Nilton De Oliveira Renno

H2O at the Phoenix Landing Site

Phoenix Ascending The Phoenix mission landed on Mars in March 2008 with the goal of studying the ice-rich soil of the planets northern arctic region. Phoenix included a robotic arm, with a camera attached to it, with the capacity to excavate through the soil to the ice layer beneath it, scoop up soil and water ice samples, and deliver them to a combination of other instruments—including a wet chemistry lab and a high-temperature oven combined with a mass spectrometer—for chemical and geological analysis. Using this setup, Smith et al. (p. 58) found a layer of ice at depths of 5 to 15 centimeters, Boynton et al. (p. 61) found evidence for the presence of calcium carbonate in the soil, and Hecht et al. (p. 64) found that most of the soluble chlorine at the surface is in the form of perchlorate. Together these results suggest that the soil at the Phoenix landing site must have suffered alteration through the action of liquid water in geologically the recent past. The analysis revealed an alkaline environment, in contrast to that found by the Mars Exploration Rovers, indicating that many different environments have existed on Mars. Phoenix also carried a lidar, an instrument that sends laser light upward into the atmosphere and detects the light scattered back by clouds and dust. An analysis of the data by Whiteway et al. (p. 68) showed that clouds of ice crystals that precipitated back to the surface formed on a daily basis, providing a mechanism to place ice at the surface. A water ice layer was found 5 to 15 centimeters beneath the soil of the north polar region of Mars. The Phoenix mission investigated patterned ground and weather in the northern arctic region of Mars for 5 months starting 25 May 2008 (solar longitude between 76.5° and 148°). A shallow ice table was uncovered by the robotic arm in the center and edge of a nearby polygon at depths of 5 to 18 centimeters. In late summer, snowfall and frost blanketed the surface at night; H2O ice and vapor constantly interacted with the soil. The soil was alkaline (pH = 7.7) and contained CaCO3, aqueous minerals, and salts up to several weight percent in the indurated surface soil. Their formation likely required the presence of water.

An analysis of the conditional instability of the tropical atmosphere

Abstract The ice phase is included in thermodynamic calculations of convective available potential energy (CAPE) for a large number of soundings in the tropical atmosphere, at both land and ocean stations. It is found that the positive-buoyancy contribution to CAPE resulting from the latent heat of fusion more than offsets the negative-buoyancy contribution due to water loading in the reversible thermodynamic process. The departure from moist neutrality in much of the tropical atmosphere exhibits a threshold in boundary-layer wet-bulb potential temperature of 22°–23°C. The corresponding sea surface temperature is approximately 26°C, close to the empirical threshold for hurricane formation, which suggests that conditional instability plays an important role in the latter phenomenon. The simultaneous presence of finite CAPE and infrequent deep convection in the tropics is tentatively attributed to the convective inhibition energy (CINE) and to the mixing process that destroys positive buoyancy in incipient ...

A Simple Thermodynamical Theory for Dust Devils

Based on the heat engine framework, a simple scaling theory for dust devils is proposed and compared to observations. This theory provides a simple physical interpretation for many of the observed characteristics of dust devils. In particular, it predicts the potential intensity and the diurnal variation of dust devil occurrence. It also predicts that the intensity of dust devils depends on the product of two thermodynamic efficiencies, corresponding respectively to vertical and horizontal temperature gradients.

Soil Diversity and Hydration as Observed by ChemCam at Gale Crater, Mars

The ChemCam instrument, which provides insight into martian soil chemistry at the submillimeter scale, identified two principal soil types along the Curiosity rover traverse: a fine-grained mafic type and a locally derived, coarse-grained felsic type. The mafic soil component is representative of widespread martian soils and is similar in composition to the martian dust. It possesses a ubiquitous hydrogen signature in ChemCam spectra, corresponding to the hydration of the amorphous phases found in the soil by the CheMin instrument. This hydration likely accounts for an important fraction of the global hydration of the surface seen by previous orbital measurements. ChemCam analyses did not reveal any significant exchange of water vapor between the regolith and the atmosphere. These observations provide constraints on the nature of the amorphous phases and their hydration.

Natural Convection as a Heat Engine: A Theory for CAPE

On many planets there is a continuous heat supply to the surface and a continuous emission of infrared radiation to space by the atmosphere. Since the heat source is located at higher pressure than the heat sink, the system is capable of doing mechanical work. Atmospheric convection is a natural heat engine that might operate in this system. Based on the heat engine framework, a simple theory is presented for atmospheric convection that predicts the buoyancy, the vertical velocity, and the fractional area covered by either dry or moist convection in a state of statistical equilibrium. During one cycle of the convective heat engine, heat is taken from the surface layer (the hot source) and a portion of it is rejected to the free troposphere (the cold sink) from where it is radiated to space. The balance is transformed into mechanical work. The mechanical work is expended in the maintenance of the convective motions against mechanical dissipation. Ultimately, the energy dissipated by mechanical friction is transformed into heat. Then, a fraction of the dissipated energy is radiated to space while the remaining portion is recycled by the convecting air parcels. Increases in the fraction of energy dissipated at warmer temperature, at the expense of decreases in the fraction of energy dissipated at colder temperatures, lead to increases in the apparent efficiency of the convective heat engine. The volume integral of the work produced by the convective heat engine gives a measure of the statistical equilibrium amount of convective available potential energy (CAPE) that must be present in the planets atmosphere so that the convective motions can be maintained against viscous dissipation. This integral is a fundamental global number qualifying the state of the planet in statistical equilibrium conditions. For the earths present climate, the heat engine framework predicts a CAPE value of the order of 1000 J kg^−1 for the tropical atmosphere. This value is in agreement with observations. It also follows from our results that the total amount of CAPE present in a convecting atmosphere should increase with increases in the global surface temperature (or the atmospheres opacity to infrared radiation).

