P23A-01
Venus Express Measurements of Whistler Mode Waves Indicative of Lightning
The occurrence of lightning in a planetary atmosphere enables chemical processes to take place that would not occur under standard temperatures and pressures. While much evidence has been reported for lightning on Venus, some searches have been negative and the existence of Venus lightning has remained controversial. A definitive test for lightning would be the confirmation of whistler-mode waves propagating from the atmosphere to the ionosphere. The Venus Express magnetometer was equipped to transmit data at up to 128 Hz to make such measurements. Herein we report observations by the Venus Express fluxgate magnetometer that reveal strong circularly polarized electromagnetic waves with frequencies near 100 Hz. The waves appear as bursts of radiation lasting 0.25 to 0.5 s. These waves have the expected properties of whistler mode signals generated by lightning discharges in the Venus clouds, and confirm the interpretation of the existence of pervasive lightning at Venus based on the observation of the electric counterpart of these waves with the Pioneer Venus Orbiter.
P23A-02
Chemical Kinetic Model for the Lower Atmosphere of Venus
A self-consistent chemical kinetic model of the Venus atmosphere at 0-47 km has been calculated for the first time. The model involves 76 reactions of 27 species. Chemical processes in the atmosphere below the clouds are initiated by photochemical products from the middle atmosphere (SO3, CO, Sn), thermochemistry in the lowest 10 km, and photolysis of S3. The sulfur bonds in OCS and Sn are weaker than the bonds of other elements in the basic atmospheric species on Venus; therefore the chemistry is sulfur-driven. Sulfur chemistry activates some H and Cl atoms and radicals, though their effect on the chemical composition is weak. The lack of kinetic data for many reactions presents a problem that has been solved using some similar reactions and thermodynamic calculations of inverse processes. Column rates of some reactions in the lower atmosphere exceed the highest rates in the middle atmosphere by two orders of magnitude. However, many reactions are balanced by the inverse processes, and their net rates are comparable to those in the middle atmosphere. The calculated profile of CO is in excellent agreement with the Pioneer Venus and Venera 12 gas chromatographic measurements and slightly above the value from the nightside spectroscopy at 2.3 μm. The OCS profile also agrees with the nightside spectroscopy which is the only source of data for this species. The model predicts 1.5 ppm of H2S confirming the questionable detection by the Pioneer Venus mass spectrometer. While the calculated mean S3 abundance agrees with the Venera 11-14 observations, a steep decrease in S_3 from the surface to 20 km is not expected from the observations. The ClSO2and SO2Cl2 mixing ratios are ~10-11 in the lowest scale height. The existing concept of the atmospheric sulfur cycles is incompatible with the observations of the OCS profile. A scheme suggested in the current work involves the basic photochemical cycle that transforms CO2and SO2into SO3, CO, and Sn and a minor photochemical cycle which forms SO2and Sn from OCS. The net effect of thermochemistry in the lowest 10 km is formation of OCS from CO and Sn. Chemistry at 30-40 km removes the downward flux of SO3 and the upward flux of OCS and increases the downward fluxes of CO and Sn. The geological cycle of sulfur remains unchanged.
P23A-03
Mars Reconnaissance Orbiter Accelerometer Experiment Results
The Mars Reconnaissance Orbiter (MRO) launched on August 12, 2005, designed for aerobraking, achieved Mars Orbital Insertion (MOI), March 10, 2006. Atmospheric density decreases exponentially with increasing height. By small propulsive adjustments of the apoapsis orbital velocity, periapsis altitude is fine tuned to the density surface that safely used the atmosphere of Mars to aerobrake over 400 orbits. MRO periapsis precessed from the South Pole at 6pm LST to near the equator at 3am LST. Meanwhile, apoapsis was brought dramatically from 40,000km at MOI to 460 km at aerobraking completion (ABX) August 30, 2006. After ABX, a few small propulsive maneuvers established the Primary Science Orbit (PSO), which without aerobraking would have required an additional 400 kg of fuel. Each of the 400 plus aerobraking orbits provided a vertical structure and distribution of density, scale heights, and temperatures, along the orbital path, providing key in situ insight into various upper atmosphere (greater than 100 km) processes. One of the major questions for scientists studying Mars is: "Where did the water go?" Honeywell's substantially improved electronics package for its IMU (QA-2000 accelerometer, gyro, electronics) maximized accelerometer sensitivities at the requests of The George Washington University, JPL, and Lockheed Martin. The improved accelerometer sensitivities allowed density measurements to exceed 200km, at least 40 km higher than with Mars Odyssey (MO). This extended vertical structures from MRO into the neutral lower exosphere, a region where various processes may allow atmospheric gasses to escape. Over the eons, water may have been lost in both near the surface and in the upper atmosphere. Thus the water balance throughout the entire atmosphere from subsurface to exosphere may both be critical. Comparisons of data from Mars Global Surveyor (MGS), MO and MRO help characterize key temporal and spatial cycles including: winter polar warming, planetary scale gravity waves, latitudinal, seasonal, and diurnal variations, and variations from perihelion to aphelion. This will validate and constrain both upper atmospheric circulation models used to understand the nature of high-altitude variability and transport processes, and engineering models used to plan future missions.
