Compositional diapirism as the origin of the low‐albedo terrain and vaporization at midlatitude on Ceres

Abstract

Recent observations of Ceres, made using the Heterodyne Instrument for the Far Infrared on board the European Space Agency's Herschel Space Observatory, have shown that water vapor is emanated from the low‐albedo midlatitude regions. Based on the supposition that some dark spots on Europa may be caused by diapirism, we explored whether Ceres' environment also can induce compositional diapirism, and whether an upwelling diapir could explain the origin and distribution of the dark spots and vaporization of Ceres. If the density of the rock that constitutes Ceres is around 2700 kg m−3, the undifferentiated crust is stable over geological time periods at less than 180 K. At a surface temperature of 170 K, a diapir can reach the surface if the crust is a few tens of kilometers thick. Alternatively, if the surface temperature is 150 K, a large contrast in viscosity would prevent diapirs from reaching the surface. The relatively moderate surface temperature in the midlatitude regions of Ceres may be the reason for the formation of dark terrains occurring only in such areas. Our finite difference calculations do not show that diapirism melts the surface ice because the temperature gradient changes very little. However, if the observed vapor emission is caused by the sublimation of fresh ice, compositional diapirism could be the mechanism that transports the fresh ice to the surface. In addition to other mechanisms such as cryovolcanism, diapirism is a valid hypothesis for explaining the dark terrain and vapor emissions of Ceres.

Ceres’ partial differentiation: undifferentiated crust mixing with a water-rich mantle

“…The critical viscosity for the development of Rayleigh-Taylor instabilities at the base of the crust is calculated as in Shoji & Kurita (2014) (see also Rubin et al 2014;Formisano et al 2016). We can confirm crustal viscosities calculated by Shoji & Kurita (2014), since they match approximately those that we calculated using the harmonic mean. Since these authors assume a higher H 2 O fraction, the rheology of the crust will be dominated by ice, and the calculation of the mixture viscosity according to the Reuss model is appropriate in their case.…”

“…The stability of Ceres' crust was studied by Shoji & Kurita (2014) and by Formisano et al (2016) for mixed ice-rock compositions by comparing the mixture viscosity with a critical viscosity for the development of Rayleigh-Taylor instabilities. Considering a composition that is dominated by phyllosilicates (see above) and calculating viscosities of each species involved, bulk viscosity can be calculated as their mean value (see, e.g., De Meer et al 2002;Grocott et al 2009;Dygert et al 2016).…”

“…Ceres' crust stability was studied by Shoji & Kurita (2014) and by Formisano et al (2016) for mixed ice-rock compositions by comparing the mixture viscosity with a critical viscosity for the development of Rayleigh-Taylor instabilities. Shoji & Kurita (2014) assumed a higher H 2 O fraction than in our study and a crust rheology dominated by ice.…”

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

“…Overturn was accomplished by compositional diapirism from a small subsurface ocean if Ceres formed 4–7 Myr after CAIs; if Ceres formed with more 26 Al, <4 Myr after CAIs, then overturn was accomplished by foundering of a thin (∼4 km) crust over a deep ocean. In contrast, Travis et al [] predict overturn by convection throughout Ceres's history, and Shoji and Kurita [] predict overturn by diapirism over Ceres's history but only near its equator. Crater counts to derive Ceres's surface age can test these predictions.…”

“…Here η ∼ 10 14 (1 − ϕ / ϕ m ) −2 ∼10 16 Pas is the viscosity of primordial, solids‐laden ice at the liquid‐ice interface, and Z 0 ∼1 km is the size of the perturbation initiating the RT instability. From equation , the timescale for a diapir of diameter 40 km and density ≈ 1500 kg m −3 to buoyantly rise 10 km by Stokes flow, assuming an ice viscosity Pas [ Shoji and Kurita , ], is about 250 years for ice at 273 K near the liquid‐ice interface, increasing to 50 Myr in ice at 170 K. While diapirs ascend to the surface, fingers of ice bearing fines and chondrules sink into the liquid layer where they melt, delivering more chondrules to the core and fines to the ocean (Figure ). RT instabilities keep generating diapirs containing liquid and fines until the entire crust has overturned, yielding a mantle of refrozen ice mixed with aqueously altered fines, all within the first ≈60 Myr of Ceres's evolution.…”