When Comet 67P/Churyumov-Gerasimenko passed by the sun in August 2015, it was expelling 300 kg of water vapor every second. A few hundred kilometers away, the European Space Agency (ESA)’s Rosetta kept pace with the celestial body.
Though recent attempts to contact the long-dark comet lander Philae have gone unanswered, scientists are still learning much from the previous data collected by Rosetta. In fact, thanks to Rosetta’s VIRTIS infrared instrument, scientists recently announced that they’ve found exposed water ice on Comet 67P’s topmost layer.
“Although water vapor is the main species observed in the coma of Comet 67P/Churyumov-Gerasimenko and water is the major constituent of cometary nuclei, limited evidence for exposed water-ice regions on the surface of the nucleus has been found so far,” write researchers in Nature. “The absence of large regions of exposed water ice seems a common finding on the surfaces of many of the comets observed so far.”
Comet 67P’s surface is “uniformly coated with dark, dehydrated, refractory and organic-rich material,” according to the researchers.
But between September and November 2014, VIRTIS scanned two regions—which appeared bright in visible light—in the comet’s Imhotep region. The researchers found that about 5% of each pixel sampling area contained pure water ice. At the sampling time, the average temperature was around -120 C.
The sampled ice was found on two debris falls, situated along the falls’ surface and at their bases.
“Although micrometer-sized pure water-ice grains are the usual result of vapor recondensation in ice-free layers, the occurrence of millimeter-sized grains of pure ice as observed on Imhotep debris falls is best explained by grain growth by vapor diffusion in ice-rich layers, or by sintering,” the researchers write. “As a consequence of these processes, the nucleus can develop an extended and complex coating in which the outer dehydrated crust is superimposed on layers enriched in water ice.”
The new find is helping scientists understand comet evolution.