13 September, 1998:
New temperature data and close-up images of the Martian
moon Phobos gathered by NASA's Mars Global Surveyor indicate the
surface of this small body has been pounded into powder by eons
of meteoroid impacts, some of which started landslides that left
dark trails marking the steep slopes of giant craters.
New temperature measurements show the surface must be
composed largely of finely ground powder at least one meter
(three feet) thick, according to scientists studying infrared
data from the thermal emission spectrometer instrument on the
spacecraft. Measurements of the day and night sides of Phobos
show such extreme temperature variations that the sunlit side of
the moon rivals a pleasant winter day in Chicago, while only a
few kilometers away, on the dark side of the moon, the climate
is more harsh than a night in Antarctica. High temperatures for
Phobos were measured at -4 degrees Celsius (25 degrees
Fahrenheit) and lows at -112 Celsius (-170 degrees Fahrenheit).
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Thermal spectrometer data indicate that the surface temperature in
the shadowed region of Phobos is -170° F (-112° C), while a few kilometers away (on the sunlit side of Phobos) the
temperature is +25° F (-4 ° C). The extreme temperature differences
indicate that the surface is composed of very small particles that lose their heat rapidly once the Sun sets. In
addition, Phobos does not have an atmosphere to hold heat during the night.
credits + more images
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A picture of the tiny moon Phobos.
The large crater is called Stickney. It is 10 km in diameter and
is nearly half the size of Phobos itself. Like its companion moon Deimos, Phobos
is believed to have once been an asteroid which was trapped by
Mar's gravity.
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The extremely fast heat loss from day to night as Phobos
turns in its seven-hour rotation can be explained if hip-deep
dust covers its surface, said Dr. Philip Christensen of Arizona
State University, Tempe, principal investigator for the
experiment on the Mars Global Surveyor spacecraft.
"The infrared data tells us that Phobos, which does not
have an atmosphere to hold heat in during the night, probably
has a surface composed of very small particles that lose their
heat rapidly once the Sun has set," Christensen said. "This has
to be an incredibly fine powder formed from impacts over
millions of years, and it looks like the whole surface is made
up of fine dust."
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| New images from the spacecraft's Mars orbiter camera show
many never-before seen features on Phobos, and are among the
highest resolution ever obtained of the Martian satellites. A
10-kilometer-diameter (six-mile) crater called Stickney, which
is almost half the size of Phobos itself, shows light and dark
streaks trailing down the slopes of the bowl, illustrating that
even with a gravity field only about 1/1000th that of the
Earth's, debris still tumbles downhill. Large boulders appear
to be partly buried in the surface material.
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This portion of Mars Orbiter Camera image 50103 shows mass movements--bright and dark streaks--on the interior
wall of
Stickney crater. credits + more images
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Infrared measurements of Phobos were made on August 7, 19
and 31 from distances ranging between 1,045-1,435 kilometers
(648-890 miles), far enough away to capture global views of the
Martian moon in a single spectrum. The instrument has been able
to obtain the first global-scale infrared spectra of Earth and
Mars in addition to the new Phobos data, bringing new insights
about the composition of these three very different worlds.
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"Of the three, Earth has the most complex infrared spectra,
primarily due to the presence of carbon dioxide, ozone and water
vapor in its atmosphere," Christensen said. "Mars, which is
much colder than Earth because of its distance from the Sun, is
less complex and shows only significant amounts of carbon
dioxide. The spectrum of Phobos, however, has little structure
because it has no atmosphere and the energy it emits is coming
entirely from its surface."
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Mars Global Surveyor is part of a sustained program of Mars
exploration, managed by the Jet Propulsion Laboratory for NASA's
Office of Space Science, Washington, DC. Lockheed Martin
Astronautics, Denver, CO, which built and operates the
spacecraft, is JPL's industrial partner in the mission. JPL is a
division of the California Institute of Technology, Pasadena,
CA.
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