Let’s talk about Phobos. We know it is a Mars moon and orbits the planet once every 7.4 hours. It has a huge impact crater called Stickney. It measures about 9 km in diameter. It’s quite large, considering that Phobos itself is 28 km wide on its longest side. But, beyond that, Phobos presents something of a mystery.
This eerily dark little world fascinates planetary scientists due to its incredibly eerie cratered and scratched surface. They also want to know if it is a solid body or a pile of floating rubble. If so, how did it get there? And, more importantly, they want to know how it became Mars’ largest satellite. All of these questions indicate that, for now, Phobos remains a mystery awaiting resolution.
Exploring Phobos up close
Recently, the European Space Agency’s Mars Express orbiter flew over Phobos as part of its regular mission. The idea was to approach this moon and bombard it with low-frequency radio waves from the onboard MARSIS instrument. There was only one problem: a typical flyby of Phobos by the spacecraft would put it too close to obtain useful MARSIS data. This is because the instrument has always done its best from a distance. The original software allowed him to study the Martian surface (and below) from about 250 kilometers away.
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The radio waves sent by MARSIS reflect primarily from an object’s surface and provide valuable information about the conditions and structures there. But, some signals actually penetrate the crust and are reflected from deeper layers. The reflections have helped scientists map the substructures of Mars and determine whether there are different layers of ice, rock, water or soil. The instrument also played a role in finding signs of liquid water on the Red Planet.
So how can MARSIS help to understand the big questions about Phobos and its origin? For the moment, scientists have two hypotheses about his past. “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 ESA Mars Express scientist Colin Wilson. “Their appearance suggests they were asteroids, but the way they orbit Mars probably suggests otherwise.”
MARSIS delivers a first glimpse
The best way to discover its origin is to look inside Phobos. Typical optical images cannot tell scientists much. But instruments capable of probing the interior of 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. He can now “see” beneath the surface of this small moon as it flies in search of structural clues.
“During this flyby, we used MARSIS to study Phobos at a distance as close as 83 km,” 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’s orbit has been refined to get us as close to Phobos as possible in a handful of flybys between 2023 and 2025, which will give us some great opportunities to try.
The data points to something beneath the landscape of Phobos
MARSIS has produced a radargram based on data captured on September 23, 2022. Essentially, the radargram depicts “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 make it a floating pile of rubble. Of course, more flybys will capture more data, which should yield more detail about what lies beneath Phobos’ crust.
The extensive 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 settle the matter. of the origin of Phobos. once 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 at Ceres with Mars
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