X marks the spot

ESA lander leaves the mark of a 'skull' on Comet 67P

Scientists have retraced Rosetta’s Philae lander's final bounce across Comet 67P.

On November 12, 2014 the European Space Agency's Rosetta probe deployed a lander to the surface of Comet 67P/Churyumov-Gerasimenko.

It was supposed to be a smooth touchdown, but it was more of a bounce.

The Philae Lander pinged across the surface of Comet 67P multiple times, before coming to rest in a cave-like structure and powering down for good. Now, six years later, scientists are finally able to pinpoint where, exactly, Philae landed — and then landed again — on the comet.

And fittingly for the time of year, the lander left its imprint in a truly spooky fashion — revealing the shape of a skull.

Skull rock.

ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; O'Rourke et al (2020)

Philae's second (unexpected) landing site is detailed in a study published Wednesday in the journal Nature. While unplanned, the bounce has revealed unprecedented details about the comet — including how it may have formed.

Ancient ice — The Rosetta probe orbited Comet 67P since August 2014, studying the comet's nucleus and coma — the halo of dust and gas it produces as it moves through space. Philae was meant to be the crowning glory of the mission, although the lander didn't quite stick its landing.

Images from Rosetta reveal Philae's bounce toward its final resting spot. The lander first touched down in a region of the comet called Agilkia, before bouncing up on a two-hour flight. It then collided with a cliff edge and fumbled its way across the comet towards a second touchdown location, according to ESA. Philae eventually came to a halt at Abydos, a sheltered, cave-like area.

Abydos valley.

Video prepared by and music composed by Gerhard Paar (Joanneum Research Forschungsgesellschaft mbH); Analysis: O'Rourke et al (2020)

The second, awkward stumble is what caught the scientists attention for this study. As it hit the comet for a second time, Philae scraped the space rock's surface, exposing a layer of ice from when the comet was first formed.

Before Philae's crash landing, this ice had never before been exposed to the radiation of the Sun. Instead, it had remained protected beneath the comet's dusty surface.

“Philae had left us with one final mystery waiting to be solved,” Laurence O’Rourke, who led the team that found Philae, said in a statement. “It was important to find the touchdown site because sensors on Philae indicated that it had dug into the surface, most likely exposing the primitive ice hidden underneath, which would give us invaluable access to billions-of-years-old ice.”

Philae's multiple landing sites.

Images: Touchdown 1: ESA/Rosetta/Philae/ROLIS/DLR; all other images: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; Analysis: O’Rourke et al (2020)

Space skulls — This newly discovered patch of raw ice can be glimpsed in the images obtained by Rosetta — oddly, the ice appears to look like the face of a skull, capturing the hearts and minds of the scientists studying it.

“The shape of the boulders impacted by Philae reminded me of a skull when viewed from above, so I decided to nickname the region ‘skull-top ridge’ and to continue that theme for other features observed,” O'Rourke said.

ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; Analysis: O’Rourke et al (2020)

Located just 30 meters away from Philae's still body, the exposed area of the raw ice measured across approximately 3.5 square meters. At the time that the images were captured, the Sun's light was directly beaming at the raw ice, which made it appear brighter than its surroundings.

The ice had not been previously exposed to the space environment — as a consequence it had never undergone space weathering. Essentially, it offers a window into what the comet looked like when it first formed.

The landing mishap also provided an opportunity for the scientists to measure just how soft the icy-dust interior is within the boulder of a comet. Their conclusion: it is very soft, frothy in fact.

“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,” O'Rourke said.

Philae's unfortunate landing could help inform future missions to other orbiting comets, the scientists say.

“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 angular in a way that has never been intended to give us these wonderful results.” Philip Heinisch, who led the analysis of the data, said in a statement.

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