View source for Martian spherules

{{Short description| Small iron oxide spherules found on Mars}}
[[File:1M132266947EFF05AMP2987M2M1.Bluberries.inside.Eagle.Crater.Sol.46.jpg|thumb|upright|Loose hematite spherules at [[Eagle (crater)|Eagle]] crater. Spherules are 3–6 mm in diameter.]]
'''Martian spherules''' (also known as '''hematite spherules''' or '''blueberries''') are small spherules (roughly spherical pebbles) that are rich in an iron oxide (grey [[hematite]], α-Fe<sub>2</sub>O<sub>3</sub>) and are found in exceedingly large numbers at [[Meridiani Planum]], a vast Martian plain.

These spherules were discovered on the [[Mars sol|Martian day]] (sol) that NASA's ''[[Opportunity rover|Opportunity]]'' rover, part of the [[Mars Exploration Rover]] program, landed at [[Meridiani Planum]]. They are grey but look bluish next to the ubiquitous rusty reds on Mars, and since the first spherules found in [[Eagle (crater)|Eagle]] crater were 3–6&nbsp;mm in diameter,
the [[Opportunity rover|''Opportunity'']] team quickly called them "blueberries".

Martian blueberries are either embedded or loose. That is, Martian blueberries are either embedded in the large body of sediments of Meridiani Planum, or they are loose blueberries that lie directly on outcrops of the sediments or lie on top soils spread over the Meridiani sediments.<ref name=Soderblom_2004 /><ref name=Grotzinger_2005 /> The size of these spherules varies by location and elevation across the Meridiani Planum.<ref name=Weitz_2006 /><ref name=Calvin_2008 /><ref name=Squyres_2009 />

Martian blueberries are rich in the iron oxide hematite, but determining how rich they are in this iron oxide has proven difficult.<ref name=Klingelhofer_2004 /><ref name=Rieder_2004 /><ref name=Christensen_2004a /><ref name=Squyres_2004 /><ref name=Morris_2006>{{cite journal | doi=10.1029/2006JE002791 | title=Mössbauer mineralogy of rock, soil, and dust at Meridiani Planum, Mars: Opportunity's journey across sulfate-rich outcrop, basaltic sand and dust, and hematite lag deposits | year=2006 | last1=Morris | first1=R. V. | last2=Klingelhöfer | first2=G. | last3=Schröder | first3=C. | last4=Rodionov | first4=D. S. | last5=Yen | first5=A. | last6=Ming | first6=D. W. | last7=De Souza | first7=P. A. | last8=Wdowiak | first8=T. | last9=Fleischer | first9=I. | last10=Gellert | first10=R. | last11=Bernhardt | first11=B. | last12=Bonnes | first12=U. | last13=Cohen | first13=B. A. | last14=Evlanov | first14=E. N. | last15=Foh | first15=J. | last16=Gütlich | first16=P. | last17=Kankeleit | first17=E. | last18=McCoy | first18=T. | last19=Mittlefehldt | first19=D. W. | last20=Renz | first20=F. | last21=Schmidt | first21=M. E. | last22=Zubkov | first22=B. | last23=Squyres | first23=S. W. | last24=Arvidson | first24=R. E. | journal=Journal of Geophysical Research: Planets | volume=111 | issue=E12 | bibcode=2006JGRE..11112S15M | doi-access=free | hdl=1893/17161 | hdl-access=free }}</ref><ref name=Calvin_2008 /><ref name=Olsen_2022>{{cite journal | doi=10.3390/min12060777 | doi-access=free | title=Searching Mass-Balance Analysis to Find the Composition of Martian Blueberries | year=2022 | last1=Olsen | first1=Rif Miles | journal=Minerals | volume=12 | issue=6 | page=777 | bibcode=2022Mine...12..777O }}</ref> (more below). The formation of blueberries required aqueous chemistry and involved flows of acidic, salty, liquid water over the Meridiani Planum and over two geological epochs.<ref name=Hynek_2002>{{cite journal | doi=10.1029/2002JE001891 | title=Geologic setting and origin of Terra Meridiani hematite deposit on Mars | year=2002 | last1=Hynek | first1=Brian M. | journal=Journal of Geophysical Research | volume=107 | issue=E10 | page=5088 | bibcode=2002JGRE..107.5088H | doi-access=free }}</ref><ref name=Squyres_2004 /><ref name=McLennan_2005 /><ref name=Tosca_2005 /><ref name=Clark_2005 /><ref name=Arvidson_2006 />

==Initial discovery==
===Discovery from orbit===
From 14 September 1997 to 29 April 1998, the ''[[Mars Global Surveyor]]'' spacecraft collected spectral data with its [[Thermal Emission Spectrometer]] (TES) instrument as the spacecraft entered its science phase orbit. During this period, TES made its first detection of crystalline gray hematite (α-Fe<sub>2</sub>O<sub>3</sub>) within [[Sinus Meridiani]], a low-albedo region. The hematite-rich surface was located in a region of [[sedimentary rock]], with spectral data indicating that hematite formed micrometer-scale particles.<ref name="Christensen2000">{{cite journal |last1=Christensen |first1=P. R. |last2=Bandfield |first2=J. L. |last3=Clark |first3=R. N. |last4=Edgett |first4=K. S. |last5=Hamilton |first5=V. E. |last6=Hoefen |first6=T. |last7=Kieffer |first7=H. H. |last8=Kuzmin |first8=R. O. |last9=Lane |first9=M. D. |last10=Malin |first10=M. C. |last11=Morris |first11=R. V. |last12=Pearl |first12=J. C. |last13=Pearson |first13=R. |last14=Roush |first14=T. L. |last15=Ruff |first15=S. W. |last16=Smith |first16=M. D. |title=Detection of crystalline hematite mineralization on Mars by the Thermal Emission Spectrometer: Evidence for near-surface water |journal=Journal of Geophysical Research: Planets |date=2000 |volume=105 |issue=E4 |pages=9623–9642 |doi=10.1029/1999JE001093 |language=en |issn=2156-2202|bibcode=2000JGR...105.9623C |s2cid=31117380 |doi-access=free }}</ref>

===Maps of surface hematite===

Between 1997 and 2002, the ''[[Mars Global Surveyor]]''{{'}}s [[Thermal Emission Spectrometer|TES]] mapped the whole planet of Mars for surface hematite levels.<ref name=harry>{{cite book |last1=McSween |first1=Harry |last2=Moersch |first2=Jeffrey |last3=Burr |first3=Devon |last4=Dunne |first4=William |last5=Emery |first5=Joshua |last6=Kah |first6=Linda |last7=McCanta |first7=Molly |title=Planetary Geoscience |date=2019 |publisher=Cambridge University Press |location=Cambridge |isbn=978-1-107-14538-2 |pages=300–306}}</ref> Figure 1a gives the [[Thermal Emission Spectrometer|TES]]'s global hematite map in low resolution. It has just one large spot covering a region with high hematite levels. This green, yellow, and red spot straddles the equator and the prime meridian in the middle of Figure 1a.  A higher resolution map of the high-hematite region is shown in Figure 1b.    
{{Gallery|align=center
| width = 360|Image:Mars Global survey of hematite by TES.png|'''Figure 1a'''. A low-resolution map across all of Mars of surface hematite levels. Data for this map was produced by the thermal emission spectrometer (TES) on the ''[[Mars Global Surveyor]]'' between 1997 and 2002. The green and red blob in the center of the map is shown in high resolution in Figure 1b. Mapped from 1997 to 2002.
|Image: Meridiani surface hematite map.jpg| '''Figure 1b'''. Map of surface hematite levels in the region around the equator and [[prime meridian]] (now called Meridiani Planum). This is a high-resolution blow-up of the central part of Figure 1a overlaid over an image of the region. Mapped from 1997 to 2002. 
}}

===Search for signs of water and life===
In the 1990s, NASA officials wanted to delineate a framework for [[Discovery Program|"faster, better, cheaper"]] exploration of Mars. In this context, the "Water Strategy" was outlined in 1995/1996.<ref name=water_strat>
Shirley, D. L. and McCleese, D. J., 1996, "Mars Exploration Program Strategy: 1995-2020," ''AIAA 96-0333, 34 th Aerospace Sciences Meeting and Exhibit, Reno, Nv.'' (available online at:
https://web.archive.org/web/20130511200249/http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/23620/1/96-0064.pdf).
</ref> High priority goals for NASA in the mid-1990s were to gather some evidence for surface water using satellite surveys and to land robotic rovers on the surface to collect detailed local evidence of water and signs of life.<ref name=water_strat />

