Mars rovers see Earth, moons and stars
By Robert Roy Britt
(SPACE.com) -- The Spirit rover on Mars took the first picture of Earth ever made from the surface of another planet. It also did a little astronomy, imaging bright stars.
It also spotted what could be a Viking Orbiter spacecraft or a meteor -- scientists aren't sure which.
The photo of Earth shows the planet as a bright dot above the horizon about an hour before sunrise. The image is not in color, though scientists say if a human stood in the same spot and looked earthward, home would probably appear pale blue.
On the other side of the planet, Opportunity captured animated images of Mars' moon Phobos eclipsing the sun. This, along with the previous image of Deimos' solar eclipse, will help astronomers pin down the small moons' orbits around the planet. Mark Lemmon, a rover science team member from Texas A&M University, said Phobos' orbital position is uncertain, with its actual route varying by about 6 miles, which is roughly the size of the moon itself.
Knowing Phobos' exact orbital path would allow satellites orbiting Mars to obtain close-up photos of the moon. Researchers do not know if the moons formed along with Mars or are captured asteroids.
Stars and streaks
Spirit is also seeing stars. The rover took nighttime images in the direction of the constellation Orion. The bright star Betelgeuse is visible in the upper right. Orion's belt, a row of three bright stars, can be seen near the bottom of the photograph.
Faint specks on the image are the result of cosmic rays hitting the camera, Lemmon said.
None of Spirit's astronomy images are part of the rover's primary mission, but by taking more of them, scientists hope to learn something about the amount of dust and water vapor in the nighttime atmosphere of Mars.
Another sky photo from Spirit shows a thin and short streak of light.
"That streak could have been a meteor," Lemmon said. Or it could have been the Viking Orbiter 2, still circling Mars long after its 1970s mission ended. Lemmon said the other nine spacecraft currently orbiting Mars -- three of which are presently in working order -- have known positions and did not create the streak.
Spirit reaches Bonneville
Meanwhile, carrying out its day job, Spirit has finally peered down into an impact crater called Bonneville. It is the first view of a good-sized impact crater on Mars ever taken from this vantagepoint, said Matt Golombek of NASA's Jet Propulsion Laboratory (JPL).
Scientists plan to use the Orion picture to help plan future astronomical observations from Mars.
The crater does not appear to harbor any sedimentary rock outcroppings, like what was found at the Opportunity landing site, Golombek said at a press conference today. Instead, the rocks around the rim appear to all be similar to rocks the rover has encountered running up to the rim. They are all thought to have been cast out by an ancient impact.
The lack of outcroppings of bedrock is somewhat of a disappointment for scientists, because it suggest there might not be any easy-to-find signs of standing water at the Spirit site. The craft has found signs of past water associated with volcanic activity, but not the sort of soggy situation revealed by Opportunity.
Spirit will explore the crater rim for a week or two before deciding whether to drive down in or move on toward the distant East Hills. The decision will be made based on both science and rover safety.
On the other side of the planet, the Opportunity rover is in the process of analyzing the "blueberry bowl," a high concentration of BB-sized spheres. Scientists are confident the spheres, which they sometimes call blueberries, formed in water, but they don't yet know their composition.
Hematite was water-generated
Spirit rover sent back this navigation camera panorama of Bonneville crater.
One of Opportunity's next tasks will be to further investigate a mineral called hematite, which is abundant on the plains that surround the shallow depression in which the robot landed. Phil Christensen of Arizona State University in Tempe said the latest infrared observations show the hematite is highly concentrated in hot spots.
"We call them the mother lode of hematite," Christensen said. The hotspots suggest the hematite has been on the plains for perhaps a billion years and has been broken up from an original rock. He also figures the hematite was long ago punched out of the landing-site crater, which contains very low quantities of the mineral.
Over the eons, some hematite has been transported back down into the shallow crater, but "that's a very slow process," he said.
On Earth, hematite usually forms in the presence of standing water. Scientists had sought to determine if water was the source of the martian hematite, which had first been detected from orbiting spacecraft and was one reason Opportunity's landing site was picked.
Given the discovery of past water at Opportunity site announced earlier this month, "I think it's fair to say the hematite also formed in water," Christensen said. His team will now try to find out how the hematite fits into the overall story of past water on Mars.
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