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Telescopes and space missions

Rosetta data deluge reveals dynamic comet with sand dunes and jets

23 Jan 2015 Tushna Commissariat
Cometary cartography: 19 mapped regions on 67P

Last November the European Space Agency’s Rosetta mission made history when its Philae lander touched down on the surface of comet 67P/Churyumov–Gerasimenko. Now, mission researchers have studied new data from a host of sensors and devices on board the main Rosetta craft that is in orbit around comet 67P. The latest data provide the closest and most detailed look at the Jupiter family comet (JFC) and tell us about its coma, shape, composition, temperature, nucleus, surface features and more. The researchers found that, in some instances, the comet differs from other JFCs encountered so far, thereby improving our knowledge of comet formation and the origin of our solar system.

The new work has been published in a special issue of the journal Science this week and includes seven new reports based on data from the orbiter.

Nicolas Thomas from the University of Bern and colleagues scrutinize data from the Optical, Spectroscopic and Infrared Remote Imaging System (OSIRIS) on board Rosetta. Their images, which cover nearly 70% of the total surface, show a variety of different structures and textures including dune and ripple-like structures, wind tails, and many active processes such as dust transport that have carved out of the comet’s features. They also see a surface riddled with fractures at different length scales, especially at the comet’s nucleus (solid centre) and surface erosion via the loss of large chunks of material. This, they conclude, means that the nucleus may have lost a large amount of matter in this way.

The researchers also identified 19 distinct regions on the comet that are separated by distinct boundaries and are grouped according to the type of terrain dominant within. The terrain itself is made up of five basic categories: dust-covered; brittle materials with pits and circular structures; large-scale depressions; smooth terrains; and exposed, more consolidated (or “rock-like”) surfaces.

Fluffy insides

Holger Sierks at the Max Planck Institute for Solar System Research in Göttingen, Germany, and colleagues also used OSIRIS to study the nucleus of 67P, which is made of dust, rock and frozen gas, Surprisingly, they found that the nucleus seems to be rather porous and fluffy, and has a bulk density less than half that of water. They also note that 67P’s unique “rubber-duck” shape posits an interesting question regarding the comet’s origin – whether its two lobes formed from two objects as a “contact binary” nearly 4.5 billion years ago, or it is a single body with a gap that evolved thanks to it losing mass. While the researchers do not have a definitive answer just yet, they point out that as both lobes have a very similar composition, a single eroded body seems more likely. However, they cannot yet rule out the possibility that 67P is the result of two similar comets forming in the same part of the solar system and then merging sometime later.

Using Rosetta’s Visible and Infrared Thermal Imaging Spectrometer (VIRTIS), Fabrizio Capaccioni at Istituto Nazionale di Astrofisica (INAF) in Rome, Italy, and colleagues found that the nucleus is covered with opaque, organic compounds – but very little water ice. This indicates that the sunlit surface of 67P is quite dehydrated. Indeed, the researchers say that this extremely dark, dry and rich comet is very different from the other JFCs studied to date, and that thanks to the presence of organic compounds on the nucleus, 67P “represents a different species in the cometary zoo”.

Samuel Gulkis at NASA’s Jet Propulsion Laboratory in Pasadena, California, and colleagues measured the temperature of 67P using the Microwave Instrument on the Rosetta Orbiter (MIRO). Their data identify the daily and seasonal patterns in the temperatures beneath 67P’s surface. They claim to have seen “fluxes of heat transport” and ice sublimation, and they suggest that most water ice is lost as it sublimates to a gas from the “neck” of the 67P comet, where plumes of gas have often been seen. The dusty covering of the comet may be several metres thick in places, and MIRO’s measurements of the surface and subsurface temperature suggest that the dust plays a key role in insulating the comet interior, helping to protect the ices thought to exist below the surface. This, according to the researchers, may play an important role in the “longevity of 67P, and probably of comets in general. The importance of measuring the temperatures below the surface of a comet – and particularly below its diurnal layer – is illustrated by these data”, they write.

Varying coma

Myrtha Hässig at the University of Bern, Switzerland, and colleagues took many measurements of the composition of the comet’s coma – the fuzzy envelope surrounding 67P’s nucleus – using the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) over many rotational periods (it takes 12.4043 h for the comet to rotate once). They saw large compositional fluctuations in the “heterogeneous coma that has diurnal and possibly seasonal variations”, and they saw that, along with water, carbon monoxide and carbon dioxide were outgassed from the surface, revealing a complex relationship between the comet’s nucleus and its coma.

Alessandra Rotundi at INAF in Rome and colleagues put together data from all of the Rosetta instruments, including the Grain Impact Analyser and Dust Accumulator (GIADA), to capture and analyse dust grains from the comet and study the dust grains’ speed, momentum and mass. Combined with data from OSIRIS, ROSINA and MIRO taken between July and September last year, the team has made a first estimate of the comet’s dust-to-gas ratio, with around four times as much mass in dust being emitted than in gas, averaged over the sunlit nucleus surface.

Hans Nilsson at the Swedish Institute of Space Physics and colleagues have used the Rosetta Plasma Consortium (RPC) instruments and scrutinized the water ions in 67P’s atmosphere to try and decipher how a magnetosphere may form around the comet. As the comet approaches the Sun, its gas-dust coma will continue to grow, and interactions with charged particles of the solar wind and ultraviolet light from the Sun will lead to the development of the comet’s ionosphere and, ultimately, the magnetosphere.

“Rosetta is essentially living with the comet as it moves towards the Sun along its orbit, learning how its behaviour changes on a daily basis and, over longer timescales, how its activity increases, how its surface may evolve, and how it interacts with the solar wind,” says Matt Taylor, ESA’s Rosetta project scientist. In the coming months, Rosetta will keep pace with 67P as it looms ever closer to the Sun – its closest approach will be in August – and the comet becomes much more active.

The research is published in Science.

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