Examining Mars' Moon Phobos in a Different Light
Jet Propulsion Laboratory
October 4, 2017
NASA's longest-lived mission to Mars has gained its first look at the
Martian moon Phobos, pursuing a deeper understanding by examining it in
The Thermal Emission Imaging System (THEMIS) camera on NASA's Mars Odyssey
orbiter observed Phobos on Sept. 29, 2017. Researchers have combined visible-wavelength
and infrared data to produce an image color-coded for surface temperatures
of this moon, which has been considered for a potential future human-mission
"Part of the observed face of Phobos was in pre-dawn darkness, part in
morning daylight," said THEMIS Deputy Principal Investigator Victoria
Hamilton of the Southwest Research Institute, headquartered in San Antonio.
Looking across the image from left to right presents a sequence of times
of day on the Martian moon, from before dawn, to sunrise, to increasing
amounts of time after dawn. This provides information about how quickly
the ground warms, which is related to the texture of the surface. As barefoot
beach walks can confirm, sand warms or cools quicker than rocks or pavement.
"Including a predawn area in the observation is useful because all the
heating from the previous day's sunshine has reached its minimum there,"
Hamilton said. "As you go from predawn area to morning area you get to
watch the heating behavior. If it heats up very quickly, it's likely not
very rocky but dusty instead."
Phobos has an oblong shape with an average diameter of about 14 miles
(22 kilometers). Cameras on other Mars orbiters have previously taken
higher-resolution images of Phobos, but none with the infrared information
available from THEMIS. Observations in multiple bands of thermal-infrared
wavelengths can yield information about the mineral composition of the
surface, as well as the surface texture.
One major question about Phobos and Mars' even smaller moon, Deimos, is
whether they are captured asteroids or bits of Mars knocked into the sky
by impacts. Compositional information from THEMIS might help pin down
Since Odyssey began orbiting the Red Planet in 2001, THEMIS has provided
compositional and thermal-properties information from all over Mars, but
never before imaged either Martian moon. The Sept. 29 observation was
completed to validate that the spacecraft could safely do so, as the start
of a possible series of observations of Phobos and Deimos in coming months.
In normal operating mode, Odyssey keeps the THEMIS camera pointed straight
down as the spacecraft orbits Mars. In 2014, the spacecraft team at Lockheed
Martin Space Systems, Denver; and NASA's Jet Propulsion Laboratory, Pasadena,
California; and the THEMIS team at Arizona State University, Tempe, developed
procedures to rotate the spacecraft for upward-looking imaging of a comet
passing near Mars. The teams have adapted those procedures for imaging
the Martian moons.
"We now have the capability of rotating the spacecraft for THEMIS observations,"
said Odyssey Project Scientist Jeffrey Plaut of JPL. "There is heightened
interest in Phobos because of the possibility that future astronauts could
perhaps use it as an outpost."
With the first observation now in hand, plans are advancing for additional
opportunities at different illumination phases of Phobos and Deimos.
"We want to get observations under all types of lighting -- fully daylit,
a small crescent, during eclipse," Hamilton said. "We hope this is the
first of several observations that will help us understand Phobos and
News Media Contact
Jet Propulsion Laboratory, Pasadena, Calif.
Karin Valentine / Robert Burnham
Arizona State University, Tempe
Southwest Research Institute, San Antonio
Laurie Cantillo / Dwayne Brown
NASA Headquarters, Washington