Let’s talk about Phobos. We know that it is a moon of Mars and that it orbits the planet once every 7.4 hours. It has a huge impact crater called Stickney. It is about 9 km in diameter. That’s pretty big, considering Phobos is 28 km on its longest side. But beyond that, Phobos presents a bit of a mystery.
This strangely dark little world fascinates planetary scientists with its incredibly strange cratered and streaked surface. They also want to know if it’s a solid body or a floating pile of debris. If so, how did it get that way? And, more importantly, they want to know how it came to be the largest satellite of Mars. All these questions indicate that, for now, Phobos remains a bit of a mystery awaiting a solution.
Exploring Phobos in the foreground
The European Space Agency’s Mars Express orbiter recently flew past Phobos as part of its regular mission. The idea was to approach this moon and bombard it with low-frequency radio waves from the MARSIS instrument on board. There was just one problem: A typical flyby of Phobos by the spacecraft would put it too close to get useful MARSIS data. This is because the instrument always did its best work from a distance. The original software allowed him to study the Martian surface (and beneath it) from about 250 kilometers away.
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An artist’s illustration of the Mars Express Orbiter on Mars. Its MARSIS instrument has been upgraded so it can study the moon Phobos. Image credit: Spacecraft: ESA/ATG Medialab; Mars: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO
The radio waves that MARSIS sends out are mostly reflected from the surface of an object and provide valuable information about the conditions and structures there. But, some signals actually penetrate the cortex and are reflected from deeper layers. The reflections helped scientists map the substructures of Mars and find out whether there are different layers of ice, rock, water or soil. The instrument also played an important role in finding signs of liquid water on the Red Planet.
So how can MARSIS help unravel the big questions about Phobos and its origin? At the moment, scientists have two hypotheses about its past. “Whether the two small moons of Mars are captured asteroids or are made of material torn from Mars during a collision is an open question,” said ESA Mars Express scientist Colin Wilson. “Their appearance suggests that they were asteroids , but the way they orbit Mars suggests otherwise.”
MARSIS offers an early look
The best way to find out its origin is to look inside Phobos. Typical optical images can only tell scientists so much. But instruments that can probe inside Phobos can reveal a lot. This is where MARSIS comes in. Thanks to a major software upgrade, MARSIS made observations during the recent close approach. Now he can “see” beneath the surface of this tiny moon as he flies past for structural clues.
“During this flyby, we used MARSIS to study Phobos from up to 83 km away,” said Andrea Cicchetti of the MARSIS team at the Italian National Institute of Astrophysics. “Getting closer allows us to study its structure in more detail and identify important features that we would never have been able to see from further away. In the future, we are confident that we could use MARSIS within 40 km. Mars Express’ orbit has been adjusted to bring us as close as possible to Phobos for a handful of flybys between 2023 and 2025, which will give us great opportunities to test it.”
MARSIS data from the Phobos flyby. The top right image shows the “radargram” acquired by MARSIS during the Phobos flyby on September 23, 2022. A radargram reveals the “echoes” created when the radio signal emitted by MARSIS bounces off something and returns to the instrument . The brighter the signal, the stronger the echo. The continuous bright line shows the echo from the moon’s surface. The lower reflections are “clutter” caused by features on the moon’s surface or, more interestingly, signs of possible structural features below the surface (e). The AC section was recorded using an earlier configuration of the MARSIS software. The new configuration was prepared during the “technical gap” and was successfully used for the first time since DF. The left and bottom right images show the path of the observation across the surface of Phobos. Credit: INAF – Istituto Nazionale di Astrofisica
The data points to something beneath the landscape of Phobos
MARSIS issued a radargram based on data captured on September 23, 2022. Essentially, the radargram represents “echoes” created when the signal from MARSIS’s 40-meter-long antenna bounced off something below the surface. This could indicate a layered structure, which could indicate that Phobos is a captured asteroid. It could also mean that there are a variety of objects inside Phobos that could turn it into a floating debris pile. Of course, more flybys will capture more data, which should provide more details about what’s hidden beneath Phobos’ crust.
Close-up studies will help scientists schedule the next Martian Moons eXploration (MMX) mission, which will land on Phobos no earlier than 2024. It will collect samples and return them to Earth in 2029. Data from these samples should help solve the question of the origin of Phobos. once and for all
For more information
A close encounter with a mysterious moon
Origin of Phobos and Deimos by the impact of a body the size of Vesta on Ceres with Mars
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