Bad Moon Rising

Categories: Feature Stories Mars
Shadow cast on cratered martian surface near western Xanthe Terra on August 26, 1999, about 2 p.m. local time, by moon Phobos. Compared to surface missions returning 10,000 or so images, the orbiters continue to provide over ten times that many.

Opportunity was able to photograph this brief eclipse of the Sun by the small Martian moon Deimos, as shown in the banner. Mars has two natural satellites, or moons, called Phobos ( Greek for "Fear") and Deimos ("Terror"). The inset right in the banner image shows the non-spherical shape of Deimos. Probably like its sister moon, it is also a captured asteroid. Deimos orbits Mars every 30 hours. Opportunity awoke Thursday morning to "Bad Moon Rising" by Creedence Clearwater Revival in honor of the eclipse.

Mars Sundial Credit: NASA JPL

Mission scientists were be able to catch a similar eclipse with the larger moon Phobos – which can blot out half the sun or more. Somewhere near the martian equator, Phobos eclipses the sun nearly every day.

Cornell’s Dr. Jim Bell, lead scientist for the Panoramic Camera, said "On Sol 39, we have confirmed that Opportunity acquired a solar eclipse by Deimos. We will also look at Phobos, as it passes across the sun. Because Phobos is closer to the surface, it blots out half the Sun. The eclipse happens quickly– in 30 to 40 seconds or so. It requires alot of timing, maybe also we will have to provide heat [to the rover] in the morning to be successful with Phobos in the next week."

Viking images of Phobos proved very valuable in determining the opacity of the night sky. Color and infrared reflectance values of the integrated disc were clues to the satellite’s chemical composition. The spectral data best fit that of carbonaceous chondrite, a carbon-rich variety of meteorite believed to represent primitive solar system material.

Rover controllers at NASA’s Jet Propulsion Laboratory (JPL), Pasadena, Calif., are planning to use the panoramic cameras on both Opportunity and Spirit for several similar events in the next six weeks. Bell expects the most dramatic images may be the one of Phobos planned for March 10.

"Scientifically, we’re interested in timing these events to possibly allow refinement of the orbits and orbital evolution of these natural satellites," Bell said. "It’s also exciting, historic and just plain cool to be able to observe eclipses on another planet at all," he said.

Depending on the orientation of Phobos as it passes between the sun and the rovers, the images might also add new information about the elongated shape of that moon.

March Phobos Eclipse taken by rover Credit: NASA JPL

Phobos is about 27 kilometers long by about 18 kilometers across its smallest dimension (17 miles by 11 miles). Deimos’ dimensions are about half as much, but the pair’s difference in size as they appear from Mars’ surface is even greater, because Phobos flies in a much lower orbit.

The rovers’ panoramic cameras observe the sun nearly every martian day as a way to gain information about how Mars’ atmosphere affects the sunlight. The challenge for the eclipse observations is in the timing. Deimos crosses the sun’s disc in only about 50 to 60 seconds. Phobos moves even more quickly, crossing the sun in only 20 to 30 seconds.

Scientists use the term "transit" for an eclipse in which the intervening body covers only a fraction of the more-distant body. For example, from Earth, the planet Venus will be seen to transit the sun on June 8, for the first time since 1882. Transits of the sun by Mercury and transits of Jupiter by Jupiter’s moons are more common observations from Earth.

From Earth, our moon and the sun have the appearance of almost identically sized discs in the sky, so the moon almost exactly covers the sun during a total solar eclipse. Because Mars is farther from the sun than Earth is, the sun looks only about two-thirds as wide from Mars as it does from Earth. However, Mars’ moons are so small that even Phobos covers only about half of the sun’s disc during an eclipse seen from Mars.

March Phobos Eclipse taken by rover Credit: NASA JPL

Phobos and the smaller, more distant satellite, Deimos, were discovered in 1877 by Asaph Hall, an astronomer at the United States Naval Observatory in Washington, D.C. Hall had been hunting for martian satellites for some time, and was about to abandon the search when he was encouraged by his wife to continue. In honor of her role in the discovery, the largest crater on Phobos was named Stickney, her maiden name. In a strange case of prescience, author Jonathan Swift wrote a century and a half earlier in his Gulliver’s Travels (1726) that " [The astronomers]..have likewise discovered two lesser stars, or satellites, which revolve about Mars."

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