Different Flavors of Blueberries

Pasadena Opportunity Sol 61

As the Opportunity rover left Eagle Crater, it took a retrospective look at the scientific treasures that its two month laboratory had presented. The rover landed in what looked from orbit to be a flat plain, but one rich in an iron oxide called hematite. No one knew exactly how the iron got there or why it was concentrated in an equatorial region the size of Oklahoma.

Hematite shows a right and left peak in the Mossbauer spectra, with a higher concentration at the crater rim than the floor. Credit: NASA/JPL/Mainz

There were hints that water may have once helped rust the iron to its oxide form, a clue that initially sparked the landing team to select these plains called Meridiani Planum. Scientists now believe a strangely spherical rock formation, which they call ‘blueberries’, is one important piece of both the hematite and ultimately the water puzzle they are trying to piece together.

According to Germany’s University of Mainz scientist, Goestar Klingelhoefer, an instrument called the Mossbauer spectrometer can help the rover trace back how the blueberries came to scatter across Eagle Crater. "The blueberries are iron-rich. Their composition is dominated by hematite. The distribution changes from basaltic (volcanic in origin) to hematite from the crater’s center to its rim." Scientists reported Friday that they have found the most hematite anywhere yet seen on Mars at the crater rim, even higher than what previously was thought to form a natural bowl of many such berries. This region, called Blueberry Bowl, had a backdrop of the lighter outcrop or bedrock, which tended to dilute the spectrometer’s reading relative to what is found at the crater rim.

To understand how martian winds, water and asteroid impacts might have strewn berries across the crater, Bethany Ehlmann of Washington University, St. Louis, has looked at five places in the south and east portion of Eagle Crater. "These five spots looked different, or make up distinct soil units," said Ehlmann.

The rover failed to escape the crater on its first try as it reached a 17 degree incline and very fine dust at the rim. After making no progress but downward trenching, the rover backed up and headed for a shallower escape.
Credit: NASA/JPL

"Their study is to discover the source: Are the berries formed from the outcrop or plains? Where does it all come from?" said Ehlmann. The grain size is one key part of answering these questions. Across the crater floor and rim, a layer of finer dust sits below these coarse spheres. While initially this might appear to be expected if wind sorted the grains and filled in the crater with spherules, but the phenomenon is somewhat curious since the fine dust is actually less dense than the overlying rocky spherules. But this geological pattern of sorting is now unlike what one finds when a shaken box of breakfast cereal will distribute heavier fruit pieces on top of finer cereal flakes. The sorting in a martian case is not so much shaking but wind.

Ehlmann points to one of the five spots called Neopolitan, because it consists of a contact point — or triple point–where three soil units meet. A dark unit borders a lighter unit next to a scuff mark from the rover’s airbags which has uncovered yet another subsurface. Ehlmann says the darker jumble of particles has fine sand underneath with angular, more basaltic character. "Some have vesicles from cooling lava. The vanilla, or lighter area, has more spherical particles and wind transport has filled in with fines."

This soil survey, according to Dr. Ray Arvidson also of Washington University, is about to expand "to verify if blueberries are strewn across the plains" between Eagle Crater and its next driving destination at a much larger (150 meter) crater called Endurance. Endurance is several tens of sols away, about 700 meters or less than a half-a-mile from Eagle.

"To understand what we now believe was a shallow, salty ocean we need a broader perspective," said Arvidson. The Meridiani region appears to once have been a "shallow, open water environment, although very acidic."

The telltale sign for the scientists that standing water shaped some rocks was cross-bedding, an inclined sediment layer indicative of soil and lapping water akin to a shoreline. "Geologists look in the field in three-dimensions," said Arvidson, "for what is called the ‘strike’ and ‘dip’ through a sediment." Like the inclines of pitch or yaw to an airplane pilot, these lateral depths slice through a rock and tell a layered story of how water shaped the deposit. Arvidson noted that stereo images from the rover can reveal strike and dip, but "it requires a fair amount of calculation."

Mars Global Surveyor image of landing site, showing Endurance Crater. Click image for larger view.
Credit: Malin Space Systems/NASA/JPL

Another work in progress, according to Arvidson, is how these volcanic glasses might have been transformed by contact with water. In a process called devitrification, slightly wet volcanic glass can crystallize. In Eagle Crater, this is mainly "crud", says Arvidson, where the glass may have eventually weathered to dust but through an intermediate semi-crystalline opal or silica with hydrated iron and sulfates. The high salt and acid content detected at parts of Eagle Crater has reached forty percent, where it mainly consists of Epsom salts or magnesium sulfate.

What has scientists intrigued are any signs of vertical reliefs, since crater walls or layered rocks tell the geological history of Mars. Between Eagle and Endurance crater spans a wide, horizontal plain but Arvidson said that Opportunity’s extended mission travelling to Endurance will seek out both the source of the blueberries and an even deeper vertical relief at Endurance.

Mission manager, Matt Wallace, concluded that both rovers were healthy. "We try to keep our finger on the pulse of vehicle health, looking for signals or markers of subtle changes and trends. Except for environmental changes (power, thermal, optical opacity and dust accumulation), there is no wear and tear on subsystems."

Related Web Pages

JPL Rovers
Spirit’s Sol images and slideshow
Opportunity image gallery and slideshow
Mars Berries Once Rich in Iron-Water
NASA’s RATs Go Roving on Mars

Water Signs
Microscopic Imager
Gusev Crater
Pancam– Surveying the Martian Scene
Mössbauer spectrometer
Alpha Proton X-ray Spectrometer