In early the 2000s, the hematite map of Figure 1b and the confirmation (from the topography mapping done by the ''[[Mars Global Surveyor]]'') that this area is a flat plain and relatively easy to land on were the decisive pieces of evidence for choosing the Meridiani Planum as one of the landing sites for NASA's two bigger 
[[Mars Exploration Rover]]s (MERs), named ''[[Opportunity (rover)|Opportunity]]'' and ''[[Spirit (rover)|Spirit]]''.<ref name=Christensen_2005>{{cite journal | doi=10.1016/j.icarus.2005.01.004 | title=Mars Exploration Rover candidate landing sites as viewed by THEMIS | year=2005 | last1=Christensen | first1=P. | last2=Ruff | first2=S. | last3=Fergason | first3=R. | last4=Gorelick | first4=N. | last5=Jakosky | first5=B. | last6=Lane | first6=M. | last7=McEwen | first7=A. | last8=McSween | first8=H. | last9=Mehall | first9=G. | last10=Milam | first10=K. | journal=Icarus | volume=176 | issue=1 | pages=12–43 | bibcode=2005Icar..176...12C }}</ref><ref>{{ Cite web|url=https://themis.asu.edu/node/5390|title=ASU/THEMIS choosing the landing site of Opportunity.}}</ref>  
The decisiveness for NASA of the hematite map of Figure 1b for choosing the landing site for ''[[Opportunity (rover)|Opportunity]]''
was because NASA was using high hematite levels as proxy evidence for large amounts of liquid water flowing in the region in the past. (Hematite only forms in the presence of liquid water in geological settings). In 2003, this high-hematite region was a high-priority place to start to search for signs of life on Mars.<ref name=Christensen_2005 /><ref name=Squyres_2003>{{cite journal | doi=10.1029/2003JE002121 | title=Athena Mars rover science investigation | year=2003 | 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. | journal=Journal of Geophysical Research: Planets | volume=108 | issue=E12 | page=8062 | bibcode=2003JGRE..108.8062S | doi-access=free }}</ref>

===New name: Meridiani Planum===

The hematite map of Figure 1b covered part of a larger area called the [[Sinus Meridiani]] by 19th-century Mars map-makers. In 2004, senior scientists for the upcoming MER [[Opportunity rover|''Opportunity'']] mission introduced the new place  name [[Meridiani Planum]]  for (roughly) the  high hematite area in Figure 1b.<ref name=Christensen_Ruff>{{cite journal | doi=10.1029/2003JE002233 | title=Formation of the hematite-bearing unit in Meridiani Planum: Evidence for deposition in standing water | year=2004 | last1=Christensen | first1=Philip R. | journal=Journal of Geophysical Research | volume=109 | issue=E8 | bibcode=2004JGRE..109.8003C | doi-access=free }}</ref>

===Discovery of spherules on the ground===

NASA'S rover ''[[Opportunity (rover)|Opportunity]]'' successfully made the "hole-in-one" landing into [[Eagle Crater]] at Meridiani Planum on January 24(PST), 2004.<ref>{{ Cite web|url=https://www.nasa.gov/feature/jpl/new-look-at-2004s-martian-hole-in-one-site|title=Hole-in-one landing of Opportunity.|work=NASA |date=21 April 2017 |last1=Greicius |first1=Tony }}</ref> On the first sol (Martian day), the rover immediately discovered thousands and thousands of small (4–6&nbsp;mm diameter) spherules lying all over the place inside [[Eagle Crater]].

Figure 2 shows a thumbnail of the view from [[Opportunity rover|''Opportunity'']]'s Pancam (panoramic camera) on the first sol. (The actual image is very large, 7838 x 2915 pixels). The lead of the [[Pancam]] team, Jim Bell, soon wrote about this view: "Scientists are intrigued by the abundance of rock outcrops dispersed throughout the crater, as well as the crater's soil, which appears to be a mixture of coarse gray grains and fine reddish grains."<ref>{{ Cite web|url=http://pancam.sese.asu.edu/001B_postcard.html|
title=Hole-in-one landing in Eagle Crater.}}</ref> Figure 3 is a detail of Figure 2 showing the grey spherules more clearly (click to enlarge).

{{Gallery|align=center
| width = 360|Image: Sol001B EagleCrater Hole in One.jpg|'''Figure 2'''. View from [[Opportunity rover|''Opportunity'']]'s Pancam on sol 1. The stainless steel column in the foreground is part of the rover's communications system. The fabrics in the foreground are deflated airbags and parts of the lander system (they are not part of the rover). In the near-middle-ground are soils compacted by the lander's airbags. The rest of the image shows red soils covered in grey spherules and the rim of Eagle Crater. A lighter rocky outcrop of the crater's rim is in the right background. Image taken on Sol 1 (2004-01-24). 
|Image: Sol001B EagleCrater Hole in One.cropped.jpg| '''Figure 3'''. This image is a detail of Figure 2. It covers a middle-ground portion of Figure 2 where the airbag compacted soils meet undisturbed soils covered in the grey spherules. Click the image to enlarge it for a better resolution of the grey spherules. Image taken on Sol 1 (2004-01-24). 
}}

