs"/>
* Magnet array were used for collecting magnetic dust particles.<ref>{{Cite web|url=https://mars.nasa.gov/mer/mission/overview/|title=Overview|website=mars.nasa.gov |date=March 22, 2024 }}</ref> <!--developed by [[Jens Martin Knudsen]]'s group at the [[Niels Bohr Institute for Astronomy, Physics and Geophysics|Niels Bohr Institute]], [[Copenhagen]].--> There were seven magnets on each rover: four magnets were carried by the Rock Abrasion Tool (RAT); two magnets (one capture magnet and one filter magnet) were mounted on the front of the rover; another magnet (sweep magnet) was mounted on the top of the rover deck in view of the Pancam.<ref>{{cite web |title=Magnet Array - NASA |url=https://mars.nasa.gov/mer/mission/instruments/magnet-array/ |website=mars.nasa.gov |access-date=20 April 2023}} {{PD-notice}}</ref> The particles were analyzed by the Mössbauer Spectrometer and [[X-ray]] Spectrometer to help determine the ratio of magnetic particles to non-magnetic particles and the composition of magnetic minerals in airborne dust and rocks that have been ground by the Rock Abrasion Tool.
* Microscopic Imager (MI) for obtaining close-up, black-and-white high-resolution images of rocks and soils, with maximum resolution of 1024 x 1024 pixels. Development was led by Ken Herkenhoff's team at the [[U.S. Geological Survey]] [[Astrogeology Research Program]]. The Microscopic Imager is a combination of a microscope and a CCD camera.<ref>{{cite web |title=Microscopic Imager (MI) - NASA |url=https://mars.nasa.gov/mer/mission/instruments/mi/ |website=mars.nasa.gov |access-date=20 April 2023}} {{PD-notice}}</ref>
* [[Rock Abrasion Tool]] (RAT), developed by [[Honeybee Robotics]], for removing dusty and weathered rock surfaces and exposing fresh material for examination by instruments on board. It was able to create a hole around 2 inches (45 millimeters) in diameter and 0.2 inch (5 millimeters) deep in a rock, and able to grind through hard volcanic rock in about two hours. RAT used three electric motors to drive rotating grinding teeth into the surface of a rock. Two grinding wheels rotate at high speeds. These wheels also rotate around each other at a much slower speed so that the two grinding wheels sweep the entire cutting area.<ref>{{cite web |title=Rock Abrasion Tool (RAT) - NASA |url=https://mars.nasa.gov/mer/mission/instruments/rat/ |website=mars.nasa.gov |access-date=20 April 2023}} {{PD-notice}}</ref>

The robotic arm was able to place instruments directly up against rock and soil targets of interest.

==Naming of ''Spirit'' and ''Opportunity''==
[[File:Sofi Collis MER.jpg|thumb|Sofi Collis with a model of Mars Exploration Rover]]
The ''Spirit'' and ''Opportunity'' rovers were named through a student essay competition. The winning entry was by Sofi Collis,<ref>[http://marsprogram.jpl.nasa.gov/mer/newsroom/pressreleases/20030608a.html Girl With Dreams Names Mars Rovers 'Spirit' and 'Opportunity']</ref> a third-grade Russian-American student from Arizona.

<blockquote>
I used to live in an orphanage. It was dark and cold and lonely. At night, I looked up at the sparkly sky and felt better. I dreamed I could fly there. In America, I can make all my dreams come true. Thank you for the 'Spirit' and the 'Opportunity.'<br />
— Sofi Collis, age 9
</blockquote>

Prior to this, during the development and building of the rovers, they were known as MER-1 Rover 1 (''Opportunity'') and MER-2 Rover 2 (''Spirit''). Internally, NASA also uses the mission designations MER-A (''Spirit'') and MER-B (''Opportunity'') based on the order of landing on Mars.

