Small iron oxide spherules found on Mars
Martian spherules (also known as hematite spherules, blueberries, &amp; 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. The size of these spherules varies by location and elevation across the Meridiani Planum.
Martian blueberries are rich in the iron oxide hematite, but determining how rich they are in this iron oxide has proven difficult. (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.
Initial discovery.
Discovery from Orbit.
The Thermal Emission Spectrometer (TES) on the orbiter "Mars Global Surveyor" first detected crystalline gray hematite (α-Fe2O3) within the Sinus Meridiani. This discovery was part of a broader effort to map Mars for minerals associated with past water.
Maps of Surface Hematite.
Between 1997 and 2002, the "Mars Global Surveyor"'s 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. 
Search for Signs of Water &amp; Life.
In the 1990s, NASA officials wanted to delineate a framework for "faster, better, cheaper" exploration of Mars. In this context, the "Water Strategy" was outlined in 1995/1996. 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.
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 Rovers (MERs), named "Opportunity" and "Spirit". 
The decisiveness for NASA of the hematite map of Figure 1b for choosing the landing site for "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.
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" mission introduced the new place name Meridiani Planum for (roughly) the high hematite area in Figure 1b.
Discovery of Spherules on the Ground.
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.
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. The loose blueberries and soils are eroded out of the underlying sediments. 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. 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:
Each of these broad steps involved multiple sub-step processes, described in the following sub-sections:
Noachian River Flows.
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.
Formation of Today's Sediments &amp; 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. This transformation probably included a significant additional deposition of high-sulfur-content material of volcanic origin. The change certainly included aqueous geochemistry that was acidic and salty, as well as rising &amp; falling water levels: Features providing evidence include cross-bedded sediments, the presence of vugs (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. Jarosite formation requires aqueous acidic conditions below pH 3.
Figures 5 and 6 show Microscopic Imager close-ups of the sediment rock matrix that appeared in a prestigious paper. 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). Figure 6 images a similar sediment outcrop surface to Figure 5. However, "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. 
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. 
At least one model of global Martian hydrology accounts for the historical shift in water flows at Meridiani Planum. 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 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.
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.
Formation of Soils &amp; Loose Spherules, Crater Degradation.
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 and about 3 billion years ago. The only water left at the plain is bound in rocks.
Erosion with water flows in earlier eras was much faster than in this arid epoch.
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.
The meteorite, gravity, and wind-driven processes work like this:
Phil Christensen outlined these processes in 2004, soon after "Opportunity" landed. Later, more in-depth research confirmed them and added details to Christensen's outline.
Blueberry Composition.
Early Blueberry Composition Results.
Early on, "Opportunitys Mössbauer spectrometer took data that determined that the iron mineral component of these spherules is dominated by hematite. However, the Mössbauer spectrometer provided no information about the mineral components of these spherules that do not"' contain iron.
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. 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 "Opportunitys mini-TES (thermal emission spectrometer) could not"' detect any silicate minerals in the spherules. 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). 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, Na2O, P2O5, SO3, and Cl) each had content above trace-level with a combined group content of 6.8 +/- 2.4 wt%, the SiO2 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" science team published three papers that studied variations in hematite spherule size. They found spherule size variation by location and elevation.
In the earliest paper, a team of "Opportunity" rover scientists reported on studies of all the soil 
materials found between the landing