Journal
JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS
Volume 123, Issue 5, Pages 1192-1202Publisher
AMER GEOPHYSICAL UNION
DOI: 10.1029/2018JE005560
Keywords
Mars atmosphere; neutral escape; pickup ions; MAVEN; particle detectors
Categories
Funding
- NASA
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The Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft arrived at Mars with the goal of determining the rates and mechanisms of atmospheric escape. Thermal hydrogen and hot oxygen escape are the two most important escape processes currently at work. Direct measurement of the escaping neutral hydrogen and oxygen atoms is impossible with current technology due to the low density and energy of escaping neutrals. However, when ionized and picked up by the solar wind, these escaping atoms can be detected by three particle detectors onboard MAVEN. By back-tracing the trajectories of measured pickup ions, constraints can be placed on the density of neutrals at altitudes not accessible by other measurement methods. In this work, pickup H+ and O+ data from the Solar Energetic Particle (SEP), Solar Wind Ion Analyzer (SWIA), and SupraThermal and Thermal Ion Composition (STATIC) instruments are used to assess the variability of neutral H and O exospheres at Mars. From an analysis of 2.5 Earth years of MAVEN data, we show that a strong H escape seasonal dependence is observed by SWIA and STATIC with inferred H escape rates as low as 3x10(25)s(-1) near aphelion and as high as 4x10(26)s(-1) near perihelion. Hot O escape rates derived from SEP, SWIA, and STATIC data imply a much less variable hot O exosphere with escape rates fluctuating by a factor of 2 around a mean value of 9x10(25)s(-1). Both escape rates are in general agreement with the most recent theoretical, modeled, and observationally inferred rates.
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