SA13A-1112 INVITED 1340h
The Solar Wind Interaction With Mars and its Implications for Atmospheric Loss
The evolution of a planetary atmosphere is determined by the balance between source processes and loss processes such as escape of atmospheric species from the top of the atmosphere. Escape mechanisms can include: (1) hydrodynamic escape, (2) Jeans, or thermal, escape, (3) photochemical escape, (4) escape via planetary ion pickup by the solar wind (or other external plasma flow), and (5) escape and loss via sputtering due energetic ion impact on an atmosphere. Several of these escape mechanisms are applicable to Mars. In particular, loss of atmospheric species associated with the solar wind interaction with this planet is known to be important. The solar wind interaction with Mars and how this interaction affects the atmospheric loss will be reviewed. How atmospheric loss mechanisms might vary over the lifetime of the solar system will also be discussed.
SA13A-1113 1340h
3D multi-fluid simulations of the ionospheric loss rates at Mars from nominal solar wind conditions to magnetic cloud events
3D multi-fluid simulations are used to investigate the behavior of solar wind protons, and ionospheric oxygen ions for a variety of solar wind conditions, ranging from quiet to storm-like conditions. Ion loss rates and solar wind precipitation rates are calculated for four different anomalous magnetic field orientations. Storms conditions are shown to produce nearly an order of magnitude increase in the outflow rate. Comparison of loss rates for different orientations of the magnetic anomalies relative to the incident solar wind direction show that the anomalies have little effect on loss rates for quiet magnetospheric conditions. But some solar wind conditions lead to dynamic reconnection of the IMF to the anomalous magnetic field, which in turn, can drive additional solar wind precipitation and ionospheric loss.
SA13A-1114 1340h
LOW-ALTITUDE IONOSPHERIC PLASMA ENERGIZATION AT MARS - ASPERA-3 FINDINGS
The Analyzer of Space Plasma and Energetic Atoms (ASPERA) on-board the Mars Express spacecraft (MEX) found that the solar wind plasma and accelerated ionospheric ions may be observed all the way down to the MEX pericenter of 270 km above the dayside planetary surface. This is very deep in the ionosphere, implying a direct exposure of the martian topside atmosphere to solar wind plasma forcing. The solar wind forcing results in an energization of ionospheric plasma. The low-altitude ion energization and outflow near Mars is surprisingly similar to the ion energization and outflow over the strongly magnetized planet Earth - from narrow "monoenergetic" ion beams to beams with a broad energy distribution. The ion outflow near the planet is in the direction of the external sheath flow, i.e. the ion energization is the result of a localized and direct solar wind momentum exchange. On the other hand the distribution of the energized plasma implies similar energization processes like that over the Earth, i.e. energization in a magnetized environment by waves and/or parallel (to B) electric fields. But the boundary conditions for Martian plasma energization is different from that of the Earth - a weak local magnetic field and penetration of solar wind plasma deep into a cold and dense ionospheric plasma. This report will be discuss results from the first year of ASPERA-3 investigations on the low-altitude energization of plasma near Mars.
SA13A-1115 1340h
Effects in the Martian Ionosphere Caused by the Solar Cycle Variation of the EUV Flux and Solar Wind Dynamic Pressure
Solar wind electrons can penetrate the Martian ionosphere outside of the regions of the strong crustal fields. Sometimes, in the northern hemisphere of Mars, the peak electron density does not follow the EUV radiation flux variations, as was found recently. The SW dynamic pressure is assumed to be high in such periods. The SW dynamic pressure reaches its maximum several years after solar activity begins to decline. Thus, following the solar activity maximum, the combination of decreasing EUV flux and increasing SW dynamic pressure may result in conditions such that the peak electron density is not correlated with the EUV radiation flux in that period. For the time period November 2000 - December 2002, electron density profiles have been derived from MGS radio occultation data. We study the correlation between the EUV flux using the E10.7 index and the adjusted peak electron density. Next, we use ACE SWEPAM data (daily averages) to study anti-correlation of the SW dynamic pressure and EUV radiation flux (E10.7 index) in this same time period. Finally, we validate the assumption that the effects of an increasing SW dynamic pressure can reduce any apparent correlation between the peak electron density and EUV radiation flux during the period following solar activity maximum.
