NASA MGS TES Press Release, September 11, 1998

MGS Thermal Emission Spectrometer Views Phobos

The Thermal Emission Spectrometer (TES) instrument on the Mars Global Surveyor has successfully observed Phobos, one of the two moons of Mars, on three separate occasions. The primary objective has been to collect infrared spectra of Phobos to study its composition and physical properties. Figure 1 gives a single TES spectrum showing the infrared energy emitted from the moon's surface at different wavelengths ranging from ~6 to 50 micrometers. The amount of energy emitted is determined mainly by the temperature of Phobos. In this figure the Phobos spectrum is compared to a modeled spectrum (smooth curve) made up of materials at different temperatures. From this comparison it is possible to determine that the surface of Phobos viewed by the TES contains temperatures that range from -170 F (~-112 °C) to +25 F (~-4 °C). The subtle differences between the Phobos spectrum and the model may be due to absorption features produced by different minerals on the surface. These data will be analyzed by the TES Science Team over the next few months to search for clues to the composition of the surface of Phobos.

Figure 2 shows the combination of data acquired jointly by the MGS Mars Orbiter Camera (MOC) and MGS Thermal Emission Spectrometer instruments. The approximate location of a single TES measurement is shown superimposed on the MOC image acquired at the same time. The TES data indicate that the surface temperature in the shadowed region of Phobos is -170 F (~-112 °C), while only several kilometers away on the sunlit side of Phobos the temperature is +25 F (~-4 °C). The extreme temperature differences between the night and day sides of Phobos indicate that the surface is composed of very small dust particles that those their heat rapidly once the Sun has set. In addition, Phobos does not have an atmosphere to help hold heat in during the night.

Following the launch of MGS in the fall of 1996, the TES had the unique opportunity to obtain the first global infrared spectra of the Earth, Mars, and now Phobos. In each case the data were collected from a great enough distance to capture global views of the planet or moon in a single spectrum. The data therefore provide a global perspective of three very different solar system objects. Figure 3 shows the infrared spectra acquired of each of these planetary bodies. The Earth has the most complex infrared spectrum, primarily due to the presence of carbon dioxide (CO₂), ozone, and water vapor (H₂O) in its atmosphere. The spectrum of Mars is less complex, showing only the presence of significant amounts of CO₂. The temperatures derived from the emitted energy show that Mars is colder than the Earth due to its greater distance from the Sun. Finally, the spectrum of Phobos has little structure due to the fact that Phobos has no atmosphere and the emitted energy is coming entirely from its surface.

The Thermal Emission Spectrometer is operated for the National Aeronautics and Space Administration by Arizona State University. The instrument was built by Raytheon Santa Barbara Remote Sensing. The Mars Orbiter Camera is operated for the National Aeronautics and Space Administration by Malin Space Science Systems. The Mars Global Surveyor Mission is managed by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. The MGS mission is operated jointly by the Jet Propulsion Laboratory and Lockheed Martin Astronautics.