Rosetta's comet sheds its coat for warmer times

Fascinating things are happening to the world’s most-watched comet, 67P/Churyumov-Gerasimenko, as it approaches the Sun. A new study from NASA and ESA scientists published in Nature reports that by January 20, 67P shed a crust of dust built up on its surface over the last four years. In fact, by the end of January – about a week from now – the study’s authors expect heat and other radiation from the Sun will strip off the comet’s crust and mantle, exposing the icy nucleus.

As the comet hurtles into increasingly warmer space, the nucleus will start to evaporate into a dull haze around it that progressively forms the iconic tail as the ambient temperature increases. On August 13, 2015, 67P is expected to achieve perihelion, its closest distance from the Sun, at 185.98 million km.

The expelled dust was collected and analyzed by the ESA Rosetta probe that is tracking the comet, specifically by its Cometary Secondary Ion Mass Analyzer. It was found to be porous and rich in sodium, indicating that it may be the origin of particulate grains often found floating in the space between planets in the Solar System. The dust’s composition also challenges older notions that such grains are dominated by silicates.

Losing the dust

The deceptively simple process of shedding dust tells astronomers a lot about the comet’s composition and how it will change over time. The comet needs to be at a particular distance from a star for stellar radiation to whip away the dust from the surface. Therefore, based on the comet’s route from the Oort Cloud and toward the center of the Solar System, astronomers can deduce under what conditions the dust was accumulated to begin with.

As the dust builds up on the surface, it forms more and more layers devoid of icy particles borrowed from the comet’s nucleus. And as the whole comet heats up, the layers at the bottom start to vaporize first and unsettle the dust from the upper layers into a haze, called the coma, around it. Moreover, dust from the lower layers also starts getting floated toward the surface as a result of the icy grains started to melt. The overall effect – as the comet approaches the Sun – is for the coma to grow larger even as dust from the lower layers replenishes the coating on the surface.

At one point, however, the strength of the Sun’s radiation becomes strong enough to expel dust faster than it can be replenished – which is what is happening to 67P at the moment (Curiously, data from various instruments shows that the ‘neck’ region of this duck-shaped comet is losing dust the fastest).

The transition from when the replenishing mechanism is active to when dust is only getting expelled is usually smooth. Sometimes, however, it can be violent if there is a hard, intervening layer between the dust and the nucleus. Such a layer could be present on 67P, too, as evinced by the fact that the Rosetta probe’s lander Philae bounced around a bit on the comet’s surface on November 12, 2014, before it could latch itself on.

Other gases

In all this time, two other instruments on board Rosetta, the Microwave Instrument for Rosetta Orbiter and Rosetta Orbiter Spectrometer for Ion and Neutral Analysis, have also been studying how the rates at which the cometary water vapor and other gases – such as carbon dioxide and carbon monoxide – being released from the nucleus vary. They have together found that while water vapor dominates the contents of the expelled matter, there are occasional spikes in the quantity of the two gases, too.

Myrtha Hässig, a NASA-sponsored scientist from the Southwest Research Institute, San Antonio, said, “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.”

Such conclusions, from the characteristics of dust erosion on the surface to its possible internal composition, as well as the more speculative ideas of cometary migration concern not just 67P but the broader class of short-period comets, many of which visit the Sun at least once every 200 years. Astronomers think their long journeys makes them well-suited to be messengers carrying non-native molecules to worlds that might not have otherwise acquired them – such as those necessary for life on Earth.