SA13A-1116 1340h
Measuring Mars' Upper Atmospheric Density from 170-260km with Electron Reflectometry: Seasonal Response & South Polar Winter Warming
Using MGS MAG/ER data and the technique of electron reflectometry, we present density measurements of Mars' nighttime upper atmosphere in the southern hemisphere between 1999 and 2004. For electron motion on open magnetic field lines connecting the solar wind to remanent crustal magnetization, reflection from magnetic gradients and absorption due to collisions with atmospheric neutrals results in a characteristic pitch angle-dependent attenuation in the electron flux, known as a loss cone. We develop a kinetic model of this interaction, assuming the validity of spherical harmonic expansions of the crustal field in regions of intense magnetization (such as Terra Sirenum), and assuming an adjustable, 2-species, isothermal atmosphere above 170km. Loss cones in the data are fitted to this model to constrain densities and temperatures in the upper atmosphere. Our range of sensitivity in altitude is based on the absorption probability of incident solar wind electrons and is ~170-260km. ~50,000 reliable measurements were made and binned by time and latitude. Major findings include 1) expected seasonal expansion/contraction of the upper atmosphere, 2) latitude dependence of densities and temperatures Showing southern winter polar warming, 3) observation of transition Altitude from CO2-dominance to O-dominance at ~200km. We present these findings and compare to the Mars Thermosphere Global Circulation Model (MTGCM) These measurements are important for improved understanding of the processes affecting Mars' upper atmosphere and of the orbital stability of future low-altitude spacecraft such as the 2005 Mars Reconnaissance Orbiter.
SA13A-1117 1340h
Observations from the ASPERA-3 ELS of Photoelectrons in the Tail of Mars
The Analyzer of Space Plasmas and Energetic Atoms (ASPERA-3) experiment is currently sampling plasma in the vicinity of Mars. ASPERA-3 determines the electron, ion, and neutral particle components of the plasma using four instruments: Electron Spectrometer (ELS), Ion Mass Analyzer (IMA), Neutral Particle Imager (NPI), and Neutral Particle Detector (NPD). The ELS instrument measures 128 logarithmically spaced samples of the electron spectrum between 1 eV and 20 keV every four seconds. Its 8% energy resolution is used to resolve the carbon dioxide photoelectron peaks which are a dominant feature in the Martian Ionosphere. These same photoelectron signatures from carbon dioxide are also detected at distances greater than 1000 km in the Martian tail, providing experimental observation of electron escape from Mars. Implications of atmospheric escape will be discussed based on these findings.
SA13A-1118 1340h
Structure of the Martian Wake
The first results from the ion mass analyzer IMA of the ASPREA-3 instrument on-board of the Mars Express spacecraft show that the martian wake consists of two different ion regimes. In the first regime ions of planetary origin form the layer adjacent to the magnetic pile-up boundary. These ions are accelerated to relatively high energies and exhibit a gradual decreasing of energy down to the planetary tail. The second plasma regime is observed in the planetary shadow. The heavy ion are accelerated to the energy of the solar wind protons. Obviously the acceleration mechanism is different for the different plasma regimes. Study of two plasma regimes in the frame referred to interplanetary magnetic field (IMF) direction (we used MGS magnetometer data to obtain the IMF orientation) clearly shows their spatial anisotropy. The monoenergetic plasma in the planetary shadow is observed only in the narrow angular sector around the positive direction of the interplanetary electric field.
SA13A-1119 1340h
Use of Martian Magnetic Field Topology as an Indicator of the Influence of Crustal Sources on Atmospheric Loss
Mars lacks a global magnetic field, and therefore the solar wind interacts directly with the atmosphere over much of the planet. Over some regions, however, Mars' crustal magnetic field is strong enough to locally stand off the solar wind to altitudes >1000 km, shielding the enclosed atmosphere. Like Earth's polar cusp regions, Martian crustal fields do not act as perfect shields for the atmosphere. At times, crustal magnetic field lines connect with the interplanetary magnetic field (IMF), providing conduits for charged particle exchange between the solar wind and lower ionosphere. These open field lines have been observed by spacecraft and predicted by models. The volume of atmosphere protected by crustal magnetic fields as well as the extent and variability of open field lines has implications for atmospheric escape to space and the energetics of the upper atmosphere. Electron and magnetic field data from the Mars Global Surveyor MAG/ER instrument provide a means of measuring the influence of crustal sources on nonthermal escape processes. The angular distribution of electrons observed by the electron reflectometer (ER) can be used to determine where and when crustal magnetic field lines are connected to the IMF. Using four years of data from the MGS mapping orbit, we have classified ER observations of electrons at 190 eV according to the topology of the magnetic field. This extended data set allows us to quantify the effects of a number of parameters that control magnetic field topology at 400 km. We will show where open and closed field lines are likely to occur, and how changes in solar wind pressure or the direction of the interplanetary magnetic field affect the locations of open field regions. From these results we will calculate the fraction of Mars' atmosphere that is shielded from the solar wind under different conditions, and the fraction of the lower ionosphere accessible through cusps. We will relate these results to the present day atmospheric escape rates in order to quantify the effect that crustal magnetic fields have on the efficiency of solar wind related atmospheric loss processes.
