Water ice, but what kind? Evidence for clathrates in 67P

By Adrienn Luspay-Kuti, research scientist, SwRI

Figure 1For decades, scientists have agreed that comets are mostly water ice, but what kind of ice — amorphous or crystalline — is still up for debate. The structure and phase of the ice is important because it tells us about how and where in the solar nebula the comet may have formed. While the Rosetta mission is nearing its end as comet 67P and its faithful spacecraft companion are headed back to the frigid outer solar system, data obtained early in the mission still holds key information about the composition and structure of the nucleus.  The analysis of previously published ROSINA data revisited by an international group of cometary scientists shows evidence that 67P’s nucleus contains crystalline water ice structures called clathrates.

The diurnal outgassing pattern observed over the southern hemisphere of 67P during the September-October 2014 time period was found to be consistent with gas release from clathrates, while gas release from amorphous ice cannot explain the observations, even when combined with recondensation and sublimation. The different time variability of methane outgassing relative to other major and minor volatile species provided the key clue to the finding that volatiles are likely stored and released as clathrates in the outgassing layers of 67P’s nucleus.Figure 2

These results based on in-situ mass spectrometer data provide an important contribution to the long-standing debate as to whether comets contain amorphous water ice with other cometary volatiles trapped inside the amorphous pores, or crystalline ices and clathrates instead. If the presence of clathrates in the nucleus means that the building blocks of 67P were themselves made out of crystalline ices and clathrates instead of amorphous ice, then 67P likely formed closer to the Sun than previously considered for Jupiter Family Comets.

“The presence of clathrates in 67P/Churyumov-Gerasimenko” by A. Luspay-Kuti et al. is published in the April 8 issue of the journal Science Advances.