Enlarge /. The close-ups highlight the bright ice that was exposed in the boulders when Philae hit them on the second touch (green box above).
The attempt by the Rosetta mission to drop the Philae lander on a comet in 2014 did not go according to plan. The harpoon mechanism that was used to hold Philae on solid ground did not work, and poor Philae jumped around and landed under a dark cliff overhang. He couldn't use his solar panels and couldn't do his job. But let's not say that Philae didn't leave its mark. Because it was like that. Literally.
To determine the value of Philae's accidental adventure, researchers worked hard to identify the spots where the vehicle hit the comet's surface. This required careful analysis of Philae's motion sensors to reconstruct their trajectory, as well as an extremely complex game of "one of these things not being the same" with before and after images of the comet's jumbled surface.
The location of the first jump was easy to find, but getting from there to his resting place was a different story. A new study by Laurence O'Rourke of the European Space Agency reveals another spot where Philae dented comet 67P. And the size of that dent actually says something remarkable about what comets are like.
The researchers eventually found what they termed the "crest of the skull" where two boulders separated by a crack appeared to have hit Philae. Upon landing, a bright spot appeared in this crevice, as if surface dust had been removed to expose water ice in the boulder. In fact, spectral data from images confirm that the bright spot is mostly water ice. While water ice makes up a significant proportion of comets – often roughly referred to as "dirty snowballs" – the surface of a comet is a layer of dust that is left behind when sunlight drives off the outermost ice so that ice is actually present.
A simple animation showing how the team believes Philae interacted with the two boulders.
The initial landing site of Philae was on a flat spot that was probably covered by a thick layer of dust. Encountering this boulder represents an interaction with something more like the interior of the comet.
The team estimates the depth of the dent left behind at about 25 centimeters. This enabled them to use the recorded speed of the 100-kilogram vehicle to calculate the robustness of the rock – or, as it turned out, a lack of it. They found that the boulder was actually as soft as fluffy snow on the earth.
This shows something that Rosetta has successfully measured: the comet is extremely porous. The high levels of water ice and CO2 ice may make you think the comet is a hard, frozen block, but about 75 percent of its volume is voids between grains of ice and dust. Without strong gravity to hold things together, comets just aren't that dense.
This is not the first estimate of the thickness of the comet obtained from the Philae landing attempt. The estimate, based on Philae & # 39; s initial touchdown crater, was much more stable and looked more like our moon's regolith, but that was likely the thick layer of dust. A drilling experiment conducted by Philae produced an almost stone-like strain estimate that far exceeded all others. However, it is not clear what material could have been drilled or if it was even accurate.
As the University of Arizona researcher Erik Asphaug writes in a summary of the new study in the journal Nature, this underscores the difficulty of missions targeting comets. For example, a sampling mission would have to think carefully about what to sample and how that material might behave.
"This is neither the first nor the last mishap on the surface of a small planetary body," writes Asphaug. "Unfortunately, the results also suggest that the best places to sample comets are not on the flat plains, but along the newly exposed ridges, cliffs, and boulders that are more difficult to land on."
And landing on a flat plain turned out to be difficult enough.
Nature, 2020. DOI: 10.1038 / s41586-020-2834-3, 10.1038 / d41586-020-02941-x (information on DOIs).