Philae's Bouncy Landing Reveals Ice As "Soft As Cappuccino Froth" On Comet 67P

Image of 67P with Philae located at the very right edge of the image in the shadow of a large boulder .ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

When the European Space Agency’s Rosetta mission reached its destination, Comet 67P/Churyumov–Gerasimenko, it carried a little lander called Philae. On November 12, 2014, it became the first human artifact to make a soft landing on a comet.

The landing was not as successful as the team had hoped and poor Philae bounced across the comet's surface before coming to rest in a position that didn’t allow enough sunlight to reach the solar-powered lander. Eventually, ESA officially gave up trying to communicate with the "lost" Philae. However, its short and glorious mission was not in vain. Almost six years later, researchers have gained some crucial insight into comets thanks to its unplanned tumbling.

Reporting in Nature, researchers have identified the second touchdown site of the lander, and made a curious discovery about the comet's icy interior exposed in a boulder Philae hit.

When Philae landed it hit the ground several times, first at its primary landing site and then where it eventually came to rest. In the second location, the lander hit the ground four times, leaving a deep imprint on an ice boulder as it scudded across the ground. The team was able to measure both the depth of the imprint (25 centimeters/9.8 inches) and how quickly it took to occur (just 3 seconds), which allowed them to estimate the compressive strength of the icy boulder. The answer is truly mind-blowing. Their calculations suggest the ice boulder is softer than freshly fallen light snow.

“The simple action of Philae stamping into the side of the crevice allowed us to work out that this ancient, billions-of-years-old, icy-dust mixture is extraordinarily soft – fluffier than froth on a cappuccino, or the foam found in a bubble bath or on top of waves at the seashore,” lead author Dr Laurence O’Rourke from ESA said in a statement.

O’Rourke led the team that found the secondary location of Philae 22 months after its landing, which he has nicknamed "skull-top ridge". Using observations from the Rosetta spacecraft and local measurements from Philae’s magnetometer ROMAP, the team was able to trace its 2-minute bumpy journey, allowing them to understand exactly what happened when Philae landed and measure the properties of the ancient ice uncovered by its bouncing.

How Philae left its mark during touchdown two. The ROMPA data was matched to locations on the surface of the comet. ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA/Rosetta/Philae/ROMAP/O’Rourke et al (2020) 

“We weren’t able to make all the measurements we planned in 2014 with Philae, so it is really amazing to use the magnetometer like this, and to combine data from both Rosetta and Philae in a way that was never intended, to give us these wonderful results,” said Philip Heinisch, who led the analysis of the ROMAP data.

The new data also shows that the boulder Philae hit is quite porous. There is a lot of empty space between the ice and dust grains in it. This matches well with general observations of the whole comet and has important implications for the future exploration of these dirty-ice celestial bodies.

“This is a fantastic multi-instrument result that not only fills in the gaps in the story of Philae’s bouncy journey, but also informs us about the nature of the comet,” said Matt Taylor, ESA’s Rosetta project scientist. “In particular, understanding the strength of a comet is critical for future lander missions. That the comet has such a fluffy interior is really valuable information in terms of how to design the landing mechanisms, and also for the mechanical processes that might be needed to retrieve samples.”

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