Mars science and telecommunications orbiter

Crustal Magnetic Fields

Present day escape processes are certainly affected by the Martian crustal fields, in the sense that some of the upper atmosphere is shielded from direct interaction with the solar wind and the associated direct planetary ion scavenging. However, recent observations by Mars Express suggest that there are some additional ion escape processes that exist because of the presence of the crustal fields. It has yet to be determined whether the associated additional losses are significant compared to the general upper atmosphere/ionosphere removal related to unmagnetized planet solar wind interaction physics.

Specifically, Mars Express UV observations showed evidence of aurora occurring in the vicinity of the strong crustal field regions, which was henceforth backed up by observations on both Mars Express and MGS of accelerated electrons moving down the field lines into the crustal field region-dominated upper atmosphere. These electrons probably excite the observed UV emissions. Associated with the inflowing electrons, Lundin et al. report (in Science, to appear shortly) outflowing "auroral ion beams", and make an analogy to outflowing (oxygen) ion beams in Earth's auroral zones that occur in response to electron precipitation. Whether the ion fluxes in these beams at Mars constitute a major escape route for Mars upper atmosphere in the form of heavy ions is not yet determined, but it certainly indicates that understanding the magnetic context of the observed escaping ions is important for diagnosing the operating physical mechanisms.

Would deep observations of the Mars magnetic field make a major difference to the above diagnosis? Certainly local (to the ion escape sites) measurements of the magnetic field context are crucial to understanding the nature of observed ion escape. It is important to determine whether the crustal fields are involved and how interplanetary conditions make a difference with or without the crustal field involvement. Deeper atmosphere observations are only critical in this respect if it is determined that auroral outflow mechanisms are significant for overall planetary ion escape. Such measurements could verify assumptions (from existing crustal field models) of the field strength where those ion outflows or other crustal field-associated ion outflows may be accelerated - below the region of existing spacecraft measurements of the fields. Their importance to present ion escape is thus dependent on how important the crustal field-related outflows turn out to be. To some extent this will be further pinned down by Mars Express ASPERA observations and their analysis. Within the time frame of the next six months the general answer may be known. Where the deep magnetic field measurements are most definitely important is in further constraining the early history of the Martian magnetic field - especially during the first 1.5 billion years of Mars' history when solar conditions should have greatly enhanced all present-day ion scavenging-related processes.

Upper Atmosphere Winds

A knowledge of the thermospheric winds (over the solar cycle and seasons, and spanning the globe) is of fundamental importance to our understanding of the Mars thermosphere-ionosphere system, and its coupling with the Mars lower atmosphere. The Mars global thermospheric circulation is thought to significantly modify the thermal and density structure about the planet (e.g. Bougher, et al., 1999, 2000; Bougher, et al., 1990). Strong horizontal (~100-300 m/sec) and vertical (~1-10 m/sec) winds are estimated, especially during the solstices, providing adiabatic heating/cooling and horizontal advection. Observed winter polar warming is attributed to this inter-hemispheric circulation (Bougher, et al., 2006; Keating, et al., 2003), and its seasonal variations owing to changing solar and aerosol heating. The distributions of light species (e.g. O, N, H, H₂, He) are surely controlled by the thermospheric circulation, resulting in winter polar night bulges of these species and corresponding nightglow emissions (e.g. Bertaux, et al., 2005). Finally, dynamical coupling with the Mars lower atmosphere, especially during dust storm events, helps regulate the thermospheric structure and dynamics. These winds also serve as a background through which upward propagating thermal tides must propagate, on their way to driving variations in the Mars lower thermosphere (~100-130 km) which affect aerobraking operations (e.g. Forbes, et al., 2002; Wilson, 2002; Withers, et al., 2003). In short, a comprehensive characterization of the Mars upper atmosphere must include neutral wind measurements in order to understand the processes that regulate density, temperature and airglow distributions, and their variations over the solar cycle and Martian seasons.

Knowledge of Mars thermospheric winds is also key to understanding some of the processes that control Mars volatile escape. Escape processes are mediated by the Mars upper atmosphere structure, which in turn is regulated strongly by the global thermospheric circulation (see above). For instance, the “thermostatic” effect of the global winds on temperatures is calculated to regulate solar cycle variations of the Mars thermospheric densities and temperatures (e.g. Bougher, et al., 1999, 2000); a reduced variation of dayside topside temperatures (from that otherwise expected) results as the solar cycle advances. Photochemical processes (e.g. O₂⁺ production and recombination) will vary as the background neutral structure changes. The time variable upper atmosphere structure (near the exobase) also serves as the seed population for O⁺ sputtering, driving both atomic and molecular escape fluxes. Furthermore, the horizontal and vertical winds also have a direct influence on the actual neutral escape fluxes. Hence, an understanding of these “present day” dynamical processes and their impact on volatile escape rates must be quantified before extrapolations to ancient Mars conditions can be made (Bougher, et al., 2004a). Mars upper atmosphere wind measurements are thus crucial to evaluating these dynamical feedbacks upon present day volatile escape rates.