Mars Express Close Fly-by Takes Radargram Of Phobos

By James Anderson •  Updated: 10/31/22 •  4 min read

The recent close encounter between the ESA Mars Express spacecraft and Phobos was the perfect time to test one of the spacecraft’s newest upgrades.

The original goal of the MARSIS instrument on the European Space Agency’s Mars Express mission was to study the internal structure of Mars. It was created for use at a typical distance of more than 250 km between the spacecraft and the planet’s surface.

However, it recently underwent a significant software upgrade that makes it possible to use it much closer up and could help to clarify the murky history of the moon Phobos.

Asaph Hall, an American astronomer, found two tiny moons orbiting Mars in 1877. He later gave them the Greek names Phobos and Deimos, which mean “fear” and “panic.” respectively.

Closer Phobos Flybys

The recent flyby allowed the MARSIS team to study Phobos from a distance of just 83 kilometres away.

“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. In future, we are confident we could use MARSIS from closer than 40 km. The orbit of Mars Express has been fine-tuned to get us as close to Phobos as possible during a handful of flybys between 2023 and 2025, which will give us great opportunities to try,”

said Andrea Cicchetti from the MARSIS team at the Italian National Institute for Astrophysics.

MARSIS Software Tweaks

Simon Wood, a Mars Express flight controller at the ESOC operations center, was in charge of implementing the new software on the ESA spacecraft. The original MARSIS software was first coded more than 20 years ago in a development environment based on Microsoft Windows 98.

“The team tested a few different variations of the software, with the final, successful tweaks uploaded to the spacecraft just hours before the flyby,”

Wood said.

To improve the amount and quality of science data sent to Earth, the new software has a number of updates that improve signal reception and on-board data processing.

Origin Mystery

 water under the martian surface

Artist’s impression of water under the martian surface. Credit: Medialab, ESA 2001

MARSIS, known for helping to find evidence of liquid water on the Red Planet, uses its 40-meter long antenna to direct low-frequency radio waves at Mars or Phobos. The majority of these waves bounce off the body’s surface, but some penetrate below the surface and are reflected at the boundaries between layers of various materials.

By looking at the reflected signals, scientists can map the structure below the surface and learn about things like the thickness and make-up of the material.

This might reveal different layers of ice, soil, rock, or water on Mars. But the inside of Phobos is still a mystery, and the upgrade to MARSIS could give us important information.

It remains unclear whether Mars’ two tiny moons are made of material snatched from Mars during a collision or made of captured asteroids.

“Their appearance suggests they were asteroids, but the way they orbit Mars arguably suggests otherwise,”

said Mars Express scientist Colin Wilson.

The team’s analysis is still in its early stages. They have, however, already spotted potential indicators of as-yet-undiscovered features beneath the moon’s surface. They are excited to see how MARSIS might help solve the mystery of where Phobos came from.

What the Radargram Reveals

marsis phobos radargram

Click image to enlarge. Credit: INAF – Istituto Nazionale di Astrofisica

The “radargram” that MARSIS captured during the flyby of Phobos on September 23, 2022, can be seen in the top-right image. The “echoes” produced when a MARSIS radio signal bounces off something and returns to the instrument are visible on a radargram. The strength of the echo increases with signal brightness.

The echo from the moon’s surface is depicted by the continuous bright line. The lower reflections are either “clutter” brought on by the moon’s surface features or, more intriguingly, they may be hints of structural features that may be present 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,”

said Carlo Nenna, MARSIS on-board software engineer. The bottom-left and left-most images display the observation’s path across Phobos’ surface.

The ESA’s initial mission to the Red Planet, the Mars Express orbiter, was launched on June 2, 2003, 19 years ago. It has spent nearly two decades exploring our neighbour Mars and revolutionizing our knowledge of its past, present, and future.

Cover Image: High-Resolution Stereo Camera onboard Mars Express image of Phobos on 7 March 2010, HRSC Orbit 7915. Credit: ESA/DLR/FU Berlin (G. Neukum), CC BY-SA 3.0 IGO

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