Mars Odyssey Captures Phobos in Infrared Light

The Thermal Emission Imaging System (THEMIS) camera on NASA’s Mars Odyssey spacecraft observed Phobos — the larger and inner of the two natural satellites of the Red Planet — on September 29, 2017.

This image combines two products from the first pointing at Phobos by Mars Odyssey’s THEMIS camera. Surface-temperature information from observation in thermal-infrared wavelengths is overlaid on a more detailed image from a visible-light observation. The left edge of the small moon was in darkness, and the right edge in morning sunlight. The distance to Phobos from Mars Odyssey during the observation was about 3,424 miles (5,511 km). Researchers will analyze the surface-temperature information from this observation and possible future THEMIS observations to learn how quickly the surface warms after sunup or cools after sundown. That could provide information about surface materials, because larger rocks heat or cool more slowly than smaller particles do. The thermal information in this image is from merging observations made in four thermal-infrared wavelength bands, centered from 11.04 microns to 14.88 microns. Image credit: NASA / JPL-Caltech / Arizona State University.

This image combines two products from the first pointing at Phobos by Mars Odyssey’s THEMIS camera. Surface-temperature information from observation in thermal-infrared wavelengths is overlaid on a more detailed image from a visible-light observation. The left edge of the small moon was in darkness, and the right edge in morning sunlight. The distance to Phobos from Mars Odyssey during the observation was about 3,424 miles (5,511 km). Researchers will analyze the surface-temperature information from this observation and possible future THEMIS observations to learn how quickly the surface warms after sunup or cools after sundown. That could provide information about surface materials, because larger rocks heat or cool more slowly than smaller particles do. The thermal information in this image is from merging observations made in four thermal-infrared wavelength bands, centered from 11.04 microns to 14.88 microns. Image credit: NASA / JPL-Caltech / Arizona State University.

Discovered along with its smaller companion, Deimos, by American astronomer Asaph Hall in 1877, Phobos has an oblong shape with an average diameter of about 14 miles (22 km).

The small moon orbits Mars at a distance of 3,721 miles (5,989 km) every 7 hours and 39 minutes. It orbits so close to the Martian surface that the curvature of the planet would obscure its view from an observer standing in Mars’ polar regions. Its orbital period is about 3 times faster than the rotation period of the Red Planet, with the unusual result among natural satellites that Phobos rises in the west and sets in the east as seen from Mars.

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.

“Although THEMIS has been at Mars for 16 years, this was the first time we have been able to turn the spacecraft around to look at Phobos,” THEMIS Mission Planner Jonathon Hill, from the School of Earth and Space Exploration at Arizona State University.

“This half-moon view of Phobos was chosen because it allowed us to observe a wide range of temperatures on the surface.”

Looking across the THEMIS 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.

“Including a predawn area in the observation is useful because all the heating from the previous day’s sunshine has reached its minimum there,” said THEMIS Deputy Principal Investigator Victoria Hamilton, a researcher at the Southwest Research Institute.

“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.”

“We want to get observations under all types of lighting — fully daylit, a small crescent, during eclipse,” she added.

“We hope this is the first of several observations that will help us understand Phobos and Deimos.”

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