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Visualizing the Surface of Venus Mark A. Bullock Southwest Research Institute, Boulder, Colorado, USA Summary An exciting prospect for major scientific advances in the geological history of Venus is surface imaging on descent through the atmosphere. The dense atmosphere provides a leisurely descent speed, valuable for allowing downlinking of images before arrival at the surface. Images of the surface on a descent probe to Venus would provide crucial information at a scale not yet explored – the 10 m to 10’s of km range. Equally important, these images would be taken at visible and near-infrared wavelengths. Comparison with Magellan radar data in making new, detailed geologic interpretations based on optical wavelengths could be extrapolated to similar regions across the planet. Here I discuss the unique optical and spacecraft dynamics issues in imaging the surface of Venus from a descent probe. Cameras must have a large dynamic range, high quantum efficiency at 1 μm, and be capable of millisecond exposure times. Nighttime images would have the highest surface contrast, but independent altimetry would be needed to account for variations in flux due to altitude-dependant surface temperature. At night, near-IR images with about the resolution of Magellan SAR images can be obtained from just beneath the clouds. Images on descent during the day would see a very bright sky but their interpretation would be less dependent upon altimetry. Images with enough contrast for scientific investigations could be obtained below approximately 15 km. Atmospheric Optical Issues Below the clouds, the primary challenges to surface imaging are very broad CO2 and H2O absorption bands longward of 0.8 μm, and Rayleigh scattering (Moroz, 2002). Below-cloud hazes may at times also degrade image quality (Esposito et al., 1983). At least five near-infrared windows in the atmospheric absorption spectrum provide access to the surface. Some of these, such as the ones at 1.02, 1.10, and 1.18 μm, have almost no absorption in them except for the far wings of nearby CO2 absorption lines. Venus’ atmosphere is 60 times more massive than Earth’s, and CO2 has four times the Rayleigh scattering cross-section of N2. The result is that Venus’ atmosphere is extremely bright at visible wavelengths. All visible wavelengths scatter appreciably, not just the blue that we are familiar with in Earth’s skies. Since blue solar light is attenuated in the clouds, the sky beneath the clouds is not blue. Due to Rayleigh scattering of longer visible wavelengths, the sky of Venus may be green or yellow. Even at 1.02 μm, the Rayleigh scattering optical depth from 45 km altitude is about 1.3. At night, the sky is lit by scattering of the glow from the surface. From 45 km the night sky is ~2.6 times brighter than the surface. During the day, sunlight at 1.02 μm penetrates the clouds and illuminates the surface. Because this light is reflected several times between the surface and clouds, the daytime sky is ~35 times brighter than the surface, as seen from 45 km. Moroz (2002) calculated the Rayleigh scattering optical depths in the 0.65, 0.86, and 1.02 μm spectral windows. He also used Venera 13 and 14 spectrophotometer data in a scattering radiative transfer model to calculate ‘visibility factors’ for the surface in each of these windows for daytime conditions. His model assumed a surface albedo of 0.1, with a solar zenith angle of 37º – the conditions of the Venera 13 descent. The visibility factor, VF was defined as VF = Fus (λ , T ) Fua (λ , T ) where Fus(λ,T) is the upward flux emitted from the surface, and Fua(λ,T) is the upward flux in the atmosphere. Its reciprocal is thus the ratio of sky brightness to surface brightness. The results of Moroz (2002) for both day and night imaging of the surface are summarized in terms of sky brightness in Table 1. Seeing the Surface from Above the Clouds The surface of Venus is most visible from above at 1.02 μm during a nighttime descent. Even at 45 km, the surface has sufficient contrast to be observed through the sky scattering if camera dynamic range is high enough and camera noise low enough. The surface is seen quite well through the cloud deck, which does not 1 250 km, limited by seeing in the Earth’s atmosphere or by the diffraction limit of the