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.
The Mars space environment is unique among solar system objects. The planet has an atmosphere but no strong intrinsic magnetic field, like those at Earth or the gas giants, and so the supersonic electrically-charged gas from the Sun, called the solar wind, comes into direct contact with the upper layers of this atmosphere. A very interesting aspect of the interaction, however, is the existence of very strong crustal magnetic fields, which hold off the solar wind in localized regions around the planet. We have used several numerical models and satellite data sets to investigate the creation of so-called pickup ions, which are charged particles from the planet's atmosphere but ionized in the overlap region with the solar wind. These particles are instantly affected by the local electric and magnetic fields in the solar wind and are rapidly accelerated. Some bombard the upper atmosphere of Mars, and we have investigated the energy deposition to the neutral gas from this process. Others escape to deep space, and we have assessed the mechanisms reponsible for this loss and the dominant ways that particles escape from Mars. The attached figure shows Mars pickup ion trajectories from our test particle simulation code, along with magnetic field intensity from a magnetohydrodynamic model. It is seen that some of the particles hit the inner boundary while others escape to deep space. We learned that the impact back on the planet's upper atmosphere is usually quite small, except during times of very strong solar wind conditions, when the energy flux can increase to be larger than that from solar EUV photons. Regarding escape, we learned that there are two main channels of loss: the central tail region and a "polar plume" region in the direction of the solar wind motional electric field. The modeling studies show these to be about equal, even though the plume population is very hard to see in the existing data sets. The new MAVEN mission will hopefully make the appropriate measurements to quantify the relative contribution of this loss channel to the total planetary ion loss rate from Mars.