P23A-04
Mars atmospheric temperature profiles from Mars Climate Sounder measurements: Retrieval algorithm and first results
Mars Climate Sounder (MCS) is a mid- and far-infrared thermal emission radiometer on board the Mars Reconnaissance Orbiter (MRO). Since Sep. 24, 2006, MCS has been taking measurements of the Martian surface and atmosphere in limb and nadir geometry with a duty cycle of ~2 profile measurements per minute. With its 8 infrared and one visible channel, MCS measures vertical profiles of atmospheric temperature, water vapor, dust and condensates from 0 to 80 km altitude with a vertical resolution of ~5 km. The retrieval of vertical profiles from the measured radiances is based on the iterative method by Chahine. We will decribe and characterize the retrieval algorithm, and present first examples of retrieved profiles of atmospheric temperature and dust opacity.
P23A-05
Observations of Hydrocarbons in the Stratospheres of Jupiter and Saturn Using Celeste
We are using Celeste, an infrared (5-25 μm) high-resolution (ΔR/R ~103-104) cryogenic ground-based grating spectrometer, at the McMath-Pierce Telescope of the National Solar Observatory at Kitt Peak, and at the NASA InfraRed Telescope Facility on top of Mauna Kea, to observe hydrocarbon line emission spectral features originating from the stratospheres of the planets Jupiter and Saturn. These observations are being performed in support of the Composite InfraRed Spectrometer (CIRS) instrument aboard the Cassini spacecraft on its mission past Jupiter and currently orbiting Saturn. Over the past few years we have recorded several ν4 lines of CH4 (1228.5 cm-1), the ν5 R(5) line of C2H2 (743.3 cm-1) and the ν9 RQ0 branch of C2H6 (822.3 cm-1) for Jupiter and Saturn on several occasions. We also have additional observations of the H2 S(1) emission line (587.0 cm-1) and several ν9 C2H4 lines (949.5 cm-1) for Jupiter. From these observations we have been able to infer molecular abundances for the given species at the pressure levels where their contribution functions are maximum (~1-10 mbar) that help constraint current photochemical models for the stratospheres of the planets. In particular we have been able to construct maps in longitude and latitude for Jupiter from the C2H2 and C2H6 data that compare favorably with the CIRS measurements, opening the door for further ground-based synoptic observations that could reflect seasonal abundance/temperature changes in the atmosphere of the planet. The current status of the observing campaign and recent modeling results will be presented.
P23A-06
Lunar Magnetism: IRMs Normalization and Impact Related Magnetization.
Models of lunar magnetism need to explain: (1) strong Natural Remanent Magnetization (NRM), indicated by IRMs normalization, in some of the returned Apollo Mare Basalts and Melt Rocks with ages from about 3.85Ae to 3.65 Ae, (2) magnetic anomalies antipodal to the young basins of a similar age, (3) the absence of major magnetic anomalies over these same basins and the presence of minor anomalies over uplifted basement. (4) strong fields with scale lengths of homogeneity of the order of kms, or less, are found over the Cayley Formations and similar material. Observation (1) has frequently been taken to require the presence of a lunar dynamo. However, this presents a dilemma. If there were a lunar dynamo at that time, why were the basin melt sheets not magnetized in the field of the dynamo. It is also an uncomfortable coincidence that the dynamo is only extant close to the time of heavy bombardment. Given these difficulties and questions of the efficiency of dynamo action in a lunar core, it is worth reexamining other possible explanations of lunar magnetism. Hood's model accounts for the antipodal anomalies, while the observations at Vredefort (Carpozen, et al., 2003) may account for the anomalies over central peaks and uplifted ring structures in major basins. Experimental work by Crawford and Schultz (1999) has demonstrated that impacts of projectiles traveling at kms/sec generate magnetic fields. Srnka et al, (1979) showed that magnetic fields can be recorded by shock with demagnetization characteristics similar to thermal remanent magnetization. The question that remains is whether all of the observed lunar magnetization can be explained directly by impact related magnetization, or indirectly through magnetization acquired in the remanent fields of material magnetized by impacts.
