Two cratered moons keep Mars company – an inner moon named Phobos and an outer moon named Deimos. Credit: NASA/JPL-Caltech/University of Arizona
Asaph Hall, an American astronomer, discovered two small moons orbiting the planet Mars in 1877. They were later named Phobos and Deimos after the Greek for “fear” and “panic.”
But excitement rather than fear and panic characterized ESA’s Mars Express spacecraft’s close encounter with Phobos on Halloween this year. The recent flyby of the larger Martian moon offered the perfect opportunity to test one of the latest improvements to the 19-year-old spacecraft.
The High Resolution Stereo Camera (HRSC) aboard ESA’s Mars Express spacecraft took this image of Phobos using the HRSC nadir channel on March 7, 2010, HRSC Orbit 7915. This image has been further photometrically enhanced to better represent the features in the dimly lit part. Resolution: about 4.4 meters per pixel. Credit: ESA/DLR/FU Berlin (G. Neukum), CC BY-SA 3.0 IGO
Thoroughly
The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument on Mars Express was originally designed to study the internal structure of Mars. As a result, it was designed to take advantage of the typical distance between the spacecraft and the planet’s surface, which is more than 250 km (155 mi).
However, it recently received a major software upgrade that allows it to be used at much closer distances. This new capability could help shed light on the mysterious origins of the moon Phobos.
Artist’s impression of water beneath the Martian surface. Credit: Illustration by Medialab, ESA 2001
“During this flyby, we used MARSIS to survey Phobos from nearly 83 km (52 miles) away,” says Andrea Cicchetti of the MARSIS team at INAF. “Getting closer allows us to study its structure in more detail and identify important features that we would never be able to see from further away.” In the future, we are confident that we can use MARSIS from less than 40 km (25 mi). Mars Express’ orbit is fine-tuned to get us as close as possible to Phobos during several flybys between 2023 and 2025, which will give us great opportunities to try.
“We didn’t know if this was possible,” said Simon Wood, the Mars Express flight controller at ESA’s ESOC operations centre, who led the uploading of the new software to the ESA spacecraft. “The team tested several different variants of the software, with the last successful setup being uploaded to the spacecraft just hours before the flyby.”
Mysterious origin
MARSIS, known for its role in detecting signs of liquid water on the Red Planet, sends low-frequency radio waves to Mars or Phobos using its 40-meter (130-foot) antenna.
Most of these waves are reflected from the surface of the body, but some pass through and are reflected at the boundaries between layers of different materials below the surface.
Researchers can map the structure below the surface and study properties such as the thickness and composition of the material by analyzing the reflected signals.
For Mars, this could reveal different layers of ice, soil, rock or water. However, the internal structure of Phobos is much more mysterious, and an upgrade to MARSIS could offer crucial insight.
The MARSIS instrument on ESA’s Mars Express spacecraft is using its newly updated software to peer beneath the surface of the Martian moon Phobos. Credit: INAF – Istituto Nazionale di Astrofisica
“Whether the two small moons of Mars are captured asteroids or made of material ripped from Mars during an impact is an open question,” says ESA Mars Express scientist Colin Wilson. “Their appearance suggests they were asteroids, but the way they orbit Mars probably suggests otherwise.”
“We are still in the early stages of our analysis,” says Andrea. “But we have already seen possible signs of previously unknown features beneath the moon’s surface. We are excited to see the role that MARSIS can play in finally solving the mystery surrounding the origin of Phobos.
What does this image show?
In the image above, the upper right section shows the “radargram” received by MARSIS during the Phobos flyby on September 23, 2022. The radargram reveals the “echo” created when the radio signal emitted by MARSIS bounces off something and returns to the tool. The brighter the signal, the more powerful the echo.
The solid bright line shows the echo from the surface of the moon. The lower reflections are either “clutter” caused by features on the moon’s surface or, more interestingly, signs of possible subsurface structural features (e).
“Section A—C was recorded using an older MARSIS software configuration,” says Carlo Nena, MARSIS onboard software engineer at Enginium, which performed the upgrade. “The new configuration was prepared during a ‘technical glitch’ and successfully used for the first time by D—F.”
The left and lower right images show the observation path across the surface of Phobos.
MARSIS is managed by the Istituto Nazionale di Astrofisica (INAF), Italy, and funded by the Italian Space Agency (ASI).
Future Exploration of Phobos
The European Space Agency (ESA) and its member states are participating in the upcoming Martian Moons eXploration (MMX) mission to land on Phobos and return a sample of its surface materials to Earth. The MMX mission, led by the Japan Space Agency (JAXA), is scheduled to launch in 2024 and return its samples to Earth in 2029. The instruments aboard Mars Express are vital for the detailed study of Phobos needed to prepare for the mission MMX.
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