Sublimation in bright spots on (1) Ceres

Abstract

The dwarf planet (1) Ceres, the largest object in the main asteroid belt with a mean diameter of about 950 kilometres, is located at a mean distance from the Sun of about 2.8 astronomical units (one astronomical unit is the Earth-Sun distance). Thermal evolution models suggest that it is a differentiated body with potential geological activity. Unlike on the icy satellites of Jupiter and Saturn, where tidal forces are responsible for spewing briny water into space, no tidal forces are acting on Ceres. In the absence of such forces, most objects in the main asteroid belt are expected to be geologically inert. The recent discovery of water vapour absorption near Ceres and previous detection of bound water and OH near and on Ceres (refs 5-7) have raised interest in the possible presence of surface ice. Here we report the presence of localized bright areas on Ceres from an orbiting imager. These unusual areas are consistent with hydrated magnesium sulfates mixed with dark background material, although other compositions are possible. Of particular interest is a bright pit on the floor of crater Occator that exhibits probable sublimation of water ice, producing haze clouds inside the crater that appear and disappear with a diurnal rhythm. Slow-moving condensed-ice or dust particles may explain this haze. We conclude that Ceres must have accreted material from beyond the 'snow line', which is the distance from the Sun at which water molecules condense.

Occator crater in color at highest spatial resolution

“…Occator is a complex crater, it exhibits a central pit (Ø ~9 km) surrounded by remnants of a central peak, and a flat floor bounded by a multitude of terraces (Nathues et al 2015a(Nathues et al , 2017a Figs 2 -9). The central pit is up to 1 km deep and hosts the brightest surface feature on Ceres named Cerealia Facula (Fig.…”

“…The surface of Ceres is peppered with hundreds of bright, carbonate-bearing areally small spots, which are less bright than those present at Occator (Nathues et al 2015a: extended data Fig. 1, Nathues et al 2016, Nathues et al 2017b.…”

“…These bright features have been commonly referred to as 'bright spots' and have been named Cerealia Facula (central bright spot) and Vinalia Faculae (secondary bright spots) (Figs 2 and 3). The bright spots populate a portion of the floor of Occator that shows evidence of a diurnal brightness variation, which was attributed to local haze (Nathues et al 2015a, Thangjam et al 2016). Spectral analysis of Dawn's Visible and Infrared Spectrometer (VIR) data revealed that these bright spots are dominated by sodium carbonate mixed with some dark ammoniated minerals (De Sanctis et al 2016).…”

“…In addition, color data with a resolution of ~35 m/pixel have been obtained for a number of selected sites (Nathues et al 2017a, b). One of the prime targets was the crater Occator, hosting the brightest surface features on Ceres (Nathues et al 2015a, Nathues et al 2017a. These bright features have been commonly referred to as 'bright spots' and have been named Cerealia Facula (central bright spot) and Vinalia Faculae (secondary bright spots) (Figs 2 and 3).…”

The Formation and Evolution of Bright Spots on Ceres

“…The average geometric albedo measured by Dawn's Visible and Infrared spectrometer (VIR) at 0.55 μm is 0.094 ± 0.008, and the average bond albedo measured with the Framing Camera (FC) clear filter is 0.034 ± 0.001 Ammannito et al, 2016;Li et al, 2016 ). Ceres' dark surface materials, whose reflectance ranges between located in the central pit and floor of Occator crater and were visible upon Dawn's approach to Ceres ( Nathues et al, 2015( Nathues et al, , 2017a. Cerealia Facula has an average Bond albedo of 0.24 ± 0.01 , and its spectra are consistent with a composition that includes significantly more abundant carbonate of different composition than the average surface (Na-rich instead of Ca, Mg-rich) as well as ammoniated salts such as NH 4 Cl or NH 4 HCO 3 .…”