Details of the Comet’s Water-Ice Cycle
Comet 67P/Churyumov-Gerasimenko is one of the most-studied comets in history because it has a spacecraft riding alongside it as it orbits the Sun. Like all other comets, it is an icy chunk made of a mixture of water and other ices plus a healthy helping of dust grains.
When a comet gets close to the Sun in its orbit — a point called perihelion — sunlight warms the nucleus and causes the ices to go from a solid to a gaseous state. That’s called “sublimation” and is similar to what dry ice does if you leave it out exposed to the Sun.
The Rosetta mission has been studying this process at Comet 67P, and found that as the comet rotates, various parts of its surface are exposed to the Sun over a 12-hour period. As they warm, water ice sublimates, and the spacecraft detected that.
However, the underlying layers remain warm owing to the sunlight they received in the previous hours, and, as a result, subsurface water ice keeps sublimating and finding its way to the surface through the comet’s porous interior.
But as soon as this “underground” water vapor reaches the cold surface, it freezes again, blanketing that patch of comet surface with a thin layer of fresh ice.
Eventually, as the Sun rises again over this part of the surface on the next comet day, the molecules in the newly formed ice layer are the first to sublimate and flow away from the comet, restarting the cycle.
From these data, it is possible to estimate the relative abundance of water ice with respect to other material. Down to a few cm deep over the region of the portion of the comet nucleus that was surveyed, water ice accounts for 10-15% of the material and appears to be well-mixed with the other constituents.
The scientists also calculated how much water vapor was being emitted by the patch that they analyzed with VIRTIS, and showed that this accounted for about 3% of the total amount of water vapor coming out from the whole comet at the same time, as measured by Rosetta’s MIRO microwave sensor.
â€œIt is possible that many patches across the surface were undergoing the same diurnal cycle, thus providing additional contributions to the overall outgassing of the comet,â€ adds Dr. Capaccioni.
The scientists are now busy analyzing VIRTIS data collected in the following months, as the comet’s activity increased around the closest approach to the Sun.
“These initial results give us a glimpse of what is happening underneath the surface, in the comet’s interior,” according to Matt Taylor, ESA Rosetta Project Scientist.