==Test rovers==
[[File:Rover testing.jpg|thumb|Rover team members simulate ''Spirit'' in a Martian sandtrap.]]
The Jet Propulsion Laboratory maintains a pair of rovers, the ''Surface System Test-Beds'' (SSTB) at its location in Pasadena for testing and modeling of situations on Mars. One test rover, SSTB1, weighing approximately {{convert|180|kg|abbr=on}}, is fully instrumented and nearly identical to ''Spirit'' and ''Opportunity''. Another test version, ''SSTB-Lite'', is identical in size and drive characteristics but does not include all instruments. It weighs {{convert|80|kg|abbr=on}}, much closer to the weight of ''Spirit'' and ''Opportunity'' in the reduced [[gravity of Mars]]. These rovers were used in 2009 for a simulation of the incident in which ''Spirit'' became trapped in soft soil.<ref name="JPL Spirit free testing">{{Cite news|url=http://www.jpl.nasa.gov/news/news.cfm?release=2009-087|title=Mars and Earth Activities Aim to Get Spirit Rolling Again|date=May 18, 2009|access-date=January 22, 2010|archive-date=August 13, 2009|archive-url=https://web.archive.org/web/20090813173623/http://jpl.nasa.gov/news/news.cfm?release=2009-087}}</ref><ref name=UniverseToday-2009-07-02>{{cite web|url= http://www.universetoday.com/33867/test-bed-rover-is-now-stuck-which-is-a-good-thing/ |title= Test-Bed Rover is Now Stuck — Which is a Good Thing! |first= Nancy |last= Atkinson |date= July 2, 2009 |access-date= March 14, 2014}}</ref><ref name=SpiritMERupdateNASA>{{cite web |url= http://www.nasa.gov/mission_pages/mer/spirit-update.html |title= Spirit Mission Manager Reports |author= NASA |access-date= March 14, 2014 }}</ref>

==Planetary science findings==
{{See also|Scientific information from the Mars Exploration Rover mission}}

===''Spirit'' Landing Site, Gusev Crater===

====Plains====
Although the Gusev crater appears from orbital images to be a dry lakebed, the observations from the surface show the interior plains mostly filled with debris. The rocks on the plains of Gusev are a type of [[basalt]]. They contain the minerals [[olivine]], [[pyroxene]], [[plagioclase]], and magnetite, and they look like volcanic basalt as they are fine-grained with irregular holes (geologists would say they have vesicles and vugs).<ref name="McSween 2004">McSween, etal. 2004. "Basaltic Rocks Analyzed by the Spirit Rover in Gusev Crater". ''Science'' : 305. 842-845</ref><ref name="Arvidson 2004">{{cite journal | author = Arvidson R. E.|display-authors=etal| year = 2004 | title =  Localization and Physical Properties Experiments Conducted by Spirit at Gusev Crater| journal = Science | volume = 305 | issue = 5685| pages = 821–824 | doi=10.1126/science.1099922 | pmid=15297662| bibcode =2004Sci...305..821A|s2cid=31102951}}</ref>
Much of the soil on the plains came from the breakdown of the local rocks. Fairly high levels of [[nickel]] were found in some soils; probably from [[meteorites]].<ref>{{cite journal | author = Gelbert R.|display-authors=etal| year = 2006 | title = The Alpha Particle X-ray Spectrometer (APXS): results from Gusev crater and calibration report | journal = J. Geophys. Res. Planets | volume = 111 | issue = E2|pages=E02S05|article-number=2005JE002555 |doi=10.1029/2005JE002555|bibcode=2006JGRE..111.2S05G| doi-access = free| hdl = 2060/20080026124| hdl-access = free}}</ref>
Analysis shows that the rocks have been slightly altered by tiny amounts of water. Outside coatings and cracks inside the rocks suggest water deposited minerals, maybe [[bromine]] compounds. All the rocks contain a fine coating of dust and one or more harder rinds of material. One type can be brushed off, while another needed to be ground off by the [[Rock Abrasion Tool]] (RAT).<ref>{{cite journal | author = Christensen P | title = Initial Results from the Mini-TES Experiment in Gusev Crater from the Spirit Rover | journal = Science | volume = 305 | issue = 5685| pages = 837–842 | doi=10.1126/science.1100564 | pmid=15297667 | date=August 2004| bibcode=2004Sci...305..837C| s2cid = 34983664 }}</ref>

