The Surface of Venus
Venus surface, geology and its relation to the interior
Venus has long and successful history of exploration. It has been studied via ground-based observations and by spacecraft. More than 20 missions, including fly-byes, orbiters, descent probes, atmospheric balloons, and landers have studied Venus. Yet many questions about Venus surface and its geology remain open.
Being very similar to Earth in such ways as size, mass, and bulk composition, Venus is very much different in conditions on it and processes in its atmosphere and interior.
Venus is located closer to the Sun (0.76 AU) but receives 1.4 times less energy than Earth because Venus is completely covered by clouds with high reflectivity. Nevertheless temperature in its atmosphere quickly increases with depth, reaching room temperature at the level of 1 bar, and almost 500 ºC at the surface level 50 km below, where the pressure is around 93 bars. The atmosphere consists primarily of carbon dioxide and has density of around 0.1 g/cm3 near the surface and produces strong green-house effect that is responsible for high surface temperature. These facts must lead to a completely different geology as compared to Earth. Indeed dry and rigid Venus crust shows no signs of plate tectonics, but it does show a rich variety of volcanic structures.
Plate tectonics on the Earth is responsible for releasing a heat from interior, but on Venus there is no such a mechanism and internal heat can be released only by either continuous volcanic eruptions or catastrophic events that destroy most of the crust. Sign of such an event we see in young age of the Venus surface, which is only about 0.5 billion years old, comparing to 3 and in some places even over 4 billion years in the case of Earth's continental crust. However, since onset of space exploration of Venus no volcanic activity has been observed.
Water, the most universal solvent, participates in almost all processes on the Earth surface resulting in mineralogical evolution. Dry extremely hot surface of Venus is dominated by primitive basaltic lavas. But was it like this ever or did Venus had liquid water some time in its past?
Knowledge about Venus evolution accumulated so far suggests that Venus and Earth were quite similar when they formed, but then the different evolution processes lead to extreme difference between conditions on these sister planets. Why and how drastic differences between Earth and Venus developed? These questions are exactly why we research Venus and support space missions to this planet.
Remote sensing is the only possibility to study Venus surface globally in foreseeable future. Remote sensing of the Venus surface is a complicated task due to Venus thick atmosphere and clouds. Together they block radiation from surface almost in whole electromagnetic spectrum except radio- and microwaves (where the atmosphere is completely transparent), and a few narrow transparency “windows” in near infra-red. These transparency “windows” give a unique opportunity to sense Venus’ surface: the surface is hot enough (≈ 740 K) to produce significant thermal flux in near infra-red, and this flux can escape to space and then can be detected at the night side of the planet.
VEX mission and VMC
The Venus Express (VEX) mission is aimed at investigations of planet's atmosphere, plasma environment and addresses some aspects of the surface physics (Svedhem, 2007). The VEX spacecraft payload consists of 7 instruments (5 of them were inherited from other missions, 2 — designed specifically for this one). VEX continues intense exploration of Venus, that was done in 1970 – 1980 by series of Venera and Pioneer spacecraft, and then in 1990 – 1994 by Magellan mission.
VEX has been launched on 9th of November 2005 and inserted into an orbit on 11th of April 2006. Since that time the spacecraft operates on polar, highly elliptical orbit (altitude in pericenter — hundreds of kilometres, altitude in apocenter — around 66000 km), with orbital period of 24 hours.
Two instruments on-board VEX are able to perform sounding in the near infra-red transparency “windows”: Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) and Venus Monitoring Camera (VMC). Together they provided the first systematic thermal mapping of the Venus surface from orbit.
VMC was developed at MPS and designed to perform imaging of the atmosphere and observations of the surface when the spacecraft is in the planet’s shadow (Markiewicz et al., 2007; Titov et al., 2012). The VMC takes images in four spectral channels, one of them (IR2) is centred at 1.01 μm at the atmosphere transparency window (e.g. Meadows and Crisp, 1996). When the spacecraft happens to be in the planet shadow (that happens near the pericentre, twice per Venus sidereal period), VMC can observe the night side of the planet. This allows mapping thermal emission from the surface. These observations are limited to ≈ ±40° latitude (where the spacecraft remains in the shadow). Thus, observations of the surface are only possible shortly (not more than 1 hour) before or after the pericentre of the orbit, corresponding to distances up to ≈ 8.5×103 km. Formal spatial resolution of these images is 1 to 6 km/px, but because the surface radiation on its way to the camera passes through the dense scattering atmosphere and cloud layer, the actual spatial resolution at the surface is about 50 km/px.
1-μm emissivity of the surface
From the Magellan radar maps (fig. 1) it is seen that the Venus surface is dominated by volcanic plains, which have been interpreted to be formed by emplacement of mafic (basaltic) lavas. This inference follows from images and the results of the in situ analysis of the soil by the Venera and Vega landers in six sites located on these plains (fig. 2). This is also supported by observations of plains morphology on high-resolution radar imagery. However, a few geologic features and units of the Venus surface could have non-basaltic, geochemically more evolved compositions. These are tesserae, mountain tops, and steep-sided domes.
Near infra-red emissivity (or reflectivity) is sensitive (as opposed to microwave one) to mineralogical composition of the surface layer. Sounding in NIR can detect different emissivity of the surface and thus different composition.
At the temperature of the surface, near infrared is located at the short-wavelength shoulder of the Planck curve, while microwaves are on the long-wavelength one. Hence near infra-red flux is much more sensitive to the temperature of the surface. Since surface temperature is decreasing with altitude (≈ 8.1 K/km), in night-side NIR images we see reversed images of the surface topography (fig. 3).