telescope. With a large adaptive optics telescope, a resolution of 20 milliarcsec is possible [e.g. (Marchis et al., 2002)], which is a surface resolution on Venus of about 5 km when it is close to Earth. Since the visibility of the surface of Venus is limited by scattering within the clouds, it should be possible to obtain Venus surface images from ground-based telescopes that have the same resolution as Venus orbital images. However, only about a third of Venus’ surface can be observed at high resolution from Earth. Venus presents the same longitudes each time it comes close to Earth, and the polar surface cannot be seen due to large atmospheric path lengths. More importantly telluric water vapor and OH limit the quality of near-IR images. significantly absorb these wavelengths. Venus’ surface has been observed from Earth-based telescopes, where contrast is created mostly by temperature differences due to topography (Lecacheux et al., 1993; Meadows and Crisp, 1996), and was mapped from orbit by Venus Express using the VIRTIS-M channel near this wavelength (Helbert et al., 2008; Mueller et al., 2008). However, the clouds scatter 1.02 μm radiation, so that viewing the surface through them is like looking through frosted glass with the light source close to the glass. Moroz (2002) showed that light from a point on the surface is scattered horizontally about 60 km within the clouds (about twice the depth of the clouds). In practice, VIRTIS-M was able to achieve about 100 km horizontal resolution. The best surface resolution that has been achieved from Earth-based telescopes is about Table 1. Venus sky brightness relative to surface brightness from several altitudes Altitude 1.02 μm night Surface Sky 1.02 μm day Surface Sky 0.85 μm day Surface Sky 0.65 μm day Surface Sky 45 1 2.6 1 38.5 1 232.6 1 2.6 x 105 30 16 8 4 2 1 1 1 1 1 1 1 2.3 1.4 0.7 0.3 0.2 0.1 1 1 1 1 1 1 23.8 13.5 6.3 3.2 2.0 1.3 1 1 1 1 1 1 196.0 138.9 33.3 9.1 3.1 1.6 1 1 1 1 1 1 6.7 x 104 6.7 x 103 526.3 66.7 12.8 6.4 Hashimoto and Sugita (2003) developed ‘declouding’ algorithms that use longer wavelengths for characterizing and removing cloud opacity. Applying this technique to the Galileo NIMS data, and accounting for surface elevation differences Hashimoto et al. (2008) concluded that intrinsic variation of emissivity at 1.02 μm was possibly due to major differences in igneous rock type. They interpreted the Galileo data to mean that the highlands were more felsic than the mafic lowland plains. The implication of this conclusion is that the highlands may be continent-like, composed of evolved felsic igneous minerals that required water to form. Mueller et al. (2008) came to the same conclusion using the 1.02 μm emissivity variations seen by VIRTIS in the southern hemisphere, although they also suggested that differences in weathering regime or in the age of surface material could also be responsible for the surface emissivity differences. More recently, Smrekar et al. (2010) noted that high surface emissivity is correlated with one of the apparently youngest hot-spot volcanoes (at least at the resolution of the Magellan altimetry data), and interpreted this to mean that fresh material, less than 2.5 My old (and possibly much younger), has erupted there. The fresh material is likely to be chemically different from most of the surface because it has not weathered in the atmosphere as much, resulting in a higher emissivity. The limited laboratory work that has been done at Venus surface temperature and pressure indicate that surface-atmosphere reactions proceed rapidly relative to geologic timescales (Fegley et al., 1995; Fegley and Prinn, 1989). Descending over one of the nine identified potential volcanic hot-spots (Stofan et al., 1995) during the day, it should be possible to tell rather quickly whether the volcano is active or has been 2 recently, from just a few images at near-visible wavelengths. Descent Vehicle Motion Another challenge to imaging the surface on descent, which is compounded by the low contrast of the surface as seen through the atmosphere, is the motion of the descent vehicle. For example, a typical camera with a 1024 x 1024 CCD might have an angular resolution of about 1 mrad/pixel. To prevent image smearing due to motion, the camera must not rotate more than 1 pixel during the exposure. Fig. 1 plots the maximum