Relation between Cold Traps and Putative Ice Deposits

One of the remarkable discoveries by the Dawn mission at Ceres are perennially shadowed regions (PSRs) and bright (ice) deposits within them. The PSRs can be delineated by computational ray-tracing based on available topographic models, and as the axis tilt of Ceres varies between 2 and 20 degrees, their extent changes periodically. The bright deposits are visible in Framing Camera images due to indirect sunlight. Existing analysis suggests these bright crater floor deposits are related to the extent of the PSRs at maximum axis tilt. However, the relation between bright deposits and the past extent of PSRs is not perfect, which may be due to errors in the current topographic model or, alternatively, of fundamental nature. We propose to improve the topographic model and repeat the shadow modeling with the improved topography to ascertain the relation between PSRs and bright deposits. In addition, we will use thermal modeling to distinguish PSRs from cold traps, as short exposures of grazing sunlight at Ceres’ distance from the Sun may not necessarily destabilize volatiles. This work will resolve the question of whether the bright deposits are indeed consistent with cold-trapped surface ice.

Methods: We will use stereophotoclinometry and stereophotogrammetry to construct improved shape models of key craters hosting PSRs that have been identified in previous studies. An existing ray-tracing model will be used to determine the extent of PSRs at current and past axis tilts. An advanced thermal model will provide maps of maximum surface temperatures within craters.

Significance: The proposed research will enhance the scientific return of data collected by the Dawn spacecraft at Ceres. In addition, the cold trapping phenomenon is relevant to Mercury and to Earth’s Moon.