Mars Express came so close to Phobos that it had to be reprogrammed to keep the Moon in focus

Let’s talk about Fear. We know that it is a moon of Mars and orbits the planet once every 7.4 hours. It has a huge impact crater called Stickney. It is about 9 km in diameter. This is quite large, considering that Phobos itself is 28 km wide on its longest side. But, beyond that, Phobos presents something of a mystery.

This strangely dark little world fascinates planetary scientists because of its surprisingly strange crater and striped surface. They also want to know if it is a solid body or a floating mass. If so, how did it come to be? And, most importantly, they want to know how the largest satellite of Mars came to be. All these questions show that, for now, Phobos remains something of a mystery awaiting a solution.

Exploring Fear up close

Recently, the European Space Agency’s Mars Express orbiter flew by Phobos as part of its routine mission. The idea was to get “up close and personal” with this moon and bombard it with low-frequency radio waves from the onboard MARSIS instrument. There was just one problem—a typical spacecraft flyby of Phobos would put it too close to get useful MARSIS data. And that’s because the instrument always did its best work from a distance. The original software allowed it to study the surface of Mars (and below it) from about 250 kilometers away.

The radio waves sent by MARSIS are mainly 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 reflect back from the deeper layers. The reflections helped scientists map the infrastructure on Mars and understand 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 us understand the big questions about Phobos and its origins? 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 removed from Mars during an impact is an open question,” said ESA Mars Express scientist Colin Wilson. “Their appearance suggests they were asteroids, but the way they orbit Mars clearly suggests something else.”

MARSIS offers an early look

The best way to learn its origins is to look inside Phobos. Standard visual images can only tell scientists so much. But the instruments that can probe inside Phobos can reveal a lot. That’s where MARSIS comes in. Thanks to a major software upgrade, MARSIS made observations during the recent close approach. He can now “see” below the surface of this small moon as he flies to look for structural clues.

“During this flyby, we used MARSIS to study Phobos from 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 spot important features that we would never be able to see from further away. In the future, we are confident that we could use MARSIS from a distance of less than 40 km. Mars Express’s orbit is set to bring us as close as possible to Phobos during a handful of flybys between 2023 and 2025, which will give us great opportunities to test.”

The data points to something beneath the landscape of Fear

MARSIS outputs a radargram based on data recorded 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 may 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 flights will capture more data, which should provide more details about what lies beneath the crust of Fear.

The nearby studies will help scientists plan the upcoming Mars Exploration Rover (MMX) mission that will land on Phobos no earlier than 2024. It will collect samples and return them to Earth in 2029. Data from these samples will help in solving the question of the origin of Fear once and for all.

For more information

A close encounter with a mysterious moon

The origin of Phobos and Deimos from the collision of a Vesta-to-Ceres-sized body with Mars