Larry Klaes ( )
Fri, 11 Sep 1998 14:40:53 -0400

Sent: Friday, September 11, 1998 2:30 PM
To: undisclosed-recipients:;

Douglas Isbell
Headquarters, Washington, DC September 11, 1998
(Phone: 202/358-1547)

Diane Ainsworth
Jet Propulsion Laboratory, Pasadena, CA
(Phone: 818/354-5011)

RELEASE: 98-164


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 three feet (one
meter) 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 25 degrees Fahrenheit (-4 degrees Celsius) and
lows at -170 degrees Fahrenheit (-112 degrees Celsius).

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

New images from the spacecraft's Mars Orbiter Camera show
many never-before seen features on Phobos, the innermost and
larger of the planet's two moons, and are among the highest
resolution pictures ever obtained of the rocky Martian satellites.
A six-mile (10-kilometer) diameter 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 1/1,000th that of the
Earth's, debris still tumbles downhill. Large boulders appear to
be partly buried in the surface material.

Infrared measurements of Phobos were made on August 7, 19 and
31 from distances ranging between 648-890 miles (1,045-1,435
kilometers), 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.

"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

The new Phobos images and thermal spectrometer measurements
are available on the Internet at: , , and at

On Monday, Sept. 14, Mars Global Surveyor begins its second
phase of aerobraking, using the friction from repeated passes
through Mars' atmosphere to lower and circularize the spacecraft's
orbit. Over the next four-and-a-half months, the spacecraft's
flight path will be lowered from the current 11.6-hour elliptical
orbit to a two-hour, nearly circular orbit over the Martian polar
caps. The magnetometer and thermal spectrometer will be turned on
through December to gather data each time the spacecraft passes
closest to Mars' surface. In addition, the radio science team
will be conducting gravity field experiments by measuring small
shifts in the spacecraft's velocity as it passes behind the planet
or is blocked from view by the Sun. The spacecraft team at NASA's
Jet Propulsion Laboratory (JPL), Pasadena, CA, and Lockheed Martin
Astronautics, Denver, is continuing to study possible options for
deployment of the spacecraft's high-gain antenna once it has
reached its low-altitude mapping orbit next spring.

Mars Global Surveyor is part of a sustained program of Mars
exploration, managed by JPL 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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