Tests quickly found that the grey spherules are rich in grey hematite.<ref name=Klingelhofer_2004>{{cite journal | doi=10.1126/science.1104653 | title=Jarosite and Hematite at Meridiani Planum from Opportunity's Mössbauer Spectrometer | year=2004 | last1=KlingelhöFer | first1=G. | last2=Morris | first2=R. V. | last3=Bernhardt | first3=B. | last4=SchröDer | first4=C. | last5=Rodionov | first5=D. S. | last6=De Souza | first6=P. A. | last7=Yen | first7=A. | last8=Gellert | first8=R. | last9=Evlanov | first9=E. N. | last10=Zubkov | first10=B. | last11=Foh | first11=J. | last12=Bonnes | first12=U. | last13=Kankeleit | first13=E. | last14=GüTlich | first14=P. | last15=Ming | first15=D. W. | last16=Renz | first16=F. | last17=Wdowiak | first17=T. | last18=Squyres | first18=S. W. | last19=Arvidson | first19=R. E. | journal=Science | volume=306 | issue=5702 | pages=1740–1745 | pmid=15576610 | bibcode=2004Sci...306.1740K | s2cid=20645172 }}</ref><ref name=Rieder_2004>{{cite journal | doi=10.1126/science.1104358 | title=Chemistry of Rocks and Soils at Meridiani Planum from the Alpha Particle X-ray Spectrometer | year=2004 | last1=Rieder | first1=R. | last2=Gellert | first2=R. | last3=Anderson | first3=R. C. | last4=BrüCkner | first4=J. | last5=Clark | first5=B. C. | last6=Dreibus | first6=G. | last7=Economou | first7=T. | last8=KlingelhöFer | first8=G. | last9=Lugmair | first9=G. W. | last10=Ming | first10=D. W. | last11=Squyres | first11=S. W. | last12=d'Uston | first12=C. | last13=WäNke | first13=H. | last14=Yen | first14=A. | last15=Zipfel | first15=J. | journal=Science | volume=306 | issue=5702 | pages=1746–1749 | pmid=15576611 | bibcode=2004Sci...306.1746R | s2cid=43214423 }}</ref><ref name="Christensen_2004a">{{cite journal | doi=10.1126/science.1104909 | title=Mineralogy at Meridiani Planum from the Mini-TES Experiment on the Opportunity Rover | year=2004 | last1=Christensen | first1=P. R. | last2=Wyatt | first2=M. B. | last3=Glotch | first3=T. D. | last4=Rogers | first4=A. D. | last5=Anwar | first5=S. | last6=Arvidson | first6=R. E. | last7=Bandfield | first7=J. L. | last8=Blaney | first8=D. L.|author8-link= Diana Blaney | last9=Budney | first9=C. | last10=Calvin | first10=W. M. | last11=Fallacaro | first11=A. | last12=Fergason | first12=R. L. | last13=Gorelick | first13=N. | last14=Graff | first14=T. G. | last15=Hamilton | first15=V. E. | last16=Hayes | first16=A. G. | last17=Johnson | first17=J. R. | last18=Knudson | first18=A. T. | last19=McSween | first19=H. Y. | last20=Mehall | first20=G. L. | last21=Mehall | first21=L. K. | last22=Moersch | first22=J. E. | last23=Morris | first23=R. V. | last24=Smith | first24=M. D. | last25=Squyres | first25=S. W. | last26=Ruff | first26=S. W. | last27=Wolff | first27=M. J. | journal=Science | volume=306 | issue=5702 | pages=1733–1739 | pmid=15576609 | bibcode=2004Sci...306.1733C | s2cid=12052805 }}</ref><ref name=Bell_2004>{{cite journal | doi=10.1126/science.1105245 | title=Pancam Multispectral Imaging Results from the Opportunity Rover at Meridiani Planum | year=2004 | last1=Bell | first1=J. F. | last2=Squyres | first2=S. W. | last3=Arvidson | first3=R. E. | last4=Arneson | first4=H. M. | last5=Bass | first5=D. | last6=Calvin | first6=W. | last7=Farrand | first7=W. H. | last8=Goetz | first8=W. | last9=Golombek | first9=M. | last10=Greeley | first10=R. | last11=Grotzinger | first11=J. | last12=Guinness | first12=E. | last13=Hayes | first13=A. G. | last14=Hubbard | first14=M. Y. H. | last15=Herkenhoff | first15=K. E. | last16=Johnson | first16=M. J. | last17=Johnson | first17=J. R. | last18=Joseph | first18=J. | last19=Kinch | first19=K. M. | last20=Lemmon | first20=M. T. | last21=Li | first21=R. | last22=Madsen | first22=M. B. | last23=Maki | first23=J. N. | last24=Malin | first24=M. | last25=McCartney | first25=E. | last26=McLennan | first26=S. | last27=McSween | first27=H. Y. | last28=Ming | first28=D. W. | last29=Morris | first29=R. V. | last30=Dobrea | first30=E. Z. Noe | journal=Science | volume=306 | issue=5702 | pages=1703–1709 | pmid=15576603 | bibcode=2004Sci...306.1703B | s2cid=20346053 | display-authors=1 }}</ref><ref name=Soderblom_2004>{{cite journal | doi=10.1126/science.1105127 | title=Soils of Eagle Crater and Meridiani Planum at the Opportunity Rover Landing Site | year=2004 | last1=Soderblom | first1=L. A. | last2=Anderson | first2=R. C. | last3=Arvidson | first3=R. E. | last4=Bell | first4=J. F. | last5=Cabrol | first5=N. A. | last6=Calvin | first6=W. | last7=Christensen | first7=P. R. | last8=Clark | first8=B. C. | last9=Economou | first9=T. | last10=Ehlmann | first10=B. L. | last11=Farrand | first11=W. H. | last12=Fike | first12=D. | last13=Gellert | first13=R. | last14=Glotch | first14=T. D. | last15=Golombek | first15=M. P. | last16=Greeley | first16=R. | last17=Grotzinger | first17=J. P. | last18=Herkenhoff | first18=K. E. | last19=Jerolmack | first19=D. J. | last20=Johnson | first20=J. R. | last21=Jolliff | first21=B. | last22=KlingelhöFer | first22=G. | last23=Knoll | first23=A. H. | last24=Learner | first24=Z. A. | last25=Li | first25=R. | last26=Malin | first26=M. C. | last27=McLennan | first27=S. M. | last28=McSween | first28=H. Y. | last29=Ming | first29=D. W. | last30=Morris | first30=R. V. | journal=Science | volume=306 | issue=5702 | pages=1723–1726 | pmid=15576606 | bibcode=2004Sci...306.1723S | s2cid=5693765 | display-authors=1 }}</ref><ref name=Squyres_2004>{{cite journal | doi=10.1126/science.1104559 | title=In Situ Evidence for an Ancient Aqueous Environment at Meridiani Planum, Mars | year=2004 | last1=Squyres | first1=S. W. | last2=Grotzinger | first2=J. P. | last3=Arvidson | first3=R. E. | last4=Bell | first4=J. F. | last5=Calvin | first5=W. | last6=Christensen | first6=P. R. | last7=Clark | first7=B. C. | last8=Crisp | first8=J. A. | last9=Farrand | first9=W. H. | last10=Herkenhoff | first10=K. E. | last11=Johnson | first11=J. R. | last12=KlingelhöFer | first12=G. | last13=Knoll | first13=A. H. | last14=McLennan | first14=S. M. | last15=McSween | first15=H. Y. | last16=Morris | first16=R. V. | last17=Rice | first17=J. W. | last18=Rieder | first18=R. | last19=Soderblom | first19=L. A. | journal=Science | volume=306 | issue=5702 | pages=1709–1714 | pmid=15576604 | bibcode=2004Sci...306.1709S | s2cid=16785189 | url=http://nrs.harvard.edu/urn-3:HUL.InstRepos:3119538 }}</ref> These tests included doing the "berry bowl" experiment (more below). 
 
The moniker "blueberries" was coined for the grey hematite spherules by the original [[Opportunity rover|''Opportunity'']] science team due to these spherules appearing bluish relative to the underlying rusty-red soils in the "natural color RGB images" analyzed.<ref name=Bell_2004 /><ref name=Calvin_2008>{{cite journal |last1=Calvin |first1=W. M. |last2=Shoffner |first2=J. D. |last3=Johnson |first3=J. R. |last4=Knoll |first4=A. H. |last5=Pocock |first5=J. M. |last6=Squyres |first6=S. W. |last7=Weitz |first7=C. M. |last8=Arvidson |first8=R. E. |last9=Bell |first9=J. F. |last10=Christensen |first10=P. R. |last11=Souza |first11=P. A. de |last12=Farrand |first12=W. H. |last13=Glotch |first13=T. D. |last14=Herkenhoff |first14=K. E. |last15=Jolliff |first15=B. L. |last16=Knudson |first16=A. T. |last17=McLennan |first17=S. M. |last18=Rogers |first18=A. D. |last19=Thompson |first19=S. D. |title=Hematite spherules at Meridiani: Results from MI, Mini-TES, and Pancam |journal=Journal of Geophysical Research: Planets |date=2008 |volume=113 |issue=E12 |pages=E12S37 |doi=10.1029/2007JE003048 |language=en |issn=2156-2202|bibcode=2008JGRE..11312S37C |url=http://nrs.harvard.edu/urn-3:HUL.InstRepos:3196278 |doi-access=free }}</ref>

==Blueberry formation==

Blueberries are either embedded in the large body of sediments of [[Meridiani Planum]] or they are loose blueberries that lie directly on outcrops of the sediments or lie on top soils spread over the Meridiani sediments.<ref name=Soderblom_2004 /><ref name=Grotzinger_2005>{{cite journal | doi=10.1016/j.epsl.2005.09.039 | title=Stratigraphy and sedimentology of a dry to wet eolian depositional system, Burns formation, Meridiani Planum, Mars | year=2005 | last1=Grotzinger | first1=J.P. | last2=Arvidson | first2=R.E. | last3=Bell | first3=J.F. | last4=Calvin | first4=W. | last5=Clark | first5=B.C. | last6=Fike | first6=D.A. | last7=Golombek | first7=M. | last8=Greeley | first8=R. | last9=Haldemann | first9=A. | last10=Herkenhoff | first10=K.E. | last11=Jolliff | first11=B.L. | last12=Knoll | first12=A.H. | last13=Malin | first13=M. | last14=McLennan | first14=S.M. | last15=Parker | first15=T. | last16=Soderblom | first16=L. | last17=Sohl-Dickstein | first17=J.N. | last18=Squyres | first18=S.W. | last19=Tosca | first19=N.J. | last20=Watters | first20=W.A. | journal=Earth and Planetary Science Letters | volume=240 | issue=1 | pages=11–72 | bibcode=2005E&PSL.240...11G }}</ref> The loose blueberries and soils are eroded out of the underlying sediments.<ref name=Golombek_2006>{{cite journal | doi=10.1029/2006JE002754 | title=Erosion rates at the Mars Exploration Rover landing sites and long-term climate change on Mars | year=2006 | last1=Golombek | first1=M. P. | last2=Grant | first2=J. A. | last3=Crumpler | first3=L. S. | last4=Greeley | first4=R. | last5=Arvidson | first5=R. E. | last6=Bell | first6=J. F. | last7=Weitz | first7=C. M. | last8=Sullivan | first8=R. | last9=Christensen | first9=P. R. | last10=Soderblom | first10=L. A. | last11=Squyres | first11=S. W. | journal=Journal of Geophysical Research: Planets | volume=111 | issue=E12 | bibcode=2006JGRE..11112S10G | doi-access=free }}</ref> Both today's embedded blueberries and the loose blueberries were formed in the sediments of [[Meridiani Planum]] by "diagenetic" processes, i.e., processes that change sediments by water-rock interactions.<ref name=Squyres_2004 /><ref name=Arvidson_2006>{{cite journal | doi=10.1029/2006JE002728 | title=Nature and origin of the hematite-bearing plains of Terra Meridiani based on analyses of orbital and Mars Exploration rover data sets | year=2006 | last1=Arvidson | first1=R. E. | last2=Poulet | first2=F. | last3=Morris | first3=R. V. | last4=Bibring | first4=J.-P. | last5=Bell | first5=J. F. | last6=Squyres | first6=S. W. | last7=Christensen | first7=P. R. | last8=Bellucci | first8=G. | last9=Gondet | first9=B. | last10=Ehlmann | first10=B. L. | last11=Farrand | first11=W. H. | last12=Fergason | first12=R. L. | last13=Golombek | first13=M. | last14=Griffes | first14=J. L. | last15=Grotzinger | first15=J. | last16=Guinness | first16=E. A. | last17=Herkenhoff | first17=K. E. | last18=Johnson | first18=J. R. | last19=Klingelhöfer | first19=G. | last20=Langevin | first20=Y. | last21=Ming | first21=D. | last22=Seelos | first22=K. | last23=Sullivan | first23=R. J. | last24=Ward | first24=J. G. | last25=Wiseman | first25=S. M. | last26=Wolff | first26=M. | journal=Journal of Geophysical Research: Planets | volume=111 | issue=E12 | bibcode=2006JGRE..11112S08A | s2cid=15596833 | doi-access=free }}</ref> The diagenetic processes not only formed embedded blueberries but also changed an original large body of sediments. Thus, blueberry formation was (broadly) a three-step process:

