This collaborative project will apply coupled state-of-the-art numerical models to consider the global response of the Martian thermosphere to energetic pickup ions. In particular, the research will study the bombardment effects of oxygen ions on composition and energetics. The neutral constituents of the atmospheric corona can be ionized, picked up by the solar wind, and ultimately returned to interact with the atmosphere, causing neutral particles to escape from Mars. This sputtering loss and associated heating effects have not yet been included in any global models to consider the coupling among the Mars system components. This project will treat the Mars environment as a single system, including pickup ions within the mass and energy budget of the Martian thermosphere. The work will apply a magneto-hydrodynamics field-based pickup ion transport model, along with a different model for the interaction between incident energetic particles and the thermosphere below the exobase, and build the sputtering and heating effects into the state-of-the-art Mars Thermosphere General Circulation Model. The escape rate estimate for direct pickup ion loss and sputtering loss will be used to advance understanding of the non-thermal processes governing atmospheric erosion, and the quantification of long-term atmospheric evolution.
This work will permit extrapolation of the history of the Martian atmosphere and climate, the possible presence of liquid water, and planetary habitability. Comparison with spacecraft measurements will constrain understanding of the Mars-solar wind interaction. The results will be important for future Mars probe spacecraft, and the research involves graduate students. For the broader community, Mars sciences capture the excitement of scientific exploration and adventure. Because this project helps to understand oxygen loss and thus water loss, it provides an ideal opportunity to advance scientific literacy through the public interest in water, and perhaps life, on Mars.
PI: Xiaohua Fang, University of Colorado Boulder Unlike Earth, Mars is not protected by a global intrinsic magnetic field. The Martian atmosphere is thus directly exposed to the solar wind, a high-speed stream of charged particles that are constantly blowing off the Sun. The main and ongoing challenge in the Mars atmospheric study is to figure out how the planet evolved from initial warm and wet conditions billions of years ago to the cold and dry present state. Within this broad context, the goal of our NSF-sponsored research is to understand the acceleration and transport of planetary particles under continuous solar wind forcing and to evaluate how this process affects the characterization of neutral profiles. The neutral atmospheric constituents can be ionized, and then be accelerated (or picked up) by the magnetic and electric fields in the solar wind. While a large amount of these energized particles are carried away by the solar wind and lost to space, an important fraction is directed by the electromagnetic forces to reenter and penetrate into the atmosphere. This bombarding process and its consequences have been overlooked for decades, as it was commonly presumed that the total energy input of these returning energetic particles was negligible when compared with the solar EUV radiation energy. Our comprehensive numerical studies under the support of this NSF grant show that the nature is much more complicated than we thought. It is true that the pickup ion bombardment is generally not sufficient to significantly affect the neutral atmosphere. However, this is not the case anymore when the Sun is at its most active and space weather gets stormy. By applying a set of state-of-the-art global models, we find that the thermospheric effects of reentering ions can change from negligible to very important when upstream solar wind conditions vary from normal to extreme. The atmospheric response under the most extreme conditions includes dramatic neutral temperature enhancement, significant neutral composition and wind changes, and increased importance of sputtering loss and possibly even thermal escape of heavy species. Besides neutral heating as well as the corresponding changes through the impact on dynamics, diffusion and mixing, and atmospheric chemistry, precipitating energetic pickup ions are capable of sputtering neutral species out of the Martian atmosphere and contributing to the hot components of the corona. All together, our findings provide an invaluable diagnostic of the Mars-solar wind interaction processes that can be tested by the new NASA Mars Atmospheric and Volatile EvolutioN (MAVEN) mission, which was launched in November 2013 and is currently orbiting Mars and starting to collect science data.
