Capability Development for Mars Missions, 15-9147
Principal
Investigator
Richard Link
Inclusive Dates: 07/01/99 - 09/28/00
Background - A model for the interaction of low-energy 0.5 - 4,000 electron volt (eV) electrons with the upper atmospheres and exospheres of solar system planets and satellites has been developed. The model describes photoelectron production within an atmosphere arising from ionization by solar extreme ultraviolet (EUV) radiation, and atomic inner core ionization and Auger electron ejection due to absorption of solar X-rays. The model was developed to investigate the penetration of shocked solar wind electrons into the atmosphere of Mars, and photoelectron production and escape from within the atmosphere. The work was performed in anticipation of funding opportunities arising from future Mars missions.
Approach - The technical approach adapted the Link [1992] solution of the Boltzmann electron transport equation to study photoelectron and solar wind electron interactions with the atmosphere and ionosphere of Mars. The work involved generalizing the existing photoionization and electron transport calculations to allow for arbitrary gas mixtures and planetary physical and orbital properties, developing a new Auger calculation to allow for heteronuclear molecules with multiple K-edges, and generating photoionization and electron impact cross-section databases for Mars. The model is the most physically comprehensive and accurate model in the nonrelativistic regime and allows SwRI, for the first time, to resolve the pitch angle distribution of electrons above 100 eV in the atmosphere of Mars, as is now being measured by the Mars Global Surveyor mission.
Accomplishments - The principal investigator has performed calculations of 1) the variation of Mars photoelectron energy spectra with solar zenith angle (0 - 90°), 2) the penetration of solar wind electrons into the Mars ionosphere for solar wind characteristic energies 5 -100 eV, and 3) photoelectron fluxes, energy fluxes, and average electron energies for Venus, Earth, Mars, Titan, Triton, and Pluto. The first scientific results from this project (see http://www.aspera-3.org/model.pdf) show that neglecting photoelectron transport (the local equilibrium approximation) is not valid above 200 kilometers on Mars, much lower than the currently assumed 300- to 350-kilometer region. This finding has serious implications for remote sensing of Mars upper atmospheric composition via far ultraviolet (FUV) emission spectroscopy, for which photoelectron impact is a major excitation source of FUV airglow. A second important scientific issue clarified by the model concerns the absence of a detectable atomic carbon peak in the Auger spectrum of Mars photoelectrons measured by the Mars Global Surveyor electron reflectometer, although the atomic oxygen peak is readily apparent. The model shows that the flux of Auger electrons resulting from inner core ionization of CO2 leading to the oxygen branch is nearly an order of magnitude larger than for the carbon branch, thus explaining these unexpected observations.
