Europe’s Mars Express spacecraft has peered deeper than ever into the Martian moon Phobos’ subsurface, finding hints of unknown structures that could be clues to the moon’s origin.
Mars Express, which is a 19-year-old veteran spacecraft orbiting Mars, came within 51.6 miles (83 kilometers) of Phobos on September 22, 2022 and was able to probe beneath the moon’s surface at the using upgraded software on his MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument.
Understanding the interior structure of Phobos could be the key to solving the mystery of its origin. “We are still at an early stage of our analysis, but we have already seen possible signs of previously unknown features under the moon’s surface,” said Andrea Cicchetti, a member of the MARSIS science team. INAF, the Italian National Institute of Astrophysics, in a statement (opens in a new tab).
Related: How Mars Moon Phobos Got Its Grooves
Mars has two moons, named Phobos and Deimos after the gods of “fear” and “panic” in Greek mythology. Unlike the major moons in our solar system, Phobos and Deimos are tiny, just 16.7 miles (27 kilometers) and 9.3 miles (15 kilometers) in diameter, respectively. They are similar in composition to C-type carbonaceous asteroids and also irregularly shaped like asteroids, leading to suspicion that they are actually rogue asteroids captured by Mars’ gravity. However, the orbits of Phobos and Deimos around the Red Planet are above Mars’ equator, and both orbits are extremely circular, suggesting that they formed around Mars. Had they been captured, one would expect them to have more elliptical orbits in different planes.
“Whether the two small moons of Mars are captured asteroids or made of material torn from Mars in a collision is an open question,” said Colin Wilson, scientist on the Space Agency’s Mars Express mission. European, in the same press release. (opens in a new tab).
MARSIS involves a 40 meter long antenna broadcasting low frequency radio waves to the surface. Most radio waves are reflected directly from the surface, but some penetrate deeper, where they encounter transitions between layers of different composition and structure, and are reflected by these boundaries. The stronger the reflection in the resulting “radargram”, the brighter the returning radio signal.
The radargram across a narrow track on Phobos shows a bright line, split in two and labeled A–C and D–F respectively. The A–C section was captured using the old MARSIS software to compare with D–F, which uses the new software and shows much more detail. The main bright line is the reflection from the surface of Phobos, but below there is evidence of fainter lines which could simply be interference, or “clutter”, from features on the surface, but they could also be caused by structures below the surface. .
MARSIS had been designed to probe the interior of Mars at an orbital distance of more than 155 miles (250 kilometers), but the recent software upgrade allows MARSIS to operate at much closer distances, enabling its use in flybys closer to the moons.
Getting even closer to Phobos will provide radargrams with even greater resolution than that obtained here. The plan over the next few years is to employ MARSIS as close as 40 kilometers (24.9 miles) to Phobos.
“Mars Express’ orbit has been refined to get us as close to Phobos as possible in a handful of flybys between 2023 and 2025,” Cicchetti said.
Mars Express isn’t the only mission centered on Phobos. In September 2024, the Japan Aerospace Exploration Agency (JAXA) plans to launch the Martian Lunar Exploration (MMX) spatialship. A bit like JAXA Hayabusa2 mission to recover samples from the near-Earth asteroid Ryugu, MMX will capture a minimum of 10 grams of regolith from the surface of Phobos. MMX will also deploy a small vagabond on the surface, before venturing to contemplate the second moon of Mars, Deimosthen return to Earth with the precious samples of Phobos which will be analyzed in the laboratories of scientists here Earth.
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