P23A-07
The Crustal Dichotomy of Mars: Geological Testing and Constraints on Geophysical Models
The term `crustal dichotomy' refers to hemispheric scale differences in crustal thickness, surface age, elevation, and morphology between the southern cratered highlands and northern lowland plains of Mars. Empirical observations of the crustal dichotomy boundary provide important constraints on the timing and mechanisms of early crustal development, which remains poorly understood after 35 years of robotic exploration. Published models include endogenic (long-wavelength mantle convection, perhaps driving plate tectonics) and exogenic (one or more giant impacts) mechanisms. The dichotomy boundary consists of Early Noachian (~4.5 to 3.92 Ga) cratered slopes and a transition zone of Early Hesperian (~3.7 to 3.6 Ga) fretted and knobby terrains between the Noachian highland plateau and Hesperian lowland plains. The old cratered slope predates and influenced the morphometry of Hesperian and younger impact craters, the drainage planform of Late Noachian valley networks, and the erosional modification of Middle to Late Noachian (~3.92 to 3.7 Ga) impact craters. No extensional faults bisect Noachian fresh or degraded craters at higher elevations on the cratered slope, and crater floor deposits are flat rather than tilted, which indicate that the boundary and the lowlands formed very early in the Noachian Period. Formation of the dichotomy in the Early Noachian is consistent with the population of buried impact craters in the lowlands, SNC isotopic data, and the crustal magnetic field data. Late Noachian plateau materials, younger volcanic rocks of the Tharsis province, and Amazonian airfall materials of the Medusae Fossae Formation later buried the old cratered slope. Development of fretted and knobby terrains in the younger plateau materials appears to be structurally controlled, but we have identified no evidence for significant fault displacement. Tectonic and erosional features of these later terrains are not coeval with the crustal dichotomy and do not provide valid model constraints for Early Noachian processes. Many published models of the crustal dichotomy have one or more of the following issues: 1) long- lived processes that extend beyond the Early Noachian Epoch, 2) formation of complex fretted and knobby terrain using one-dimensional tensile stress, 3) use of Hesperian faults and landforms as model constraints, 4) limited applicability beyond the study area, and 5) insufficient testing to reject alternative endogenic hypotheses. Future modeling of the crustal dichotomy should focus on rapid endogenic mechanisms operating within the Early Noachian Epoch.
P23A-08
Gravity Field and Interior Structure of Saturn from Cassini Observations
We discuss the sources for a determination of Saturn's external gravitational potential, beginning with a Pioneer 11 flyby in September 1979, two Voyager flybys in November 1980 for Voyager 1 and August 1981 for Voyager 2, four useful close approaches by the Cassini orbiter in May and June 2005, and culminating in an extraordinary close approach for Radio Science in September 2006. Results from the 2006 data are not yet available, but even without them, Cassini offers improvements in accuracy over Pioneer and Voyager by a factor of 37 in the zonal coefficient J2, a factor of 14 in J4, and a factor of 5 in J6. These improvements are important to our understanding of the internal structure of Saturn in particular, and to solar and extrasolar giant planets in general. Basically, Saturn can be modeled as a rapidly rotating planet in hydrostatic equilibrium. Consistent with the limited data available, we express the density distribution as a polynomial of fifth degree in the normalized mean radius β = r/R over the real interval zero to one, where R is the radius of a sphere with density equal to the mean density of Saturn. Then the six coefficients of the polynomial are adjusted by nonlinear least squares until they match the measured even zonal gravity coefficients J2,J4,J6 within a fraction of a standard deviation. The gravity coefficients are computed from the density distribution by the method of level surfaces to the third order in the rotational smallness parameter. Two degrees of freedom are removed by applying the constraints that (1)~the derivative of the density distribution is zero at the center, and (2)~the density is zero at the surface. Further, a unique density distribution is obtained by the method of singular value decomposition truncated at rank three. Given this unique density distribution, the internal pressure can be obtained by numerical integration of the equation of hydrostatic equilibrium, expressed in terms of the single independent parameter β. By means of this technique, a pressure of 3~Mbar is indicated at about half the distance to the surface, consistent with a phase transition from molecular to metallic hydrogen at 50% depth. However, a similar integration of the mass continuity equation does not use up all the mass. Mathematically this results in a point- mass core of about 10 Earth masses, although in reality the core must be sufficiently large to have a physically reasonable mean density. Our results are robust against the relatively large uncertainty in Saturn's rotation period.