The close-ups highlight the bright ice exposed in the boulders when Philae struck them during its second touchdown (green box above).
Enlarge / The close-ups spotlight the intense ice uncovered within the boulders when Philae struck them throughout its second landing (inexperienced field above).

The Rosetta mission’s try to drop the Philae lander on a comet in 2014 didn’t go in accordance with plan. The harpoon mechanism meant to stay Philae to terra-not-quite-firma didn’t work, and poor Philae ended up bouncing round and touchdown below a darkish cliff overhang, unable to deploy its photo voltaic panels and full its duties. However let it not be stated that Philae failed to go away its mark. As a result of it did. Fairly actually.

To extract worth from Philae’s unintentional journey, researchers have labored arduous to establish the spots the place the craft impacted the floor of the comet. This required painstaking evaluation of Philae’s movement sensors to reconstruct its trajectory, together with a terrifically complicated recreation of “certainly one of these items isn’t just like the others” performed with before-and-after photos of the comet’s jumbled floor.

The positioning of the preliminary bounce was straightforward sufficient to search out, however the path from there to its resting place was one other story. A brand new research led by the European House Company’s Laurence O’Rourke reveals one other spot the place Philae dented comet 67P. And the scale of that dent truly tells us one thing exceptional about what comets are like.

Dented

Researchers finally discovered a spot they dubbed “skull-top ridge” the place a pair of boulders separated by a crevice appeared to have met Philae. After the touchdown, a shiny spot appeared in that crevice, as if floor mud had been eliminated to show water ice within the boulder. And certainly, spectral knowledge from imagery confirms that the intense spot is essentially water ice. Whereas water ice makes up a considerable portion of comets—which are sometimes considerably rudely known as “soiled snowballs”—a comet’s floor consists of a layer of mud left behind as daylight drives off the outermost ice, so truly seeing ice there’s telling.

A easy animation to point out how the crew thinks Philae interacted with the pair of boulders.

Philae’s preliminary landing location was in a flat spot seemingly coated by a thick layer of that mud. Its encounter with this boulder represents an interplay with one thing extra much like the comet’s inside.

The crew estimates the depth of the dent it left behind at about 25 centimeters. Utilizing the recorded velocity of the 100-kilogram craft, this allowed them to calculate the boulder’s sturdiness—or lack thereof, because it seems. They discovered that the boulder was truly about as comfortable as fluffy snow on Earth.

Blended measurements

This illustrates one thing that Rosetta efficiently measured: the comet is extraordinarily porous. The excessive water ice and CO2 ice content material may make you assume the comet is a tough, frozen block, however round 75 p.c of its quantity is void house in between grains of ice and dirt. With out robust gravity to drag issues collectively, comets simply aren’t that dense.

This isn’t the primary estimate of the comet’s materials energy to return out of the Philae touchdown try. The estimate based mostly on Philae’s preliminary landing crater was considerably sturdier, extra much like the regolith of our Moon, however that was seemingly the thick mud layer. A drilling experiment carried out by Philae returned a virtually rock-like stretch estimate, far surpassing any others. Nevertheless it’s not clear what materials that would have been drilling, or if it was even correct.

However as College of Arizona researcher Erik Asphaug writes in a abstract accompanying the brand new research within the journal Nature, this all highlights the problem of missions focusing on comets. A pattern assortment mission, for instance, must consider carefully about what to pattern and the way that materials may behave.

“This isn’t the primary misadventure to happen on the floor of a small planetary physique, nor will or not it’s the final,” Asphaug writes. “Sadly, the findings additionally counsel that the very best locations to pattern comets won’t be the flat plains, however alongside the newly uncovered ridges, cliffs and boulder piles, that are harder to land on.”

And touchdown on a flat plain proved troublesome sufficient.

Nature, 2020. DOI: 10.1038/s41586-020-2834-3, 10.1038/d41586-020-02941-x (About DOIs).

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