SA12A-01 INVITED 10:20h
Using Magnetic Induction to Explore Liquid Water in icy Satellites
Magnetic field observations made by Galileo in the vicinity of Europa, Ganymede and Callisto showed that strong magnetic induction fields emanate from these moons in response to the rotating magnetic field of Jupiter. Further analyses have shown that the magnetic induction fields are consistent with the presence of subsurface liquid oceans in these moons. Many independent lines of geological and geophysical evidences support this conclusion for Europa and Ganymede. For example, the very young age (< 20 my) of Europa's surface and the isostatic relaxation of recent impact craters suggest that Europa's surface is renewed by processes operating in a low viscosity region beneath the surface. Similarly, the formation of pull-apart bands on Europa and Ganymede are facilitated by the presence of a liquid or a very low viscosity material beneath a thin crust. The induction signature from Callisto, however, is a complete surprise. We will discuss the present status and future plans of magnetic induction studies devoted to exploring liquid water in the icy Galilean satellites. Next, we will discuss how one could use the magnetic induction to explore the interiors of icy satellites in other solar system bodies such as Titan and other icy moons of Saturn. We will discuss how the uncertainty in the determination of the thickness of the ocean could be minimized by making magnetic induction measurements at multiple frequencies. Finally, we discuss how the orbital measurements of the magnetic field could be combined with observations made on the surfaces of the icy moons to yield a more robust data set which can be inverted uniquely for ocean conductivity and thickness.
SA12A-02 INVITED 10:35h
Space Weather Connections to Clouds and Climate
There is now a considerable amount of observational data and theoretical work pointing to a link between space weather and atmospheric electricity, and then between atmospheric electricity and cloud cover and precipitation, which ultimately affect climate and the biosphere. Studies so far have been largely confined to the Earth, but may be applicable to all planets with clouds in their atmospheres. The current density Jz, that is the return current flowing downward through clouds in the global circuit, is modulated by the galactic cosmic ray flux; by solar energetic particles; by the dawn-dusk polar cap potential difference; and by the precipitation of relativistic electrons from the radiation belts. The flow of Jz through clouds generates unipolar space charge, which is positive at cloud tops and negative at cloud base. This charge attaches to aerosol particles, and affects their interaction with other particles and droplets. Ultrafine aerosol particles are formed around ions and are preserved from scavenging on background aerosols, and preserved for growth by vapor deposition, by space charge at the bases and tops of layer clouds. There is electro-preservation of both ultrafines and of existing CCN that leads to increases in CCN concentration, and increases in cloud cover and reduction in both droplet size and precipitation by the `indirect aerosol effect'. For cold clouds and larger aerosol particles that act as ice forming nuclei, the rate of scavenging of the IFN by large supercooled droplets varies with space charge. Changes in space weather affect both ion production and Jz in planetary atmospheres. In addition, changes in cosmic ray flux affect conductivity within thunderclouds and may affect the output of the thundercloud generators in the global circuit. Thus all four processes, (a) ion-induced nucleation, (b) electro-preservation of leading to increases in CCN concentration and the indirect aerosol effect, (c) contact ice nucleation affecting the production of ices, (d) cosmic ray effects on the generators of the global circuit, are potential links between space weather and life on planets.
http://www.utdallas.edu/dept/physics/Faculty/tinsley/tinsley.htm
SA12A-03 INVITED 10:50h
ASPERA-3 First Results on Energetic Neutral Atom Imaging at Mars
The ASPERA-3 instrument onboard ESA Mars Express mission comprises four instruments, two energetic neutral atom (ENA) sensors and an electron and ion spectrometer. The Neutral Particle Imager (NPI) provides measurements of the integral ENA flux in the energy range 0.1 - 60 keV with no mass and energy resolution but a comparatively high angular resolution of $4.6\deg\times11.5\deg$. The Neutral Particle Detector (NPD) provides measurements of the ENA flux in the energy range 0.1 - 10 keV, resolving velocity and mass (H and O) with a coarse angular resolution of $5\deg\times30\deg$. The ENA detection technique is based on the atom - surface interaction. We review the latest ASPERA-3 results on ENA measurements at Mars focusing on three types of observations (1) hydrogen atoms backscattered from the Martian upper atmosphere, (2) ENA occultation, and (3) possible observations of oxygen ENAs. NPD sensors detected intensive fluxes of hydrogen atoms at an energy of 1 keV from the nadir direction around pericenter (approximately 270 km). The atoms originate from the solar wind protons precipitating on the upper atmosphere and going through a cascade of charge exchange - stripping reactions and finally elastically scattered back. This interpretation is confirmed by local measurements by the ASPERA-3 ion spectrometer of solar wind protons at heights as low as pericenter. ENA images of this component indicate that the solar wind can reach large areas of the upper atmosphere delivering mass, energy, and momentum to atmospheric atoms. ENA resulting from the charge - exchange of the undisturbed solar wind on the exosphere outside the Martian magnetosheath can propagate through the upper atmosphere at the terminator and experience scattering within a large solid angle. The fluxes of these scattered ENAs are observed in the sunward direction deep inside the eclipse resembling occultation geometry. The profile of the signal can be used to define atmospheric parameters at the terminator region. Finally, fluxes of very slow atoms recorded in NPD time-of-flight (TOF) spectra may result from low energy oxygen atoms (down to the instrument lower energy limit for oxygen, approximately 300 eV) emitted from altitudes of few hundred km.
