Europe’s Mars Express spacecraft has looked deeper than ever before into the Martian moon’s subsurface, finding evidence of unknown structures that could provide clues to the moon’s origin.
Mars Express, a spacecraft with 19 years of experience in orbiting Mars, came within 51.6 miles (83 kilometers) of Phobos on September 22, 2022 and was able to use updated software on its MARSIS instrument ( Mars Advanced Radar for subsurface and ionospheric sounding).
Understanding the internal structure of Phobos could hold the key to solving the mystery of its origin. “We are still in the early stages of our analysis, but we have already seen possible signs of previously unknown features beneath the lunar surface,” Andrea Cicchetti, a member of the MARSIS science team at INAF, Italy’s National Institute for Astrophysics, said in a statement (opens in new tab).
Related: How the Mars Moon Phobos got its grooves
Mars has two moons, named after the gods of “fear” and “panic” in Greek mythology, Phobos and Deimos. Unlike our solar system’s large moons, Phobos and Deimos are tiny, only 16.7 miles (27 kilometers) and 9.3 miles (15 kilometers) in diameter, respectively. They have a composition similar to carbonaceous C-type asteroids and are also irregularly shaped like asteroids, which has led to suspicions that they may actually be rogue asteroids captured by Mars’ gravity. However, the orbits of Phobos and Deimos around the red planet pass over the equator of Mars, and both orbits are extremely circular, suggesting that they formed around Mars. If they had been captured, one would have expected them to have more elliptical orbits in different planes.
“Whether the two small moons of Mars are captured asteroids or consist of material ripped from Mars during a collision is an open question,” said Colin Wilson, scientist on the European Space Agency’s Mars Express mission, in the same statement (opens in new tab).
MARSIS includes a 40-meter-long antenna that beams low-frequency radio waves onto the surface. Most radio waves are reflected back directly from the surface, but some penetrate deeper, where they encounter transitions between layers of different composition and structure and are bounced off 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 AC and D-F, respectively. The AC section was captured with 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 underneath there is evidence of fainter lines that could just be interference or “noise” from features on the surface, but could also be caused by subsurface structures.
MARSIS was designed to study the interior of Mars from an orbital distance of more than 250 kilometers, but the recent software upgrade allows MARSIS to operate at much closer ranges, enabling its use on close moon flybys.
Getting even closer to Phobos will yield radargrams with even greater resolution than achieved here. It is planned to deploy MARSIS up to 40 kilometers (24.9 miles) from Phobos in the next few years.
“Mars Express’s orbit has been fine-tuned to get us as close as possible to Phobos during a handful of flybys between 2023 and 2025,” Cicchetti said.
Mars Express isn’t the only mission focused on Phobos. The Japanese Aerospace Exploration Agency (JAXA) plans to launch in September 2024 Mars Moon Exploration (MMX) spacecraft. Similar to JAXA Hayabusa2 Mission to take samples from the near-Earth asteroid RyuguMMX will capture at least 10 grams of regolith from the surface of Phobos. MMX will also provide a small one rover to the surface before heading out for a long look at Mars’ second moon, Deimosand then return to Earth with the valuable Phobos samples to be analyzed here in the scientists’ laboratories Earth.
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