acceptable rotation rate about any spacecraft axis as a function of exposure time. The three lines show this maximum rate for camera with 0.5, 1.0, and 2.0 mrad/pixel. With the 1024 x 1024 CCD format, these correspond to fields of view of approximately 29, 59, and 117º. The figure shows that for high resolution images of the Venus surface with no smearing, either low vehicle motion or fast exposures are required. For example, for a camera with a 59º field of view, an exposure time of less than 10 msec is required to avoid image smearing if the vehicle is rotating about any axis faster than ~7º/s. Lorenz (2010) reviewed the available data on all planetary atmospheric probes. The information on vehicle motion for descent through the Venus atmosphere is shown in Table 2. Venera 11 and 12 and the Pioneer Venus probes all jettisoned their parachutes before reaching the base of the clouds. All exhibited motion that was too violent for obtaining unsmeared images with an exposure time of 10 ms, although in general the amplitude of the oscillations decreased with increasing depth into the atmosphere. With the motion experienced by these Venus probes, exposures of a few msec would have been necessary to obtain sharp images of the surface. Figure 1. Venus descent imaging: The maximum rotation rate about any axis of the spacecraft before image smearing occurs, as a function of exposure time, for 1024x1024 pixels, 60º FOV. With a field of view of 60º, a 1024 x 1024 camera would obtain images with a spatial resolution of 50 m at 45 km, and 9 m at 8 km. One effective way to obtain sharp images and at the same time increase the signal to noise ratio is to perform n x n binning. 2 x 2 binning would reduce the resolution to 100 m at 45 km and 18 m at 8 km. But it would also allow longer exposure times, and reduce noise by a factor of 2. The resolution of images acquired by a 1024 x 1024 format camera with 1 mrad/pix is shown below in Table 3 for several altitudes beneath the clouds. Note that the spatial resolution from just below the clouds is about the same as Magellan’s best at 75 m/px, so these images would be of 2700 km2 of the Venus surface in near-visible at a resolution comparable to the highest resolution Magellan SAR images. Table 2. Empirical data on Venus descent probe motion from Lorenz (2010) Spacecraft Rotation Rate RPM Period Angle of Attack Venera 11 10º/s 1.7 2.5 s 7º Venera 12 PV Large Probe PV Small Probes 20º/s 10º/s 3.3 1.7 1.5 s 1.1 s 1-2 s 8º 0-8º At 1.02 μm, the emitted flux from the cooler highlands is lower than the flux from low lying regions. At night, variations in the flux from the surface are dominated by temperature differences due to altitude. Since the flux goes as the fourth power of temperature, the flux emitted from mountain tops at 660 K is 1.6 times lower than flux from the lowest regions at 750 K. A 10% surface 3 emissivity variation would be masked by variations in altitude of about 9 km. Therefore, to make the most of nighttime descent images, altimetry of the images areas at high resolution is essential to separate altitude effects from intrinsic emissivity differences. Altimetry of day side images would also be of value to geologic interpretation of the scene. Reflected sunlight at 1.02 μm is about 1/3 the magnitude of emitted flux at this wavelength, so daytime images have both a reflected and emitted component. Table 3. Typical Venus Descent Image Resolution Altitude FOV 60º Resolution 1024x1024 Resolution 2x2 binning 45 30 16 8 4 2 1 52.0 km 34.6 km 18.5 km 9.2 km 4.6 km 2.3 km 1.2 km 50.8 m 33.8 m 18.1 m 9.0 m 4.5 m 2.2 m 1.1 m 101.6 m 67.6 m 36.2 m 18.0 m 9.0 m 4.4 m 2.2 m The uncompressed volume of a 16 bit 1024 x 1024 image is 16 Mb, or about 8 Mb with minimal compression. Terminal velocity of a descent probe in the deep Venus atmosphere is about 10 m/s. UHF propagates almost unattenuated in the Venus atmosphere (unlike S and particularly X band). At 100 Kb/s, the last image that could be transmitted before landing would be from an altitude of 800 m. With a field of view of 60º, features as small as 90 cm in the 1 km2 around the landing site could be discerned. The Talk I will present simulations of the Venus surface as seen at 1.02 μm by a probe descending beneath the clouds during both the night and day. 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