Issue Date: August 3, 2015 | Web Date: July 30, 2015
Philae Probe Sniffs Out Comet's Chemistry
Last November, the world watched with bated breath as the European Space Agency’s Rosetta mission landed a probe for the first time on the surface of a comet. Now a series of seven papers in the journal Science provide new data and analyses from that mission to the 2.7-mile-long, dumbbell-shaped comet 67P/Churyumov-Gerasimenko, often abbreviated as 67P.
Among the results of imaging, radar, and other experiments, two papers reveal the molecular makeup of 67P, including the observation of four organic molecules not previously observed on a comet and the presence of a radiation-induced polymer (Science 2015, DOI: 10.1126/science.aab0673 and 10.1126/science.aab0689).
The washing machine-sized Philae probe had a rough landing. On its first touchdown, the systems required to tether the probe to the comet’s icy, solid core failed. So instead, Philae bounced several times, colliding with the rim of a crater before coming to its final landing spot more than a kilometer away, says mission scientist Jens Biele at the German Aerospace Center in Cologne, Germany.
Because of these landing glitches, several of the mission’s scientific operations failed, including a plan to drill into the comet’s interior and send the samples to two mass spectrometers on the lander, Ptolemy and COSAC.
Luckily, however, both mass spectrometers have an emergency “sniff” mode, says Fred Goesmann, who leads the COSAC experiment. This permits the analysis of molecules that have passively percolated into the apparatus through venting pipes.
Using this sniff mode, COSAC detected 16 organic compounds, including four that have never before been observed on a comet: methyl isocyanate, acetone, propanal, and acetamide. The findings reinforce the view that comets boast a rich inventory of organic molecules, comments Steve Charnley, a molecular astrophysicist at NASA who wasn’t involved in the work. It also furthers the origin-of-life theory that comets striking Earth during the prebiotic era may have supplied organic molecules that helped life evolve on our planet, he says.
Data acquired using the second mass spectrometer, Ptolemy, revealed that the comet contains organic polymers, most likely from the radiation-induced polymerization of formaldehyde, says Ian P. Wright, who leads the Ptolemy experiment. Ptolemy also detected a significant amount of carbon dioxide, a molecule that was not observed using COSAC.
This and other inconsistencies in what the two spectrometers measured—for example, COSAC found a high abundance of nitrogen-containing compounds, while Ptolemy detected none—likely “says something about the heterogeneity of the comet,” Wright says. Or the inconsistencies could have to do with the 180° difference in relative orientations of the mass spectrometers, Goesmann says. COSAC, which faced the comet surface, might have measured comet dust billowing up, while Ptolemy, which faced outward, might have measured elements of the haze around the comet’s surface.
In the meantime, Wright says the Philae mission team is busy trying to solve communication problems with the probe—the radio signal is fickle—so that they can upload new command sequences that would permit the probe to learn more about 67P.
“The results already in hand are an incredible achievement, especially considering the landing challenges and the fact that these analyses come from only about two minutes of data acquisition,” comments Michael Mumma, the director of the Goddard Center for Astrobiology, at NASA. “This work shows the very tip of the cornucopia of rewards that will ensue if they can get the damn thing working again.”
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