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Module 11: Venus the Sulphurous Greenhouse
Activity 2:
Observing the Surface of
Venus
Summary:
In this Activity, we will investigate
(a) exploring Venus by radar,
(b) landing on Venus, and
(c) the surface of Venus
- cratering, volcanism, tectonic activity
(a) Exploring Venus by Radar
Venus’ thick covering of clouds rules out the use of
optical astronomy.
However radio waves travel through clouds, and so we
can use the techniques of radar astronomy to send radar
signals to Venus (either from Earth, or from an orbiting
radar transmitting space probe) and analyse them on
their return.
Without radar astronomy, we would not even know how
fast the surface of Venus rotates.
The Arecibo 305 Metre Radio Telescope, formed out of a
natural limestone sinkhole in Puerto Rico, can be used as a
powerful radar transceiver* for planetary studies.
* transmits & receives radar signals
Radar astronomy is used to measure accurately the
distance to Solar System objects by using the “round-trip
time” for the signal (to accuracy of 1 microsecond)
- and is also important for navigation on space missions.
Using Doppler shift techniques, radar astronomy has
been used to determine the rotation rates of Mercury
and Venus.
Radar astronomy can also be used to map surface
features of Solar System targets.
Radar imaging can directly resolve surface features
on the Moon:
(not to scale!)
But Venus is much further away, and so only low resolution
maps of Venus are achieved by radar imaging from Earth.
Relatively high resolution radar mapping of the surface
of Venus came about once radar-equipped space probes
could be placed in orbit around Venus.
Radar imaging of Venus transformed our view of our
closest planetary neighbour:
The first orbital radar mapping of Venus was carried out
by Pioneer 12, the Pioneer Venus Orbiter.
Pioneer 12 entered orbit around Venus in 1978, and
made radar maps of 93% of the planet’s surface. Designed
to carry out an 8 month mission, it in fact stayed in operation
until October 1992!
Radar mapping was also
carried out by the USSR
Venera 15 & 16 probes,
launched in 1983, and then
the US Magellan spacecraft,
between 1990 and 1994,
mapped the surface of
Venus to a resolution of
100m.
Magellan was the first
space probe to be launched
by the Shuttle program.
Magellan
This 3-kilometer
resolution radar map is
a composite of Magellan
images compiled
between 1990 and
1994. Gaps were filled
in by the Earth-based
Arecibo Radio
Telescope.
Ishtar Terra highlands featuring Maxwell Montes,
the largest mountain on Venus
In this false-colour map,
red represents
mountains, while blue
represents valleys.
On a radar map, bright regions correspond to terrain which
is highly reflective to radar  rough terrain,
and dark regions correspond to terrain which does not reflect
radar well  smooth terrain.
So radar maps of Venus can look rather strange, but
then, so can radar maps of Earth:
This image of Mt. Rainier,
Washington USA, planet
Earth, was produced by the
Spaceborne Radar
Laboratory which flew on
the Space Shuttle
Endeavour in 1994.
As with all fields of astronomical imaging, it takes
training and practice to interpret such images
correctly.
(b) Landing on Venus
Crafts that land on Venus have to contend with the extreme
temperature (hot enough to melt lead and zinc) and crushing
pressure at its surface.
The first spacecraft to land on Venus* was Soviet Venera 7,
which arrived at the surface on 15 December 1970 and
transmitted data for 23 minutes. This was followed by
Venera 8, which landed on 22 July 1972. Neither of these
spacecrafts, however, had cameras so no images were
returned.
* One could argue that Venera 3 was the first spacecraft to reach Venus –
impacting the surface (rather than landing) on 1 March 1966. The
communications systems failed however and no data was transmitted.
The first spacecraft to return images from Venus was
Venera 9, which arrived at the surface on 22 October 1975
and operated for 53 minutes. It transmitted a single image:
Three days later, Venera 10 arrived and also managed to
transmit an image in the 65 minutes it remained operational.
Images were returned from a total of four of the Soviet
Venera spacecrafts (Venera 9, 10, 13 & 14) which landed
on Venus between the 1970s and early 1980s, and
managed between them to conduct experiments, collect
data and take pictures for typically an hour before being
put out of commission by the extreme conditions.
They carried out radioactive testing on rocks at their landing
sites, finding what appeared to be basaltic at some locations
and granitic at others - both types of rock which are originally
formed from lava.
Lens cap!
Venera 13 survived for 2h7mins and took 14 images on 1 March 1982
(c) The Surface of Venus
The surface of Venus
is fairly flat:
20% lowland plains,
70% rolling uplands,
and
10% highlands.
With the exception of the three highland regions, elevation
differences are only 2 to 3km.
