September 11, 1998
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.msss.com/ 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.