James R. Murphy

Mars atmospheric dynamics as simulated by the NASA Ames General Circulation Model: 1. The zonal‐mean circulation

This is the first in a series of papers that will discuss Mars atmospheric dynamics as simulated by the NASA Ames General Circulation Model (GCM). This paper describes the GCMs zonal-mean circulation and how it responds to seasonal variations and dust loading. The results are compared to Mariner 9 and Viking observations, and the processes responsible for maintaining the simulated circulation are discussed. At the solstices the zonal-mean circulation consists of a single cross-equatorial Hadley circulation between 30°S and 30°N. For relatively modest dust loadings (τ=0.3), the associated peak mass flux is 100 × 108 kg s−1 at northern winter solstice and 55 × 108 kg s−1 at southern winter solstice. At both seasons, westerlies dominate the winter hemisphere, and easterlies dominate the summer hemisphere. Maximum zonal winds occur near the model top (∼47 km) and are about the same at both seasons: 120 m s−1 in the winter hemisphere and 60 m s−1 in the summer hemisphere. Mean surface westerlies of 10–20 m s−1 are predicted at the middle and high latitudes of the winter hemisphere, as well as in the summer hemisphere near the rising branch of the Hadley circulation. The latter has the structure of a “jet” and is particularly strong (>20 m s−1) at northern winter solstice. With increasing amounts of dust (up to τ=5), the zonal mean circulation at northern winter solstice intensifies and gives no indication of a negative feedback. Dust can easily double the mass flux of the Hadley circulation. In the solstice simulations, the mean meridional circulation is the main dynamical contributor to the heat and momentum balance; the eddies play a relatively minor role. There is no evidence in these simulations for a polar warming. At the equinoxes the zonal mean circulation is more Earth-like and consists of two roughly symmetric Hadley cells with westerly winds in the mid-latitudes of each hemisphere and easterlies in the tropics. The simulated zonal winds are about half as strong as they are at solstice. However, the strength of the mean meridional circulation is much less than at solstice and averages between 5 and 10 × 108 kg s−1. At these seasons, the eddies and mean circulation make comparable, but opposing, contributions to the heat and momentum balances.

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.

Orbital change experiments with a Mars general circulation model

Abstract We use a Mars general circulation model to examine the effect of orbital changes on the planet’s general circulation and climate system. Experiments are performed for obliquities ranging from 0° to 60° for two different longitudes of perihelion. Each experiment simulates a full Mars year assuming a fixed atmospheric dust distribution and fixed amount of CO 2 in the atmosphere/cap system. We find that global mean surface temperatures and pressures decline with increasing obliquity due to the increasing extent of the winter polar caps. The seasonal CO 2 cycle and intensity of the solstice circulation amplify considerably with increasing obliquity such that global dust storms are likely at both solstices. The most significant feature of the high obliquity solstice circulations is the development of an intense low-level jet associated with the return branch of the Hadley circulation. Model surface stresses are used to map regions of preferred dust lifting, which are defined in terms of an annual deflation potential. For the present obliquity, the model-predicted regions of high deflation potential are in good agreement with Cantor et al.’s (2001 , J. Geophys. Res. 106 , 23653–23688) observations, which gives us some confidence in the model’s ability to predict where lifting might occur when Mars’ orbit parameters are different than they are today. In general we find that the dust lifting potential increases sharply with obliquity and is greatest at times of high obliquity when perihelion coincides with northern summer solstice. Over an obliquity cycle, the model global annual deflation potential ranges from several tenths of a millimeter at 0° obliquity to almost 15 mm at 60° obliquity. Much higher values are possible when the atmosphere is very dusty. We find a strong correlation between the deflation potential and surface thermal inertia: regions of high deflation potential correspond to regions of high thermal inertia (high rock abundance), and regions of low deflation potential correspond to regions of low thermal inertia (high dust/sand abundance). Furthermore, while the regions of preferred lifting (high deflation potential) expand somewhat with increasing obliquity and dust loading, the central parts of Tharsis, Arabia, and Elysium show no tendency for significant lifting at any obliquity or longitude of perihelion. These regions may therefore be very old and represent net long-term sinks for atmospheric dust. It is the topography of the planet, through its influence on surface pressure and wind systems, which ultimately determines where dust accumulates. Finally, as was found by Fenton and Richardson (2001 , J. Geophys. Res. 106 , 32885–32909), we find no tendency for the development of east-southeasterly winds at the Pathfinder site for any of our orbital change experiments. This suggests that the ancient wind regime discussed by Greeley et al. (2000 , J. Geophys. Res. 105, 1829–1840) was produced by other factors, such as polar wander.

