[meteorite-list] Martian 'Blueberries' Really Pieces of Meteorites?

[Ron Baalke]

http://news.nationalgeographic.com/news/2014/02/140224-mars-blueberries-water-meteorite-space-science/

Martian "Blueberries" Really Pieces of Meteorites?

The famed "blueberries" of Mars discovered by NASA rovers might just be 
asteroid impact debris.

Scientists have proposed a number of theories to explain the countless 
gray-blue spherical rocks on Mars.

Joseph Bennington Castro
for National Geographic
February 24, 2014

The famed "blueberry" rocks discovered on Mars by NASA's Opportunity rover 
are not geological evidence of a history of ancient water on the red planet, 
a group of scientists now argue.

Instead, they propose that the tiny spherules are actually remnants of 
small meteorites that broke up in Mars's atmosphere. (See also "Mars Blueberries.")

In 2004, NASA's Opportunity rover found countless gray-blue spherical 
rocks on Mars. Scientists proposed a number of theories - from volcanic 
eruptions to shockwaves caused by meteor impacts - to explain the tiny 
spheres (nicknamed blueberries by the discovery team), which contained 
large amounts of the iron-bearing mineral hematite.

The discovery team suggested that the blueberries formed when groundwater 
flowed through porous rocks, the way similar hematite-carrying spheres 
formed on Earth. The water flow sparks chemical reactions that cause the 
iron minerals to precipitate into small, layered balls.

When it was announced, this "concretion" model explaining the blueberries' 
formation seemed to solidify the case for water flowing on Mars in ancient 
eons.

In the journal Planetary and Space Science, however, planetary scientists 
at the University of Hawaii in Honolulu suggest that a meteorite model 
better explains various physical properties of the blueberries.

"None of the physical properties of the spherules match the concretion 
model," says geophysics researcher Anupam Misra, who led the study. "But 
the meteorite theory explains all of their properties."

Concretion vs. Meteorite

The biggest issue with the concretion model is that it cannot explain 
the narrow sizes of the Martian blueberries, Misra says.

On Mars, most of the hematite rocks are about 0.16 inches (4 millimeters) 
in diameter, and no larger than 0.24 inches (6.2 millimeters). By contrast, 
Earth spherules exhibit a large range of sizes, not limited to only a 
quarter of an inch.

These size differences make sense if the Mars rocks have a cosmic origin, 
Misra says. If a small meteorite hit Mars's atmosphere, it would break 
up into smaller pieces that would shower the surface of the planet. Just 
a few of these impacts could produce the many blueberries discovered on 
Mars. The researchers calculated that a meteorite 1.6 inches (4 centimeters) 
in diameter could scatter a thousand 0.16-inch spherules over a large 
area.

The scientists also suggest that the blueberries aren't as old as previously 
thought. According to the concretion model, the slow erosion processes 
that uncover hematite rocks would indicate that the blueberries are thousands 
of years old. But the apparent shininess of some of the Mars blueberries 
suggests they may be relatively young and haven't undergone much physical 
weathering.

A recent meteorite event would produce such shiny rocks, and could have 
impacted NASA's rovers. Images of the Opportunity and Spirit rovers show 
what appear to be micro-blueberries on top of the machines, as well as 
a circular burn mark from impact, Misra says.

A Controversial Claim

Not everyone agrees with the scientists' critique of the concretion model. 
"There are some pretty gross inaccuracies about what they're claiming," 
says Brenda Bowen, a geologist at the University of Utah who has studied 
concretions on Earth.

For one thing, the environment on Earth is very different from that of 
Mars. If the blueberries on Mars are concretions, their sizes would be 
limited by water availability, the size of pores in the sediment rock, 
and the reactants available in the environment, Bowen says.

Bowen also notes that the shininess of the rocks doesn't mean they're 
young. "When we look at deposits formed from hematite concretions on Earth, 
some are coated in dust, some in sand, and some are nicely polished," 
she says. "When the wind blows, you can get this kind of polishing from 
abrasion."

Sky Is Falling?

Though Bowen thinks the concretion theory is correct, she doesn't rule 
out the possibility of a meteorite explanation for at least some of the 
blueberries.

However, Timothy Glotch does.

"These spherules didn't form from some kind of high-temperature event, 
as you would expect from something melting in the Martian atmosphere," 
says Glotch, a geoscientist at Stony Brook University in New York.

Analysis of the blueberries using tools onboard Opportunity suggests the 
spherules formed from a low-temperature process, he says, adding that 
the resolution of the supposed rover-impact images is too low to definitively 
identify micro-blueberries or impact burns.

Whatever the case, a cosmic origin for the Mars blueberries wouldn't significantly 
change our understanding of the red planet, since other evidence points 
to a watery past for Mars, Misra says. But it would suggest that future 
scientific tools sent to Mars might need to be more robust to survive 
possible meteorite showers.

"If you are designing instruments to be used on Mars, those instruments 
need to be able to take impacts from small meteorites," Misra says.