A close encounter with a mysterious moon

In 1877, American astronomer Asaph Hall discovered two small moons circling the planet Mars, later named Phobos and Deimos from the Greek for “fear” and “panic”.

But it was excitement, rather than fear and panic, that characterized the close encounter with Phobos made by ESA’s Mars Express spacecraft in the run-up to Halloween this year. The recent flyby of the largest Martian moon offered the perfect opportunity to test one of the latest upgrades to the 19-year-old spacecraft.

The MARSIS instrument on Mars Express was originally designed to study the internal structure of Mars. As a result, it was designed for use at the typical distance between the spacecraft and the planet’s surface—more than 250 km.

However, it recently received a major software upgrade that allows it to be used at much closer distances, which could help shed light on the mysterious origins of the Phobos moon.

“During this flyby, we used MARSIS to study Phobos from 83 km.” says Andrea Cicchetti from the MARSIS team at INAF. “Getting closer allows us to study its structure in more detail and detect 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 has been refined 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.”

“We didn’t know if this was possible,” says Simon Wood, Mars Express flight controller at ESA’s ESOC operations centre, who oversaw the upload of the new software to ESA’s spacecraft. “The team tested a few different variations of the software, with the final, successful modifications being uploaded to the spacecraft a few hours before the flight.”

Mysterious origins

MARSIS, famous for its role in discovering signs of liquid water on the Red Planet, sends low-frequency radio waves to Mars or Phobos using its 40-meter-long antenna.

Most of these waves are reflected from the surface of the body, but some travel and are reflected at the boundaries between layers of different materials below the surface.

By examining the reflected signals, scientists can map the structure below the surface and study properties such as the thickness and composition of the material.

For Mars, this could reveal different layers of ice, soil, rock or water. But the internal structure of Phobos is more of a mystery, and the upgrade to MARSIS could provide important insight.

“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,” says ESA Mars Express scientist Colin Wilson. “Their appearance suggests they were asteroids, but the way they orbit Mars clearly suggests something else.”

“We are still at an early stage in our analysis,” says Andrea. “But we’ve already seen potential signs of previously unknown features beneath the moon’s surface. We’re excited to see the role MARSIS can play in solving the mystery surrounding the origin of Phobos.”

What does this picture show?

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 solid light line shows the echo from the lunar surface. The lower reflections are either “clutter” caused by features on the lunar surface, or, more interestingly, signs of possible structural features below the surface (e).

“Section A–C was recorded using an older configuration of the MARSIS software,” says Carlo Nenna, MARSIS software engineer at Enginium, which is implementing the upgrade. “The new configuration was prepared during the ‘tech gap’ and was successfully used for the first time by the DF.”

The images on the left and bottom right show the path of the observation on the surface of Phobos.