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| Titel |
Near infrared imaging of the surface of Venus and implications for crustal composition |
| VerfasserIn |
N. Müller, J. Helbert |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250025442
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| Zusammenfassung |
| Venus Express is an ESA spacecraft orbiting Venus since April 2006. The instrument VIRTIS
acquires multispectral images in the range from 0.2 to 5 μm wavelength. An analysis of
VIRTIS images at the wavelengths of the atmospheric window at 1.02 μm shows evidence for
variation of surface emissivity on the southern hemisphere [Mueller et al. 2008]. Inferred
surface emissivity is correlated to some extend with morphological units identified from radar
images of the NASA/JPL Magellan mission [Tanaka et al. 1997]. Alpha and Phoebe
Regios are highlands mostly composed of tessera terrain, which is defined as a
region strongly deformed by compressive and extensional tectonism in at least two
directions. In comparison to lowland plains and other less tectonized highlands, these
regions generally emit less thermal radiation, which implies lower emissivity. A
recent analysis of NIR data from the Galileo fly-by in 1990 finds, that highland
regions on Venus on average have a lower emissivity than lowlands [Hashimoto et al.
2008]. As a significant part of Venus highlands in the area observed by Galileo is
composed of tessera, this observation is consistent with the observation of Mueller et al.
[2008].
In situ measurements by the Venera and Vega landers are at most places consistent with
basaltic surface composition. The hypsometry of Venus is unimodal. Inferred lava viscosity
of most volcanic features is low, consistent with basaltic composition. All these observations
hint towards a crust mostly composed of basalt [Basilevsky et al 1997]. However, no landing
site was on tessera terrain, tessera are hypsometrically elevated and the morphology is
dominated by tectonic deformation. Among other arguments this leads to the hypothesis that
tessera highlands crust is more abundant in feldspar and silica, comparable to lunar
highlands or continents on Earth [Nikolaeva et al., 1992]. NIR mapping supports
this hypothesis, although other interpretations of the NIR data can not be ruled
out.
Generation of felsic crust is unlikely under the current climatic and tectonic regime on
Venus. The lunar highland crust is believed to be a remnant of an magma ocean [Taylor
1974]. Enrichment in silica as in the continental crust of Earth requires recycling of water
into the mantle [Campbell and Taylor 1984]. The surface of Venus is extremely dry and
Venus and crustal recycling by plate tectonics does not operate at present. Any
crust with felsic bulk composition had to be created during the early history of the
planet. In a stratigraphic analysis tessera terrain predates all units it is in contact
with [Ivanov and Head 1996]. Tessera terrain is defined by an extensive history of
tectonic deformation. Assuming that tessera highlands indeed represent less dense
crustal blocks created early in the history of Venus, implications arise from their
persistence on the surface of Venus regarding resurfacing mechanism, crustal recycling
and thermal evolution. If tessera highlands are enriched in silica relative to basalt
this implies existence of a primordial ocean on Venus [Hashimoto et al. 2008]. In
either case Venus would even more closely resemble the Earth-Moon system than
previously assumed, making Venus an excellent subject for general studies of earth-like
planets.
Basilevsky, A. T.,et al. (1997), The Resurfacing History of Venus, in Venus II, pp.
1047–1084. Hashimoto, et al. (2008), Galileo Near Infrared Mapping Spectrometer (NIMS)
Data Suggests Felsic Highland Crust on Venus, JGR, in press. Ivanov, M. A., et al. (1996),
Tessera terrain on Venus: A survey of the global distribution, characteristics, and relation to
surrounding units from Magellan data, JGR, 101, 14,861–14,908. Mueller, N., et al. (2008), Venus
surface thermal emission at one micrometer in VIRTIS imaging observations - evidence for
variation of crust and mantle differentiation conditions, JGR , in press. Nikolaeva, O. V., et al.
(1992), Evidence on the crustal dichotomy, pp. 129–139, Venus Geology, Geochemistry, and
Geophysics - Research results from the USSR. Tanaka, K. L., et al. (1997), Physiography,
Geomorphic/geologic Mapping and Stratigraphy of Venus, in Venus II, pp. 667–694. Taylor, S. R.
(1974), Geochemical Evolution of the Moon, LPI Contributions, 195, 7–9. Taylor, S. R., and I. H.
Campbell (1983), No water, no granites - No oceans, no continents, GRL, 10, 1061–1064. |
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