Mars science and telecommunications orbiter

Circulation

There is a scarcity of observational data sufficient to characterize Martian atmospheric dynamics. The temperature field has been the only global measurement used to characterize the circulation (Leovy, 2001). In fact, only Mars Global Surveyor has really been able to provide more than sparse profile measurements (Conrath, et al., 2000; Smith, et al., 2001) although the main instrument used for this purpose (the Thermal Emission Spectrometer, TES) was not specifically designed for that goal.

Within that context, our current vision of the circulation is largely based on numerical simulations performed using General Circulation Models (see Forget, et al., 1999; Haberle, et al., 1993; Hourdin, et al., 1993; Wilson and Hamilton, 1996). These studies have shown that the global circulation is characterized by an extended Hadley circulation modulated and modified by several kinds of waves propagating through the atmosphere, as illustrated in Figure 3.

Of particular importance are the thermal tides of diurnal or semi-diurnal period which are excited by the near-surface diurnal cycle and propagate through the atmosphere with increasing amplitude as the atmospheric density decreases with altitude (Wilson and Hamilton, 1996; Zurek, et al., 1992). The tidal waves are thought to be the primary phenomenon controlling the dynamics of the atmosphere above 50 km, although very few observations are available. With regard to comparative meteorology, the traveling planetary waves observed at high- and mid-latitudes in winter are of primary interest since they are counterparts of the mid-latitude low and high pressure weather systems that control the weather in Europe and North America. Compared to Earth, one key characteristic of the Mars atmosphere is the vertical extent of most meteorological phenomena. On Earth, the stratosphere confines the Hadley cell and most planetary waves to the troposphere, below 20 km. On Mars, many structures extend vertically up to the thermosphere (120 km), as suggested by MGS aerobraking density measurements (Keating, et al., 1998).

Of great importance to the specific goals of MSTO is the simultaneous measurement of the winds and trace gases. Such measurements will allow us to infer the time and spatial variation of various surface sources and sinks. Methane is the obvious trace gas of interest, but CO would also be interesting to measure and there may be others as well.

recommended Measurements

Composition

Measurements of the detailed molecular composition of the atmosphere should be made over the altitude range from the surface to 100 km with sufficient sensitivity to provide detectivity levels of parts per billion or less, and at 1/2 scale height (4 - 5 km) vertical resolution. These measurements should also achieve high enough spatial resolution to reveal horizontal variability at a scale of 10 km to aid in identifying localized sources of variable species. The measurements should be of adequate sensitivity to allow determination of the relative isotopic abundances ¹²C/¹³C, ¹⁶O/¹⁷O/¹⁸O, H/D and ³²S/³⁴S in relevant constituent species with sufficient accuracy to permit unambiguous discrimination between biogenic and abiogenic origins.

High sensitivity (in terms of measurable relative concentrations) in remote sensing detection and quantification of the trace molecular constituents of the atmosphere from orbit requires recording high resolution, near-infrared absorption spectra in the solar occultation mode. For such observations to be of sufficient sensitivity to allow "unambiguous interpretation", (i.e., identification of species and quantitative analysis at detection levels well below current upper limits for the molecular species of interest) the spectral resolution must be at least one, and preferably two, orders of magnitude higher than has been employed at Mars to date.

This can be readily illustrated with reference to typical oscillator strengths for molecules of interest at these wavelengths: For "expected" species of interest (CH₄, H₂CO, H₂S, etc.) the strongest transitions in the rotation-vibration region are ~ 10^-19 cm^-1cm^-2molec^-1. In an atmosphere with a base pressure of 6mb and scale height 12 km, the column cross section is ~2x10²³ molecules cm^-2. Consider an instrument with a spectral resolution of 1cm^-1 (FWHM) and a noise level equivalent to 1% of the background radiance level. An absorption feature equal to the noise level (i.e., a signal to noise of 1) would have an equivalent width (integrated absorption) of 10^-2 cm^-1. In this case, molecular spectral transitions of the strongest lines would correspond (for nadir viewing) to uniform mixing at a relative concentration of 2x10^-7. Clearly, trace species present at mixing ratios of parts per billion would not be detectable, and either the spectral resolution would have to be improved by several orders or the effective path length increased, or both. For the wide variety of candidate species under consideration, the required sensitivity is probably best achieved by exploiting the source intensity and amplified path length advantages of the solar occultation technique. The observations should be made in conjunction with nadir-looking measurements by, for example, a tunable sub-mm radiometer at selected frequencies and in specific locations as determined by the results of the broad coverage, molecular trace species inventory from the infrared instrument. This combination would allow localized sources of spatially variable species to be identified and provide insight into their origin.

Circulation

Except for the surface measurements performed by the Viking landers and Pathfinder, winds on Mars have never been directly measured from spacecraft observations. Typically, winds have been derived from the temperature field using the thermal gradient wind approximation or similar, more sophisticated, techniques that assume zero velocity at the surface. However, these estimates may be far from accurate on Mars because of the near-surface winds driven by the strong diurnal cycle, and because of the large amplitude of the waves above 40 km and the difficulty of accounting for the complex interactions among the different wind components. For instance, a very limited number of Doppler shift measurements of the CO lines have been obtained using Earth-based radio-telescopes and interferometers (Lellouch, et al., 1993; Lellouch, et al., 1991; Moreno, et al., 1999; Moreno, et al., 2001). These measurements suggest that retrograde winds around 60 km dominate at almost all latitudes, even around equinox (Jegou, et al., 2000). This disagrees strongly with thermal wind estimates based on MGS TES data as well as with theoretical GCM predictions. The enigma raised by these remote measurements suggests that new data, obtained from direct measurements, is essential to understand processes controlling the dynamics of the Martian atmosphere.

The wind measurements should maximize the latitudinal coverage (0˚to ≥ |70˚|), in order to observe winter jets and baroclinic waves. The horizontal resolution needs to be smaller than the scale of the free atmosphere structures (i.e., ≤ 5˚ latitude). The vertical coverage should be from the surface to 120 km altitude, with resolution better than 10 km. The accuracy of the wind speed measurements should be better than 20 m/s