A New Model for the Origin of Chlorine on Enceladus

Abstract:

One of the great discoveries at Enceladus was finding salt in plume ice grains. Postberg et al. (2009) inferred that these grains have a sodium chloride concentration of 0.05-0.2 mol/kg H₂O. A key cosmochemical question is why this concentration for Enceladus’ ocean rather than a higher concentration like that in Earth’s ocean? Previously, Glein & Shock (2010) assumed that Enceladus started with a chlorine inventory similar to CI chondrites. However, the abundances of halogens in chondrites have been revised significantly downward (Clay et al., 2017), and we have learned from comet 67P that there are also volatile carriers of halogens in the outer solar system. We also know much more about the internal structure of Enceladus (e.g., Hemingway & Mittal, 2019) than we did a decade ago. These results motivate a revisiting of Enceladus’ chlorine cosmochemistry. I have attempted to quantify how much Cl is in the rocky core, the ocean, and the ice shell, as well as how much Cl was lost to space due to plume outgassing. The analysis suggests that the chlorine inventory of Enceladus is of order 10¹⁷ kg and is dominated by the salty liquid water ocean. I have also made bottom-up models that could explain how Enceladus acquired its Cl inventory. These models assume that Enceladus formed from mixtures of ice and rock, each with distinct chlorine abundances. Interestingly, I have found that the observationally derived Cl inventory can be reproduced by assuming that Enceladus formed as an ice-rock mixture containing a solar bulk abundance of chlorine. Therefore, a simple scenario of complete condensation of Cl seems to be emerging, which makes sense theoretically because nebular condensation temperatures for Cl (>160 K) are not significantly lower than that of water ice (~180 K). If Enceladus did indeed accrete a solar abundance of chlorine, then its ocean may represent a fairly concentrated reservoir of isotopically solar chlorine. The solar ratio of ³⁷Cl/³⁵Cl is currently not well constrained, but a future mission to Enceladus may provide a means to measure this cosmochemically important ratio.