Even the highland–lowland elevation differences are only
about 12km, compared to 20km*on Earth and 25km on
Mars.
* Mt. Everest reaches slightly under 9km above sea level,
and the Marianas trench descends 11km below sea level.
Map of the terrain, false-coloured to emphasize altitude differences:
the northern hemisphere is relatively mountainous,
with uncratered upland plateaus
the southern hemisphere
is mainly rolling, cratered, lava plains
Which parts of the surface of Venus would you expect
to be the oldest
- the plateaus in the north, or the lava plains in the
south?
The presence of cratering on the southern lowland plains
gives us a clue.
We expect significant cratering to indicate relatively old
surfaces, perhaps dating from the era of intense
bombardment in the early Solar System.
The plateaus in the north are relatively uncratered,
indicating that the surface has been reworked there, by
volcanic flows or tectonic activity, in more recent times.
So we conclude that the cratered southern lowlands
represent a more ancient venusian surface than do the
highlands.
This is opposite to the situation here on Earth:
here the lowlands, which are our ocean basins, are
the newest part of the terrestrial surface.
Venus is a rocky world, illuminated by orange sunlight:
- picture sent back by Venera 13, which landed
on Venus in 1982. Part of the Venera lander is visible here
• Cratering on Venus
More cratering is evident on Venus than on Earth, but
as we have noted before, other influences including plate
tectonics and biological effects have probably acted to
wipe out or obscure much of the terrestrial cratering record.
Planetary scientists study cratering on Venus to learn
more about the planet, but also to learn more about
cratering on Earth. Both planets have similar
accelerations due to gravity, and both have substantial
atmospheres.
This Magellan radar image of
Lavinia Planitia is again falsecoloured to emphasize
differences in elevation, with red
indicating high- and blue
indicating low-lying ground.
3 large meteorite impact craters,
diameters of 35 to 65 km, are
surrounded by bright (rough)
ejecta, and appear to be partially
filled with dark (smooth) material,
probably lava risen to the surface
through fractures produced by the
impacts.
Ejecta from the original
impact litter the surrounds of
the craters, and come out as
light, bright areas in a radar
map because they are rough
terrain. They are coded in the
red-orange-yellow range,
because they occur high up
on the sides of the craters.
Lava flows are predominantly
smooth, so they appear dark
in a radar map. They are coded
blue, because they occur
deep down on the crater floor.
Compared to the Moon and Mercury, Venus has a much
smaller number of craters, and in particular lacks very
small craters.
The latter is probably due to the very dense atmosphere
of Venus.
Think about why this might not favour very small craters,
then click here to see if you agree with our explanation.
• Volcanism on Venus
From what we know about the surface of Venus, it is largely
dominated by volcanism.
Thousands of volcanoes dot the surface, which is also marked
by lava flows
of up to
thousands of
kilometers
in length
- responsible
for wiping
out much of
the cratering
record on
Venus.
Maat Mons, an 8-kilometer volcano, is shown in this threedimensional perspective reconstruction of the surface of Venus,
as seen from a viewpoint 560 kilometers north of Maat Mons at an
elevation of 1.7 kilometers. Lava flows extend for hundreds of
kilometers across the fractured plains shown in the foreground,
to the base of Maat Mons.
In order for us to
more clearly see
detail, NASA 3D
reconstructions
are usually
exaggerated
by a factor
of 22.5 in
height!
Here is a similar image, this time of Gula Mons, a 3 km high
volcano:
… but here is the same volcano, without the factor of 22.5
exaggeration in height!
As you can see, exaggerating the vertical scale helps us
see detail, especially as the terrain all appears to have
similar colouring.
However it can give us a false sense of how steep
the terrain really is.
Volcanoes on Venus are shield volcanoes, which form
from particularly fluid lava and have extremely shallow slopes.
Shield volcanoes are common on Earth too - the largest
terrestrial volcano,
Mauna Loa in
Hawaii, is also a
shield volcano.
Below is a radar image of one fairly common type of
venusian volcanic feature which showed up in the
Magellan images.
Known as a “venusian tick”, it represents a volcano about
32 kilometers wide at the summit with ridges and valleys
radiating down its sides lending it an insect like
appearance.
Other volcanoes on Venus form as pancake-shaped
domes up to 25 km across and 2 km high,
presumably as the product of very thick sticky lava.
These volcanoes have no counterpart on Earth.
How Venus manages to produce both lava which is
free-flowing enough to extend thousands of
kilometres, plus lava viscous enough to form the
pancake-shaped domes that we have just seen, is an
unresolved issue!
Sometimes the lava has not erupted through the surface at
all, but instead has formed large circular or oval bulges
hundreds of kilometers across, called coronae.