SA13A-1120 1340h
Magnetic Oscillations at Mars During the October 2003 Solar Storm
The powerful x-class superflare which occurred on the Sun on October 28, 2003 had important effects on the plasma environments of both the Earth and Mars. We present here observations of the effects at Mars from the Mars Global Surveyor (MGS) Magnetometer/Electron Reflectometer experiment. In particular we focus on the changes in the nature of the magnetic oscillations observed during the passage of the solar storm. We find that the types of plasma oscillations observed at the MGS mapping altitude of 400 km are similar to the oscillations observed in the magnetosheath (which is normally at altitudes higher than 1000 km). This result is consistent with other observations that show that the magnetosheath altitude is lowered during the solar storm. We also observe relatively strong oscillations in the nightside tail region where such oscillations are rarely observed. Such observations have implications for atmospheric loss since they raise the possibility that large portions of the atmosphere are exposed to the solar wind and are hence subject to pickup escape during such large storm events.
SA13A-1121 1340h
Search for Gas/Dust Escape From Phobos and Deimos Using MGS MAG/ER Observations
More than 600 elliptical aerobraking and science phasing orbits made by Mars Global Surveyor (MGS) early in the mission provide unprecedented coverage of the solar wind in the vicinity of the Martian moons Phobos and Deimos, as well as their orbits. We have performed a comprehensive survey of electron and magnetic field perturbations in the solar wind to search for possible signatures of solar wind interaction with dust or gas escaping from the moons. Contrary to previous reports based on PHOBOS-2 spacecraft data we find that the solar wind perturbations are distributed quite equally over the spatial area covered by MGS and there are no clustering of perturbations near Phobos, Deimos, their orbits, or their wakes. The solar wind perturbations are most likely caused by the multiple bow shock crossings, foreshock turbulence, and hot flow anomalies. We conclude that the density of the gas/dust escaping the moons must be too low to induce detectable electron and magnetic field perturbations in the solar wind. In essence we do not find any evidence for outgassing/dust escape from the Martian moons.
SA13A-1122 1340h
Oxidant Enhancement in Martian Dust Devils and Storms: I. Storm Electric Fields and Electron Dissociative Attachment
Laboratory studies, computer simulations, and desert field tests indicate that aeolian dust transport can generate atmospheric electricity via contact electrification or "triboelectricity". In convective structures such as dust devils or storms, grain stratification (leading to charge separation) gives rise to an overall electric dipole moment to the aeolian feature, similar in nature to the dipolar electric field generated in terrestrial thunderstorms. Previous simulation studies indicate that this storm electric field on Mars can approach the ambient breakdown field strength of 20 kV/m. Noteable, in terrestrial dust devils, coherent dipolar electric fields have been measured to near 20 kV/m. Given the expected electrostatic fields in Martian dust devils and storms, electrons in the low pressure CO2 gas can be energized via electric fields to values exceeding the electron dissociative attachment energy of both CO2 and H2O, resulting in the formation of new chemical products CO and O- and OH and H- within the storm. Using a collisional plasma physics model, we present a calculation of the CO/O- and OH/H- reaction and production rates. We demonstrate that these rates vary geometrically with the ambient electric field, with substantial production of dissociative products when fields approach breakdown levels of 20-30 kV/m. These storm-related chemical products are key ingredients for the generation of oxidants which can ultimately affect the habitability of Mars, as discussed in the following companion presentation.
