A 67P Churyumov-Gerasimenko update by Melanie Davies FRAS

In September 2019, at a major planetary sciences conference (EPSC-DPS), scientists lead by Jean-Baptiste Vincent presented evidence of curious bouncing boulders on Comet 67P/Churyumov-Gerasimenko. EPSC-DPS is the joint meeting of the European Planetary Science Congress and the Division of Planetary Sciences, and last year was held in Geneva, Switzerland.

From August 2014 to September 2016 Comet 67P had an orbiting companion: Rosetta. Operated by the European Space Agency (ESA), Rosetta’s OSIRIS camera captured around 76,000 high resolution images of the comet surface. This vast amount of images is still being investigated and analysed, so we can better understand how this comet evolved at perihelion passage – its closest approach to the Sun.

Comet 67P/Churyumov-Gerasimenko was likened to a rubber duck without a beak! It has two distinctive rugged lobes with a smooth ‘neck’ in between. It was on this neck that many structural changes were identified as the comet warmed up; cracks and fissures, cliff falls, and the appearance of new boulder fields. Jean-Baptiste Vincent and his team discovered a new example of a bouncing boulder of particular interest – a rock measuring about 10 metres wide that appeared to have fallen during a cliff collapse. Vincent’s bouncing boulder was discovered by analysing before and after images of this region from 2015 and 2016 respectively.

The first image (left) provides a reference view of the comet, along with a close-up of the region under study. The smaller insets on the right show before and after images of the region containing the bouncing boulder. Impressions of the boulder have been left in the the comet’s surface material as it bounced to a halt. It is thought to have fallen from a nearby cliff, which is about 50m high. The graphic at the bottom illustrates the path of the boulder as it bounced across the surface. Image credit: ESA/Rosetta/MPS for OSIRIS Team; Analysis: J-B. Vincent et al (2019)

“We think it fell from the nearby 50 m-high cliff, and is the largest fragment in this landslide, with a mass of about 230 tonnes,” explained Vincent of the DLR Institute for Planetary Research, Berlin.

“So much happened on this comet between May and December 2015 when it was most active, but unfortunately because of this activity we had to keep Rosetta at a safe distance. As such we don’t have a close enough view to see illuminated surfaces with enough resolution to exactly pinpoint the ‘before’ location of the boulder.”

By studying boulder movements like this in different parts of the comet, scientists can determine the mechanical properties of both the falling rocks and the loosely consolidated surface material on which it lands: the comet’s regolith. Reliable estimates of the mass and gravity of 67P can be used to determine things like impact speed and compression strength of a falling rock, depending on its survival, fragmentation and the size of its impact crater. Bouncing boulders are especially useful for this.

The structure of this comet is in general very weak compared with the ice and rocks we are familiar with on the terrestrial planets and moons in the Solar System. Boulders on Comet 67P are around one hundred times weaker than freshly packed snow!