Getting to know Rosetta’s comet
Rosetta is revealing its host comet as having a remarkable array of surface features and with many processes contributing to its activity, painting a complex picture of its evolution.
In a special edition of the journal Science, initial results are presented from seven of Rosetta’s 11 science instruments based on measurements made during the approach to and soon after arriving at Comet 67P/Churyumov–Gerasimenko in August 2014.
The familiar shape of the dual-lobed comet has now had many of its vital statistics measured: the small lobe measures 2.6 × 2.3 × 1.8 km and the large lobe 4.1 × 3.3 × 1.8 km. The total volume of the comet is 21.4 km3 and the Radio Science Instrument has measured its mass to be 10 billion tonnes, yielding a density of 470 kg/m3.
By assuming an overall composition dominated by water ice and dust with a density of 1500–2000 kg/m3, the Rosetta scientists show that the comet has a very high porosity of 70–80%, with the interior structure likely comprising weakly bonded ice-dust clumps with small void spaces between them.
The OSIRIS scientific camera, has imaged some 70% of the surface to date: the remaining unseen area lies in the southern hemisphere that has not yet been fully illuminated since Rosetta’s arrival.
The scientists have so far identified 19 regions separated by distinct boundaries and, following the ancient Egyptian theme of the Rosetta mission, these regions are named for Egyptian deities, and are grouped according to the type of terrain dominant within.
Five basic – but diverse – categories of terrain type have been determined: dust-covered; brittle materials with pits and circular structures; large-scale depressions; smooth terrains; and exposed more consolidated (‘rock-like’) surfaces.
Much of the northern hemisphere is covered in dust. As the comet is heated, ice turns directly into gas that escapes to form the atmosphere or coma. Dust is dragged along with the gas at slower speeds, and particles that are not travelling fast enough to overcome the weak gravity fall back to the surface instead.
Some sources of discrete jets of activity have also been identified. While a significant proportion of activity emanates from the smooth neck region, jets have also been spotted rising from pits.
The gases that escape from the surface have also been seen to play an important role in transporting dust across the surface, producing dune-like ripples, and boulders with ‘wind-tails’ – the boulders act as natural obstacles to the direction of the gas flow, creating streaks of material ‘downwind’ of them.