Mars Express peers beneath the surface of Phobos

The MARSIS instrument on ESA's Mars Express spacecraft 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.

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

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“During this flyby, we used MARSIS to study Phobos from as close as 83 km,” says Andrea Cicchetti from the MARSIS team at INAF. “Getting closer allows us to study its structure in more detail and identify important features we would never have been able to see from further away."

The top-right image shows the ‘radargram’ acquired by MARSIS during the flyby of Phobos on 23 September 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 more powerful the echo.

The continuous bright line shows the echo from the moon’s surface. The lower reflections are either ‘clutter’ caused by features on the moon’s 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. The new configuration was prepared during the ‘technical gap’ and successfully used for the very first time from D—F.

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

MARSIS is operated by the Istituto Nazionale di Astrofisica (INAF), Italy, and funded by the Italian Space Agency (ASI).