* Formation of the original body of sediments;
* Diagenetic transformation of the original sediments to produce today's sediments and embedded blueberries;
* Erosion of top layers of the sediments to form top soils and loose blueberries.

Each of these broad steps involved multiple sub-step processes, described in the following sub-sections:

===Noachian river flows===

[[File:THEMIS.thermal.interia.map.with.rivers.png|thumb|'''Figure 4'''. Thermal inertia map of the southern half of Meridiani Planum and a region to the south. Dried-up river valleys are visible in the region to the south.<ref name=Fergason_2006 /> They end at the plain. The map was made from orbit by the THEMIS instrument.<ref>{{cite book | doi=10.1007/978-0-306-48600-5_3 | chapter=The Thermal Emission Imaging System (Themis) for the Mars 2001 Odyssey Mission | title=2001 Mars Odyssey | year=2004 | last1=Christensen | first1=Philip R. | last2=Jakosky | first2=Bruce M. | last3=Kieffer | first3=Hugh H. | last4=Malin | first4=Michael C. | last5=Jr | first5=Harry Y. Mcsween | last6=Nealson | first6=Kenneth | last7=Mehall | first7=Greg L. | last8=Silverman | first8=Steven H. | last9=Ferry | first9=Steven | last10=Caplinger | first10=Michael | last11=Ravine | first11=Michael | pages=85–130 | isbn=978-94-015-6958-3 }}</ref>]]

Prior to the formation of Meridiani's defining sediments, in the wet [[Noachian]] (named for the biblical Noah) more than about ~3.7 billion years ago, liquid water was present and plentiful enough to form river channels that bought and deposited large quantities of basaltic silt to the current Meridiani region.<ref name=Edgett_Parker>{{cite journal | doi=10.1029/97GL02840 | title=Water on early Mars: Possible subaqueous sedimentary deposits covering ancient cratered terrain in western Arabia and Sinus Meridiani | year=1997 | last1=Edgett | first1=Kenneth S. | last2=Parker | first2=Timothy J. | journal=Geophysical Research Letters | volume=24 | issue=22 | pages=2897–2900 | bibcode=1997GeoRL..24.2897E | s2cid=128370670 }}</ref><ref name=Christensen_2000>{{cite journal | doi=10.1029/1999JE001093 | title=Detection of crystalline hematite mineralization on Mars by the Thermal Emission Spectrometer: Evidence for near-surface water | year=2000 | last1=Christensen | first1=P. R. | last2=Bandfield | first2=J. L. | last3=Clark | first3=R. N. | last4=Edgett | first4=K. S. | last5=Hamilton | first5=V. E. | last6=Hoefen | first6=T. | last7=Kieffer | first7=H. H. | last8=Kuzmin | first8=R. O. | last9=Lane | first9=M. D. | last10=Malin | first10=M. C. | last11=Morris | first11=R. V. | last12=Pearl | first12=J. C. | last13=Pearson | first13=R. | last14=Roush | first14=T. L. | last15=Ruff | first15=S. W. | last16=Smith | first16=M. D. | journal=Journal of Geophysical Research: Planets | volume=105 | issue=E4 | pages=9623–9642 | bibcode=2000JGR...105.9623C | doi-access=free }}</ref><ref name=Hynek_Phillips>{{cite journal | doi=10.1130/0091-7613(2001)029<0407:EFEDOT>2.0.CO;2 | issn=0091-7613 | year=2001 | volume=29 | page=407 | title=Evidence for extensive denudation of the Martian highlands | last1=Hynek | first1=Brian M. | last2=Phillips | first2=Roger J. | journal=Geology | issue=5 | bibcode=2001Geo....29..407H }}</ref> The dried river valleys are easily seen in thermal inertia images taken in orbit by [[2001 Mars Odyssey|''Mars Odyssey'']] and reproduced in Figure 4 (click on it for higher resolution).<ref name=Fergason_2006>{{cite journal | doi=10.1029/2006JE002735 | title=High-resolution thermal inertia derived from the Thermal Emission Imaging System (THEMIS): Thermal model and applications | year=2006 | last1=Fergason | first1=Robin L. | last2=Christensen | first2=Philip R. | last3=Kieffer | first3=Hugh H. | journal=Journal of Geophysical Research: Planets | volume=111 | issue=E12 | bibcode=2006JGRE..11112004F | doi-access=free }}</ref> The river valleys seen in Figure 4 terminate abruptly as they flow into the Meridiani's massive formation of sediments.