SA12A-04 INVITED 11:05h
Latest Results From ASPERA and Implications for Atmospheric Loss
Mars does not have a strong global intrinsic magnetic field and, therefore, the solar wind can flow close to the planets in high neutral density regions. Because of the formed direct interaction between the atmosphere/exosphere and the solar wind, the ionized atmospheric neutrals can be picked up by the solar wind. Charge exchange between the solar wind protons and planetary neutrals, instead, produce energetic neutral hydrogen atoms (H-ENA) which are the manifestation of the direct interaction between the solar wind and planetary neutrals. Picked-up planetary O$^+$ ions in turn form energetic neutral oxygen atoms (O-ENA) via charge exchange process. The ion escape, H-ENAs, O-ENAs and electrons are currently measured at Mars by ASPERA-3 (Analyzer of Space Plasmas and Energetic Atoms) instrument onboard MarsExpress orbiter. The measurements started in January 2004. ASPERA-3 contains four individual subunits. Ions are measured by IMA (Ion Mass Analyzer), energetic neutral atoms by NPI (Neutral Particle Imager) and NPD (Neutral Particle Detector), and electrons by ELS (Electron Spectrometer) instruments. In this presentation we (1) Summarize some of the basic new measurements associated with Martian atmospheric loss made by ASPERA-3/MarsExpress instrument, and (2) Use global Mars-solar wind interaction models to interpret the measurements.
SA12A-05 INVITED 11:20h
Mars Atmospheric Evolution : What Can Dynamical Simulations Tell Us?
The history of the martian atmosphere and climate over time cannot be properly understood without knowing the role of loss of water and other volatiles to space. Furthermore, the martian climate system is an integrated one, from below the surface to above the exobase. Thus, volatile exchange and loss rates cannot be properly investigated without determining the role of the upper atmosphere and its coupling below (e.g. surface-atmosphere interactions, dynamics and energetics, dust storms) and influences above (e.g. solar wind interaction). Dynamical models (General Circulation Models) for the entire martian atmosphere ($\sim$0-250 km) are beginning to be developed and exercised that address global energetics, chemistry, and dynamics. These models capture the key processes coupling the Mars lower and upper atmospheres, basic photochemistry, as well as solar wind interaction processes. Important volatile loss processes include : (a) solar wind stripping (i.e. pick-up ion loss), (b) photochemical loss (e.g. dissociative recombination of O$_2$+), (c) thermal loss (i.e. Jeans escape of light species), and (d) impact ejection of the atmosphere (i.e. sputtering). Each of these processes depends on the intensity of solar EUV radiation, which affects thermospheric temperatures and densities, ionospheric properties, exosphere structure, and ultimately the fluxes of escaping atoms and ions. General Circulation Models (GCMs) provide the global context in which to understand present day escape processes and extrapolate these processes into the past for ancient solar and martian conditions. Here we consider the effects of higher solar EUV fluxes of the ancient sun upon the martian thermosphere-ionosphere structure. A reasonable characterization of this atmospheric structure, and an understanding of the underlying process that drive its variations, provide the foundation upon which escape rates can be estimated over Mars history. The combination of key spacecraft measurements (e.g. exobase temperatures and densities, ionization rates, hot atom distributions, pick-up ion production rates) and detailed models (e.g. GCMs, ionospheric models, sputtering codes, MHD codes) are needed to quantify present-day volatile escape rates. Model extrapolation of these escape rates into the past and integration of these rates over time yields an estimate of the volatile (e.g. water) loss over most of martian history.
