Stable, hydrogen-burning, M dwarf stars make up about 75% of all stars in the Galaxy. They are extremely long-lived, and because they are much smaller in mass than the Sun (between 0.5 and 0.08 M(Sun)), their temperature and stellar luminosity are low and peaked in the red. We have re-examined what is known at present about the potential for a terrestrial planet forming within, or migrating into, the classic liquid-surface-water habitable zone close to an M dwarf star. Observations of protoplanetary disks suggest that planet-building materials are common around M dwarfs, but N-body simulations differ in their estimations of the likelihood of potentially habitable, wet planets that reside within their habitable zones, which are only about one-fifth to 1/50th of the width of that for a G star. Particularly in light of the claimed detection of the planets with masses as small as 5.5 and 7.5 M(Earth) orbiting M stars, there seems no reason to exclude the possibility of terrestrial planets. Tidally locked synchronous rotation within the narrow habitable zone does not necessarily lead to atmospheric collapse, and active stellar flaring may not be as much of an evolutionarily disadvantageous factor as has previously been supposed. We conclude that M dwarf stars may indeed be viable hosts for planets on which the origin and evolution of life can occur. A number of planetary processes such as cessation of geothermal activity or thermal and nonthermal atmospheric loss processes may limit the duration of planetary habitability to periods far shorter than the extreme lifetime of the M dwarf star. Nevertheless, it makes sense to include M dwarf stars in programs that seek to find habitable worlds and evidence of life. This paper presents the summary conclusions of an interdisciplinary workshop (http://mstars.seti.org) sponsored by the NASA Astrobiology Institute and convened at the SETI Institute.

The Mars Global Surveyor (MGS) z -axis accelerometer has obtained over 200 vertical structures of thermospheric density, temperature, and pressure, ranging from 110 to 170 kilometers, compared to only three previous such vertical structures. In November 1997, a regional dust storm in the Southern Hemisphere triggered an unexpectedly large thermospheric response at mid-northern latitudes, increasing the altitude of thermospheric pressure surfaces there by as much as 8 kilometers and indicating a strong global thermospheric response to a regional dust storm. Throughout the MGS mission, thermospheric density bulges have been detected on opposite sides of the planet near 90°E and 90°W, in the vicinity of maximum terrain heights. This wave 2 pattern may be caused by topographically-forced planetary waves propagating up from the lower atmosphere.

Abstract. The NASA Ames Mars General Circulation Model is used to interpret selected results from the Mars Pathfinder atmospheric structure instrument/meteorology (ASI/MET) experiment. The present version of the model has an improved soil thermal model, a new boundary layer scheme, and a correction for non-local thermodynamic equilibrium effects at solar wavelengths. We find good agreement with the ASI/MET entry data if the dust observed at the Pathfinder site is assumed to be distributed throughout the lowest five to six scale heights. This implies that the dust is globally distributed as well. In the lower atmosphere the inversion between 10 and 16 km in Pathfinder's entry profile is likely due to thermal emission from a water ice cloud in that region. In the upper atmosphere (above 50 km), dynamical processes, tides in particular, appear to have a cooling effect and may play an important role in driving temperatures toward the CO2 condensation temperature near 80 km. Near-surface air temperatures and wind directions are well simulated by the model by assuming a low surface albedo (0.16) and moderately high soil thermal inertia (336 SI). However, modeled tidal surface pressure amplitudes are about a factor of 2 smaller than observed. This may indicate that the model is not properly simulating interference effects between eastward and westward modes.

The vertical propagation of nonmigrating (i.e., longitude‐dependent or non–Sun‐synchronous) solar diurnal and semidiurnal tides into the thermosphere of Mars is investigated through numerical simulation. The waves are generated in the NASA Ames Mars general circulation model (MGCM) through solar radiative, topographic, and nonlinear processes using a comprehensive physics package and including a diurnal cycle. At an altitude near 70 km, zonal wave number decompositions of the diurnal and semidiurnal tidal fields are performed, and each wave component is extended from 70 to 250 km using a linear steady state global scale wave model for Mars (Mars GSWM). Conditions representative of aerocentric longitudes Ls = 30 (near equinox) and Ls = 270 (Southern Hemisphere summer solstice) are considered. Modeled total relative density variations of order ±10–40% near 125 km are analyzed in terms of the zonal wave numbers (ks) seen from the Sun‐synchronous perspective of the Mars Global Surveyor (MGS) accelerometer experiment, and yield reasonable agreement in amplitude and phase with the density measurements. The model indicates the two most important waves responsible for ks = 3 to be the eastward‐propagating diurnal and semidiurnal oscillations with zonal wave numbers s = 2 (∼15–40%) and s = 1 (∼8%), respectively. The eastward‐propagating diurnal component with s = 1 (∼15%) and the semidiurnal standing (s = 0) oscillation (∼4–23%) are concluded to be the main contributors to the ks = 2 longitudinal density variation seen from the Mars Global Surveyor (MGS). The standing (s = 0) diurnal oscillation (∼4–5%) and the westward‐propagating semidiurnal component with s = 1 (∼5–8%) emerge as the most likely contributors to ks = 1. Other waves that may make important secondary contributions include the westward‐propagating semidiurnal oscillations with s = 3 (∼4–6%) and s = 4 (∼3–9%). In addition, above 100 km the wind and temperature fields associated with the above waves represent ∼15–30% perturbations on the Sun‐synchronous wind and temperature fields driven in situ by EUV and near‐IR solar radiation absorption. Nonmigrating tides primarily arise from zonal asymmetries in wave forcing associated with Mars' topography; our results show for the first time that the dynamical effects of Mars' topography extend throughout the atmospheric column to Mars' exobase (∼200–250 km).

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