====Dust====
The dust in Gusev Crater is the same as dust all around the planet. All the dust was found to be magnetic. Moreover, ''Spirit'' found the [[magnetism]] was caused by the mineral [[magnetite]], especially magnetite that contained the element [[titanium]]. One magnet was able to completely divert all dust hence all Martian dust is thought to be magnetic.<ref>{{cite journal | author = Bertelsen P. | year = 2004 | title = Magnetic Properties on the Mars Exploration Rover Spirit at Gusev Crater | journal = Science | volume = 305 | issue = 5685| pages = 827–829 | doi=10.1126/science.1100112 | pmid=15297664|display-authors=etal| bibcode=2004Sci...305..827B| s2cid = 41811443 }}</ref>  The spectra of the dust was similar to spectra of bright, low thermal inertia regions like [[Tharsis]] and Arabia that have been detected by orbiting satellites. A thin layer of dust, maybe less than one millimeter thick covers all surfaces. Something in it contains a small amount of chemically bound water.<ref>Bell, J (ed.)  ''The Martian Surface''. 2008. Cambridge University Press. {{ISBN|978-0-521-86698-9}}</ref><ref>{{cite journal | author = Gelbert R. | year = 2004| title = Chemistry of Rocks and Soils in Gusev Crater from the Alpha Particle X-ray Spectrometer | journal = Science | volume = 305 | issue = 5685| pages = 829–32| doi=10.1126/science.1099913|display-authors=etal | pmid=15297665| bibcode=2004Sci...305..829G| s2cid = 30195269}}</ref>

====Columbia Hills====
[[File:MER STS-107 Memorial.JPG|thumb|''Spirit'' contains a memorial to the crew of the [[Space Shuttle Columbia|Space Shuttle ''Columbia'']]'s [[STS-107]] 2003 mission, which disintegrated during reentry.]]  
As the rover climbed above the plains onto the Columbia Hills, the mineralogy that was seen changed.<ref>{{cite journal | author = Arvidson R. | year = 2006 | title = Overview of the Spirit Mars Exploration Rover Mission to Gusev Crater: Landing Site to Backstay Rock in the Columbia Hills | url =https://authors.library.caltech.edu/50489/1/jgre2029.pdf | journal = Journal of Geophysical Research | volume = 111 | issue = E2| page = E02S01 | doi = 10.1029/2005je002499 |display-authors=etal | article-number = 2005JE002499 | bibcode=2006JGRE..111.2S01A| hdl = 2060/20080026038 | doi-access = free }}</ref><ref>{{cite journal | author = Crumpler L. | year = 2005 | title = Mars Exploration Rover Geologic Traverse by the Spirit Rover in the Plains of Gusev Crater, Mars | journal = Geology | volume = 33 | issue = 10| pages = 809–812 | doi=10.1130/g21673.1|display-authors=etal| bibcode = 2005Geo....33..809C}}</ref>  Scientists found a variety of rock types in the Columbia Hills, and they placed them into six different categories. The six are: Clovis, Wishbone, Peace, Watchtower, Backstay, and Independence. They are named after a prominent rock in each group. Their chemical compositions, as measured by APXS, are significantly different from each other.<ref>{{cite journal | author = Squyres S. | year = 2006 | title = Rocks of the Columbia Hills | journal = J. Geophys. Res. Planets | volume = 111 | issue = E2| pages = n/a |display-authors=etal| article-number = 2005JE002562 | doi = 10.1029/2005JE002562 | bibcode = 2006JGRE..111.2S11S }}</ref>  Most importantly, all of the rocks in Columbia Hills show various degrees of alteration due to aqueous fluids.<ref>{{cite journal | author = Ming D. | year = 2006 | title = Geochemical and mineralogical indicators for aqueous processes in the Columbia Hills of Gusev crater, Mars | journal = J. Geophys. Res. | volume = 111 | issue = E2| pages = n/a | doi=10.1029/2005je002560 | bibcode=2006JGRE..111.2S12M|display-authors=etal| article-number = 2005JE002560 | hdl = 1893/17114 | hdl-access = free }}</ref>
They are enriched in the elements phosphorus, sulfur, chlorine, and bromine—all of which can be carried around in water solutions. The Columbia Hills' rocks contain basaltic glass, along with varying amounts of olivine and [[sulfate]]s.<ref name="Schroder 2005">Schroder, C., et al. (2005) European Geosciences Union, General Assembly, Geophysical Research abstr., Vol. 7, 10254, 2005</ref><ref>{{cite web |last=Christensen |first=P.R. |year=2005 |title=Mineral Composition and Abundance of the Rocks and Soils at Gusev and Meridiani from the Mars Exploration Rover Mini-TES Instruments AGU Joint Assembly, 23-27 May 2005 |url=http://www.agu.org/meetings/sm05/waissm05.html |publisher=Agu.org |access-date=January 16, 2012 |archive-date=May 13, 2013 |archive-url=https://web.archive.org/web/20130513050221/http://www.agu.org/meetings/sm05/waissm05.html }}</ref>
The olivine abundance varies inversely with the amount of sulfates. This is exactly what is expected because water destroys olivine but helps to produce sulfates.

