P11A-1248
Isotope Effects on the Photochemical Escape of O from Mars
Dissociative recombination of O2+ ions is the most important photochemical escape mechanism for oxygen atoms from Mars at the current epoch. We construct here models of the Martian thermosphere/ionosphere for low and high solar activities, and determine the rate of O2+ dissociative recombination (DR) as a function of altitude for each model. Using a Monte Carlo method, we compute the probabilities of escape for 16O and 18O atoms and compare them to see if there is a significant isotope effect. The escape probability for 18O atoms is expected to be less than that of 16O atoms, partly because 18O is released with less energy in DR of 34O2+ than that of 16O in DR of 32O2+, and its escape energy is larger. This isotope effect, which is inherent in the mechanism, is independent of the fractionation that exists between the homopause and the exobase. The energy distribution of O produced in O2+ DR is determined by the values of the ion and electron temperatures at a given altitude, and by the channels by which the DR proceeds. This energy distribution will be computed using a Monte Carlo method as a function of altitude from 100 to 400 km. Because the isotopes of O are found not to be fractionated in the Martian atmosphere relative to earth, a significant reservoir of O that can exchange with the atmosphere is implied. We also compute the total rates of photochemical escape of 16O from the Martian atmosphere for high and low solar activity models and compare them to the results of the many other investigators who have examined this escape mechanism.
P11A-1249
3D Study of Suprathermal Oxygen Atoms in Mars Upper Thermosphere and Exosphere over the Range of Limiting Conditions
The dynamics of energetic particles flowing through the Martian upper atmosphere has been studied. Most of the production of hot atomic oxygen occurs deep in the day-side thermosphere of Mars, where dissociative recombination (DR) of the O2+ ion is the dominant source. The study of an upper atmosphere is complicated by the change in the flow regime from a thermospheric collisional to an exospheric collisionless domain. To understand the Martian exosphere, it is then highly desirable to employ a global kinetic model that includes a self-consistent description of both thermospheric and exospheric regions. In this study, a combination of our 3D Direct Simulation Monte Carlo (DSMC) model and the 3D Mars Thermosphere General Circulation Model (MTGCM) (Bougher et al. 2006; Tenishev et al. 2008; Valeille et al. 2008) was used to describe self-consistently the exosphere and the upper thermosphere. Along with maps of ion production by photoionization (PI), charge exchange (CE) and electron impact (EI), the DSMC model provides for the first time density and temperature profiles, return fluxes and atmospheric loss rates of suprathermal exospheric oxygen as functions of the latitude and longitude for all cases considered. To present a complete description of this physical problem, several of the most limiting cases spanning spatial and temporal domains were examined. Along with solar activity variability (solar minimum and maximum), the influences of position on the planet and of different seasons (orbital position, dust activity) are also investigated and their relative contribution to the atmospheric loss is shown to be of the same order. Support for this work comes from NASA Mars Fundamental Research grant NNG05GL80G.
P11A-1250
Energy and Momentum Relaxation in Collisions of Fast H Atoms with the Atmospheric H and O Gas
Collisions of energetic H and D atoms with the thermalized H and O atmospheric gas are investigated. Accurate potential energy surfaces for H + H and H + O interaction are used in our quantum mechanical computations of the differential and total cross-sections of elastic collisions of hydrogen atoms. Collisions of hydrogen isotopes are also investigated. Simplified fitting formulas, describing general features of calculated collisional cross-sections, are discussed. Energy spectra of the secondary energetic atoms produced in recoil collisions are calculated. Rates of the momentum and energy relaxation of energetic H atoms are computed for collisional energies between 0.01eV and 10 keV. Energy relaxation of energetic H atoms produced in the charge-exchange collisions of solar wind ions in the upper Martian atmosphere is analyzed. Impact of H + H and H + O collisions on a formation of the escape flux of hydrogen atoms and their isotopes is investigated.
