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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
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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
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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
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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.
The simulations used a radiative transfer model
that treats multiple scattering in an inhomegeneous
atmosphere with the Venera 13 and 14 data
(Moshkin et al., 1983). The improved Magellan
topography of (Rappaport et al., 1999) was used
for determining the emitted nighttime surface flux.
During the day, the contrast is much lower, but the
confounding
effect
of
altitude-dependent
temperature is reduced.
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