A close encounter with a mysterious moon

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

But it was excitement rather than fear and panic that marked ESA’s Mars Express spacecraft’s close encounter with Phobos in the run-up to Halloween this year. The recent flyby of Mars’ larger moon provided the perfect opportunity to test out one of the 19-year-old spacecraft’s latest upgrades.

The MARSIS instrument on Mars Express was originally developed 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.

But it recently received a major software upgrade that allows it to be used at much shorter ranges and could help shed light on the mysterious origin of the moon Phobos.

“During this flyby, we used MARSIS to study Phobos from a distance of up to 83 km,” says Andrea Cicchetti of the MARSIS team at INAF. “By getting closer, we can examine its structure more closely and identify important features that we could never have seen from a greater distance. We are confident that in the future we will be able to use MARSIS from a distance of less than 40 km. Mars Express’s orbit has been fine-tuned to get us as close as possible to Phobos during a handful of flybys between 2023 and 2025, which will give us great opportunities to try it.”

“We didn’t know if this was possible,” says Simon Wood, flight controller for Mars Express at ESA’s ESOC Operations Center, who oversaw the uploading of the new software to ESA’s spacecraft. “The team tested a few different variations of the software, with the last successful tweaks uploaded to the spacecraft just hours before the flyby.”

Mysterious origins

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

Most of these waves are reflected from the body’s surface, but some travel through and are reflected at subsurface interfaces between layers of different materials.

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

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

“Whether the two small moons of Mars are captured asteroids or consist of material ripped from Mars in a collision is an open question,” says ESA Mars Express scientist Colin Wilson. “Their appearance suggests they’re asteroids, but the way they orbit Mars might suggest otherwise.”

“We are still in the early stages of our analysis,” says Andrea. “But we’ve already seen possible signs of previously unknown features beneath the lunar surface. We are excited to see what role MARSIS may play in finally solving the mystery of the origin of Phobos.”

What does this picture show?

The upper right image shows the “radargram” captured by MARSIS during the flyby of Phobos on September 23, 2022. A radargram shows 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 bright line shows the echo from the lunar surface. The lower reflections are either “noise” caused by features on the lunar surface or, more interestingly, indications of possible subsurface structural features (e).

“Sections AC were recorded with an older configuration of the MARSIS software,” says Carlo Nenna, MARSIS On-Board Software Engineer at Enginium, who is implementing the upgrade. “The new configuration was prepared during the ‘technical gap’ and was used successfully for the first time by D-F.”

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