P11A-1251
A possible formation process of outer lobes of Double Layered Ejecta craters on Mars
Martian impact ejecta are famous for their morphologies suggesting ejecta would be formed by radial ground- hugging flows in the late stage of the impact. The atmosphere [Schultz, 1992] and/or the subsurface volatiles [Carr et al., 1977] have been suggested as causes of fluidization. Examining the process to generate and emplace the radial flow would allow us to understand the nature of the entrained fluid. Double Layered Ejecta (DLE), one of the major subclasses of martian ejecta, have many unique features. Most enigmatic is the presence of two distinct layers of ejecta: a thick inner lobe and a thin outer lobe. The striking differences between the two lobes suggest that two different processes occur independently during ejecta emplacement, a case implausible by a single ballistic trajectory. In this study, based on the hypothesis that an impact- induced vortex ring modifies surface materials in the late stage of the impact to produce the outer lobe, the volumes of displaced particles by the vortex ring were measured in laboratory experiments to compare the volumes of the outer lobes. We utilized the experimental situation of a vortex ring impacting on a particle layer. Two dimensionless numbers based on particle size (Shields' and Reynolds number) in lab fall within the same ranges as those on Mars [Suzuki et al., 2007], it is thus possible to compare the relationships between volumes and Γ, a parameter expressing the strength of a vortex ring. We fit the dependence of Vdisplaced on Gamma using a power law Vdisplaced = a Γb and we found b = 1.25 ± 0.17. The volumes of the outer lobes were measured, selecting 7 fresh craters larger than 5km in diameter in the survey area (0N-60N, 90E-150E). In the case of impact cratering, the vortex strength Γ can be scaled with the crater diameter [Barnouin-Jha and Schultz, 1998] as Γ ∝ D⅔. Using also a power law Vouter = c Γd, we obtained d = 1.42 ± 0.24 for the volume of outer lobes of DLE. As the power indices of Γ fall in the same range in lab and on Mars, we conclude that atmospheric vortex ring is a plausible phenomenon explaining the outer lobe of the DLE and we propose a scenario for the formation of these craters.
P11A-1252
Planetary Mass Spectrometry: From Atmospheres to the Solar Wind
Measurement of the bulk composition and isotopic fractionation of planetary atmospheres yields essential information about their origin, evolution and eventual loss. In particular, measurements from below the homopause out through the exosphere to the solar wind (e.g., at Mars and Venus) or to a planetary magnetosphere (e.g., at Titan) are essential to understanding processes driving atmospheric evolution and loss. Over the past three years we have developed and tested prototypes of two types of time-of-flight (TOF) mass spectrometers capable of the comprehensive measurements needed to settle questions of atmospheric evolution and loss. The first is the Ion Neutral Mass Spectrometer (INMS) which uses electrostatic mirrors to artificially lengthen and focus ion flight times, allowing the spectrometer to reach mass resolution needed to separate key compounds such as N2 and CO or isotopologues such as 13C17O16O vs. 12C18O16O (dM = 0.0033 amu). In order to boost sensitivity and signal to noise the INMS is also capable of storing and concentrating gases for prolonged analysis. The INMS prototype has demonstrated mass resolution > 15,000 M/dM and sensitivity for noble gas isotopes at 1 part per billion. The second spectrometer is the hot plasma composition analyzer (HPCA), a carbon-foil based instrument with heritage from the CAPS Cassini Ion Mass Spectrometer but with significantly improved mass resolution and sensitivity. The HPCA is designed to measure ion fluxes impinging on and escaping from planetary atmospheres. Together the INMS and HPCA cover the broad range of ion species and velocity space needed for investigation of planetary atmospheres and ionospheres and their interactions with the space environment.
P11A-1253
Pre-recessional (Ls 160-200) Polar Water Ice Clouds at the Martian South Pole: Potential Tracers of East-West Asymmetry?
