Strange structure below Mars' moon Phobos

Europe's Mars Express probe looks deeper into the subsurface of the Martian moon Phobos, revealing never-before-seen structures that could reveal the celestial body's origins.

Close-up photo of the moon Phobos. (Photo: ESA)

With 19 years of orbiting Mars, the Mars Express spacecraft approached Phobos at a distance of 83 km on September 22, 2022 and probed below the surface of this satellite using upgraded software on the MARSIS (Mars) instrument. Advanced Radar for Subsurface and Ionosphere Sounding), Space reported on October 31. "We are still in the early stages of analysis, but we see signs of unknown structures beneath the surface of the moon Phobos ," said Andrea Cicchetti, member of the MARSIS scientific research team at the Institute of Astrophysics. Italian National Office (INAF), said.

Mars has two moons named Phobos and Deimos after the gods of "fear" and "panic" in Greek mythology. Unlike the large moons in the solar system, Phobos and Deimos are very small, only 27 and 15km wide. They have a structure similar to C-type asteroids and an unusual shape, leading to speculation that they may be wandering asteroids attracted by the gravity of Mars. However, both the orbits of Phobos and Deimos around the red planet are above the equator and extremely circular, suggesting they formed around Mars. If sucked in, they will have elliptical orbits in distinct planes.

MARSIS is equipped with a 40m long antenna that transmits low-frequency radio waves to the surface of Phobos. Most radio waves are reflected back directly, but some penetrate deeper. There, they encounter transitions between different layers of components and structures before reflecting back. The stronger the reflection, the brighter the transmitted radio signal. Radar charts from Phobos show that there is a strange structure under the moon's surface.

MARSIS was designed to probe the interior of Mars from orbit at a distance of more than 250 km, but a recent software upgrade allows MARSIS to operate at much closer distances. Getting closer to Phobos will provide a radar chart with greater resolution. The plan for the near future is to fly MARSIS within 40 km of Phobos on approaches between 2023 and 2025.

Mars Express isn't the only mission focusing on Phobos. In September 2024, the Japan Aerospace Exploration Agency (JAXA) plans to launch the Martian Moon eXploration (MMX) spacecraft. MMX will take at least 10 grams of Phobos's surface soil and bring it back to Earth for scientists to analyze.