Марсианские гематитовые сферы в Meridiani Planum

Small iron oxide spherules found on Mars

Martian spherules (also known as hematite spherules, blueberries, & Martian blueberries) are small spherules (roughly spherical pebbles) that are rich in an iron oxide (grey hematite, α Fe2O3) and are found at Meridiani Planum (a large plain on Mars) in exceedingly large numbers. These spherules were discovered on the Martian day that NASA's Mars Exploration Rover Opportunity landed at Meridiani Planum. (At NASA's Mission Control building that was January 24, 2004.) They are grey but look bluish next to the ubiquitous rusty reds on Mars, and since the first spherules found in Eagle Crater were 3–6 mm in diameter,
the 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. (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. This discovery was part of a broader effort to map Mars for minerals associated with past water.

Between 1997 and 2002, the Mars Global Surveyors TES mapped the whole planet of Mars for surface hematite levels. Figure 1a gives the 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.

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 mission introduced the new place name Meridiani Planum for (roughly) the high hematite area in Figure 1b.

NASA'S rover Opportunity successfully made the "hole in one" landing into Eagle Crater at Meridiani Planum on January 24(PST), 2004. On the first sol (Martian day), the rover immediately discovered thousands and thousands of small (4–6 mm diameter) spherules lying all over the place inside Eagle Crater. Figure 2 shows a thumbnail of the view from 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." Figure 3 is a detail of Figure 2 showing the grey spherules more clearly (click to enlarge). Tests quickly found that the grey spherules are rich in grey hematite. These tests included doing the "berry bowl" experiment (more below). The moniker "blueberries" was coined for the grey hematite spherules by the original Opportunity science team due to these spherules appearing bluish relative to the underlying rusty red soils in the "natural color RGB images" analyzed.

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. The dried river valleys are easily seen in thermal inertia images taken in orbit by Mars Odyssey and reproduced in Figure 4 (click on it for higher resolution). The river valleys seen in Figure 4 terminate abruptly as they flow into the Meridiani's massive formation of sediments.

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. Jarosite formation requires aqueous acidic conditions below pH 3. 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. The hematite formed into spherules by concretion (when minerals came out of solution). The concretion process to form spherules of hematite probably occurred by diffusion of the hematite through the sedimentary rock matrix.

The period of rising and falling aquifer levels ceased, and no water flowed on Meridiani Planum thereafter. Although, when this happened is poorly understood. Estimates include around 3.5 billion years ago

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.

In 2021, neuroscientist Rhawn Gabriel Joseph and others, including Rudolph Schild, co authored a paper accepted by the predatory journal 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", noting that the paper's first author had previously concluded from similar photographic analysis to have seen "fields of skulls" on Mars.