Sublimation experiments to reproduce ammonium and chloride deposits under Cerean surface conditions

 IntroductionSublimation experiments performed in vacuum chambers have proven helpful for understanding the processes that occur on the surfaces of volatile-rich celestial bodies without significant atmospheres [1, 2, 3]. In a multitude of celestial bodies where water ice is not found today, there are indications that it was present in the past. On dwarf planet Ceres, located in the asteroid belt between Mars and Jupiter, indicators of what might have been liquid water are significant. In addition to hydrated minerals, salt deposits have been detected at the surface [4, 5], which is difficult to explain without the presence of water. These deposits can be interpreted as remnants of former saline ice reservoirs or ascending brines and may substantially alter the physical properties of the Cerean regolith [6, 7]. It is still in debate how salt-rich ice was transported to the surface of Ceres. Cryovolcanism or hydrothermal processes near the surface may be possible processes [8, 9]. Plausible heat sources for melting near-surface ice could be decaying radionuclides in the interior of Ceres or heat generation from impact events during Ceres' history. MethodsIn a vacuum sublimation chamber, environmental conditions similar to those on the Cerean surface are reproduced, and the conditions under which brines can form salt crusts on a regolith analogue are investigated. The focus is on the parameters of salt composition and the scenarios of how salty ice and regolith analogue interacted in a low-temperature and low-pressure environment. In particular, Na+, NH4+, Cl-, and CO32- based salts were used since they are observed in specific areas of the Cerean surface. We use two salt mixtures with different ratios of the selected ions and prepare three experimental setups:The Ceres regolith analogue is placed in a transparent sample container, saturated with brine, precooled to a solid sample at 190 K, and mounted in a sublimation chamber. The regolith analogue is saturated with liquid brine and placed in the sample container without being precooled. The dry regolith analogue is covered with a layer of salty ice particles