Search for water outgassing of (1) Ceres near perihelion^★

¹ Institut UTINAM UMR 6213, CNRS, Université Bourgogne Franche-Comté, OSU THETA, BP 1615, 25010 Besançon Cedex, France
e-mail: rousselot@obs-besancon.fr
² European Southern Observatory, Alonso de Còrdova 3107, Vitacura Casilla 19001, Santiago, Chile
³ Space sciences, Technologies & Astrophysics Research (STAR) Institute, University of Liège, Liège, Belgium
⁴ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
⁵ Planetary Science Institute, 1700 E Fort Lowell Road, Tucson, AZ 85719, USA
⁶ Earth, Planetary and Space Sciences, University of California, Los Angeles, 405 Hilgard Avenue, Los Angeles, CA 90095-1567, USA
⁷ Aix-Marseille Université, CNRS, Laboratoire d’Astrophysique de Marseille, Marseille, France
⁸ Department of Earth, Atmospheric and Planetary Sciences, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
⁹ School of Electrical Engineering, KTH Royal Institute of Technology, Stockholm, Sweden
¹⁰ Thirty-Meter-Telescope, 100 West Walnut St, Suite 300, Pasadena, CA 91124, USA
¹¹ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650011, PR China

Received: 19 April 2019
Accepted: 13 June 2019

Abstract

Context. (1) Ceres is the largest body in the main asteroid belt and one of the most intriguing objects in the solar system, in part because of the discovery of water outgassing by the Herschel Space Observatory (HSO) and its still-debated origin. Ceres was the target of NASA’s Dawn spacecraft for 3.5 yr, which achieved a detailed characterization of the dwarf planet. The possible influence of the local flux of solar energetic particles (SEP) on the production of a Cerean exosphere and water vapor has been suggested, in addition to the sublimation of water ice that depends on the temperature, meaning the heliocentric distance.

Aims. We used the opportunity of both the perihelion passage of (1) Ceres in April 2018, and the presence of Dawn in its vicinity (for measuring the SEP flux in real time) to check the influence of heliocentric distance and SEP flux on water outgassing.

Methods. We searched for OH emission lines near the limb of Ceres in the near-UV with the UVES spectrograph mounted on the 8-m ESO Very Large Telescope. Two spectra were recorded when Ceres was close to its perihelion, in February 2018, and with Dawn spacecraft orbiting Ceres. It was possible to simultaneously measure energetic particles around Ceres at the time of our observations.

Results. Our observations did not permit detection of OH emission lines to a very high sensitivity level. This level is estimated to correspond to a global water production rate of Q_H2O ∽ 2 × 10²⁶ molecules s⁻¹, similar to the water production rate derived from HSO observations. The solar energetic particles flux measured around Ceres was negligible at the time of these observations.

Conclusions. Our observations support the idea that heliocentric distance (i.e., the sublimation of water ice) does not play a major role in the water emission from Ceres. This production rate could be either related to SEP events or to other mechanisms, possibly of endogenic origin.