===Formation of today's sediments and embedded spherules===

From around the late-[[Noachian]]/early-[[Hesperian]] to sometime around 3.5 billion years ago, the layered sediments deposited in the earlier [[Noachian]] epoch were transformed.<ref name=Arvidson_2006 /> This transformation probably included a significant additional deposition of high-sulfur-content material of volcanic origin.<ref name=Clark_2005 /> The change certainly included aqueous geochemistry that was acidic and salty, as well as rising & falling water levels: Features providing evidence include cross-bedded sediments, the presence of [[vug]]s (cavities), and embedded hematite spherules that cut across sediment layers, additionally the presence of large amounts of [[magnesium sulfate]] and other sulfate-rich minerals such as [[jarosite]] and chlorides.<ref name=Squyres_2004 /><ref name=Clark_2005>{{cite journal | doi=10.1016/j.epsl.2005.09.040 | title=Chemistry and mineralogy of outcrops at Meridiani Planum | year=2005 | last1=Clark | first1=B.C. | last2=Morris | first2=R.V. | last3=McLennan | first3=S.M. | last4=Gellert | first4=R. | last5=Jolliff | first5=B. | last6=Knoll | first6=A.H. | last7=Squyres | first7=S.W. | last8=Lowenstein | first8=T.K. | last9=Ming | first9=D.W. | last10=Tosca | first10=N.J. | last11=Yen | first11=A. | last12=Christensen | first12=P.R. | last13=Gorevan | first13=S. | last14=Brückner | first14=J. | last15=Calvin | first15=W. | last16=Dreibus | first16=G. | last17=Farrand | first17=W. | last18=Klingelhoefer | first18=G. | last19=Waenke | first19=H. | last20=Zipfel | first20=J. | last21=Bell | first21=J.F. | last22=Grotzinger | first22=J. | last23=McSween | first23=H.Y. | last24=Rieder | first24=R. | journal=Earth and Planetary Science Letters | volume=240 | issue=1 | pages=73–94 | bibcode=2005E&PSL.240...73C }}</ref><ref name=McLennan_2005>{{cite journal | doi=10.1016/j.epsl.2005.09.041 | title=Provenance and diagenesis of the evaporite-bearing Burns formation, Meridiani Planum, Mars | year=2005 | last1=McLennan | first1=S.M. | last2=Bell | first2=J.F. | last3=Calvin | first3=W.M. | last4=Christensen | first4=P.R. | last5=Clark | first5=B.C. | last6=De Souza | first6=P.A. | last7=Farmer | first7=J. | last8=Farrand | first8=W.H. | last9=Fike | first9=D.A. | last10=Gellert | first10=R. | last11=Ghosh | first11=A. | last12=Glotch | first12=T.D. | last13=Grotzinger | first13=J.P. | last14=Hahn | first14=B. | last15=Herkenhoff | first15=K.E. | last16=Hurowitz | first16=J.A. | last17=Johnson | first17=J.R. | last18=Johnson | first18=S.S. | last19=Jolliff | first19=B. | last20=Klingelhöfer | first20=G. | last21=Knoll | first21=A.H. | last22=Learner | first22=Z. | last23=Malin | first23=M.C. | last24=McSween | first24=H.Y. | last25=Pocock | first25=J. | last26=Ruff | first26=S.W. | last27=Soderblom | first27=L.A. | last28=Squyres | first28=S.W. | last29=Tosca | first29=N.J. | last30=Watters | first30=W.A. | journal=Earth and Planetary Science Letters | volume=240 | issue=1 | pages=95–121 | bibcode=2005E&PSL.240...95M | display-authors=1 }}</ref><ref name=Tosca_2005>{{cite journal | doi=10.1016/j.epsl.2005.09.042 | title=Geochemical modeling of evaporation processes on Mars: Insight from the sedimentary record at Meridiani Planum | year=2005 | last1=Tosca | first1=N.J. | last2=McLennan | first2=S.M. | last3=Clark | first3=B.C. | last4=Grotzinger | first4=J.P. | last5=Hurowitz | first5=J.A. | last6=Knoll | first6=A.H. | last7=Schröder | first7=C. | last8=Squyres | first8=S.W. | journal=Earth and Planetary Science Letters | volume=240 | issue=1 | pages=122–148 | bibcode=2005E&PSL.240..122T }}</ref><ref name=Herkenhoff_2004>{{cite journal | doi=10.1126/science.1105286 | title=Evidence from Opportunity's Microscopic Imager for Water on Meridiani Planum | year=2004 | last1=Herkenhoff | first1=K. E. | last2=Squyres | first2=S. W. | last3=Arvidson | first3=R. | last4=Bass | first4=D. S. | last5=Bell | first5=J. F. | last6=Bertelsen | first6=P. | last7=Ehlmann | first7=B. L. | last8=Farrand | first8=W. | last9=Gaddis | first9=L. | last10=Greeley | first10=R. | last11=Grotzinger | first11=J. | last12=Hayes | first12=A. G. | last13=Hviid | first13=S. F. | last14=Johnson | first14=J. R. | last15=Jolliff | first15=B. | last16=Kinch | first16=K. M. | last17=Knoll | first17=A. H. | last18=Madsen | first18=M. B. | last19=Maki | first19=J. N. | last20=McLennan | first20=S. M. | last21=McSween | first21=H. Y. | last22=Ming | first22=D. W. | last23=Rice | first23=J. W. | last24=Richter | first24=L. | last25=Sims | first25=M. | last26=Smith | first26=P. H. | last27=Soderblom | first27=L. A. | last28=Spanovich | first28=N. | last29=Sullivan | first29=R. | last30=Thompson | first30=S. | journal=Science | volume=306 | issue=5702 | pages=1727–1730 | pmid=15576607 | bibcode=2004Sci...306.1727H | s2cid=41361236 | url=https://zenodo.org/record/1230854 | display-authors=1 }}</ref><ref name=MarionCatling2010>{{cite journal | doi=10.1016/j.icarus.2009.12.003 | title=Modeling aqueous perchlorate chemistries with applications to Mars | year=2010 | last1=Marion | first1=G.M. | last2=Catling | first2=D.C. | last3=Zahnle | first3=K.J. | last4=Claire | first4=M.W. | journal=Icarus | volume=207 | issue=2 | pages=675–685 | bibcode=2010Icar..207..675M }}</ref> Jarosite formation requires aqueous acidic conditions below pH 3.<ref name=McLennan_2005 /><ref name=Tosca_2005 />

Figures 5 and 6 show Microscopic Imager close-ups of the sediment rock matrix that appeared in a prestigious paper.<ref name=Squyres_2004 /> Figure 5 illustrates the four physical constituents of sediment outcrop: (i) the sedimentary layers containing a lot of basaltic sand particles; (ii) the embedded hematite spherules; (iii) fine-grained, sulfate-rich cement (in most parts of the outcrop); (iv) vug cavities (that are thought to be molds for crystals of, for example, hydrated sulfates).<ref name=Squyres_2004 /> Figure 6 images a similar sediment outcrop surface to Figure 5. However, [[Opportunity rover|''Opportunity'']]'s Rock Abrasion Tool abraded this surface. Such abrasions showed that (a) the sediment layers are very soft and easy to cut, and (b) the hematite spherules have uniform internal structures.<ref name=Squyres_2004 /><ref name=Grotzinger_2005 /><ref name=Calvin_2008 /><ref name=Chan_2012 /> 
{{Gallery|align=center
| width = 360|Image:1M130760791EFF0454P2933M2M1.jpg| '''Figure 5'''. Close-up of sediment matrix with embedded hematite spherules at Eagle Crater. The central (partially embedded) spherule is 3.7 mm in diameter. The image covers an approximate 32 mm x 32 mm area. It was taken on Sol 29 (2004-02-24). 
|Image:1M131212783EFF0500P2959M2M1.jpg| '''Figure 6'''. Close-up of sediment matrix and embedded hematite spherules abraded by [[Opportunity rover|''Opportunity'']]'s Rock Abrasion Tool. This Eagle Crater image covers an approximate 32 mm x 32 mm area. It was taken on Sol 34 (2004-02-29).
}}

The diagenetic transformation (i.e., change by water-rock interactions) to today's sediments involved a significant shift in water flows in the region. The inflows from rivers became less, and the dominant water movements in the sediments became vertical with rising and falling aquifer levels.<ref name=Squyres_2004 /><ref name=Clark_2005 /> 
At least one model of global Martian hydrology accounts for the historical shift in water flows at Meridiani Planum.<ref name=Andrews_Hanna>{{cite journal | doi=10.1038/nature05594 | title=Meridiani Planum and the global hydrology of Mars | year=2007 | last1=Andrews-Hanna | first1=Jeffrey C. | last2=Phillips | first2=Roger J. | last3=Zuber | first3=Maria T. | journal=Nature | volume=446 | issue=7132 | pages=163–166 | pmid=17344848 | bibcode=2007Natur.446..163A | s2cid=4428510 }}</ref> This model links Meridiani's change in water flows to activity in the volcanic Tharsis region. With the vertical aquifer flows, it is believed that (playa) lakes repeatedly formed and disappeared as the aquifer levels rose and fell.<ref name=Squyres_2004 /><ref name=Squyres_2004a>{{cite journal | doi=10.1126/science.1106171 | title=The Opportunity Rover's Athena Science Investigation at Meridiani Planum, Mars | year=2004 | last1=Squyres | first1=S. W. | last2=Arvidson | first2=R. E. | last3=Bell | first3=J. F. | last4=BrüCkner | first4=J. | last5=Cabrol | first5=N. A. | last6=Calvin | first6=W. | last7=Carr | first7=M. H. | last8=Christensen | first8=P. R. | last9=Clark | first9=B. C. | last10=Crumpler | first10=L. | last11=Marais | first11=D. J. Des | last12=d'Uston | first12=C. | last13=Economou | first13=T. | last14=Farmer | first14=J. | last15=Farrand | first15=W. | last16=Folkner | first16=W. | last17=Golombek | first17=M. | last18=Gorevan | first18=S. | last19=Grant | first19=J. A. | last20=Greeley | first20=R. | last21=Grotzinger | first21=J. | last22=Haskin | first22=L. | last23=Herkenhoff | first23=K. E. | last24=Hviid | first24=S. | last25=Johnson | first25=J. | last26=KlingelhöFer | first26=G. | last27=Knoll | first27=A. H. | last28=Landis | first28=G. | last29=Lemmon | first29=M. | last30=Li | first30=R. | journal=Science | volume=306 | issue=5702 | pages=1698–1703 | pmid=15576602 | bibcode=2004Sci...306.1698S | s2cid=7876861 | display-authors=1 }}</ref><ref name=McLennan_2005 /><ref name=Tosca_2005 /> (The dry area around Utah's [[Great Salt Lake]] is a playa.)