SA13A-1123 1340h
Oxidant Enhancement in Martian Dust Devils and Storms II
The failure of the Viking Life Sciences Experiments to find organics on Mars has been suggested as being due to the presence of oxidants. In particular, hydrogen peroxide (H$_2$O$_2$) has been proposed as the most likely oxidizer of the surface of Mars. H$_2$O$_2$ was detected in 2003, and the measured mixing ratio, 20-30 ppb, agrees well with global photochemical models. However, this abundance of H$_2$O$_2$ is perhaps not large enough to account for the above Viking result. In this presentation, we will discuss a new mechanism that can produce substantially greater abundance of H$_2$O$_2$. Delory et al. (see previous companion presentation) have shown that triboelectric fields in martian dust devils and storms may be near 20 kV/m, and have determined the production rates of OH and O- from dissociation of H$_2$O and CO$_2$ via electric field driven electrons. Using a chemical model, we calculate that the abundance of H$_2$O$_2$ due to electrochemistry in dust devils and storms greatly exceeds that produced photochemically. Since the aeolian processes must have been prevalent throughout the martian geologic history, this effect of H$_2$O$_2$ enhancement, together with the large UV radiation reaching the martian surface, implies that the martian surface and near-surface environment are unlikely to be hospitable to life.
SA13A-1124 1340h
Considerations for a Radar System to Detect an Ocean Underneath the Icy Shell of Europa
The detection of an ocean underneath Europa is one of the primary objectives of the Jupiter Icy Moons Orbiter (JIMO) mission. An orbiting surface penetrating radar has the potential of providing that measurement thus yielding information regarding the possibility of life support on Europa. Radars in the MHz range have successfully monitored the kilometer-deep ice shelves of Greenland and Antarctica, including the detection of Lake Vostok (and others) below an ice sheet thickness of about 4 km. The performance of a radar system orbiting Europa will be subject to several potential complications and unknowns. Besides ionospheric dispersion and the actual depth of the ocean, which is estimated between 2 and 30 km, major unknowns affecting radar performance are the temperature profile, the amount of salt and other impurities within the ice crust as well as the surface roughness. These impurities can in part be produced at the highly irradiated surface by magnetospheric interactions and transported downward into the ice crust by geologic processes. The ionospheric interference must also be modeled from effects of these interactions on production of the thin neutral atmosphere and subsequent ionization of the neutrals. We investigated these uncertainties through radar simulations using different surface and ice characteristics over a frequency range from 10 to 50 MHz. The talk will present results from these simulations discussing potential limitations.
SA13A-1125 1340h
Habitability in High Radiation Environments: The Case for Gaia at Europa
In the paper of Cooper et al. (2001) we concluded, in relation to our work on magnetospheric irradiation of Europa and the other icy galilean moons of Jupiter, that 'icy satellites with significant heat, irradiation, and subsurface water resources may provide common abodes for life throughout the universe'. This expanded the original proposal of Chyba (2000) and his later works that radiolytic production of oxidants and simple hydrocarbons on Europa's icy surface could support evolution and survival of life within a Europan subsurface ocean. In the general case of icy planets and moons the radiation environment does not have to interact directly with the surface but could also provide energy for life through radiation-induced chemistry in thick atmospheres chemically coupled to icy surfaces with hydrocarbon reservoirs as on Titan. The Gaia model for Earth implies that the entire planet operates with atmospheric, geologic, and geochemical processes conducive to life. Essential requirements for Gaia are an oxidizing atmospheric environment at planetary surfaces, where oxidants like molecular oxygen are produced by radiation processes (mediated by photosynthetic chemistry on Earth but more directly produced by radiolysis on Europa), reservoirs of liquid water and hydrocarbons on or below the surface, other reduced materials in the interior, and geologic processes which drive chemical exchange between the chemically oxidized surface and reduced interior environments. At Europa a thin oxygen atmosphere is observed and arises from magnetospheric interaction, and there is much evidence for active resurfacing likely related to solid-state convection and diapiric processes within a thick crust of soft ice overlying a liquid ocean. These processes on Europa are analogous to that of the tectonic conveyer belt that continually recycles carbon, oxygen, and other essential materials for life between the atmosphere, surface, and interior on Earth. The ice crust at Europa could be thin and support more direct and rapid chemical exchange between the highly irradiated surface and the ocean, but this is not required for life since deep convection can accomplish the same exchange over thousands to millions of years. Hydrocarbons are likely present both from moon formation and later delivery to the surface by impacts of cometary bodies. More recent work from Galileo suggests strong associations between spatial distributions of brine-like materials on Europa's surface and geologic structures related to convection in the ice crust, tidal heating, and the underlying ocean. The effect of the brines on convection may be analogous to thermahaline circulation in the terrestrial oceans. The detected hydrated sulfates (including briny salts and sulfuric acid hydrates) on Europa's surface can at least in part be attributed to input of iogenic sulfur from the Jovian magnetosphere and radiolytic processing. The needed conveyer belt process within Europa could then be substantially driven by surface interaction with the magnetosphere, i.e. there could be radiation-driven geology, and this could make a critical contribution to astrobiological habitability within Europa. In the sense of Gaia and with reference to Edgar Allan Poe's famous work, Europa may have a tell-tale beating heart, and future missions such as the Jupiter icy Moons Orbiter (JIMO) will need to survive, look through, and exploit the local magnetospheric, ionospheric, and atmospheric environments to sense its physical, chemical, and electromagnetic presence. References: Cooper, J. F., et al., Icarus, 149, 133-159, 2001; Chyba, C. F., Nature, 403, 381, 2000.
