Venus Surface Thermal Emission Observed by VIRTIS on Venus Express

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1 Venus Surface Thermal Emission Observed by VIRTIS on Venus Express N. Müller, J. Helbert, G. Hashimoto, C. Tsang, S. Erard, G. Piccioni, P. Drossart The VIRTIS-VEX Team

2 Near Infrared Spectral Windows Highlands Lowlands Topography /km Radiance / W m -2 μm -1 sr -1

3 Atmospheric Emission Topography /km Radiance / W m -2 μm -1 sr -1

4 Data Processing 1.02 μm 1.02 μm VIRTIS 1.40 μm μm Magellan Radiance / W m -2 μm -1 sr -1 Flux / W m -2 μm -1 Temperature / K

5 Characterizing 'normal' surface emission

6 Mosaic of VIRTIS images

7 VIRTIS Brightness derived Topography Translate brightness to topography with empirical relation Galileo NIMS Carlson et al. 1993

8 Magellan Topography Improvements of PDS version: - Magellan ephemerides [Konopliv et al. 1996] - altimeter data reprocessed [Rappaport et al 1999] - distribution: ftp://voir.mit.edu/pub/mg_3003/ Smoothed to 120 km spatial resolution

9 Relative Brightness Relative Brightness - shown is brightness relative to average brightness biased for topography Relation to emissivity - atmosphere attenuates emissivity signal - brightness variation is lower estimate of emissivity variation Surface composition - Vega 2 / Venera 14, basalts - Venera 8/13, high in alkalines

10 NIR and Radar Emissivity Anomalies Radiothermal anomaly - areas higher than 4 to 5 km show very low radar emissivity NIR emissivity - some anomaly might be expected - not enough high regions covered Systematic problem - this approach assumes that there is no trend of emissivity with altitude - radiative transfer modelling required

11 Tessera terrain with high albedo Galileo NIMS observation Hashimoto et al Felsic composition (Feldspar-Silica rich) - will result in low emissivity compared to basalt - other high albedo materials? Composition analogue to - lunar highlands? - terrestrial continents? Altimetry less reliable? - steep slopes difficult - bias to too low altitudes will produce this apparent emissivity effect - bias found in Magellan stereo image mapping [Howington- Kraus et al. 2002]

12 Altimetry 100 km Cloud layer blurs image of surface comparable to convolution with a 90 km FWHM gaussian [Hashimoto & Imamura 2001] Pixel errors (i. e. outlying radar footprints) affect larger area preliminiary analysis indicates that outlying radar footprints are not the cause of the tessera anomalies

13 High relative brightness Flow fields (fluctus) - Cavilaca - Juturna -Kaiwan - Mylitta Volcanic edifices - Imdr regio - Phoebe regio - Themis regio Coronae - Shiwanokia - Shulamite -Selu - Quetzalpetlatl / Boala

14 Lava-flows, Quetzalpetlatl / Boala Corona Goldstone Radar Image [Kratter et al. 2007] Interpretations active volcanism unlikely? unusual composition, e.g. ultra-mafic?

15 Emissivity and Surface Age Tessera emplacement of tessera rock is thought to predate any other unit i.e. stratigraphically old Lava flows. no craters on the unit including Quetzalpetlatl stratigraphically young Gravimetry Themis, Phoebe regios are similar to terrestrial hotspot regions, e.g. Hawaii indicator of recent activity? Radar dark parabolas no clear signature

16 Summary Observations VIRTIS one micron brightness is correlated with Magellan altimetry residual brightness, not accounted for by topography, is partly correlated with geomorphological units from Magellan radar imaging tessera often dark (high albedo) majority of plains have brightness close to global average some (young) lava flows, volcanoes and coronae flanks are bright (low albedo) Possible Interpretations 1. variation of composition: felsic tessera (e.g. granite, anorthosite), widespread mafic plains (e.g. basalt), sporadic ultramafic volcanism (e.g. pikrite, komatiite) 2. varying states or modes of chemical weathering Discussion Brightness is highly sensitive to surface temperature / topography Magellan topography may contain biased errors at tessera terrain unexpected and non-random surface temperature variations?

17 Outlook Work in progress surface windows at 1.10 and 1.18 μm three point spectra of the surface 0.85, 0.90 μm windows imaged by VIRTIS VIS channel but much too noisy so far Surface emissivity so far only brightness corrected for topography radiative transfer modelling required for emissivity [Hashimoto et al. 2008] Recommendations for future instruments / missions targeting the surface Less spectral resolution than VIRTIS, better signal to noise at 6 to 10 bands 0.85, 0.90, 1.02, 1.10 and 1.18 μm: surface windows [Baines et al. 2000] 1.31 μm for cloud correction 1.40 μm as additional dark correction more windows at 1.55, 1.74, 2.30 μm, not used by us but possibly improve surface emissivity retrieval spatial resolution of 20 to 40 km per pixel reliable altimetry of tessera terrains stereo imaging? frequent observations of the same areas, long mission duration / instrument lifetime more global coverage