McLennan and his students constructed a geochemical model that generates hematite within a context like the Meridiani sediment.<ref name=McLennan_2005 /><ref name=Tosca_2005 /><ref>{{cite journal | doi=10.1029/2007JE003019 | title=Fe oxidation processes at Meridiani Planum and implications for secondary Fe mineralogy on Mars | year=2008 | last1=Tosca | first1=Nicholas J. | last2=McLennan | first2=Scott M. | last3=Dyar | first3=M. Darby | last4=Sklute | first4=Elizabeth C. | last5=Michel | first5=F. Marc | journal=Journal of Geophysical Research | volume=113 | issue=E5 | bibcode=2008JGRE..113.5005T | doi-access=free }}</ref><ref>{{cite journal | doi=10.1038/NGEo831 | title=Origin of acidic surface waters and the evolution of atmospheric chemistry on early Mars | year=2010 | last1=Hurowitz | first1=Joel A. | last2=Fischer | first2=Woodward W. | last3=Tosca | first3=Nicholas J. | last4=Milliken | first4=Ralph E. | journal=Nature Geoscience | volume=3 | issue=5 | pages=323–326 | bibcode=2010NatGe...3..323H | url=https://resolver.caltech.edu/CaltechAUTHORS:20100526-081809534 }}</ref>

The hematite formed into spherules by [[concretion]] (when minerals came out of solution).<ref name=Squyres_2004 /><ref name=Chan_2012 /><ref>Bell, J (ed.) ''The Martian Surface''. 2008. Cambridge University Press. {{ISBN|978-0-521-86698-9}}</ref><ref>{{Cite news|url=http://www.astrobio.net/news-exclusive/dining-on-diamonds/|title=Dining on Diamonds |date=2004-08-05|work=[[Astrobiology Magazine]]|language=en-US|access-date=23 January 2017 |archive-url=https://web.archive.org/web/20151001135532/http://www.astrobio.net/news-exclusive/dining-on-diamonds/ |archive-date=2015-10-01 |url-status=usurped}}</ref> 
The concretion process to form spherules of hematite probably occurred by diffusion of the hematite through the sedimentary rock matrix.<ref name=Chan_2012>{{cite book | doi=10.2110/pec.12.102.0253 | chapter=Characteristics of Terrestrial Ferric Oxide Concretions and Implications for Mars | title=Sedimentary Geology of Mars | year=2012 | last1=Chan | first1=Marjorie A. | last2=Potter | first2=Sally L. | last3=Bowen | first3=Brenda B. | last4=Parry | first4=W.T. | last5=Barge | first5=Laura M. | last6=Seiler | first6=Winston | last7=Petersen | first7=Erich U. | last8=Bowman | first8=John R. | pages=253–270 | isbn=978-1-56576-312-8 }}</ref>

===Formation of soils and loose spherules, crater degradation===

The period of rising and falling aquifer levels ceased, and no water flowed on Meridiani Planum thereafter.<ref name=Arvidson_2006 /><ref name=Golombek_2014>{{cite journal | doi=10.1002/2014JE004658 | title=Small crater modification on Meridiani Planum and implications for erosion rates and climate change on Mars | year=2014 | last1=Golombek | first1=M. P. | last2=Warner | first2=N. H. | last3=Ganti | first3=V. | last4=Lamb | first4=M. P. | last5=Parker | first5=T. J. | last6=Fergason | first6=R. L. | last7=Sullivan | first7=R. | journal=Journal of Geophysical Research: Planets | volume=119 | issue=12 | pages=2522–2547 | bibcode=2014JGRE..119.2522G | s2cid=34067844 | url=https://resolver.caltech.edu/CaltechAUTHORS:20181127-133838350 }}</ref> Although, when this happened is poorly understood. Estimates include around 3.5 billion years ago <ref name=Arvidson_2006 /> and about 3 billion years ago.<ref name=Golombek_2014 /> The only water left at the plain is bound in rocks.<ref name=Clark_2005 />

Erosion with water flows in earlier eras was much faster than in this arid epoch.<ref name=Golombek_2014 />
However, erosion did not stop. Other much slower erosional processes continued and became the primary agents of change to the plain. This slower change was and is driven by meteorite impacts, the wind, and gravity. Over the hard-to-grasp eon of around three billion years, meteorite impacts and the wind formed the sandy top soils and loose hematite spherules and sorted these into the layered soil bedforms that we can now see.<ref name=Golombek_2006 /><ref name=Golombek_2014 /><ref name=fenton>{{cite journal | doi=10.1016/j.aeolia.2014.11.004 | title=Late Amazonian aeolian features, gradation, wind regimes, and Sediment State in the Vicinity of the Mars Exploration Rover Opportunity, Meridiani Planum, Mars | year=2015 | last1=Fenton | first1=Lori K. | last2=Michaels | first2=Timothy I. | last3=Chojnacki | first3=Matthew | journal=Aeolian Research | volume=16 | pages=75–99 | bibcode=2015AeoRe..16...75F }}</ref><ref name="Sullivan_2011"/>

[[File:Sol1162B.Rim.Victoria.Crater.and.plain.at.Meridiani.Planum.ejecta.decay.detail.jpg|thumb|'''Figure 7'''. Detail showing hematite spherules eroding out of blocks of sediment ejecta. It is better to click and enlarge this. Notice the increased surface density of loose spherules lying in rings around the small clocks of sediment ejecta. This image is cropped out of a larger image taken on Sol 1162 (2007-05-01).]]

The meteorite, gravity, and wind-driven processes work like this:
* Over billions of years, meteorite impacts created many craters on the plain.
* There were enough small (5 to 30 m diameter) craters created in the eon of around three billion years to cover, on average, the whole plain once.<ref name=Olsen_2021 /> Although, each small crater degraded and disappeared in about 25 million years or less, and only about 0.7% of the plain's area is presently covered in small craters.<ref name=Golombek_2014 /><ref name=Olsen_2021 />
* Each meteorite impact produces large numbers of blocks of sediment material in the crater rim and as ejecta around the crater.
* Most of the initial sediment blocks project above the surrounding material (by a few centimeters or more) and are exposed to saltating sand (i.e., wind-driven, bouncing sand).
* Meridiani sediment matrix is soft and easy-to-erode.<ref name=Squyres_2004 /><ref name=Clark_2005 /> It erodes about 30 to 300 times faster than other regions of Mars (such as Gusev Crater).<ref name=Golombek_2006 /><ref name=Golombek_2014 /> (Although, this arid erosion is much slower than erosion with water flows.)
* The saltating sand erodes the soft, easy-to-erode parts of the sediment matrix in the projecting blocks. 
* These blocks are either completely eroded or erode until they become smooth and no longer project into saltating sand.
* This block erosion creates dust particles and turns embedded spherules into loose spherules.
* The dust particles are blown off the plain and become part of the global dust.
* The sulfates preferentially turn into dust and are transported off the plain by the wind.
* The larger basalt sand particles, spherules fragments, and hematite spherules remain in place on the plain.
* Wind, gravity, and size-sorting created the soil bedforms from the basaltic sands, spherule fragments, and spherules. 
* With the aid of gravity and wind, the original (small) crater holes are gradually filled in (with material from eroded rim blocks and other local erosion material), and the plain is returned to a flat state.

Phil Christensen outlined these processes in 2004, soon after [[Opportunity rover|''Opportunity'']] landed.<ref name=Christensen_2004a/> Later, more in-depth research confirmed them and added details to Christensen's outline.<ref name=Golombek_2006 /><ref name=Golombek_2014 /><ref name=fenton /><ref name=Olsen_2021>{{cite book | doi=10.2514/6.2021-4037 | chapter=Iron Oxide Harvesting on Mars | title=Ascend 2021 | year=2021 | last1=Olsen | first1=Rif M. | isbn=978-1-62410-612-5 }}</ref>

==Blueberry composition==

===Early Blueberry composition results===
Early on, [[Opportunity rover|''Opportunity'']]'s Mössbauer spectrometer took data that determined that the iron mineral component of these spherules is dominated by hematite.<ref name=Klingelhofer_2004 /><ref name=Morris_2006 /> However, the Mössbauer spectrometer provided no information about the mineral components of these spherules that do '''not''' contain iron.