MGS Radio Science electron density profiles: Interannual variability and implications for the Martian neutral atmosphere

density of 7.3–8.5 � 10 4 cm � 3 is also measured during solar moderate conditions at Mars. Strong wave number 2–3 oscillations in peak heights are consistently observed as a function of longitude over the 2 Martian years. These observed ionospheric features are remarkably similar during aphelion conditions 1 Martian year apart. This year-to-year repeatability in the thermosphere-ionosphere structure is consistent with that observed in multiyear aphelion temperature data of the Mars lower atmosphere [Clancy et al., 2000; Smith, 2004]. Coupled Mars general circulation model (MGCM) and Mars thermospheric general circulation model (MTGCM) codes are run for Mars aphelion conditions, yielding mean and longitude variable ionospheric peak heights that reasonably match RS observations. A tidal decomposition of MTGCM thermospheric densities shows that observed ionospheric wave number 3 features are linked to a nonmigrating tidal mode with semidiurnal period (s = 2) and zonal wave number 1 (s = � 1) characteristics. The height of this photochemically determined ionospheric peak should be monitored regularly. INDEX TERMS: 5435 Planetology: Solid Surface Planets: Ionospheres (2459); 5409 Planetology: Solid Surface Planets: Atmospheres—structure and dynamics; 6225 Planetology: Solar System Objects: Mars; KEYWORDS: ionosphere, Mars, thermosphere

Polar warming in the Mars thermosphere: Seasonal variations owing to changing insolation and dust distributions

Received 13 July 2005; revised 16 December 2005; accepted 27 December 2005; published 27 January 2006. [1] Warming of the martian lower thermosphere (100–130 km) at north polar latitudes near the perihelion/winter solstice (Ls = 270) was recently observed. No analogous warming has been observed within the south polar thermosphere during its aphelion/winter season (Ls 90). Detailed global model simulations are required to investigate the physical processes driving these seasonal variations. New simulations are conducted for conditions approximating the atmosphere during these Mars Global Surveyor (MGS) and Odyssey (ODY) aerobraking periods. Strong northern winter polar warming features are calculated near 120 km, yielding nightside mean temperatures 10–15 K warmer than observed ODY values. No southern winter polar warming trend is simulated; however, nightside mean temperatures are 20– 30 K warmer than observed by MGS. The stronger interhemispheric circulation during northern winter is clearly driven by stronger insolation and dust heating near perihelion, resulting in subsidence and warmer temperatures in the northern polar night. Citation: Bougher, S. W., J. M. Bell, J. R. Murphy, M. A. Lopez-Valverde, and P. G. Withers (2006), Polar warming in the Mars thermosphere: Seasonal variations owing to changing insolation and dust distributions, Geophys. Res. Lett., 33, L02203, doi:10.1029/2005GL024059.

Mars Global Ionosphere‐Thermosphere Model: Solar cycle, seasonal, and diurnal variations of the Mars upper atmosphere

A new Mars Global Ionosphere-Thermosphere Model (M-GITM) is presented that combines the terrestrial GITM framework with Mars fundamental physical parameters, ion-neutral chemistry, and key radiative processes in order to capture the basic observed features of the thermal, compositional, and dynamical structure of the Mars atmosphere from the ground to the exosphere (0–250 km). Lower, middle, and upper atmosphere processes are included, based in part upon formulations used in previous lower and upper atmosphere Mars GCMs. This enables the M-GITM code to be run for various seasonal, solar cycle, and dust conditions. M-GITM validation studies have focused upon simulations for a range of solar and seasonal conditions. Key upper atmosphere measurements are selected for comparison to corresponding M-GITM neutral temperatures and neutral-ion densities. In addition, simulated lower atmosphere temperatures are compared with observations in order to provide a first-order confirmation of a realistic lower atmosphere. M-GITM captures solar cycle and seasonal trends in the upper atmosphere that are consistent with observations, yielding significant periodic changes in the temperature structure, the species density distributions, and the large-scale global wind system. For instance, mid afternoon temperatures near ∼200 km are predicted to vary from ∼210 to 350 K (equinox) and ∼190 to 390 k (aphelion to perihelion) over the solar cycle. These simulations will serve as a benchmark against which to compare episodic variations (e.g., due to solar flares and dust storms) in future M-GITM studies. Additionally, M-GITM will be used to support MAVEN mission activities (2014–2016).

Modeling the Martian dust cycle and surface dust reservoirs with the NASA Ames general circulation model

[1]xa0We employ the NASA Ames Mars general circulation model (GCM) to investigate the dust lifting mechanisms responsible for the observed Martian dust cycle and the net surface response to the combined influence of dust lifting and deposition. This GCM includes lifting, transport, and deposition of radiatively active dust. Two dust lifting mechanisms are accounted for: wind stress lifting and dust devil lifting. A “baseline” simulation is presented and shown to compare well to available spatial and temporal observations of atmospheric opacity, wind stress dust lifting events, and atmospheric temperatures recorded during a nonglobal dust storm year. Multiple simulations were conducted to explore the models sensitivity to a wide range of dust lifting parameters (the functional dependence of surface dust flux on wind stress, the wind stress threshold for lifting, etc.) Model results robustly suggest that wind stress lifting produces the peak in atmospheric dust load during southern spring and summer and that dust devils maintain the background haze of atmospheric dust during northern spring and summer. These results are consistent with previously published conclusions. Dust devil and wind stress lifting contribute equally to the simulated total amount of dust lifted annually during nonglobal dust storm years. The simulated spatial pattern of annual net deflation/deposition suggests that the low thermal inertia regions (Tharsis, Arabia, and Elysium) are not currently net dust accumulation regions. This net deflation is the result of dust devil dust lifting, suggesting that dust devils could play an important role in the present-day pattern of surface dust reservoirs.

