• Mars Express spacecraft bounced radio waves off Phobos
  • The spacecraft came within 51.6 miles of Phobos on Sept. 22
  • MARSIS instrument was used to study Phobos

Europe's Mars Express has found potential unknown structures beneath the planet's moon, Phobos, and the finding may provide clues to its origin story.

Mars Express is a 19-year-veteran spacecraft that came within 51.6 miles of Phobos on Sept. 22. The spacecraft used the opportunity to peer into the moon's surface using upgraded software on its MARSIS instrument (Mars Advanced Radar for Subsurface and Ionosphere Sounding).

"We are still at an early stage in our analysis, but we have already seen possible signs of previously unknown features below the moon's surface," said Andrea Cicchetti, who is a member of the MARSIS science team at the Italian National Institute for Astrophysics (INAF), in a statement.

The "radargram" from Phobos shows two colored lines labeled A–C and D–F.

"Section A—C was recorded using an older configuration of the MARSIS software," Carlo Nenna, MARSIS on-board software engineer at Enginium, said. "The new configuration was prepared during the 'technical gap' and successfully used for the very first time from D—F."

The two moons of Mars -- Phobos and Deimos -- are named after the gods of "fear" and "panic" in Greek mythology by American astronomer Asaph Hall, who discovered them in 1877.

Originally, the MARSIS instrument was meant to study the internal structure of Mars. It was, therefore, designed to be used at the typical distance between the spacecraft and the planet's surface, which was more than 155 miles.

"During this flyby, we used MARSIS to study Phobos from as close as 83 km," Cicchetti from the MARSIS team, said. "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."

Cicchetti said the team was confident they could use MARSIS from as close as 40 km, and "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 has a 40-meter-long antenna that beams low-frequency radio waves down to the surface. While most of them are reflected directly back, some penetrate deeper where the transitions between layers of different compositions and structures are reflected by these structures, according to

In another mission, the Martian Moon eXploration (MMX) spacecraft will target Phobos to uncover its secrets. Led by the Japanese Space Agency (JAXA), the mission is scheduled to launch in 2024, and return samples from Phobos surface material to Earth in 2029.

European Space Agency noted instruments on board the Mars Express have played an important role in the detailed study of Phobos, which was necessary to prepare for the MMX mission.

Image: Martian moon, Phobos, captured by the Mars Odyssey spacecraft in infrared using its Thermal Emission Imaging System (THEMIS). NASA/JPL-Caltech/ASU/NAU