P42A-01
Major Rifts on Venus: Lithospheric Properties and Formation of Parga and Hecate Chasmata
Parga and Hecate Chasmata are complex rift systems each approximately 10,000 km in length. Each chasmata has multiple branches at varying angles to the main trend, and over a hundred associated coronae. Their admittance signatures are distinct from the surrounding plains, and from each other. At Hecate Chasmata, admittance signatures and estimated elastic thicknesses are relatively uniform along several of the rift segments, while at Parga, most of the rift segments display variable admittance signatures. In addition, at Hecate, thinner elastic lithosphere correlates with coronae that are stratigraphically younger than the rift. The correlation between lithospheric properties and rift segments at Hecate suggests that rift formation at this location may be influenced by lithospheric properties, as occurs on Earth. Applying simple models of rift formation based on uniform regional extension (Buck, 1991) and comparing with estimated crustal and elastic thicknesses shows that both these rifts should be considered as wide rifts, which is favored for a strong crustal rheology. This is consistent with their average widths of approximately 150 km and even wider zones of fractures. The implication of the wide rift designation is that spreading velocities are on the order of 1 cm/yr, which is faster than might be expected for a stagnant lid convective regime. Agreement between models, lithospheric properties and stratigraphy at Hecate suggests that uniform extension may explain rifting at Hecate. However Parga does not display such correlations, possibly indicating modification since rifting occurred. The relationship between coronae and rifts remains enigmatic, and could have a significant effect on lithospheric properties.
P42A-02
Venus NIR Surface Emissivity estimated from VIRTIS on Venus Express Observations
Spectral window regions close to 1μm allow for the transfer of thermal radiation from the surface through the atmosphere of Venus. Results of radiative transfer modeling are used to invert VIRTIS images at 1.02, 1.10 and 1.18μm for thermal emission of the surface. Local atmospheric transmittance is derived from the VIRTIS band at 1.31μm. Several hundreds of VIRTIS images covering in total most of the southern hemisphere of Venus have thus been analyzed and stacked for improvement of signal to noise ratio. The results of this approach are to some extent ambiguous since neither surface emissivity nor surface temperature are well known. Furthermore aerosols or a gradient of absorbing gaseous constituents near the surface might affect the interpretation. But neglecting any effects of the near surface atmosphere and assuming parameters of the radiative transfer model within reasonable ranges it is possible to estimate either surface emissivity or surface temperature. Temperature of surface and atmosphere is mostly a function of altitude, no large diurnal, seasonal or latitudinal variations are expected in the lower atmosphere. The lapse rate is constrained by the adiabatic lapse rate. A hint for global average of surface emissivity is given by the dominance of probably basaltic volcanic plains on the southern hemisphere. It is however imaginable that temperature dependant weathering leads to a trend of emissivity with altitude similar to that seen in the Magellan radiothermal emissivity observations. Regardless of any global variations of lapse rate or emissivity with altitude, spatial variations of emissivity independent from topography can be examined by assuming constant emissivity and fitting surface temperature accordingly to the global relation of thermal emission to topography. This spatial variation of thermal emission is assumed to be due to variation of surface emissivity and shows correlation with some geological features known from Magellan radar images. In the Lada Terra region large lava streams, Cavillaca - and Juturna Fluctus, show increased emissivity with respect to neighboring regions of the same altitude. Other large lava streams in the region show a similar but less obvious relative emissivity. Large areas of tessera terrain on the contrary consistently show relative low emissivity. While this variation of emissivity might be related to content of mafic minerals it is also conceivable that weathering and thus age of the terrain in question is responsible. The accumulation of observations by VIRTIS during the Venus Express mission will allow us to study with a larger database these correlations and to further disentangle possible atmospheric from surface contributions and thus to increase understanding of surface composition as well as of composition and temperature of the atmosphere in contact with the surface.
P42A-03
The Structure of the Venus Neutral Atmosphere from the Radio Science Experiment VeRa on Venus Express
The Venus Express Radio Science Experiment VeRa is sounding the Venus neutral atmosphere and ionosphere using the spacecraft radio subsystem in the oneway radio link mode. An Ultrastable Oscillator (USO) provides a high quality onboard frequency reference source for the derivation of electron density profiles in the ionosphere and profiles of pressure, temperature and neutral number density of the neutral atmosphere. The measurement configuration allows an altitude resolution of only a few hundred metres from the cloud deck at about 40 km to approximately 100 km. Three occultation seasons could be covered in the first two years of the Venus Express mission resulting in a data set of about 140 profiles of the neutral atmosphere. The polar orbit of Venus Express provides the opportunity to study the atmosphere at all planetocentric latitudes under varying illumination conditions. Special attention will be given to day-night variations of the thermal structure and the temperature distribution at high polar latitudes on both hemispheres ("cold collar region") and signal absorption effects caused by the H2SO4 vapour.
P42A-04
Venus Express Observes a New Type of Subcritical Shock With Pure Kinematic Relaxation
Collisionless shocks are present in the vicinity of many astrophysical objectssuch as supernova remnants, space jets, stars and planets immersed in the supersonic flow of star winds. Collisionless shocks are the most powerful cosmic rays accelerators and are responsible for the emission of gamma-ray bursts. Understanding of the shock structure is crucial for understanding of the processes of the redistribution of the upstream flow energy into accelerated particles and formation of downstream thermalised distribution. We report first observations (by Venus Express) of subcritical shocks that does not fit the well-established classical structure classification. It is shown that its abnormal structure is due to kinematic collisionless relaxation of downstream ions. The spatial gyrophase mixing leads to the formation of a downstream thermalised distribution, instead of various instabilities.This type of subcritical shock with kinematic relaxation was never discussed before in theoretical models.