The limited radar images we have of Venus cannot
confirm whether any volcanic activity continues.
However there is some indirect evidence - for example,
of lightning strikes possibly concentrated over volcanic
regions, and fluctuations in the level of sulphur dioxide
in the atmosphere - which suggest that volcanic activity is
still happening on Venus.
Perhaps future missions to Venus will manage to resolve
the issue by imaging a venusian volcano in the act of
erupting!
• Tectonic Activity on Venus
While no large-scale plate tectonics of the scale of that
on Earth appears to be happening on Venus, the lava
flows indicate convective flows in its mantle, as on Earth.
Limited tectonic activity of a sort does seem to occur,
creating patterns of ridges and rift valleys in the lowlands,
and much of the highland mountain regions appear to
have been produced by compression of the crust.
One theory suggests that periodically sections of the hot
flexible crust (kept that way by the thick atmosphere) may
sink into the mantle and new crust may be formed from
lava flows and volcanism - a periodic resurfacing of large
sections of the planet.
Gula Mons
Sif Mons
700 km long rift valley
(Computer reconstruction, with vertical scale again exaggerated)
We’ll delay modelling the evolution of Venus so that
we can compare it directly to that of Mars in a later
Activity.
In the meantime, when you have finished this Activity,
use the CD-ROM which accompanies the Universe
textbook to view simulated fly-pasts of venusian
volcanoes, rift valleys and craters - in the Animations &
Videos section.
In the same section there is also a video showing
views of the Arecibo telescope.
In the next Activity we will go on to look at the Earth’s
other neighbour, Mars.
Image Credits
Arecibo
http://www.naic.edu/about/photos/aoviews/tescop.jpg
Venera 13 surface view of Venus
http://nssdc.gsfc.nasa.gov/image/planetary/venus/venera13-left.jpg
NASA: Pioneer 12
http://loenardo.jpl.nasa.gov/msl/QuickLooks/pioneer12QL.html
NASA: Venus globe
http://nssdc.gsfc.nasa.gov/image/planetary/venus/venusglobe.jpg
NASA: Earth globe
http://pds.jpl.nasa.gov/planets/welcome/earth.htm
NASA: Just passing by
http://antwrp.gsfc.nasa.gov/apod/ap980501.html
NASA: Impact Craters
http://pds.jpl.nasa.gov/planets/gif/ven/dsnall.gif
Mauna Loa, Hawaii,US Geological Survey, J.D. Griggs
http://wwwhvo.wr.usgs.gov/maunaloa/4303062_M.jpg
Image Credits
NASA:
Ovda Regio
http://pds.jpl.nasa.gov/planets/gif/ven/ovdan.gif
Topographic Map of Venus from Pioneer Venus (Mercator Projection)
http://nssdc.gsfc.nasa.gov/image/planetary/venus/pvo_topo_mercator.jpg
Beneath Venus' Clouds
http://antwrp.gsfc.nasa.gov/apod/ap960924.html
Complex Craters, Venus
http://cass.jsc.nasa.gov/images/simp/simp_S05TN.gif
Computer-generated surface view of Maat Mons
http://nssdc.gsfc.nasa.gov/image/planetary/venus/mgn_maat_mons.jpg
Computer generated surface view of Gula Mons.
http://nssdc.gsfc.nasa.gov/image/planetary/venus/mgn_gula_mons.jpg
Gula Mons (to realistic scale)
http://nssdc.gsfc.nasa.gov/image/planetary/venus/mgn_gula_mons_squish.jpg
Image Credits
A Venusian Tick,
http://antwrp.gsfc.nasa.gov/apod/ap950823.html
Computer generated surface view of Alpha Regio
http://nssdc.gsfc.nasa.gov/image/planetary/venus/mgn_alpha_regio.jpg
Computer generated surface view of a corona, tentatively named Idem-Kuva.
http://nssdc.gsfc.nasa.gov/image/planetary/venus/mgn_corona_idem-kuva.jpg
Now return to the Module 11 home page, and
read more about the surface of the Venus in the
Textbook Readings.
Hit the Esc key (escape)
to return to the Module 11 Home Page
Atmospheres and Craters
As we will see when we investigate meteorites on
Earth in a later Activity, Solar System debris striking
a planetary atmosphere largely “burns up” before it
hits the surface.
As the atmosphere of Venus is very dense compared
to that of the Moon and Mercury or even Earth, the
thick atmosphere probably tends to destroy small debris
before it can reach the surface, leading to a lack of very
small craters.
Also, even somewhat larger projectiles probably tend to
break up into smaller fragments when they hit the venusian
atmosphere, and those fragments in turn are likely to burn
up.
Click here to return to the Activity!