As of March 2020, NASA’s Mars Odyssey spacecraft has captured six images of Mars’ satellite Phobos. The orbiter’s infrared camera, Thermal Emission Imaging System (THEMIS), is used to measure temperature changes that provide insight into the physical properties and composition of this Red Planet moon.
Chronologically, the types of Phobos represent the growing, waning, and full phases of the moon. On February 25, 2020, Phobos was observed during a lunar eclipse, when the shadow of Mars completely blocked sunlight from reaching the lunar surface. This provided some of the lowest temperatures measured on Phobos to date.
The lowest measured temperature was around –123 degrees Celsius. On March 27, 2020, Phobos was seen emerging from an eclipse while the surface was still warming up.
All infrared images of the camera are colored and superimposed on the visible THEMIS images taken simultaneously. An exception is the eclipse image, which is superimposed on a computer-generated visible image of what Phobos would look like if it were not completely in shadow.
It is one of the two satellites of Mars (along with Deimos). It was discovered by the American astronomer Asaf Hall in 1877. Phobos orbits at an average distance of 6006 km from the surface of Mars and 2.77 radius of Mars from the center of the planet (9 400 km). This is 41 times less than the distance between the centers of the Earth and the Moon (384,400 km). Phobos makes a full revolution in 7 hours 39 minutes 14 seconds, which is about three times faster than the rotation of Mars around its own axis. As a result, in the Martian sky, Phobos rises in the west and sets in the east. Phobos is about 25 km across.