The Clovis group is especially interesting because the [[Mössbauer spectrometer]] (MB) detected [[goethite]] in it.<ref>Klingelhofer, G., et al. (2005) Lunar Planet. Sci. XXXVI abstr. 2349</ref> Goethite forms only in the presence of water, so its discovery is the first direct evidence of past water in the Columbia Hills's rocks. In addition, the MB spectra of rocks and outcrops displayed a strong decline in olivine presence,<ref name="Schroder 2005" />
although the rocks probably once contained much olivine.<ref>Morris, S., et al. Mossbauer mineralogy of rock, soil, and dust at Gusev crater, Mars: Spirit's journal through weakly altered olivine basalt on the plains and pervasively altered basalt in the Columbia Hills. J. Geophys. Res.: 111</ref>  Olivine is a marker for the lack of water because it easily decomposes in the presence of water. Sulfate was found, and it needs water to form.
Wishstone contained a great deal of plagioclase, some olivine, and [[anhydrate]] (a sulfate). Peace rocks showed [[sulfur]] and strong evidence for bound water, so hydrated sulfates are suspected. Watchtower class rocks lack olivine consequently they may have been altered by water. The Independence class showed some signs of clay (perhaps montmorillonite a member of the smectite group). Clays require fairly long term exposure to water to form.
One type of soil, called Paso Robles, from the Columbia Hills, may be an evaporate deposit because it contains large amounts of sulfur, [[phosphorus]], [[calcium]], and iron.<ref>Ming, D., et al. 2006 Geochemical and mineralogical indicators for aqueous processes in the Columbia Hills of Gusev crater, Mars. J. Geophys. Res.111</ref>
Also, MB found that much of the iron in Paso Robles soil was of the oxidized, Fe<sup>3+</sup> form.
Towards the middle of the six-year mission (a mission that was supposed to last only 90 days), large amounts of pure [[silica]] were found in the soil. The silica could have come from the interaction of soil with acid vapors produced by volcanic activity in the presence of water or from water in a hot spring environment.<ref name="nasa4">{{cite web |url=http://www.nasa.gov/mission_pages/mer/mer-20070521.html |title=NASA - Mars Rover Spirit Unearths Surprise Evidence of Wetter Past |publisher=nasa.gov |access-date=May 25, 2015 |archive-date=March 8, 2013 |archive-url=https://web.archive.org/web/20130308054606/http://www.nasa.gov/mission_pages/mer/mer-20070521.html }}</ref>

