On November 17, 2019, ESA’s Mars Express orbiter made a close flyby of Phobos, the larger and inner of the two natural satellites of Mars. The probe tracked Phobos as the tiny moon passed in front of the spacecraft’s High Resolution Stereo Camera, capturing Phobos’ motions and surface in detail.

Phobos is an unusual satellite, orbiting closer to its planet than any other moon in the Solar System. It orbits Mars about 6,000 km (3,700 miles) from the surface and completes an orbit in just 7 hours and 39 minutes.

The moon has dimensions of 26 x 22 x 18 km (16.2 x 13.7 x 11.2 miles) and a very lumpy appearance. It also has impact craters and grooves on its surface.

A number of these craters — created as Phobos was hit by small bodies and rocky debris during its travels through space — can be seen in the new images from Mars Express.

“The largest of these is Stickney crater, which can be seen towards the center of the frame and measures 10 km (6.2 miles) across,” said members of the Mars Express team.

“Also visible are a number of linear marks and furrows that resemble long, deep grooves or scratches.”

“The origin of these features is uncertain: they may have been carved out by debris rolling across the moon’s surface, or created as the moon was pulled in different directions by tidal forces driven by its parent planet.”

These new images show Phobos at a number of angles (it can be seen rotating and slowly lightens up before it begins to darken again), and are very useful for scientists.

“Different shadows are cast as the Sun’s position changes relative to the target object: this illuminates and highlights the surface features and enables calculations of feature height, depth and relief, and reveals much about the roughness, porosity and reflectivity of the surface material itself,” the researchers said.

“A phase angle of zero degrees occurs when the Sun is directly behind the observer. In this alignment, all of the light illuminating Phobos hits the surface vertically and is thus largely reflected back into space, causing the target object to brighten up noticeably, as seen in the animation, and shadows to disappear.”

“The lowest phase angle in this animation is not precisely zero, but 0.92 degrees.”

“This arrangement — of the Sun, Mars Express and Phobos where the latter is observed at a phase angle of near zero — is very rare, and happens only three times a year at most.”

“Other chances to achieve a phase angle of under one will not occur until April and September 2020.”

“As such, Mars Express takes every opportunity to view this small and intriguing moon from this angle, to shed light on its properties, behavior, possible origin, orbital characteristics and location in space — and to probe its potential as a mission destination.”