[[File:Xpe pubeng approved 032304 color berry bowl-B060R1 br.jpg|thumb|'''Figure 8'''. Site of "berry bowl" experiment. The two sampling targets are next to each other. Image taken on Sol 48 (2004-03-13).]]
The "berry bowl" experiment took [[alpha particle X-ray spectrometer]] (APXS) readings of two sampling targets just centimeters apart: One had no (zero or one) spherules in the spectrometer's field of view (FOV), while the other had around 25 spherules in the FOV. Figure 8 shows the adjacent "berry bowl" sampling targets. The APXS results indicated there was noticeably more iron in the target with ~25 spherules relative to the target with 0 or 1 spherules. Based on this and similar experiments, several unreviewed conference abstracts claimed (deliberately not cited here) 
that hematite dominated the composition of the spherules and some published papers cited these conference claims. However, there were reasons to be cautious. The instruments detected mixed signals from sampling targets that included signals not only from the spherules but also from dust and rock (in the "berry bowl" experiment) or dust and soils (in other composition data collections). In 2006, Morris et al.<ref name=Morris_2006 /> showed that the methods used by some researchers to pick out the spherule composition signal from the dust and soil signals were flawed and that such methods could do no more than constrain the iron oxide content of the spherules to between 24 wt% and 100 wt% (that is, almost no constraint at all).

===Later Blueberry composition results===
A 2008 paper published the result of a clever experiment that showed [[Opportunity rover|''Opportunity'']]'s mini-TES (thermal emission spectrometer) could '''not''' detect any silicate minerals in the spherules.<ref name=Calvin_2008 /> This non-detection constrained silicate levels in spherules to less than 10 wt% and probably below 8 wt%. This result is  helpful since the APXS data shows a strong anti-correlation between silicates and iron oxide in the spherules -
so low silicate levels indicate high iron oxide levels.

A recent paper used the mini-TES's non-detection of silicates and some improved data analysis methods to find over 340,000 allowable standard oxide chemical compositions for the spherules (allowable = consistent with the silicate non-detection).<ref name=Olsen_2022 /> The lowest and highest weight percentages for the iron oxide content in these allowable spherule compositions were, respectively, 79.5 wt% and 99.8 wt%. While, for the large majority of the allowable compositions, the iron oxide contents in the spherules were between 85 wt% and 96 wt%; further, the nickel content was always close to 0.3 wt%, a group of five standard oxides (MgO, Na<sub>2</sub>O, P<sub>2</sub>O<sub>5</sub>, SO<sub>3</sub>, and Cl)  each had content above trace-level with a combined group content of 6.8 +/- 2.4 wt%, the SiO<sub>2</sub> levels ranged between 8 wt% and 0 wt%, and the other eight APXS standard oxides had either 0 wt% content or only trace level content.

==Size of Blueberries==

The [[Opportunity rover|''Opportunity'']] science team published three papers that studied variations in hematite spherule size.<ref name=Weitz_2006 /><ref name=Calvin_2008 /><ref name=Squyres_2009>Squyres, S. W., et al.,  2009,  "Exploration of Victoria Crater by the Mars Rover Opportunity," ''Science'',  1058-1061. https://doi.org/10.1126/science.1170355</ref> They found spherule size variation by location and elevation.

In the earliest paper, a team of [[Opportunity rover|''Opportunity'']] rover scientists reported on studies of all the soil 
materials found between the landing site in [[Eagle Crater]] to the location on sol 552 of the rover's traverse (between [[Endurance (crater)|Endurance Crater]] and [[Victoria Crater]]).   
They found that in a sample of 696 blueberries, disregarding any non-spherical blueberries from the sample, the blueberries' average major axis to be about 2.87&nbsp;mm (just over one-tenth inch). They also discovered that blueberries found within soils are typically smaller than blueberries found in the outcrops. They noted the size of the blueberries tends to decrease with decreasing latitude.<ref name=Weitz_2006>{{cite journal |last1=Weitz |first1=C. M. |last2=Anderson |first2=R. C. |last3=Bell |first3=J. F. |last4=Farrand |first4=W. H. |last5=Herkenhoff |first5=K. E. |last6=Johnson |first6=J. R. |last7=Jolliff |first7=B. L. |last8=Morris |first8=R. V. |last9=Squyres |first9=S. W. |last10=Sullivan |first10=R. J. |title=Soil grain analyses at Meridiani Planum, Mars |journal=Journal of Geophysical Research: Planets |date=2006 |volume=111 |issue=E12 |pages=n/a |doi=10.1029/2005JE002541 |language=en |issn=2156-2202|bibcode=2006JGRE..11112S04W }}</ref>

The [[Opportunity rover|''Opportunity'']] team found many fragmented blueberries and suggested the fracturing occurred after spherule formation. They believe the fracturing either be from meteoric impacts or the "same process" that "fractured the outcrop". However, the team notes this would not explain the presence of the smallest hematite spherules detected. The smallest are close to perfectly spherical and therefore cannot be explained by fracturing or erosion.<ref name=Weitz_2006 /> The [[Opportunity rover|''Opportunity'']] team also found that blueberries uncovered by the [[Rock Abrasion Tool]] aboard Opportunity were 4.2 +/- 0.9&nbsp;mm (0.16 inches) major axis length at Eagle Crater and 4.5 +/- 0.6&nbsp;mm at Endurance crater, about 2.2 +/- 0.5&nbsp;mm (0.087 inches) at Vostok and about 3.0 +/- 0.2&nbsp;mm (0.12 inches) at [[Naturaliste (crater)]]. Those found in "the plains" south of Endurance Crater were smaller (1-2mm or 0.04-0.08 inches) than those of Eagle and Endurance craters.<ref name=Weitz_2006 />

The second paper studying spherule size extended the study area 2–3&nbsp;km further south on the plains to Victoria Crater.<ref name=Calvin_2008 /> This paper reported similar observations to the first but went further to suggest  the observed size variation might be due to sampling different sediment stratigraphic levels at different locations. Additionally, it suggested simple variations in diagenetic conditions were linked to changes in spherule size.

The third paper made systematic size measurements of hematite spherules embedded in the walls of [[Victoria Crater]] at different heights.<ref name=Squyres_2009 /> ([[Victoria Crater]] is a large and deep crater.) These measurements showed a clear variation of spherule size with elevation within the sediments of [[Meridiani Planum]]. The smaller spherules were higher up, the larger ones lower down. The lowest spherules near the bottom of [[Victoria Crater]] had similar diameters to the spherules in [[Eagle Crater]], and the elevations of these distant locations were nearly equal.<ref name=Squyres_2009 />

No papers were written on spherule size that covered areas of the rover's traverse south from [[Victoria Crater]] to the enormous [[Endeavour Crater]]. However, searches of the archive of the images taken by the rover's Microscopic Imager show some of the largest blueberries photographed are close to the rim of [[Endeavour Crater]] (see Figure 11).

{{Gallery|align=center
| width = 240|Image:1M132266947EFF05AMP2987M2M1.Bluberries.inside.Eagle.Crater.Sol.46.jpg|'''Figure 9'''. Loose hematite spherules on an outcrop of sediments at Eagle Crater. At this location, most spherule diameters were 4 - 6 mm;<ref name=Squyres_2004 /> in this image, the range is 3 - 6 mm. Image taken on Sol 46 (2004-03-10).
|Image:1M208968375EFF748BP2957M2M1.small.blueberries.sol.910.jpg|'''Figure 10'''. Small, loose hematite spherules on soils about 500 m northwest of Victoria Crater. In this image, most spherules have 1 - 2 mm diameters. A few have diameters under 1 mm, and the largest is 2.5 mm x 4 mm. Image taken on Sol 910 (2004-08-15).
|Image:1M365124236EFFBKN5P2996M2M1.large.blueberries.sol.2669.close.to.Endeavour.rim.jpg|'''Figure 11'''. A few loose hematite spherules on an outcrop of sediments about 200 m from Endeavour Crater. The largest spherule has a diameter of 8.3 mm - this is one of the biggest photographed anywhere by ''Opportunity.'' Image taken on Sol 2669 (2011-07-28).
}}

==Numbers of Blueberries and loose spherule surface density==

There are no peer-reviewed published estimates of the number of loose hematite spherules on Meridiani's soils or embedded hematite spherules in the plain's sediments. However, the reader can sense how mind-boggling big those numbers are with a photograph of an area of soil with a typical surface density of the hematite spheres. Such a photograph ''has'' been published.<ref name=Calvin_2008 />

Figures 12 and 13 are true-color and false-color versions of the photo.<ref name=Calvin_2008 /> The spherules are easier to see in the published false-color version (Figure 23).<ref name=Calvin_2008 /> Click on it to enlarge it. The sampling target of Figures 12 & 13 had 29% coarse hematite coverage. The range of coverage among similar targets was 10% to 40%.<ref name=Calvin_2008 /> These targets were sampled over a wide area, between Sol 70 (2004-04-04) and Sol 999 (2007-11-15).

