Jeffery Lee Hollingsworth

General circulation model simulations of the Mars Pathfinder atmospheric structure investigation/meteorology data

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 Pathfinders 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.

A Reappraisal of The Habitability of Planets around M Dwarf Stars

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.

Forced Stationary Planetary Waves in Mars's Winter Atmosphere

Abstract Mariner 9 and Viking spacecraft observations provided evidence for planetary-scale, wavelike disturbances in the Mars winter atmosphere. Possible sources of the wave activity are dynamical instabilities, for example, barotropic and / or baroclinic instabilities. Other candidate sources are forced. quasi-stationary planetary waves—waves that arise predominantly via zonally asymmetric surface properties. The authors attempt to model aspects of the wave activity, focusing on forced planetary waves in representative wintertime atmospheres for Mars, by applying a spherical linear primitive equations model. Basic states representing relatively “nondusty” and “highly dusty” conditions near winter solstice allow wavenumber 1 and 2 disturbances to propagate meridionally and vertically about the jet. Higher wavenumbers are strongly vertically trapped. Stationary waves during winter in the northern and southern extratropics differ strongly in amplitude, phase, and dominant horizontal wave pattern. Northern ...

Radio occultation measurements of forced atmospheric waves on Mars

Mars Global Surveyor performed a series of radio occultation experiments in December 1998, resulting in 36 profiles of the neutral atmosphere in late northern spring (Ls = 74.1°–77.3°). The measurements are confined in latitude (64.6°–67.2°N) and local time (0321–0418), but their distribution in longitude is fairly uniform. We used least squares spectral analysis to characterize the zonal structure of the atmosphere and constructed longitude-height cross sections of both temperature and geopotential. Zonal variations of temperature exceed 12 K near the surface but are much smaller (2–3 K) at higher altitudes. Zonal variations of geopotential are ∼200 m throughout the vertical range of the measurements. These patterns of temperature and geopotential appear to be stationary relative to the surface with little day-to-day variation within the 7-sol span of the measurements. We relied heavily on Mars general circulation models (GCMs) for guidance in understanding these data. Stationary planetary waves are responsible for some aspects of the temperature and geopotential fields, particularly at pressures exceeding 100–200 Pa. On the basis of strong similarities between a GCM simulation and the observations, we conclude that the disturbance takes the form of a planetary wave train excited by Alba Patera. The data also include the signature of non-Sun-synchronous thermal tides, which produce a pattern that appears to be stationary when sampled at fixed local time. Comparison between a GCM simulation and the measured geopotential field provides evidence for the presence of the resonantly enhanced, diurnal, wave-1 Kelvin mode.

Albedo of the south pole on Mars determined by topographic forcing of atmosphere dynamics

The nature of the martian south polar cap has remained enigmatic since the first spacecraft observations. In particular, the presence of a perennial carbon dioxide ice cap, the formation of a vast area of black ‘slab ice’ known as the Cryptic region and the asymmetric springtime retreat of the cap have eluded explanation. Here we present observations and climate modelling that indicate the south pole of Mars is characterized by two distinct regional climates that are the result of dynamical forcing by the largest southern impact basins, Argyre and Hellas. The style of surface frost deposition is controlled by these regional climates. In the cold and stormy conditions that exist poleward of 60° S and extend 180° in longitude west from the Mountains of Mitchel (∼ 30° W), surface frost accumulation is dominated by precipitation. In the opposite hemisphere, the polar atmosphere is relatively warm and clear and frost accumulation is dominated by direct vapour deposition. It is the differences in these deposition styles that determine the cap albedo.

Mars Global Surveyor aerobraking: Atmospheric trends and model interpretation

Abstract Mars Global Surveyor (MGS) recently obtained coordinated lower-atmosphere (thermal and dust) measurements and simultaneous upper atmosphere accelerometer data (densities, scale heights and temperatures) for the purpose of safely aerobraking the spacecraft toward its mapping orbit (Keating et al. 1998). Much useful scientific information was also gleaned that describes the coupling of these atmospheric regions during this Phase I aerobraking period (September 1997–March 1998; Ls = 184–300). The major features of this aerobraking data are presented, and its trends elucidated in order to: (1) illustrate the aerobraking environment experienced by the spacecraft, and (2) decompose the processes responsible for the atmospheric variations observed. Coupled general circulation models of the Mars lower and upper atmospheres are exercised to investigate the solar-orbital, seasonal, wave, and dust variations observed during MGS aerobraking. The precession of the MGS periapsis position during Phase I permits longitudinal, latitudinal, local time, and vertical variations of the thermosphere to be monitored. Future aerobraking activities at Mars will benefit greatly from this MGS aerobraking data and its model interpretation.