Mars Water-Ice Clouds and Precipitation

Phoenix Ascending The Phoenix mission landed on Mars in March 2008 with the goal of studying the ice-rich soil of the planets northern arctic region. Phoenix included a robotic arm, with a camera attached to it, with the capacity to excavate through the soil to the ice layer beneath it, scoop up soil and water ice samples, and deliver them to a combination of other instruments—including a wet chemistry lab and a high-temperature oven combined with a mass spectrometer—for chemical and geological analysis. Using this setup, Smith et al. (p. 58) found a layer of ice at depths of 5 to 15 centimeters, Boynton et al. (p. 61) found evidence for the presence of calcium carbonate in the soil, and Hecht et al. (p. 64) found that most of the soluble chlorine at the surface is in the form of perchlorate. Together these results suggest that the soil at the Phoenix landing site must have suffered alteration through the action of liquid water in geologically the recent past. The analysis revealed an alkaline environment, in contrast to that found by the Mars Exploration Rovers, indicating that many different environments have existed on Mars. Phoenix also carried a lidar, an instrument that sends laser light upward into the atmosphere and detects the light scattered back by clouds and dust. An analysis of the data by Whiteway et al. (p. 68) showed that clouds of ice crystals that precipitated back to the surface formed on a daily basis, providing a mechanism to place ice at the surface. Laser remote sensing from Mars’ surface revealed water-ice clouds that formed during the day and precipitated at night. The light detection and ranging instrument on the Phoenix mission observed water-ice clouds in the atmosphere of Mars that were similar to cirrus clouds on Earth. Fall streaks in the cloud structure traced the precipitation of ice crystals toward the ground. Measurements of atmospheric dust indicated that the planetary boundary layer (PBL) on Mars was well mixed, up to heights of around 4 kilometers, by the summer daytime turbulence and convection. The water-ice clouds were detected at the top of the PBL and near the ground each night in late summer after the air temperature started decreasing. The interpretation is that water vapor mixed upward by daytime turbulence and convection forms ice crystal clouds at night that precipitate back toward the surface.

Electric and magnetic signatures of dust devils from the 2000–2001 MATADOR desert tests

[1] Dust devils are significant meteorological phenomena on Mars: They are ubiquitous, continually gardening the Martian surface, and may be the primary atmospheric dustloading mechanism in nonstorm seasons. Further, dust grains in the swirling dust devils may become electrically charged via triboelectric effects. Electrical effects associated with terrestrial dust devils have been reported previously, but these were isolated measurements (electric fields only) with no corroborating measurements. To study the fluid and electrical forces associated with dust devils, NASA’s Human Exploration and Development of Space (HEDS) enterprise sponsored a set of desert field tests with a suite of mutually compatible and complementary instruments in order to determine the relationship between electric, magnetic, and fluid forces. The project (originally a selected flight project) was entitled ‘‘Martian ATmosphere And Dust in the Optical and Radio’’ (MATADOR). In this work, we present a number of interesting examples of the electromagnetic nature of the dust devil. We also describe potential hazards of the dust devil and how similar devil- and storm-related forces on Mars might affect any human occupation. INDEX TERMS: 6225 Planetology: Solar System Objects: Mars; 3304 Meteorology and Atmospheric Dynamics: Atmospheric electricity; 3379 Meteorology and Atmospheric Dynamics: Turbulence; 3394 Meteorology and Atmospheric Dynamics: Instruments and techniques; 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); KEYWORDS: triboelectricity, electric fields, dust devils, magnetic fields, atmospheric electricity

Radiative‐convective model with an explicit hydrologic cycle: 1. Formulation and sensitivity to model parameters

A hydrological cycle is explicityl included in a one-dimensional radiative-convective equilibrium model which is coupled to a “swamp” surface and tested with various cumulus convection schemes: the hard and soft convective adjustment schemes, the Kuo scheme, the Goddard Institute for Space Studies (GISS) (1974) model 1 scheme, the GISS (1983) model 2 scheme, and the Emanuel scheme. The essential difference between our model and other radiative-convective models is that in our model the moisture profile (but not cloudiness) is interactively computed by the cumulus convection scheme. This has a crucial influence on the computation of the radiative fluxes throughout the atmosphere and therefore on the models sensitivities. Using the Emanuel scheme, we show that the climate equilibrium is very sensitive to cloud microphysical processes. Clouds with high precipitation efficiency produce cold and dry climates. Clouds with low precipitation efficiency lead to moist and warm climates. Since climate equilibrium can be very sensitive to the cloud microphysical processes, any cumulus convection scheme adequate for use in general circulation models (GCMs) should be strongly based on them. The cumulus convection schemes currently in use in GCMs bypass the microphysical processes by making arbitrary moistening assumptions. We suggest that they are inadequate for climate change studies.

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.

Convective circulations induced by surface heterogeneities

A simple theory for convective circulations induced by surface heterogeneities is proposed. The theory is based on the thermodynamics of heat engines and provides a simple physical explanation for the general characteristics of circulations forced by surface inhomogeneities in sloping terrains. In particular, the theory is applied to a mesoscale circulation induced by deforestation. It predicts that the intensity of the mesoscale convective circulation forced by deforestation depends on the difference of the near-surface nonadiabatic temperature and humidity between the forest and cleared regions and on the depth of the convective boundary layer. The theory was successfully tested against observations made during a field experiment in the Amazon forest and a nearby clearing.