The moons of Mars and Jupiter meet

Since arriving in orbit of the red planet in 2003, Mars Express has routinely observed the planet’s geology. It also monitors the planet’s two moons, Phobos and Deimos, to learn more about their surface composition, particularly Phobos’ mysterious ridged terrain and finally the moons’ enigmatic origins. This includes tracking their orbits.

The moons experience strong tidal forces from Mars, causing their orbits to fluctuate constantly. Phobos orbits extremely close to Mars, only 6000 km away, and is moving towards the planet, while Deimos is moving away from it.

Because of the brightness of Mars compared to these small bodies, measuring their orbits from Earth is difficult.

The coincidental orientation of Deimos, which passed in front of Jupiter on February 14, 2022, allowed for a more accurate determination of Deimos’ position and orbit. That is, by measuring the duration of occlusion – when the light of one celestial body is blocked by another – the orbit can be calculated.

Such an alignment is extremely unusual, since Deimos must be exactly in the orbital plane of Jupiter’s moons for the alignment to occur.

The animated sequence of 80 High Resolution Stereo Camera (HRSC) images shows the small and irregularly shaped 15 km wide Moon’s bumpy surface as it passed in front of Jupiter. Jupiter’s moons appear as small white dots due to their distance of almost 750 million km from Mars Express. This amazing distance is five times the distance between the earth and the sun.

The animation first shows Deimos passing in front of the icy moon Europa. The largest moon in the solar system, Ganymede, will then be obscured. Gas giant Jupiter, appearing as a large white spot in the center, then disappears behind Deimos.

Deimos then covers the extremely active volcanic moon Io, which is similar in size to Earth’s moon. Finally, the crater moon Callisto disappears behind Deimos.

Deimos appears to move up and down in the animation as it rotates to position the HRSC camera due to the small wobbles of Mars Express. The movement of the solar panels, located 12 m from the spacecraft, and two long radar antennas also contribute to the small vibrations.

Jupiter and its three largest icy moons will be visited by the ESA-led JUpiter ICy moons Explorer (Juice) mission, scheduled for launch in 2023 and arrival in the Jovian system in 2031. Juice will conduct flybys of the moons Ganymede, Callisto and Europa to study their surface and interior, which are thought to harbor oceans. These observations will help study the conditions for the emergence of life in our solar system and the formation of planets

After mapping the alignment with Jupiter, Mars Express turned its gaze to the moment when Deimos was blocked by its larger brother Phobos, measuring about 27 km along its longest axis. The animation is composed of 19 HRSC images acquired on March 30, 2022, when Phobos was 12 km from the camera. From this perspective, it’s difficult to see the size difference between the Martian moons because Deimos is farther from the camera at 28 km away.

Much is still unknown about the formation and composition of the Martian moons. Future missions such as the JAXA-led Martian Moon eXploration Mission (MMX) with contributions from ESA will help to understand more. MMX will observe Deimos and place a lander on Phobos to collect and return a sample from the surface.

Precise orbital measurements, such as those provided by occlusions, are required to pinpoint the precise positions of moons for future missions such as MMX and Juice.

14 years of Mars Express observations of the moons, including occultations with other Solar System objects, have allowed a 1-2 km correction in our knowledge of where the moons are located, with an accuracy of the order of a few hundred metres.

In June 2022, Europa’s orbit data for the Juice mission was refined by the occultation of a star. This occultation was predicted using ESA’s Gaia mission.

The new images show that Mars Express continues to illuminate Mars, the immediate vicinity of the red planet, and beyond.

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