ESA penetrates the inner structure of the moon Phobos

Oct. 28 () –

ESA’s Mars Express mission’s recent flyby of the moon Phobos provided the perfect opportunity to test one of the last spacecraft upgrades in its 19 years.

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

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

“During this flyby, we used MARSIS to study Phobos from as close as 83 km away,” he says. it’s a statement Andrea Cicchetti from the MARSIS team at INAF (Istituto Nazionale di Astrofisica). “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 farther away. In the future, we are confident we could use MARSIS closer than 40 km. Mars Express’s orbit has been adjusted to bring us closer as far as possible to Phobos during a handful of flybys between 2023 and 2025, which will give us great opportunities to try.”

“We didn’t know if this was possible,” says Simon Wood, Mars Express flight controller at ESA’s ESOC operations center, who oversaw the loading of the new software onto the ESA spacecraft. “The team tried a few different variations of the software, and the final successful adjustments were uploaded to the spacecraft just hours before the flyby.”

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

Most of these waves are reflected by 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 structure below the surface and study properties such as material thickness and composition.

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 update to MARSIS could offer important information.

“Whether Mars’ two small moons are captured asteroids or made of material ripped off Mars during a collision is an open question,” says ESA Mars Express Scientist Colin Wilson. “Their appearance suggests they were asteroids, but the way they orbit Mars arguably suggests otherwise.”

“We are still at an early stage of our analysis,” says Andrea. “But we have already seen possible signs of previously unknown features below the surface of the moon. We are excited to see the role that MARSIS could play in finally solving the mystery surrounding the origin of Phobos.”

The top image shows the “radargram” acquired by MARSIS during the flyby of Phobos 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 bright line shows the echo from the moon’s surface. The bottom reflections are ‘clutter’ caused by features on the moon’s surface (b) or, more interestingly, signs of possible structural features below the surface (e).

“Section A_C was registered with a previous configuration of MARSIS software”, says Carlo Nenna, MARSIS Embedded Software Engineer at Enginium, which is rolling out the update. “The new configuration was prepared during the ‘tech gap’ and was successfully used for the first time since D_F.”

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