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 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&nbsp;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.

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. 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.

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&nbsp;mm diameter) spherules lying all over the place inside Eagle Crater.

Figure 2 shows a thumbnail of the view from Opportunitys 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

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.

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. 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.

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

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. 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 site in Eagle Crater to the location on sol 552 of the rover's traverse (between 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.

The 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. 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. 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.

See also

References

  • 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
  • [http://www.scienzainrete.it/contenuto/articolo/Ce-vita-su-Marte] relations to cyanobacteria and terrestrial stromatolites.