Odyssey Thermal Images

Mars Odyssey Captures New Thermal Images of Phobos

Astronomers using the Thermal Emission Imaging System (THEMIS) camera on NASA’s Mars Odyssey orbiter have captured new infrared images of Phobos, the larger and inner of the two natural Martian satellites.

These images from the THEMIS camera on NASA’s Mars Odyssey orbiter show Phobos during different phases: waxing, waning and full. Image credit: NASA / JPL-Caltech / ASU / NAU.

Phobos is an unusual satellite, orbiting closer to its planet than any other moon in the Solar System.

The moon has an oblong shape with an average diameter of about 22 km (14 miles). It also has impact craters and grooves on its surface.

It orbits Mars about 6,000 km (3,700 miles) from the surface and completes an orbit in just 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 planet, with the unusual result among natural satellites that Phobos rises in the west and sets in the east as seen from Mars.

The origin of Phobos and the second Martian moon, Deimos, has been debated for decades. The question is whether these objects were asteroids captured intact by Mars gravity or whether the small satellites formed from an equatorial disk of debris, as is most consistent with their nearly circular and co-planar orbits.

“As far as Phobos goes its origins are enigmatic,” said Dr. Christopher Edwards, a researcher in the Department of Astronomy and Planetary Science at Northern Arizona University.

“The orbit it is in is not very stable, and some scientists have proposed that the moon has been destroyed and reformed multiple times because of its orbital position.”

“It also turns out that the orbit’s exact geometry makes it hard to capture — so some teams have proposed it is derived from Mars. How that happened is not clear, either!”

“Perhaps it’s from a big meteorite impact that ejected material into the orbit, and the material grouped together to form Phobos.”

“So that’s why we’re looking for the physical properties of the surface, which might help identify locations where we could see the primary composition and not just the fine-grained dust.”

Dr. Edwards and colleagues used the THEMIS instrument to capture images from about 6,000 km (3,700 miles) above the surface of Phobos to measure temperature variations during different phases: waxing, waning and full.

“An image taken on December 9, 2019, shows the surface of Phobos at its maximum temperature, 27 degrees Celsius (81 degrees Fahrenheit),” they said.

“An image taken on February 25, 2020, shows Phobos while in eclipse, where Mars’ shadow completely blocked sunlight from reaching the moon’s surface. This event resulted in some of the coldest temperatures measured on Phobos to date, with the coldest being about minus 123 degrees Celsius (minus 189 degrees Fahrenheit).

“On March 27, 2020, Phobos was observed exiting an eclipse, when its surface was still warming up.”

“From these new images we’re seeing that the surface of Phobos is relatively uniform and made up of very fine-grained materials,” Dr. Edwards said.

“These observations are also helping to characterize the composition of Phobos, which appears to be mostly basaltic.”

“Future observations will provide a more complete picture of the temperature extremes on the moon’s surface.”