Chuck Bednar for redOrbit.com – Your Universe Online
Comet 67P/Churyumov–Gerasimenko has five basic but diverse categories of terrain type, as well as 19 distinct geomorphological boundaries, according to early data obtained by the ESA’s Rosetta mission and published in a special edition of the journal Science.
Several new studies based on initial results from seven of the comet-landing probe’s 11 science instruments have been published this week, the agency announced on Thursday. The preliminary findings, made during Rosetta’s approach to Comet 67P/C-G and shortly following its arrival in August 2014, include details about its surface features and its ongoing activity.
Measurements of the duck-shaped, dual-lobed comet revealed that the smaller portion measures 2.6 × 2.3 × 1.8 km and the larger one measures 4.1 × 3.3 × 1.8 km. The comet has a total volume of 21.4 km3; its mass is 10 billion metric tons; and it has a density of 470 kg/m3.
Furthermore, by assuming that its overall composition is primarily water, ice, and dust with a density of 1500-2000 kg/m3, mission scientists have determined that the comet has a very high porosity between 70 percent and 80 percent, and that its interior structure is probably made from weakly-bonded clumps of ice and dust with small void spaces separating them.
The terrain and regions
Thanks to the efforts of Rosetta’s OSIRIS camera, roughly 70 percent of the surface has been imaged, leading to the discovery of five terrain types: dust-covered; brittle materials with pits and circular structures; large-scale depressions; smooth terrains; and exposed, consolidated rock-like surfaces. Only parts of the southern hemisphere have yet to be imaged.
The 19 distinct regions of the comet have, in keeping with the ancient Egyptian theme of the Rosetta mission, been named for Egyptian deities. They have been grouped based on the type of terrain dominant within: Ma’at, Ash and Babi (dust-covered); Seth (brittle materials); Hatmehit, Nut and Aten (depressions); Hapi, Imhotep and Anubis (smooth); and Maftet, Bastet, Serqet, Hathor, Anuket, Khepry, Aker, Atum and Apis (rock-like).
The majority of the northern hemisphere is covered in dust, the ESA said. As comet 67P/C-G becomes heated, ice turns directly into gas, which escapes to form its atmosphere, or coma. Dust is carried along with the gas at slower speeds, and particles not travelling fast enough to overcome the weak gravity fall to the surface instead.
It’s not that kind of coma
In one of the studies, researchers from the Southwest Research Institute (SwRI) in San Antonio and their colleagues report that the coma is far less homogenous than expected, and that there is significant variation in the comet’s outgassing over time.
“If we would have just seen a steady increase of gases as we closed in on the comet, there would be no question about heterogeneity of the nucleus,” explained lead author and SwRI postdoctoral researcher Dr. Myrtha Hässig. “Instead we saw spikes in water readings, and a few hours later, a spike in carbon dioxide readings This variation could be a temperature effect or a seasonal effect, or it could point to the possibility of comet migrations in the early solar system.”
Dr. Hunter Waite, a program director and planetary scientist at SwRI, added, “Our whole concept of the variability of volatile release at comets will change based on this paper, which will have significant impact on our understanding of comet formation and evolution.”
In another study, researchers reported that Rosetta’s Microwave Instrument for Rosetta Orbiter (MIRO) detected a significant increase the amount of water “pouring” out of 67P/C-G. From June through August 2014, the amount of H2O being released into space by the comet increased tenfold, to the equivalent of 40 ounces every second, according to NASA researchers.
“To be up close and personal with a comet for an extended period of time has provided us with an unprecedented opportunity to see how comets transform from cold, icy bodies to active objects spewing out gas and dust as they get closer to the sun,” explained Sam Gulkis, principal investigator of the MIRO instrument at NASA’s Jet Propulsion Laboratory in Pasadena, California.
A third paper describes evidence for carbon-based molecules on the surface of 67P/C-G that was gathered using the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument. Based on previous studies, the authors of that paper explained that they expected to see signs of more complex molecules, including alcohols, carboxylic acids and nitrogen-containing amines.
However, data obtained from VIRTIS suggests that the surface of the comet is actually made up mostly of simpler hydrocarbons – a discovery which could radically impact our understanding of how carbon-based molecules first developed and spread through our solar system.
“Comets have always surprised humanity. C-G seems to be no exception,” study co-author Murthy Gudipati, a part-time senior research scientist at the University of Maryland Institute for Physical Science and Technology, said in a statement.
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