Geochemistry, thermal evolution, and cryovolcanism on Ceres with a muddy ice mantle

Abstract:

Ceres is a geophysical puzzle: observations with the Dawn spacecraft have revealed a seemingly old surface saturated with craters, and a shape close to that determined by [1] suggestive of a homogeneous, unevolved interior. These findings strongly contrast with pre-Dawn observations of products of aqueous alteration on Ceres' surface [2], and of water vapor emanating from Ceres [3], as well as with Dawn images of bright regions on the surface, all suggestive of past and ongoing geological activity.

We present a model of Ceres' interior that may reconcile these observations. Following [4], we assume that Ceres accreted ice and chondritic rock (both micron-sized rock fines and millimeter-sized chondrules), and that micron-sized fines stayed suspended in liquid. We have carried out geophysical and thermal evolution simulations using a code modified from [5,6], whose outcomes suggest that aqueously altered grains were emplaced on Ceres’ surface during the first tens of Myr of its evolution. We have also performed geochemical simulations using the PHREEQC code [7] of the interaction between pure liquid water and assemblages of chondritic elemental and mineral composition [8,9]. Their outcomes suggest that Ceres’ unusual surface mineralogy is consistent with aqueous alteration at T ≥ 200^oC. This requires an early ocean formed by heating from 26Al decay.

Thermal evolution simulations, including insulating fines, yield present-day temperatures at the core-mantle boundary of 240-250 K, just warm enough for chloride brines to persist and be freezing today [10]. Ongoing freezing may over-pressurize brine pockets, driving cryovolcanic outflow whose surface expression may have been observed by Dawn at Ceres’ ‘bright spots’. These outflows may be contributing to the water vapor being produced at Ceres.

[1] Drummond et al. (2014) Icarus 236, 28-37

[2] Milliken & Rivkin (2009) Nat. Geosc. 2, 258-261

[3] Küppers et al. (2014) Nature 505, 525-527

[4] Travis et al. (2015) 46th LPSC, abstract 2360

[5] Desch et al. (2009) Icarus 202, 694-714

[6] Neveu et al. (2015) JGR:Planets 120, 123-154

[7] Parkhurst & Appelo (2013) http://pubs.usgs.gov/tm/06/a43

[8] Wasson & Kallemeyn (1988) Proc. R. Soc. Lond. A 325, 535-544

[9] Howard et al. (2011) GCA 75, 2735-2751

[10] Barduhn & Manudhane (1979) Desalination 28, 233-241