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-Fe2O3) and are found in exceedingly large numbers at Meridiani Planum, a vast Martian plain.
These spherules were discovered on the Martian day (sol) that NASA's 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 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 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-Fe2O3) 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.
Maps of surface hematite
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.
Search for signs of water and 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.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). 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:
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
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 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. 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 & 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).Bell, J (ed.) The Martian Surface. 2008. Cambridge University Press. The concretion process to form spherules of hematite probably occurred by diffusion of the hematite through the sedimentary rock matrix.
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. 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: 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. 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. 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. It erodes about 30 to 300 times faster than other regions of Mars (such as Gusev Crater). (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.