ESA’s Rosetta orbiter has made the first unambiguous detection of solid organic matter in dust grains ejected by comet 67P/Churyumov-Gerasimenko.
This false-color four-image mosaic comprises images taken from a distance of 28.7 km from the center of Comet 67P/Churyumov-Gerasimenko on 3 February 2015. The mosaic measures 4.2 x 4.6 km. Image credit: ESA / Rosetta / NAVCAM / CC BY-SA IGO 3.0.
While organics had already been detected in situ on the surface of 67P/Churyumov-Gerasimenko by instruments on-board the Philae lander and from orbit by Rosetta’s ROSINA instrument, those were both in the form of gases resulting from the sublimation of ices.
By contrast, Rosetta’s COmetary Secondary Ion Mass Analyser (COSIMA) has made its detections in solid dust.
A paper detailing the result was published in the journal Nature and is available online.
“We report the in situ detection of solid organic matter in the dust particles emitted by comet 67P/Churyumov-Gerasimenko,” the authors said.
“The carbon in this organic material is bound in very large macromolecular compounds, analogous to the insoluble organic matter found in the carbonaceous chondrite meteorites.”
The presence of solid carbonaceous matter was only ever hinted at in previous comet missions, which flew by their targets at high speed and, as a result, disrupted the particles, making characterization challenging.
But Rosetta is orbiting 67P/Churyumov-Gerasimenko and can catch dust particles moving at low speed.
“By July 2016, COSIMA had detected more than 27,000 particles in the vicinity of comet 67P/Churyumov–Gerasimenko,” the scientists said.
“Of them, more than 200 particles have been analyzed. They show various morphologies and mineral compositions, inferred from time-of-flight secondary ion mass spectrometry (TOF-SIMS) analyses.”
“We present findings on the organic content of two representative particles, named Kenneth and Juliette.”
Both particles are larger than 100 μ m in size. Kenneth was collected between 11 and 12 May 2015, while Juliette was collected between 23 and 29 October 2015.
False-color image of two of the dust grains collected and analyzed by Rosetta’s COSIMA instrument, named Kenneth and Juliette, which show the signature of carbon-based organics. Image credit: ESA / Rosetta / COSIMA Team / MPS / CSNSM / UNIBW / TUORLA / IWF / IAS / ESA / BUW / MPE / LPC2E / LCM / FMI / UTU / LISA / UOFC / vH&S / Fray et al, doi: 10.1038/nature19320.
“Our analysis reveals carbon in a far more complex form than expected. It is so complex, we can’t give it a proper formula or a name,” said co-author Dr. Hervé Cottin, of the Université Paris Est Créteil et Université Paris Diderot in France.
The carbon is found to be mixed with other previously reported elements such as sodium, magnesium, aluminum, silicon, calcium and iron.
It is bound in very large macromolecular compounds similar to the insoluble organic matter found in carbonaceous chondrite meteorites that have fallen to Earth, but with a major difference: there is much more hydrogen found in the comet’s samples than in meteorites.
But as this kind of meteorite is associated with reasonably well-processed parent bodies such as asteroids, it is reasonable to assume that they lost their hydrogen due to heating.
By contrast, comets must have avoided such significant heating to retain their hydrogen, and therefore must contain more primitive material.
From analyses of meteorites and lab simulations, the researchers were also expecting to identify a wide diversity of organic material in Rosetta’s comet, ranging from very small molecules to heavy organics.
Although Rosetta’s ROSINA and Philae’s PTOLEMY and COSAC instruments detected numerous low-molecular weight volatile organic molecules, COSIMA only saw very large carbon-bearing macromolecules in the dust particles, with nothing in between.
This suggests potentially different sources for the lightweight volatile and heavier refractory carbonaceous material detected in the comet.
“Although we cannot know if the organics seen in these dust particles were created in the interstellar medium before the protoplanetary nebula came together, or in the protoplanetary disk during early Solar System formation, COSIMA’s dust grains are certainly witnesses to early formation processes, including that of the comet itself,” said lead author Dr. Nicolas Fray, also from the Université Paris Est Créteil et Université Paris Diderot.
“These particles have remained pristine and untouched for billions of years until they were released in the days or weeks before being ‘caught’ by COSIMA,” said senior author Dr. Martin Hilchenbach, COSIMA principal investigator at the Max-Planck Institute for Solar System Research in Germany.
“The results add to the growing picture that comet 67P/Churyumov-Gerasimenko contains some of the most primitive material from our Solar System’s early history.”
_____
Nicolas Fray et al. High-molecular-weight organic matter in the particles of comet 67P/Churyumov–Gerasimenko. Nature, published online September 7, 2016; doi: 10.1038/nature19320