{{Gallery|align=center
| width = 360|Image:Sol532B P2456 1 True RAD.jpg|'''Figure 12'''. This (approximate true color) image is a surface hematite sampling target taken by ''Opportunity''{{'}}s  Pancam on Sol 532.<ref name=Calvin_2008 /> This target was measured to have 29% coarse hematite coverage. Image taken on Sol 532 (2005-07-02).        
|Image:Sol532B P2456 1 False L257.jpg|'''Figure 13'''. This is a false color version of Figure 12. The hematite spherules are easier to resolve in false color. Image taken on Sol 532 (2005-07-02).   
}}

The parts of the plain [[Opportunity rover|''Opportunity'']] studied are ''not'' special: Compared to the rest of Meridiani Planum, they do not have high surface hematite levels. To see this, look at the plain's surface hematite map (Figure 1b) and the small blue line (labeled OT ) indicating the route of [[Opportunity rover|''Opportunity'']]'s Traverse of the plain.

The mind-bogglingness of the number of loose hematite spherules hits when Figures 12 & 13 are extrapolated to the plain's whole surface area (about 150,000&nbsp;km<sup>2</sup><ref name=Christensen_Ruff />): 150,000&nbsp;km<sup>2</sup> is close to 2/3's the area of the main island of Japan ([[Honshu]]) and also 72% the area of the main island of the UK ([[Great Britain]]), it is also bigger than the land areas of [[List of U.S. states and territories by area|30 of the 50 states of the USA]].

The number of embedded spherules (in the plain's sediments) is probably much higher than the number of loose spherules (on soils).<ref name=Olsen_2021 /> Since (1) the estimates of the erosion depth of original sediment needed to produce the loose spherules are less than 1 meter,<ref name=Squyres_2004a /><ref name=Soderblom_2004 /><ref name=Golombek_2006 /> while (2) the typical depths of the plain's sediments are several hundred meters.<ref name=Arvidson_2006 />

==Shiny Blueberries without dust==

[[File:The Mystery of the Sparkling Spheres.jpg|thumb|'''Figure 14'''. Shiny spherules in the walls of a newly dug trench. Image taken in February 2004.]]
The image at right (Figure 14) shows shiny hematite blueberries. The shininess and the position of these blueberries are unusual. The rover [[Opportunity rover|''Opportunity'']] dug a trench into the top soils that lie over the Meridiani Planum's sediments. Figure 14 shows a wall to the newly dug trench with (partially uncovered) soil-embedded blueberries. 
Soil-embedded blueberries are rare. Size-sorting tends to position loose blueberries on or very near the surface of soil bedforms. Almost all photographed blueberries were exposed to the atmosphere and are now covered in a layer of Mars's dust.<ref name=Morris_2006 /> The layers of dust take away the shininess of the blueberries. The blueberries inside the trench are dust-free because the interiors of soil beds are largely dust-free.<ref name=Sullivan_2011>{{cite journal | doi=10.1029/2010JE003625 | title=Cohesions, friction angles, and other physical properties of Martian regolith from Mars Exploration Rover wheel trenches and wheel scuffs | year=2011 | last1=Sullivan | first1=R. | last2=Anderson | first2=R. | last3=Biesiadecki | first3=J. | last4=Bond | first4=T. | last5=Stewart | first5=H. | journal=Journal of Geophysical Research | volume=116 | issue=E2 | bibcode=2011JGRE..116.2006S }}</ref> Without the dust, these blueberries are shiny.

==Blueberries on Earth==

===Earth analogs===

Researchers from the [[University of Utah]] have explored the similarities between the blueberries and spherical concretions discovered within "Jurassic [[Navajo Sandstone]]" in southern Utah. They have concluded Mars must have had previous ground water activity to form the blueberries. However, they do note the spherules are more spherical in the Martian sample due to the lack of "joints, fractures, faults, or other preferential fluid paths", unlike the Utah sample.<ref>{{cite journal |last1=Chan |first1=Marjorie A. |last2=Beitler |first2=Brenda |last3=Parry |first3=W. T. |last4=Ormö |first4=Jens |last5=Komatsu |first5=Goro |title=A possible terrestrial analogue for haematite concretions on Mars |journal=Nature |date=17 June 2004 |volume=429 |issue=6993 |pages=731–4 |doi=10.1038/nature02600 |pmid=15201902 |bibcode=2004Natur.429..731C |s2cid=4424335 }}</ref> A team of researchers from Japan studied the spherules found in Utah as well as spherules that were later discovered in Mongolia, in the [[Gobi]]. They found evidence that the concretions found in these locations are first formed as "spherical calcite concretions" in sandstone. Acidic water rich in iron then dissolve the calcite leaving behind the iron rich (hematite) spherule. This leads to the conclusion that the blueberries may have formed early in Mars's history when the atmosphere was more dense by the same process.<ref>{{cite journal |last1=Yoshida |first1=H. |last2=Hasegawa |first2=H. |last3=Katsuta |first3=N. |last4=Maruyama |first4=I. |last5=Sirono |first5=S. |last6=Minami |first6=M. |last7=Asahara |first7=Y. |last8=Nishimoto |first8=S. |last9=Yamaguchi |first9=Y. |last10=Ichinnorov |first10=N. |last11=Metcalfe |first11=R. |title=Fe-oxide concretions formed by interacting carbonate and acidic waters on Earth and Mars |journal=Science Advances |date=1 December 2018 |volume=4 |issue=12 |article-number=eaau0872 |doi=10.1126/sciadv.aau0872 |pmid=30525103 |pmc=6281427 |language=en |issn=2375-2548|bibcode=2018SciA....4..872Y }}</ref>

==See also==
* {{annotated link|Meridiani Planum}}
* {{annotated link|Opportunity rover|''Opportunity'' rover}}
* {{annotated link|Mars Global Surveyor}}
* {{annotated link|2001 Mars Odyssey}}
* {{annotated link|Moqui Marbles}}
* {{annotated link|List of surface features of Mars imaged by Opportunity|List of surface features of Mars imaged by ''Opportunity''}}

==References==
{{reflist}}

==External links==
*[http://www.sciencemag.org/cgi/content/summary/306/5702/1697 ''Science'' magazine special issue on MER ''Opportunity'' initial results]
*[http://marsrovers.jpl.nasa.gov/newsroom/pressreleases/pressreleases-2004.html Press releases] from JPL relating to Mars spherules: [http://marsrovers.jpl.nasa.gov/newsroom/pressreleases/20040204a.html Initial press release], [http://marsrovers.jpl.nasa.gov/newsroom/pressreleases/20040204a.html composition results]
* [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2005LPI....36.2026G&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=44fdafdf2530369 Morphological Investigations of Martian Spherules, Comparisons to Collected Terrestrial Counterparts]. Full text [http://www.lpi.usra.edu/meetings/lpsc2005/pdf/2026.pdf PDF].
* [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2007LPI....38.1922B&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=44fdafdf2530369 Sedimentary Concretions vs. Impact Condensates: Origin of the Hematitic Spherules of Meridiani Planum, Mars]. Full text [http://www.lpi.usra.edu/meetings/lpsc2007/pdf/1922.pdf PDF].
* [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2007LPI....38.2053A&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=44fdafdf2530369 Ironstone Concretions - Analogs to Martian Hematite Spherules]. Full text [http://www.lpi.usra.edu/meetings/lpsc2007/pdf/2053.pdf PDF].
* [http://www.utah.edu/unews/releases/04/jun/marsmarbles.html Moqui Marbles] relations to Martian Spherules.
* [https://www.sciencedaily.com/releases/2004/06/040617082028.htm ScienceDaily.com] on Moqui marbles and Martian spherules
* [http://www.scienzainrete.it/contenuto/articolo/Ce-vita-su-Marte] relations to cyanobacteria and terrestrial stromatolites.

{{Features visited by Opportunity}}
{{Mars}}
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[[Category:Mars|Spherules]]
[[Category:Rocks on Mars]]