After traveling through the solar system for 10 years, ESA’s Rosetta finally made its rendezvous with the comet 67P/Churyumov-Gerasimenko on August 6. Less than a month later, NASA's ultraviolet imaging spectrograph has returned with the first science results of such an up-close mission to a comet. The instrument, named Alice, revealed information about the comet’s color, surface ice content, and preliminary data about compounds within the comet’s atmosphere—technically known as its coma.
Alice revealed that 67P/C-G’s coma included oxygen and hydrogen, which isn’t entirely shocking as comas are generally regarded to be composed of water vapor and dust. What was surprising was that the surface of the comet’s nucleus did not appear to have large amounts of ice. As comets approach the sun's heat, the ice is turned into water vapor. But 67P/C-G is still over 500 million kilometers away from the sun, which should be far enough away for the ice to remain intact. This could be a clue that the comet had been in close proximity with our sun or a different star at some point during its history.
"We’re a bit surprised at just how unreflective the comet’s surface is and how little evidence of exposed water-ice it shows," Alice's principle investigator Alan Stern told NASA. Alice's data also showed 67P/C-G is blacker than coal when viewed at UV wavelengths, which is described as fairly unusual.
“As the mission progresses, we will continue to search for surface ice patches and ultraviolet color and composition variations across the surface of the comet,” stated co-investigator Lori Feaga.
During its mission on 67P/C-G, Alice will continue to gather data regarding the composition of the nucleus and coma. Niche conditions required for certain elements will help astronomers pinpoint where the comet formed. For instance, neon will only be present within the nucleus if the comet was formed in an area that is 16K degrees (-257 C or -430 F). The presence or absence of such elements will also yield information about the thermal environmental conditions the comet has experienced throughout its life.