Composition and structure of the Martian surface at high southern latitudes from neutron spectroscopy

[1]xa0Neutron spectroscopy data acquired by Mars Odyssey are analyzed to determine the abundance and depth of near-surface water ice as a function of latitude in the southern hemisphere as well as the inventory of CO2 in the south polar residual cap. The surface is modeled as a semi-infinite, water-rich permafrost layer covered by desiccated material, which is consistent with theoretical models of ground ice stability. Latitude-dependent parameters, water abundance and depth, are determined from zonally averaged neutron counting data. Spatial mixing of the output of neutrons from regions within the footprint of the spectrometer is modeled, and asymmetrical features such as the residual cap are included in the analysis. Absorption of thermal neutrons by major elements other than hydrogen is found to have a significant influence on the determination of water abundance. Poleward of −60°, the water-rich layer contains 60% ± 10% water by weight (70% to 85% by volume) and is covered by less than 15 g/cm2 ± 5 g/cm2 of dry material. The volume fraction of water is generally higher than can be accommodated in the pore space of surface soils, which implies that water vapor diffusion processes alone cannot explain the observations. Alternatives for the formation of the water-rich layer are discussed. Results of our analysis of the residual-cap CO2 inventory support conclusions that the atmosphere is not buffered by a larger reservoir of surface CO2 at the poles and that Mars total CO2 inventory is well represented by the present atmospheric mass.

Martian and terrestrial dust devils: Test of a scaling theory using Pathfinder data

The Mars Pathfinder meteorological station recorded wind, pressure, and temperature fluctuations that have been interpreted as dust devils: warm-core vortices that form at the bottom of connective plumes. We apply a scaling theory [Renno et al., 1998], developed to explain terrestrial dust devil observations, to test the validity of this interpretation and to provide a simple physical interpretation of the general characteristics of Martian dust devils. The theory is based on the thermodynamics of heat engines and predicts the central pressure and the wind speed of the connective vortices. Our findings are as follows: For the best documented candidate event (sol 25), observed wind and pressure fluctuations are consistent with those predicted by the model and hence strengthen the interpretation of this case as a dust devil. A number of other candidates, less well documented, however, also are consistent with passage of a dust devil over or near the lander. Temperature fluctuations observed on other sols permit dust devils an order of magnitude larger than the ones measured by the meteorology package. The strongest dust devils predicted by our theory have a central pressure deficit of about 50 Pa and wind speed of about 60 m s−1. The strongest dust devils are capable of lofting dust and hence support the interpretation of selected Pathfinder images as showing the passage of dust devils within sight of the lander.

Results of the Imager for Mars Pathfinder windsock experiment

The Imager for Mars Pathfinder (IMP) windsock experiment measured wind speeds at three heights within 1.2 m of the Martian surface during Pathfinder landed operations. These wind data allowed direct measurement of near-surface wind profiles on Mars for the first time, including determination of aerodynamic roughness length and wind friction speeds. Winds were light during periods of windsock imaging, but data from the strongest breezes indicate aerodynamic roughness length of 3 cm at the landing site, with wind friction speeds reaching 1 m/s. Maximum wind friction speeds were about half of the threshold-of-motion friction speeds predicted for loose, fine-grained materials on smooth Martian terrain and about one third of the threshold-of-motion friction speeds predicted for the same size particles over terrain with aerodynamic roughness of 3 cm. Consistent with this, and suggesting that low wind speeds prevailed when the windsock array was not imaged and/or no particles were available for aeolian transport, no wind-related changes to the surface during mission operations have been recognized. The aerodynamic roughness length reported here implies that proposed deflation of fine particles around the landing site, or activation of duneforms seen by IMP and Sojourner, would require wind speeds >28 m/s at the Pathfinder top windsock height (or >31 m/s at the equivalent Viking wind sensor height of 1.6 m) and wind speeds >45 m/s above 10 m. These wind speeds would cause rock abrasion if a supply of durable particles were available for saltation. Previous analyses indicate that the Pathfinder landing site probably is rockier and rougher than many other plains units on Mars, so aerodynamic roughness length elsewhere probably is less than the 3-cm value reported for the Pathfinder site.