Emissivity of the soils depends not only on their mineralogical composition but also on the grain size. Winds near the surface could redistribute dust and thus smoothing differences in composition. If the wind speed depends on altitude, grain size might also depend on altitude. These effects will decrease contrasts of emissivity between various units.
Atmosphere in transparency “windows” is not completely transparent. In particular, there is gaseous absorption and scattering in clouds. Therefore for analysis we need radiative transfer modelling, results of which we compare with the observational data.
We searched for compositional differences not globally but within rather small region which has the appropriate objects of the study and is well covered by the VMC images. This is the area South-West (SW) of Beta Regio. Here there is a relatively small but distinct massif of tessera terrain, Chimon-mana Tessera, the surface emissivity of which we try to determine and compare with that of the adjacent plains. About 1000 km to the north, among the plains, there is a relatively small volcano, Tuulikki Mons, whose morphology (gentle slopes and extended outskirts of lava flows) are indicative of basaltic composition e.g. Head, 1992).
From analysis of VMC data for this area we made the following conclusions.
- The night-side VMC images provide reliable information on spatial variations of the NIR thermal emission of the Venus surface, which potentially may be interpreted in terms of geological characteristics of the studied area, including possible compositional differences between the geologic units.
- Our calculations for the area SW of Beta Regio showed that 1-μm emissivity of Chimon-mana tessera surface material is by 15—35 % lower than that of relatively fresh supposedly basaltic lavas of plains and volcanic edifices. This is consistent with the hypothesis that the tessera material is not basaltic and may be felsic. These results are in agreement with the results of Helbert et al. (2008), Mueller et al. (2008), Hashimoto et al. (2008), and Gilmore, Mueller, et al. (2011) and with early suggestions of Nikolaeva et al. (1992). If the felsic nature of Venusian tesserae is confirmed in further studies, this may have important implications for geochemical environments in early history of Venus, indirectly supporting a hypothesis of water-rich early Venus (e.g. Kasting et al., 1984; Kasting, 1988; Grinspoon and Bullock, 2003).
- We have found that the surface materials of plains in the study area are very variegated in their 1-μm emissivity, which probably reflects variability of their local geologic histories, mostly the degree of chemical weathering with less weathered materials showing higher emissivities. Future studies in the areas of geologically more homogeneous plains would be helpful in proving this suggestion.
- We have also found a possible decrease of the calculated emissivity at the top of Tuulikki Mons volcano which, if real, may be due to different (more felsic?) composition of volcanic products on the volcano summit comparing to its slopes. This suggestion seems to be supported by the observation that at the volcano summit there is a steep-sided dome. More evolved lavas in the latest stages of evolution of basaltic magma chambers are rather typical for magmatism of Earth (e.g. McBirney, 2006).
Details of this work are presented in Basilevsky et al. (2012).
Search for ongoing volcanism
Since near infra-red flux is very sensitive to the temperature of the surface, such observtaions give possibility to detect a hot lava on the surface. Since VMC has observed significant part of the Northern hemisphere of Venus, we used these data to search for hot spots at the surface, which might mean presence of a hot (fresh) lava and on-going volcanic activity. Therefore we have estimated the possibility of detecting the lava fields of various sizes and shapes by VMC observations. We consider different aspects of the search of the on-going volcanic activity from observations taken by the Venus Monitoring Camera (VMC) 1 micron channel onboard of Venus Express. Here our emphasis is the areas of Maat Mons volcano and its vicinities which based on analysis of the MGN SAR images shows evidence of geologically very young volcanism.
- Analysis of VMC images taken in 12 observation sessions during the time period from 31 Oct 2007 to 15 Jun 2009 did not reveal any suspicious high-emission spots which could be signatures of the on-going volcanic eruptions.
- We compared this time sequence of observations with the history of eruptions of volcano Mauna Loa, Hawaii, in the 20th century. This comparison shows that if Maat Mons volcano had the eruption history similar to that of Mauna Loa, the probability to observe an eruption in these VMC observation sequences would be about 8 %, meaning that the absence of detection does not mean that Maat is not active in the present epoch (fig 4).
- These estimates give probability to have lava field in the camera’s field of view but do not consider the effect of absorption and blurring of the thermal radiation coming from Venus surface by the planet atmosphere and clouds, which decreases detectability of thermal signature of fresh lavas. To assess the role of this effect we simulated NIR images of the study area with artificially added lava flows having surface temperature 1000 K and different areas. These simulations showed that 1 km2 lava flows should be marginally seen by VMC. Increase of the lava surface area to 2 – 3 km2 makes them visible on the plains and increase of the area to 4 – 5 km2 makes them visible even in deep rift zones. Elongation of lava fields in general increases these values. However, for typical length to width ratios of about 10 the decrease of contrast is not significant, but becomes significant for extremely long fields with aspect ratio more than 1000 (fig. 5).
- Typical individual lava flows on Mauna Loa are a few km large, however, they often have been being formed during weeks to months and the instantaneous size of the hot flow surface was usually much smaller. Thus the detection probability is significantly lower than 8 %, but it is probably far from negligible. Our consideration suggests that further search of Maat Mons and other areas including young rift zones with VMC, in particular, makes sense and should be continued.
- More effective search could be done if observations simultaneously cover most part of the night side of Venus for relatively long (years) time of continuous observations.
Details of this work are presented in Shalygin et al. (2012).
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