SA13A-1126 1340h
Modeling Titan's Upper Atmosphere with 1-D and 3-D Models:
Titan, the 2nd largest moon in the solar system, contains a substantial atmosphere like present-day Earth. However, its unique nitrile and hydrocarbon composition appears analogous to that of a pre-biotic Earth (Yung, Allen and Pinto 1984). Finally, Titan's position within Saturn's magnetosphere provides a unique plasma environment for further investigation. Given all of these qualities, studying this satellite through numerical models represents an intriguing endeavor in comparative planetary atmospheres. The arrival of Cassini-Huygens at Titan in late October will provide data crucial to better understanding the vertical chemical and thermal structures. In anticipation of this data, a 1-D model of Titan's upper atmosphere has been developed. Currently, this model includes solar heating, coupled photochemical and thermal conduction routines, and a full radiative transfer code of HCN rotational cooling. Also, magnetospheric forcing has been incorporated making use of offline calculations from the Cravens ionospheric model. Results from the 1-D model will be presented, including temperature and density profiles for various solar and Saturn magnetospheric conditions. In addition, preliminary simulations from a new 3-D Titan TGCM will be presented, although the model remains in an early stage of development.
http://data.engin.umich.edu/tgcm_planets_archive/thermo.html
SA13A-1127 1340h
Sugars, Alcohols, and Cometary Astrobiochemistry
Radio and IR observations have revealed that a rich organic chemistry exists in comets and in a variety of interstellar regions. Among the organic molecules detected are acids, alcohols, aldehydes, ketones, and nitriles. The simplest sugar, glycolaldehyde, has been reported (Hollis et al., ApJ, 2000, 540, L107), as has an amino acid, glycine (Kuan et al., ApJ, 2003, 593, 848; but see Hollis et al., ApJ, 2003, 588, 353). Gas-phase reactions to produce many of these molecules are not well understood, and solid-phase chemistry is thought to make an important contribution. To better understand organic chemistry in cold cosmic environments, we have performed photo- and radiation chemical experiments on icy materials at 10 - 100 K. Gas-phase molecules are frozen in a vacuum chamber, and then exposed to either MeV protons or vacuum-UV photons to mimic cosmic-ray bombardment or cosmic-UV exposure, respectively. Changes in ice composition are followed in situ with IR spectroscopy. In this AGU presentation we will describe our latest results for glycolaldehyde, as well as a few prebiological organics. Solid-state IR spectra and reaction pathways will be presented, and predictions will be made for the chemical composition of selected Solar System objects. -- This research is funded through NASA's Planetary Atmospheres and SARA programs, and through the NASA Astrobiology Program under RTOP 344-53-51-01 to M. J. Mumma (NASA GSFC).
SA13A-1128 1340h
Formation of Organic Molecules in Photo-evaporating Pre-planetary Disks
A significant fraction of stars forming today are born in dense OB associations such as Orion. Photo-evaporation by the UV radiation of nearby O and B stars can destroy disks around low-mass stars on $10^5-10^6$ yr time-scales, dramatically affecting the formation of planets. However, the very same UV radiation that destroys the disks may also encourage the formation of organic molecules in these disks. UV photolysis of ices has been shown to produce solid-phase organic molecules, from simple CO and CH$_4$ to amino acids and alcohols (e.g., Bernstein et al 2002). If these compounds -- and the circumstellar disk in which they are formed -- can survive photo-evaporation, then they could play a role as biotic precursors. We have modeled the formation and loss rates for simple organic molecules produced in UV-irradiated circumstellar disks. The stability of these molecules depends on their location within the disk and the time delay between disk formation and the onset of photo-evaporation. We will identify the parameters of circumstellar disks which are most likely to allow formation and retention of organic compounds.