Possible Ceres bow shock surfaces based on fluid models

Abstract

The hot electron beams that Dawn detected at Ceres can be explained by fast‐Fermi acceleration at a temporary bow shock. A shock forms when the solar wind encounters a temporary atmosphere, similar to a cometary coma. We use a magnetohydrodynamic model to quantitatively reproduce the 3‐D shock surface at Ceres and deduce the atmosphere characteristics that are required to create such a shock. Our most simple model requires about 1.8 kg/s, or 6 × 1025/s water vapor production rate to form such a shock. Such an estimate relies on characteristics of the solar wind‐Ceres interaction. We present several case studies to show how these conditions affect our estimate. In addition, we contrast these cases with the smaller and narrower shock caused by a subsurface induction. Our multifluid model reveals the asymmetry introduced by the large gyroradius of the heavy pickup ions and further constrains the IMF direction during the events.

Exosphere-mediated migration of volatile species on airless bodies across the solar system

“…Thus, even the molecules moving fast enough to escape Vesta and Ceres will most likely photodissociate prior to escaping their gravitational pull. In this sense, Ceres and Vesta behave similarly to comets in the Solar System, which eject water molecules that ultimately photodissociate prior to escaping, interact with the local electromagnetic field, and stream antisunward, forming an ion tail; this is consistent with the detected plasma bow shock at Ceres (Jia et al, 2017). However, these molecules are moving very fast (minimum speeds of five and two atmospheric scale heights per hour for Ceres and Vesta respectively), and generally leave the atmosphere/exosphere prior to photodissociating.…”

The Putative Cerean Exosphere

“…Evidence of haze on Ceres (Nathues et al 2015;Thangjam et al 2016) is controversial . However, the detection of energetic electrons on three successive spacecraft orbits after a solar proton event can be interpreted as the by-product of a transient atmosphere (Russell et al 2016;Jia et al 2017;Villarreal et al 2017).…”

Water Group Exospheres and Surface Interactions on the Moon, Mercury, and Ceres

“…One of several possible explanations for such events would be a bow shock from the interaction of a solar energetic particle (SEP) event with an atmosphere. Jia et al (2017) modeled such a situation, finding that about 6 × 10 25 s −1 (2 kg/s) water vapor production would be required to form such a shock. found a correlation between detections of an exosphere and the inferred presence of solar energetic protons (SEP) at Ceres.…”