A generation ago, Astronomers thought of comets as simple things – huge dirty snowballs of rock and ice with a few organic chemicals thrown in. But after six months orbiting comet 67P/Churyumov–Gerasimenko, the unmanned Rosetta probe has shown them to be far more complex and active than previously thought.
To mark a special issue of Science, the European Space Agency (ESA) has released some preliminary findings from the data sent back by seven of the 11 instruments on the Rosetta comet orbiter. Far from being a static homogeneous collection of ice and stone, 67P shows a varied collection of terrains and processes that change their behavior as the comet moves closer to the Sun.
The most striking things about 67P is the shape it presented as Rosetta approached. Instead of a compact ball or an irregular cylinder, the comet turned out to be a "rubber duck" with a small lobe measuring 2.6 x 2.3 x 1.8 km (1.6 x 1.4 x 1.1 mi) and the larger one 4.1 x 3.3 x 1.8 km (2.5 x 2.0 x 1.1 mi). Linking them is a narrow neck marked by a 500 m (1.640 ft) crack running parallel to it. According to ESA, similar cracks on 67P are due to stresses caused by the comet heating and cooling, though whether the larger crack has a similar origin or is due to stress that may one day split the comet has yet to be determined.
This neck and its crack ties in with another mystery – the origin of the comet. The composition of the two lobes are identical, but question remains about whether the neck is the result of erosion or the fusion of two very similar comets sharing a similar origin.
Another curious fact about the comet is that it's a lot spongier than expected. At 10 billion tonnes (11 billion tons), 67P may not seem very light, but its volume is 21.4 km3 (5.1 mi3), which works out to a density of 470 kg/m3. This means it's 70 to 80 percent porous with an internal structure of ice dust and small voids.
The 70 percent of comet's surface that's been mapped so far shows remarkable variety, with 19 regions named after Egyptian deities and showing very different terrains. Some are covered with dust expelled from the interior as ice boils away in jets and falls back in dune-like ripples. Other areas are made up of brittle materials, and then there are large depressions, and smooth areas. A particularly curious feature are the "goosebumps" found on very steep cliff faces, the origin of which is still unknown.
But it wasn't just the comet itself that Rosetta was revealing. The famous coma surrounding comets is well known, but Rosetta demonstrated that there's more enveloping 67P. So far, instruments have shown that large amounts of dust as gas is being blown out of the comet to the point where whenever the it circles away from the Sun it ends up with a temporary shroud of dust orbiting it. In addition, there's more than just sublimated water jetting out, there's also carbon dioxide and carbon monoxide.
One aspect of all this outgassing is that 67P is not only developing an atmosphere, but as the solar wind the Sun’s UV radiation strikes the gas atoms and ionizes them, a magnetosphere as well. ESA says that this effect will become more pronounced as 67P draws closer to the Sun.
"Rosetta is essentially living with the comet as it moves towards the Sun along its orbit, learning how its behavior 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. "We have already learned a lot in the few months we have been alongside the comet, but as more and more data are collected and analysed from this close study of the comet we hope to answer many key questions about its origin and evolution."