Martian spherules (also known as blueberries due to their blue hue in false-color images released by NASA) are the abundant spherical hematite inclusions discovered by the Mars rover Opportunity at Meridiani Planum on the planet Mars in 2004.
The Mars Global Surveyor Thermal Emission Spectrometer (TES) first detected “crystalline hematite (α‐Fe2O3)” within Sinus Meridiani from orbit. The presence of hematite was confirmed after the Opportunity rover landed in Meridiani Planum. The moniker "blueberries" was coined by the original science team due to the hematite appearing blue relative to the surrounding media in the "natural color RGB images" analyzed.
A number of straightforward geological processes can yield round shapes, including accretion under water, meteor impacts, or volcanic eruptions. The principal investigator, Steve Squyres, indicates they could alternately be concretions, or accumulated material, formed by minerals coming out of solution as water diffused through rock.
Researchers at NASA’s Jet Propulsion Laboratory studied all the soil environments observed by Opportunity, and therefore studied the blueberries from Eagle Crater to Endurance (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 mm (just over 1/10th inch). They also discovered blueberries that are 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. 
The JPL 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 note 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.
NASA’s JPL also found that blueberries uncovered by the Rock Abrasion Tool aboard Opportunity were about 4 mm (0.16 inches) semi-major axis length at Eagle Crater and Endurance crater, about 2.2mm (0.087 inches) at Vostok, and about 3.0 mm (0.12 inches) at Naturaliste (crater). Those found in “the plains” were smaller (1-2mm or 0.04-0.08 inches) than those of Eagle and Endurance craters.
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. 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.
Interpretation as fungus
In 2021, neuroscientist Rhawn Gabriel Joseph co-authored a paper published in Advances in Microbiology which highlighted a series of Opportunity photos of spherules which appeared to show the objects "expand in size, or conversely, change shape, move to new locations, and/or wane in size and nearly disappear". The paper concluded that the images strongly supported the idea that the spherules were fungal in nature, Joseph coining the term "Martian mushrooms" to describe them.
Biologist PZ Myers dismissed the paper as "an extraordinary claim that requires better evidence than an analysis of photographic morphology", noting that Joseph had previously concluded from similar photographic analysis to have seen "fields of skulls" on Mars.
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- Calvin, W. M.; Shoffner, J. D.; Johnson, J. R.; Knoll, A. H.; Pocock, J. M.; Squyres, S. W.; Weitz, C. M.; Arvidson, R. E. et al. (2008). "Hematite spherules at Meridiani: Results from MI, Mini-TES, and Pancam" (in en). Journal of Geophysical Research: Planets 113 (E12): E12S37. doi:10.1029/2007JE003048. ISSN 2156-2202. Bibcode: 2008JGRE..11312S37C. http://nrs.harvard.edu/urn-3:HUL.InstRepos:3196278.
- "Mars Exploration Rover Mission: Press Releases". http://mars.nasa.gov/mer/newsroom/pressreleases/20040204a.html.
- "Dining on Diamonds - Astrobiology Magazine" (in en-US). Astrobiology Magazine. 2004-08-05. http://www.astrobio.net/news-exclusive/dining-on-diamonds/.
- Weitz, C. M.; Anderson, R. C.; Bell, J. F.; Farrand, W. H.; Herkenhoff, K. E.; Johnson, J. R.; Jolliff, B. L.; Morris, R. V. et al. (2006). "Soil grain analyses at Meridiani Planum, Mars" (in en). Journal of Geophysical Research: Planets 111 (E12): n/a. doi:10.1029/2005JE002541. ISSN 2156-2202. Bibcode: 2006JGRE..11112S04W.
- Chan, Marjorie A.; Beitler, Brenda; Parry, W. T.; Ormö, Jens; Komatsu, Goro (17 June 2004). "A possible terrestrial analogue for haematite concretions on Mars". Nature 429 (6993): 731–4. doi:10.1038/nature02600. PMID 15201902. Bibcode: 2004Natur.429..731C.
- Yoshida, H.; Hasegawa, H.; Katsuta, N.; Maruyama, I.; Sirono, S.; Minami, M.; Asahara, Y.; Nishimoto, S. et al. (1 December 2018). "Fe-oxide concretions formed by interacting carbonate and acidic waters on Earth and Mars" (in en). Science Advances 4 (12): eaau0872. doi:10.1126/sciadv.aau0872. ISSN 2375-2548. PMID 30525103. Bibcode: 2018SciA....4..872Y.
- Delbert, Caroline (7 May 2021). "Scientists Believe These Photos Show Mushrooms on Mars. Not So Fast.". Popular Mechanics. https://www.popularmechanics.com/space/moon-mars/a36356445/mushrooms-on-mars-nasa-photos-life-on-mars/.
- Joseph, Rhawn Gabriel. "(PDF) Fungi on Mars? Evidence of Growth and Behavior From Sequential Images" (in en). Advances in Microbiology. https://www.researchgate.net/publication/351252619_Fungi_on_Mars_Evidence_of_Growth_and_Behavior_From_Sequential_Images.
- Ryan, Jackson. "No, NASA photos are not evidence of fungus growing on Mars, sorry" (in en). CNET. https://www.cnet.com/news/sorry-nasa-photos-are-not-evidence-of-fungus-growing-on-mars/.
- Science Magazine special issue on MER Opportunity initial results
- Press releases from JPL relating to Mars spherules: Initial press release, composition results
- Morphological Investigations of Martian Spherules, Comparisons to Collected Terrestrial Counterparts. Full text PDF.
- Sedimentary Concretions vs. Impact Condensates: Origin of the Hematitic Spherules of Meridiani Planum, Mars. Full text PDF.
- Ironstone Concretions - Analogs to Martian Hematite Spherules. Full text PDF.
- Moqui Marbles relations to Martian Spherules.
- ScienceDaily.com on Moqui marbles and Martian spherules
-  relations to cyanobacteria and terrestrial stromatolites.
Original source: https://en.wikipedia.org/wiki/ Martian spherules. Read more