Zusammenfassung
We modeled several thermal evolution scenarios for Ceres to explore
the nature of large, wet protoplanets and to predict current-day
evidence that might be found by close inspection, such as by the
Dawn mission. The density for Ceres is near 2.1, suggesting a water
content between 17% and 27% by mass. Short-and long-lived radioactive
nuclide heating is considered. Even if only long-lived radionuclide
heating is assumed, the water ice in Ceres melts quickly and a water
mantle forms, but an approximately 10-km crust does not melt. The
circulating warm water would alter the silicates. As heat is lost
by conduction through the frozen crust, water begins to freeze out
at the base of the crust. Solid-state convection begins and transports
more heat as well as perhaps material dissolved or entrained in the
water to or near the surface. Ceres' water layer eventually (but
perhaps not entirely) freezes, forming a layered density structure
with perhaps some liquid water remaining today. Our differentiated
models are in agreement with the recently measured difference between
the equatorial and polar radii. We find that Ceres' existence and
evolution depend critically on it containing water at formation,
and this depends strongly on the combination of when it accreted
and the amount of Al-26 present in the pre-Ceres ∼ 1-km-sized
objects; slightly more Al-26 or earlier accretion produces a dry
Vesta-like object. Melting and freezing plus mineralization would
lead to several dimensional changes over time, creating topographic
features, zones of weakness, and perhaps disruptions in the crust.
Nutzer