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

MARTIAN MOON PHOBOS HIP-DEEP IN POWDER 

     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 
dust." 

     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 
surface." 

     The new Phobos images and thermal spectrometer measurements 
are available on the Internet at:  http://www.jpl.nasa.gov  , 
http://photojournal.jpl.nasa.gov/  ,
http://www.msss.com/ and at http://emma.la.asu.edu  

     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.