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Soviet findings from Phobos and Mars.

Soviet Findings from Phobos and Mars

Of the two Soviet Phobos spacecraft sent last year toward Mars and its larger moon Phobos, the first failed en route due to an incorrect ground command and the second reached its Mars-circling orbit but operated there successfully for less than two months. Even so, Soviet and Western scientists writing in the Oct. 19 NATURE report some intriguing preliminary findings from the truncated mission.

One group puts the overall density of Phobos at about 1.95 grams per cubic centimeter, less than the 2.2 g/cc determined in 1976 from measurements by the two U.S. Viking spacecraft. The researchers note that the change results from more accurate tracking of the spacecraft, which improved knowledge of Phobos' position in its orbit by "an order of magnitude."

Spectral measurements suggest that the potato-shaped moon, no more than 25 kilometers at its largest dimension, resembles a class of meteorites called carbonaceous chondrites. Their densities appear closer to the earlier estimate of Phobos' density. Scientists say that the new, lower number suggests either that Phobos has a porous interior or that its interior contains a significant amount of ice of some type.

Though Phobos 2 never got close enough to send its landing craft down to sample the surface, infrared instruments obtained measurements in orbit from as close as 190 km away, showing differences in the spectral and thermal properties of surface areas less than 2 km across. Some scientists have suggested that fine, rocky grains cover the moon's entire surface, but the Phobos researchers note that while some areas may indeed represent deposits of fine material, others seem more solid, like "localized exposures of consolidated rock."

An instrument called an infrared spectrometer accumulated more than 36,000 spectra, not only of Phobos, but also of Mars. They cover more than 25 percent of a Martian area that includes the huge canyon called Valles Marineris, as long as the United States is wide, and the large volcanoes atop an uplift known as the Tharsis Montes.

The spectra show variations of as much as 20 percent in the amount of hydrated minerals on the Tharsis volcanoes. Furthermore, the data suggest that the slopes of the volcanoes appear consistently richer in such minerals than do the "much dryer" surrounding plains.

The two Viking landers measured the elements on the Martian surface, but only those in the fine materials within a few feet of where the craft touched down. Phobos 2 provided much broader measurements from orbit by recording gamma rays emitted from the surface in response to solar and galactic cosmic rays. (The Soviet Union's Mars 5 craft made similar measurements, though with less precision, in 1974.)

The readings presumably represent a mixture of surface material and underlying bedrock, but one researcher suggests that it should be possible to separate the two components. "The closest analogue of the rock on Earth," according to one of the Phobos mission teams, "has been found to be subalkaline basalt occurring on oceanic seabeds."
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Publication:Science News
Date:Oct 28, 1989
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