The Mars Express just got up close and personal with Phobos

Flying closely can give scientists clues to the history of the Red Planet's largest moon.
A closeup of Phobos, one of Mars' two moons. ESA/DLR/FU Berlin

Mars is lucky enough to have two confirmed moons, and both have some scary names. Deimos, the smaller of the two moons, is named for the Roman god of dread. Phobos is larger, and its name comes from from the Greek words for “fear” or “panic.”

However, excitement and joy reigned supreme when the European Space Agency’s (ESA) Mars Express spacecraft closely encountered the Red Planet’s larger moon. The flyby in September allowed the scientists to test one of the 19-year-old spacecraft’s newest tools.

[Related: Two NASA missions combined forces to analyze a new kind of marsquake.]

Aboard the Mars Express is the MARSIS instrument, which was originally designed to study Mars’ internal structure. NASA’s Jet Propulsion Laboratory (JPL), the University of Rome, and the Italian Space Agency (ASI) built it so that it would be used more than 155 miles away from the surface of Mars, or the typical distance between the Red Planet’s surface and the spacecraft. A major software upgrade allows the Mars Express to travel closer to a celestial body’s surface. This update could shed light on the moon Phobos’ mysterious origin by peering inside the moon.

“During this flyby, we used MARSIS to study Phobos from as close as [about 51 miles],” Andrea Cicchetti from the MARSIS team at INAF said in a statement. “Getting closer allows us to study its structure in more detail and identify important features we would never have been able to see from further away. In future, we are confident we could use MARSIS from closer than [about 24 miles]. The orbit of Mars Express has been fine-tuned to get us as close to Phobos as possible during a handful of flybys between 2023 and 2025, which will give us great opportunities to try.”

The MARSIS instrument on ESA’s Mars Express spacecraft uses its recently upgraded software to peer beneath the surface of the martian moon Phobos. INAF – Istituto Nazionale di Astrofisica.

MARSIS is famous for its role in discovering signs of liquid water on Mars in 2005. It sends low-frequency radio waves to Phobos or Mars with a 131-foot-long antenna. Most of the waves are reflected off the surface, but some travel through, reflecting at the boundaries between layers of different materials below the moon’s surface.

[Related: What is a ‘Martian flower’?]

Studying the reflected signals can help scientists map the structure below the surface, revealing the thickness and composition of the material, among other features. The waves can also show evidence of different water, rock, ice, or soil layers. However, more mysteries lie in the internal structure of Phobos, and the MARSIS upgrade could help solve the puzzle.

“Whether Mars’ two small moons are captured asteroids or made of material ripped from Mars during a collision is an open question,” ESA Mars Express scientist Colin Wilson said in a statement. “Their appearance suggests they were asteroids, but the way they orbit Mars arguably suggests otherwise.”

“We are still at an early stage in our analysis,” Cicchetti added. “But we have already seen possible signs of previously unknown features below the moon’s surface. We are excited to see the role that MARSIS might play in finally solving the mystery surrounding Phobos’ origin.”

MARSIS is operated by the Istituto Nazionale di Astrofisica (INAF) in Italy and is funded by the ASI. The ESA and its Member States are part of the upcoming Martian Moons eXploration (MMX) mission to land on Phobos and return a sample of its surface materials to Earth. The MMX mission is led by the Japanese Space Agency (JAXA) and is scheduled to launch in 2024 and return its samples to Earth in 2029.