Preliminary interpretation of the REMS pressure data from the first 100 sols of the MSL mission

We provide a preliminary interpretation of the Rover Environmental Monitoring Station (REMS) pressure data from the first 100 Martian solar days (sols) of the Mars Science Laboratory mission. The pressure sensor is performing well and has revealed the existence of phenomena undetected by previous missions that include possible gravity waves excited by evening downslope flows, relatively dust-free convective vortices analogous in structure to dust devils, and signatures indicative of the circulation induced by Gale Crater and its central mound. Other more familiar phenomena are also present including the thermal tides, generated by daily insolation variations, and the CO2 cycle, driven by the condensation and sublimation of CO2 in the polar regions. The amplitude of the thermal tides is several times larger than those seen by other landers primarily because Curiosity is located where eastward and westward tidal modes constructively interfere and also because the crater circulation amplifies the tides to some extent. During the first 100 sols tidal amplitudes generally decline, which we attribute to the waning influence of the Kelvin wave. Toward the end of the 100 sol period, tidal amplitudes abruptly increased in response to a nearby regional dust storm that did not expand to global scales. Tidal phases changed abruptly during the onset of this storm suggesting a change in the interaction between eastward and westward modes. When compared to Viking Lander 2 data, the REMS daily average pressures show no evidence yet for the 1–20 Pa increase expected from the possible loss of CO2 from the south polar residual cap.

An interpretation of Martian thermospheric waves based on analysis of a general circulation model

Planetary-scale longitudinal variations in density observed by the Mars Global Surveyor accelerometer in the 125 km region can be qualitatively reproduced by the NASA Ames Mars general circulation model in the 80 km altitude region, but only when locations having specific local times are used in the analysis. If the model results are averaged over all local times, the high-altitude longitudinal variations nearly disappear, leaving only a small stationary wave 1 pattern, consistent with theory and previous modeling studies. This analysis suggests that the observed wavelike structures are a result of sampling tidal modes at a limited range of local times, rather than by topographically forced Rossby waves as previously suggested.

Kelvin wave propagation in the upper atmospheres of Mars and Earth

The propagation characteristics of a diurnal Kelvin wave (DKW) in the atmosphere of Mars and a 3-day Kelvin wave in the terrestrial atmosphere are compared and interpreted for Northern Hemisphere summer conditions (Ls = 90 and July, respectively). Significant horizontal wind and temperature perturbations are shown to accompany these oscillations in the thermospheres of both planets: ∼10–40 ms−1 and 10–25 K on Earth and 20–70 ms−1 and 10–30 K on Mars. Molecular dissipation on both planets serves to latitudinally broaden the thermospheric response, and to induce meridional wind maxima at the poles. On Earth, mean zonal winds asymmetric about the equator are found to locally (in altitude) distort the latitudinal shapes of the Kelvin wave fields by coupling into modes which tend to remain confined to the level of excitation, and which are equatorially trapped. On Mars the first asymmetric eastward propagating mode with vertical wavelength of about 60 km is very efficiently induced by the asymmetric zonal mean winds in the middle atmosphere; this wave component propagates well into the thermosphere and accounts for much global asymmetry seen in the wave fields.

A comparison of MGS Phase 1 aerobraking radio occultation data and the NASA Ames Mars GCM

The NASA Ames Mars general circulation model (MGCM) is used to investigate how different phenomena in the Martian climate system combine to produce temperature profiles measured by the Radioscience Instrument (RS) aboard Mars Global Surveyor (MGS) in early 1998. When integrated with dust amounts consistent with Thermal Emission Spectrometer (TES) observations, and with a topography data set determined from the Mars Orbiting Laser Altimeter (MOLA), modeled temperature profiles are within 5–10 K of observations. Modeled and measured profiles in the winter subtropics are warmer than pure radiative-convective considerations suggest, providing further evidence of strong dynamical warming associated with descent in the Hadley cell. South of 40°S, the best fit to the data is obtained by confining airborne dust to below 2 scale heights in this region. The differences in the strength of low-level inversions found in the MGCM and in the RS data indicate the presence of either a low-level radiative cooling mechanism such as isolated dust clouds or water-ice clouds, or a dynamical mechanism such as low-level gravity wave drag. When initialized with a mean surface pressure that gives the best fit to Viking lander measurements, the model matches RS surface pressure data to within 0.1 mbar. This consistency is a further validation of the MGCM and shows how GCMs can be used to interpret measurements of surface pressure taken at different locations using different methods.