Coatings on Rocks Around a Young Crater
Weird coatings on rocks beside a young Martian crater remain puzzling after a preliminary look at
data from examination of the site by NASA's Opportunity rover.
The rover spent six weeks investigating the crater called "Concepción" before resuming its long journey
this month. The crater is about 10 meters (33 feet) in diameter. Dark rays extending from it, as seen
from orbit, flagged it in advance as a target of interest because the rays suggest the crater is young.
The rocks ejected outward from the impact that dug Concepción are chunks of the same type of bedrock
Opportunity has seen at hundreds of locations since landing in January 2004: soft, sulfate-rich sandstone
holding harder peppercorn-size dark spheres like berries in a muffin. The little spheres, rich in iron,
gained the nickname "blueberries."
Lots of Rocks at This Crater Have the Coating
"It was clear from the images that Opportunity took on the approach to Concepción that there was strange
stuff on lots of the rocks near the crater," said Steve Squyres of Cornell University, Ithaca, N.Y., principal
investigator for Opportunity and its twin rover, Spirit. "There's dark, grayish material coating faces of the
rocks and filling fractures in them. At least part of it is composed of blueberries jammed together as close as
you could pack them. We've never seen anything like this before."
Opportunity used tools on its robotic arm to examine this unusual material on a rock called "Chocolate Hills."
In some places, the layer of closely packed spheres lies between thinner, smoother layers. "It looks like a
blueberry sandwich," said Matt Golombek, a rover science-team member at NASA's Jet Propulsion Laboratory,
Initial analysis of the coating's composition does not show any obvious component from whatever space rock hit
Mars to dig the crater, but that is not a surprise, Golombek said. "The impact is so fast, most of the impactor
vaporizes," he said. "Thin films of melt get thrown out, but typically the composition of the melt is the stuff
that the impactor hit, rather than the impactor material."
Impactor Debris or Common Joint Filling?
The composition Opportunity found for the dark coating material fits at least two hypotheses being evaluated, and
possibly others. One is that the material resulted from partial melting of blueberry-containing sandstone from the
energy of the impact. Another is that it formed from filling of fractures in this type of rock before the impact occurred.
"It's possible that when you melt this rock, the sandstone melts before the blueberries do, leaving intact blueberries
as part of a melt layer," Squyres said. "As an alternative, we know that this type of rock has fractures and that the
sandstone can dissolve. Long ago, water flowing through fractures could have dissolved the sandstone and liberated
blueberries that fell down into the fracture and packed together. In this hypothesis, the impact that excavated the
crater did not play a role in forming this material, but split rocks along fractures so the material is exposed on the exterior like a coating."
This Material Was Not Found At Other Craters?
Golombek said, "One consideration that jumps out is that we've been driving around this part of Mars for six years and
never seen this stuff before, then we get to this young crater and it's coating rocks all around the crater. Sure looks
like there's a connection, but it could just be a coincidence."
The observation that the rocks thrown from the crater have not yet eroded away much is evidence that the crater is young,
confirming the suggestion from the dark rays. Squyres said, "We're not ready to attach a number to it, but this is really
young. It is the youngest crater we've ever seen with Opportunity and probably the youngest either rover has seen."
Why The Crater Has Dark Rays
One question Opportunity's visit did answer was about the dark rays: "We wondered before getting to Concepción why the rays
are dark," Golombek said. "We found out that the rays are areas with blocks of light-toned sandstone ejected from the crater.
They look dark from orbit because of the shadows that the blocks are casting when the orbital images are taken in mid-afternoon."
Since departing Concepción on March 9, Opportunity has driven 614 meters (2,014 feet) farther along the route to its long-term
destination at Endeavour Crater, about 19 kilometers (12 miles) in diameter and still at a drive distance of more than 12 kilometers (7 miles).
Back On the Road to Endeavor!
Squyres said, "We're on the road again. We have a healthy rover and we have enough power for substantial drives. We want to get
to Endeavour with a healthy rover. It takes a compelling target for us to stop and study. And Concepción was a compelling
target." JPL, a division of the California Institute of Technology in Pasadena, manages the Mars Exploration Rover Project
for the NASA Science Mission Directorate, Washington.
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|This image from the panoramic camera on NASA's Mars Exploration Rover Opportunity shows a rock called "Chocolate Hills," which
the rover found and examined at the edge of a young crater called "Concepción."
The rover used the tools on its robotic arm to examine the texture and composition of target areas on the rock with and without the
dark coating. The rock is about the size of a loaf of bread. Initial analysis was inconclusive about whether the coating on the
rock is material that melted during the impact event that dug the crater. This view is presented in false color, which makes some
differences between materials easier to see. It combines three separate images taken through filters admitting wavelengths of
750 nanometers, 530 nanometers and 430 nanometers. Opportunity took the image during the 2,147nd Martian day, or sol,
of the rover's mission on Mars (February 6, 2010). Image by NASA/JPL-Caltech/Cornell University. Enlarge image.
|This image from the microscopic imager on NASA's Mars Exploration Rover Opportunity shows
details of the coating on a rock called "Chocolate Hills," which the rover found and examined at the edge of a young crater called "Concepción."
The rover took this image during the 2,150th Martian day, or sol, of its mission on Mars (Febrary 9, 2010). This target patch on Chocolate Hills is called "Aloya."
The view covers an area about 3 centimeters (1.2 inches) across. The color comes from imaging the same area with the panoramic camera and is false color to highlight differences in materials.
The coating includes a layer in which peppercorn-size spheres nicknamed "blueberries" are packed densely. Image by NASA/JPL-Caltech/Cornell University. Enlarge image.