After ''Spirit'' stopped working scientists studied old data from the Miniature Thermal Emission Spectrometer, or [[Mini-TES]] and confirmed the presence of large amounts of [[carbonate]]-rich rocks, which means that regions of the planet may have once harbored water. The carbonates were discovered in an outcrop of rocks called "Comanche."<ref>{{cite journal|url=https://www.sciencedaily.com/releases/2010/06/100603140959.htm |title=Outcrop of long-sought rare rock on Mars found |journal=Science |volume=329 |issue=5990 |pages=421–424 |doi=10.1126/science.1189667 |pmid=20522738 |date=June 4, 2010 |access-date=October 25, 2012|bibcode=2010Sci...329..421M |last1=Morris |first1=R. V. |last2=Ruff |first2=S. W. |last3=Gellert |first3=R. |last4=Ming |first4=D. W. |last5=Arvidson |first5=R. E. |last6=Clark |first6=B. C. |last7=Golden |first7=D. C. |last8=Siebach |first8=K. |last9=Klingelhofer |first9=G. |last10=Schroder |first10=C. |last11=Fleischer |first11=I. |last12=Yen |first12=A. S. |last13=Squyres |first13=S. W. |s2cid=7461676 |doi-access=free |url-access=subscription }}</ref><ref>{{cite journal |author1=Morris Richard V. |author2=Ruff Steven W. |author3=Gellert Ralf |author4=Ming Douglas W. |author5=Arvidson Raymond E. |author6=Clark Benton C. |author7=Golden D. C. |author8=Siebach Kirsten |author9=Klingelhöfer Göstar |author10=Schröder Christian |author11=Fleischer Iris |author12=Yen Albert S. |author13=Squyres Steven W. | year = 2010 | title = Identification of Carbonate-Rich Outcrops on Mars by the Spirit Rover | journal = Science | volume =  329| issue = 5990| pages =  421–424| doi = 10.1126/science.1189667 | pmid=20522738|bibcode=2010Sci...329..421M |s2cid=7461676 |doi-access=free }}</ref>

''Spirit'' found evidence of slight weathering on the plains of Gusev, but no evidence that a lake was there. However, in the Columbia Hills there was clear evidence for a moderate amount of aqueous weathering. The evidence included sulfates and the minerals goethite and carbonates which only form in the presence of water. It is believed that Gusev crater may have held a lake long ago, but it has since been covered by igneous materials. All the dust contains a magnetic component which was identified as magnetite with some titanium. Furthermore, the thin coating of dust that covers everything on Mars is the same in all parts of Mars.

===''Opportunity'' Landing Site, Meridiani Planum===
[[File:PIA17759-MarsOpportunityRover-SelfPortrait-20140106.jpg|thumb|left|Self-portrait of [[Opportunity (rover)|''Opportunity'']] near [[Endeavour (crater)|Endeavour Crater]] on the surface of [[Mars]] (January 6, 2014).]]
[[File:PIA21497capetrib.jpg|thumb|[[Cape Tribulation (Mars)|Cape Tribulation]] southern end, as seen in 2017 by ''Opportunity'' rover]]
The ''Opportunity'' rover landed in a small crater, dubbed "Eagle", on the flat plains of Meridiani. The plains of the landing site were characterized by the presence of a large number of small [[Martian spherules|spherules]], spherical [[concretion]]s that were tagged "blueberries" by the science team, which were found both loose on the surface, and also embedded in the rock. These proved to have a high concentration of the mineral [[hematite]], and showed the signature of being formed in an aqueous environment. The layered bedrock revealed in the crater walls showed signs of being sedimentary in nature, and compositional and microscopic-imagery analysis showed this to be primarily with composition of [[Jarosite]], a ferrous sulfate mineral that is characteristically an [[evaporite]] that is the residue from the evaporation of a salty pond or sea.<ref>{{cite journal | author = Squyres S. | year = 2004 | title = The Opportunity Rover's Athena Science Investigation At Meridiani Planum, Mars | journal = Science | volume = 306 | issue = 5702| pages = 1698–1703 | doi=10.1126/science.1106171 | pmid=15576602|display-authors=etal| bibcode=2004Sci...306.1698S| s2cid = 7876861 }}</ref><ref>{{cite journal | author = Squyres S. | year = 2006 | title = Overview of the Opportunity Mars Exploration Rover Mission to Meridiani Planum: Eagle Crater to Purgatory Dune | journal = Journal of Geophysical Research | volume = 111 | issue = E12| pages = E12S12 | doi=10.1029/2006je002771 | bibcode=2006JGRE..11112S12S|display-authors=etal| hdl = 1893/17165 | hdl-access = free }}</ref>

The mission has provided substantial evidence of past water activity on Mars. In addition to investigating the "water hypothesis", ''Opportunity'' has also obtained astronomical observations and atmospheric data.
The extended mission took the rover across the plains to a series of larger craters in the south, with the arrival at the edge of a 25-km diameter crater, Endeavour Crater, eight years after landing. The orbital spectroscopy of this crater rim show the signs of [[phyllosilicate]] rocks, indicative of older sedimentary deposits.