CRISM observations of the south polar region during the pre-spring recession period (Ls 160-200) show
tenuous water ice signatures mixed with CO2 ice signatures over the polar cap between latitudes [1]. Water
ice has been identified using the 1.5 micron absorption band, which is discernable within the CO2 1.435
micron band complex due to the narrow nature of the CO2 ice absorptions [2, 3]. A surface deposit of water
ice is not favored due to the apparent fine grained nature of the water ice, which is indicated by the weak 1.5
micron absorption band, and the disappearance of the clouds prior to retreat of the south polar CO2 ice
beneath them.
CRISM observations show the pre-recessional water ice clouds appear to go through a steady growth phase
from Ls 160-190 and have a short terminal phase from Ls 200-205 [1, 2]. The clouds never quite encircle the
whole pole, and never penetrate the 80 degree latitude line. They are strongest at, and appear to originate
from, the 90-150 degree meridian range.
From Mars GCM simulations, similar patterns appear to originate from east-west asymmetries in the early
spring circulation and low-level thermal environments of the high-latitude/polar regions of the southern
hemisphere that arise due to asymmetries of large-scale orography and its effects on the atmosphere and
climate.
We are currently investigating the optical thickness of the water ice clouds using CRISM full resolution
emission phase function observations and intend to examine the H2O absorption bands at 3.2 microns to
further elucidate the cloud properties.
The potential to trace asymmetric polar atmospheric fluxes using these water ice clouds as tracers may lead
to a better understanding of the enigmatic cryptic region and the displacement of the south pole residual cap
[5].
[1] Brown, A.J. (2007) Fall AGU abstract P33A-1016
[2] Brown, A.J. and Calvin, W. JGR in preparation
[3] Langevin, Y. et al. (2006) JGR 112 doi:10.1029/2006JE002841
[4] Hollingsworth, J.L. et al. (2008) 3rd Mars Atmosphere Workshop
[5] Colaprete, A. et al. (2005) Nature 435 184-188
http://abrown.seti.org
P11A-1254
MGCM simulations of the 2001 Mars global dust storm using synthesized MGS dust opacity
We have synthesized all available Mars Global Surveyor (MGS) observations of the 2001 global dust storm
on Mars in order to produce the best possible description of the evolution of column optical depth and active
dust lifting regions. MGS data include MOC daily global weather maps, MOC dust (visible) optical depth
measurements, TES measurements of atmospheric temperature and 9-micron dust opacity, and MHSA
measurements of middle atmosphere temperatures. Together these allow for the juxtaposition of temperature
and opacity fields with visual imagery to enable a more comprehensive assessment of storm development.
Known limitations of TES observations result in significant spatial gaps in data, especially at high latitudes
and in regions with very high dust opacity. NASA/NOAA Mars General Circulation Model (MGCM) simulations
of the storm using synthesized opacity datasets will be presented. We have found that allowing the MGCM to
self-consistently advect a radiatively active dust distribution, suitably constrained by the observed column
opacity, allows a realistic simulation of the atmospheric temperature evolution. The Hadley circulations and
the thermal tides play a prominant role in structuring the aerosol field.
