Scientists with the European Space Agency's Rosetta mission may have had a scare when the Philae lander bounced off of the surface of comet 67P/Churyumov-Gerasimenko, but the bumpy touchdown actually had a silver lining: It allowed them to take measurements in two separate spots instead of one.
Now, in a suite of papers published in the journal Science, Philae researchers have started to sketch out the comet's physical and chemical profile -- that one spot is covered in fluffy, clumpy sediments while another is caked in a hard crust; that the comet's head is porous but fairly uniform in composition; and that there are a number of organic molecules, including four that have never before been detected on a comet.
The results, from the first spacecraft to perform a soft-landing on a comet, are just beginning to reveal an unprecedented up-close-and-personal view of this icy little world -- though there's still much more to learn, mission scientists said.
The flurry of seven papers are "more or less just a collection of pixels of a picture -- but I think you have to take a step back and look at all the different pixels to see if it actually gives you a picture," said physicist Fred Goesmann, who led experiments looking at organic compounds on the comet. "It hasn't really emerged yet what the big picture looks like."
When the Philae lander, released by the comet-chasing Rosetta spacecraft, first tried to touch down on Nov. 12 in an area later named Agilkia, it could not stick the landing. The anchoring harpoons and retro-rockets did not work and the lander bounced off the comet.
Luckily, Philae was not lost to space -- after hitting a crater rim and bouncing again, it eventually landed in a spot later dubbed Abydos. Unfortunately, it ended up on its side so that several of its instruments could not be deployed, and mostly out of the sun so that it had only three days to examine the comet before the solar-powered spacecraft's batteries ran out of power.
However, there was an up-side to the rough landing: It gave the researchers a chance to examine two spots on the comet's surface instead of just one -- and those two sites, Agilkia and Abydos, turned out to be quite different.
By analyzing bouncing-trajectory data from the spacecraft's two landing spots, the scientists found that the initial site of the bounce, Agilkia, was very soft, with a compressive strength of 1 kilopascal, while the compressive strength of the ground at Abydos topped 2 megapascals -- making it roughly 2,000 times harder. It was so hard, in fact, that a thermal sensor rod could not be hammered into the surface.
Agilkia, it seems, was covered in soft, fluffy sediment made of conglomerates of particles of dust and ice, in a layer that probably went roughly 10 inches deep, covering a harder surface beneath. Abydos' surface, on the other hand, has apparently been sintered by the sun's rays into a tough crust that probably feels something like the pumice found on Earth, said physicist Jens Biele, a mission scientist who previously served as Philae's payload manager and who led the study of the two impacts.
"The mechanical properties, how hard it is, how soft it is … [are] something you can only get in situ; you must touch or land on the comet," Biele said. "You cannot get it from orbit, from the big Rosetta spacecraft instruments."
Hard though that crust may be, the interior of the rubber-duck-shaped comet's head appears to be a porous mix of dust and ice, said Wlodek Kofman, principal investigator for an experiment that passed radio waves through the comet between Philae and Rosetta to measure the comet's inner properties. The relative homogeneity within 67P, he said, helps to further dispel the idea that comets are "rubble piles" of disparate material.
"This means, most probably, that this comet's insides are very pristine," Kofman said. This material probably condensed out of the dust that formed the young solar system -- making it a fossil from its early history.
While a temperature-sensing experiment was not able to penetrate the hardened crust at Abydos, the spacecraft did find that the daily temperature swings on the comet were from about minus 296 degrees to minus 226 degrees Fahrenheit.
"One of the puzzles about comets is, how does a little piece of ice produce such a big cometary phenomenon?" said Tilman Spohn, principal investigator for the temperature-sensing experiment.
Two experiments also detected organic compounds on 67P/Churyumov-Gerasimenko. One, the cometary sampling and composition experiment, found signs of 16 organic compounds, including four that had never before been detected in a comet. The other, known as the Ptolemy experiment, also found a radiation-caused formaldehyde polymer at the surface, which is interesting because of its potential relationship to prebiotic chemistry, said Ian Wright, principal investigator for the Ptolemy instrument.
"Formaldehyde is a really interesting molecule because of the fact that it's implicated in formation pathways for things like sugars, carbohydrates and interesting molecules like that," Wright said.
Many scientists think that the chemical building blocks for life on Earth might have been delivered by a comet -- and 67P's contents are doing little to dissuade researchers from that train of thought.
"You've got the ingredients of the backbone of the DNA molecule," said Monica Grady, a planetary scientist at the Open University who also worked on the Ptolemy experiment.
"We've found the building blocks of life are there in this comet, which is really, really great news," she added.
The mission team has not heard from Philae for weeks since it woke up from hibernation and sent a few messages back to Earth, but the researchers say they have hope that they'll touch base with the little spacecraft again.
"We'd love to do more measurements -- we just can't, because we can't command Philae," Goesmann said.
Frustrating as it is not to hear from Philae, he and other Rosetta researchers said they're still pleased with the data they have.
"We got one mass spectrum," Goesmann said of the suite of compounds his experiment found, "and that, I still think, is the sexiest mass spectrum in the world."
Some pointed out another silver lining: If Philae had landed in its intended spot on Agilkia, it would have been fried by the sun many months ago.
"If we'd stayed where we'd landed, we'd have been dead a long time ago," Wright said.