SA12A-06 INVITED 11:35h
The Role of Ionosphere/Thermosphere Coupling Processes in the Escape of Species from Mars
We discuss coupled ionosphere/thermosphere models of Mars and implications for the photochemical escape processes of the atomic species O, C and N. Escape also occurs by ion outflow, and the relative rates of ion loss are determined by ion-neutral chemistry. The escape flux of ions has been computed by a number of investigators, including Ma et al. and S. Brecht, and some measurements are available from the Phobos spacecraft. The relative escape rates of ions depend on the composition of the ionosphere, which is determined by ion-neutral chemistry. Photochemical escape of atoms often occurs by processes that involves ions, such as dissociative recombination of O$_2^+$, N$_2^+$, CO$^+$, and NO$^+$, which yields fragments of various energies, many of which exceed the escape energy for Mars. Ions other than N$_2^+$ are formed mostly or partially by ion-molecule reactions. Except for NO$^+$, the ions may be destroyed by ion-molecule reactions also. The ratio of ion-molecule reactions to dissociative recombination depends on the presence or absence of neutral species with which the ions can react. At high altitudes, the densities of neutral species is smaller than at lower altitudes. Therefore, above the ``exobase", dissociative recombination may be more important. Since many ions react with H$_2$, its density profile is important in determining the photochemical escape of heavy ions. Earlier in the history of Mars, the atmosphere may have been more reducing. A larger abundance of H$_2$ would decrease the densities of O$^+$, N$_2^+$, CO$^+$, and CO$_2^+$, which react with H$_2$. We model the ionosphere and photochemical escape mechanisms for the higher solar fluxes and more reducing atmosphere of early Mars.
SA12A-07 INVITED 11:50h
Solar Wind Interactions with Planetary Atmospheres
When considering the issue of life on Mars, a variety of issues come to mind. One of them is the evolution of the atmosphere of the planet. Planets with an intrinsic magnetic field are protected for the most part from the solar wind, leaving just the solar radiation flux as method for modifying the atmospheric and ionospheric chemistry. In the case of Mars, there is no planet wide magnetic field. There are conjectures that at one time Mars did indeed have a significant magnetic field. If it did the dynamo has long since died. This situation means that the atmosphere/ionosphere of Mars is in direct contact with solar wind plasma and fields. Further, the bow shock of Mars does not protect it very well from incoming energetic particles. This situation means that unlike Earth, Mars loses it ionosphere/atmosphere by scavenging via electric fields from the solar wind. Further, the ionosphere/atmosphere also experiences an additional source of energy/heat. Simulations have shown that the deposition energy can approach and in some cases exceed that of the solar EUV flux. Thus the atmosphere is subject heating even on the night side because of the large ion gyroradius. In this talk the physics of this situation will be discussed. It will be coupled with the idea that Mars may have lost it's water via the atmospheric loss mechanism driven by the solar wind interactions. The consequences for life are subject to conjecture, but it is clear that these enhanced loses and heating had a profound affect on the atmosphere/ionosphere of Mars.
SA12A-08 INVITED 12:05h
Cosmic Ray and Solar Energetic Particle Interactions with the Surface of Mars and the Moon
Important factors that determine the suitability of a planetary environment to spawn life as we know it are the availability of water and the level of biologically destructive radiation near the surface. Earth satisfies both requirement with ample margin. Although Mars cannot presently support life on or above its surface because of a lack of stable water and intense fluxes of UV from the Sun, very little is known about conditions beneath its surface. A complete evaluation of the cosmic-ray and solar energetic particle-induced radiation environment near the Martian surface has not been conducted. In this talk we will present the radiation dose from neutrons between 0 and 6 MeV in the low-altitude circular orbit of Mars Odyssey, and on and just below the surface of Mars from measurements made using the Mars Odyssey Neutron Spectrometer. These doses will be compared to similar ones derived for the Moon from the Lunar Prospector Neutron Spectrometer. This dose is controlled by the water-equivalent-hydrogen content of surface soils and the thickness of the atmosphere, the dose decreases with increasing atmospheric thickness and soil water content. As a result for Mars, the dose rate is least for latitudes poleward of +55 degrees (the water content is high and the topography is low), followed closely by that at latitudes poleward of -55 degrees (both the water content and topography are high), and is highest within the Tharsis plateau where the topography is high and the water content is low.