http://www.gfdl.noaa.gov/~rjw/
P11A-1255
Hygroscopic Salts on Mars
We present preliminary results on the influence of a salt-rich regolith in the water cycle of Mars. Global climate modeling shows that the relative humidity on the Martian surface often reaches values above the deliquescence point of salts that are common components of the regolith. At the deliquescence point, these salts will absorb atmospheric water vapor and form a saturated, transient liquid solution that is stable under a range of temperatures. If atmospheric temperatures fall below the eutectic point of the solution, the later will freeze in the pore space of the regolith, thereby resulting in a net transport of water from the vapor phase in the atmosphere, to the solid state in the regolith. This simple model partially accounts for some the distribution of water on the Martian surface as revealed by Mars Odyssey, in particular, we find that:
P11A-1256
Formation of zonal jets by moist convection on gas-giant planets
All four gas-giant planets, Jupiter, Saturn, Uranus and Neptune exhibit multiple banded zonal jets. Jupiter and Saturn also show equatorial superrotation while Uranus and Neptune show equatorial subrotation. The formation mechanisms of these zonal jets are largely unknown. Moist convection, which generates small-scale turbulence, has been proposed as a energy source to drive the zonal jets. However, this hypothesis has not been adequately tested. Moreover, current numerical models generally produce the same equatorial jet direction for all four giant planets. These models therefore fail to provide a single mechanism that can simultaneously explain the equatorial jet on both the gas giants (Jupiter/Saturn) and ice giants (Uranus/Neptune). Here, we present 3D global numerical simulations using the MITgcm that include water vapor. Condensation, and the associated latent heating, occurs whenever the relative humidity exceeds 100%. The locations of heating in our model evolve with (and are determined by) the flow; unlike many previous studies, our forcing scheme does not impose any zonal symmetry on the system. In our simulations, the moist convection and its associated latent heating generates numerous eddies that drive multiple zonal jets. The dynamics in our simulations are self-maintained and, for plausible water abundances, produce roughly 20 jets with equatorial superrotation on Jupiter and Saturn and 3 jets with equatorial subrotation on Uranus and Neptune. The jet widths are controlled by the Rhines scale. These simulated jets resemble the observed jet patterns on the giant planets. Our simulations thus suggest that moist convection plays an important role in pumping jets on giant planets, and moreover provides a possible coherent explanation for superrotation on Jupiter/Saturn and subrotation on Uranus/Neptune. The moist-convection events in our simulations also bear an encouraging resemblance to observed moist-convection events on Jupiter and Saturn.
P11A-1257
Correlative Analysis of the Interaction of a Large Red Oval with the Great Red Spot and Oval BA in May - August 2008: Local Meteorology
We acquired visible, near- and mid-infrared observations via a coordinated global campaign to observe the close encounter between the Great Red Spot and Oval BA, involving Hubble Space Telescope (HST), NASA/InfraRed Telescope Facility (IRTF)(NSFCAM2/MIRSI), Telescopio Nazionale Galileo (TNG)/NICS (with adaptive optics), Very Large Telescope (VLT)/VISIR, NOAJ/Subaru/COMICS and UKIRT/UIST (with tip-tilt). Although initial results indicate that changes in the albedo of the visible cloud deck and thermal field in the troposphere recovered shortly after the passage of the large red anticyclonic oval between the GRS and Oval BA, at near-infrared wavelengths, changes in the atmosphere are still occurring (as of this writing). The interaction started late June 2008. The small red oval, drifting eastward toward the GRS, was entrained in the peripheral flow south of the GRS. After being squeezed between the GRS and Oval BA, the elongated large red oval emerged, with part of it following a spiral path as it was entrained in the northern flow around the GRS, while another portion reformed itself slightly north of its pre-interaction latitude, indicating that the nature of the passage may have occurred at higher altitudes. By 10 July 2008, remnants of the red oval were still recognizable as a distinct feature as high-altitude particles in the near infrared. The red oval and Oval BA continued to drift eastward of the GRS. By 27 July 2008, the GRS and Oval BA were still observable at as relatively bright, discrete features in the reflected sunlight, with only the GRS showing a bright 4.78-micron annulus. However, the pre-encounter 4.78-micron bright annulus of the large red oval was not detectable. We shall present correlative analysis of the local meteorology of the interaction in terms of variations of the wind field, spectral composition, and tropospheric properties and compare with similar properties prior to the current interaction.
P11A-1258
Atmospheric Escape from GL876d
GL876d is a ~7.5 Earth-mass planet surrounding an M-star with low-to-intermediate level of chromosphere activity at an orbit closer than the habitable zone of the star. The moderate EUV flux from its star and its large mass make the planet's upper atmosphere relatively constrained. The planet's large mass also suggests a strong intrinsic magnetic field, which could resist the stellar wind at large distances. Both aspects make it promising for GL876d to maintain its atmosphere. We will report both the observational and the theoretical studies on the potential atmosphere of GL876d. In addition, we will study the conditions necessary for planetary atmospheres to be stable against atmospheric escape over long geological timescales.