==See also==
* [[Aeolis quadrangle]]
* [[Syd Lieberman]] (Official storyteller of the Mars Exploration Mission)

==References==
{{Refbegin}}
* Portions of this article are adopted from [http://mars.jpl.nasa.gov/mer/overview/ NASA/JPL MER article].
* Additional information was adapted from [http://marsrovers.jpl.nasa.gov/home/index.html the MER homepage]
{{Refend}}
{{Reflist|colwidth=30em}}

==Further reading==
* ''Roving Mars: Spirit, Opportunity, and the Exploration of the Red Planet'' by [[Steve Squyres]] (published August 2005; {{ISBN|1-4013-0149-5}})
* ''Postcards from Mars: The First Photographer on the Red Planet'' by Jim Bell (published November 2006; {{ISBN|0-525-94985-2}})
* {{cite journal |last1=Squyres |first1=Steven W. |last2=Arvidson |first2=Raymond E. |last3=Baumgartner |first3=Eric T. |last4=Bell |first4=James F. |last5=Christensen |first5=Philip R. |last6=Gorevan |first6=Stephen |last7=Herkenhoff |first7=Kenneth E. |last8=Klingelhöfer |first8=Göstar |last9=Madsen |first9=Morten Bo |last10=Morris |first10=Richard V. |last11=Rieder |first11=Rudolf |last12=Romero |first12=Raul A. |title=Athena Mars rover science investigation |journal=Journal of Geophysical Research: Planets |date=December 2003 |volume=108 |issue=E12 |page=8062 |doi=10.1029/2003JE002121 |bibcode=2003JGRE..108.8062S |language=en |display-authors=3|doi-access=free }}
* [https://web.archive.org/web/20071104030752/http://www-robotics.jpl.nasa.gov/projects/projectPublications.cfm?Project=1 Technical papers by JPL Robotics Engineers]
* [http://www.pcworld.idg.com.au/index.php/id;563338166;pp;1 Interview: The driver behind NASA's Mars Rovers from Australian PC World] {{Webarchive|url=https://web.archive.org/web/20081005013722/http://www.pcworld.idg.com.au/index.php/id;563338166;pp;1 |date=October 5, 2008 }}

==External links==
{{commons category|Mars Exploration Rovers}}
* [http://marsrovers.jpl.nasa.gov/home/index.html NASA JPL's MER website]
* [https://solarsystem.nasa.gov/resources/2370/mars-exploration-rovers-3d-model/ Mars Exploration Rovers 3D Model]
* [https://web.archive.org/web/20070801222207/http://solarsystem.nasa.gov/missions/profile.cfm?MCode=MER_A ''Spirit'' Mission Profile]
* [https://web.archive.org/web/20040106162221/http://www.solarsystem.nasa.gov/missions/profile.cfm?MCode=MER_B ''Opportunity'' Mission Profile]
* [https://web.archive.org/web/20100527044955/http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/12837/1/01-1164.pdf Mars Exploration Rover project, NASA/JPL document NSS ISDC 2001 27 May 2001]
* [http://www.sciencemag.org/content/vol305/issue5685/index.dtl ''Science'', 6 August 2004] - Scientific papers from the first phase of the ''Spirit'' mission
* [https://web.archive.org/web/20160305021707/http://hobbiton.thisside.net/rovermanual/ Mars Rover Manual]: Centralized resource for all publicly released rover technical details
* [http://an.rsl.wustl.edu/ MER Analysts Notebook] (access to the MER scientific data set)
* [http://pancam.sese.asu.edu Official PanCam True Color Images Gallery]
* [https://web.archive.org/web/20110709161905/http://www.lyle.org/~markoff/ Rover image gallery]
* [http://areo.info/mer/ Nonofficial Daily PanCam Color Image Gallery]
* [http://www.planetary.org/explore/space-topics/space-missions/mer-updates/ Archive] of MER progress reports by A.J.S. Rayl at planetary.org

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