Lecture #17:
The Main Point
• Surface of Venus:
• Radar Mapping Concepts.
Radar has been used to map the
surface of Venus, and we have found
a relatively “young” surface with
abundant evidence for volcanic,
tectonic, erosional, and impact
processes at work.
• The Magellan mission.
• Surface properties.
• Reading:
– Chapter 9.5.
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Recall: The surface of Venus is obscured from view at visible to
infrared wavelengths by a thick layer of clouds.
But light at radio wavelengths (~1 mm to ~1 m) penetrates the clouds.
Radar (an acronym for RAdio Detection And Ranging) is used to
study the surface of Venus remotely.
Radio sensors are either passive or active.
– Passive systems simply record radio energy emitted from a surface.
– Active systems also record the reflected radio waves they transmit.
Radar mapping of Venus (and Earth) uses active systems:
– Arecibo radio telescope.
– Spacecraft like Magellan or Venera.
– All transmit radio waves, then measure the reflected radio energy.
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Scattering of Radio Waves
Radar Mapping
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• Smooth surfaces
scatter radio waves
back to the
transmitter like a
mirror (specular).
• Rough surfaces
scatter radio waves
in all directions
(diffuse).
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Interpretation of Radar Images
The Magellan Mission
• Named after the 16th century
Portuguese explorer who
first circumnavigated the Earth.
• Launched from the Space Shuttle in May 1989.
• Mapped Venus from Sept. 1990 to Oct. 1994:
Radar images are different from optical
images and can be somewhat counterintuitive. For most geologic targets, the
strength of the radar echo depends
primarily on the roughness of the surface
(bright = rough, dark = smooth).
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Radar image of Venus
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Covered 98% of Venus' surface with a resolution of ~ 1 km.
Measured surface topography to ~100 m vertical resolution.
Measured global gravity field to high accuracy.
Provided data for insights on surface geologic processes and the nature of
the planet's interior.
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Geology of Venus: Magellan Results
A Relatively Simple
Spacecraft!
• Most of it is just a large
satellite dish with a
transmitter and receiver.
• Used spare parts from
Voyager and other Earth
and planetary missions.
• Cost: about $650 million
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Venus Topography: Most of the surface is at the same
narrow range of elevations (vast lava plains).
Geology of Venus: Magellan Results
• Abundant evidence of past volcanism.
• Nearly 1000 impact craters > 1 km discovered:
– Crater density implies ~750 million year age.
– Some small areas could be much more recent.
– No "ancient" surface preserved.
• Abundant evidence of tectonism:
– But no clear evidence of plate tectonics like Earth.
• Some evidence for erosion:
– dunes, wind streaks, lava channels.
% of surface area
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Venus Global Topography
Volcanism on Venus
• Magellan data show vast expanses of low-lying lava plains.
• Two large "continents":
• Aphrodite (Africa-sized).
• Ishtar (Australia-sized): highest point 11 km (Maxwell Montes).
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• Hundreds of thousands of volcanoes have
been discovered on Venus!
• Volcanism is widespread and planetwide.
• Vast “mare-like” lava plains dominate.
• But many individual cones and mountains.
• Also: evidence for subsurface magma
intrusions.
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Sapas Mons ~300 km diameter
Impact Craters on Venus
• Nearly 1000 craters > 1 km diameter have
been discovered in Magellan images:
– Very few craters smaller than 10 km.
– Impactors smaller than 1 km in size burn up!
– Craters 10 km to 30 km have irregular shapes.
"Pancake" domes
each ~25 km
• Relative Age Dating (counting craters)
suggests a ~uniform surface age of ~750
million years since the last resurfacing.
Cinder cone field
each ~1 km
Volcano and flows
~100 km across
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Three large craters
(largest ~50 km across)
Impact Craters on Venus
• About 1000 found > 1 km.
• Pretty evenly distributed around the surface (uniform age?)
Dickinson crater
(diameter ~70 km)
Golubkina
Crater
(diameter ~ 30 km)
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Stress fractures and cracks
(region is ~40 km across)
Tectonism on Venus
• Numerous patterns of ridges and cracks
seen in Magellan images:
– Evidence of tectonic forces.
– Probably driven by convection in the interior.
Crater ripped apart
by tectonic forces
(diameter ~37 km)
• Evidence for mountain belts and rift valleys.
• No direct evidence for Earth-like plate
tectonics, however.
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Wind streak
~5 km volcano
Erosion and Transport on Venus
• Evidence for wind transport in Magellan
images:
– Wind streaks.
– Dunes.
– Surface winds are weak, but the atmosphere is
very dense.
• Long, thin channels, but there’s no water!
– Erosion by flowing lava.
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Lava channel
~200 km long
~2 km wide
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“The Tick”
A volcano?
(size ~50 km)
Plus oddball landforms!
• Several categories of geologic features found in
Magellan images that are not familiar on Earth
– Coronae.
– Arachnoids.
– Mixed terrains.
• Impact, then volcanism, then tectonism... ???
Circular “corona” feature
(~200 km across. Formed by
rising “blobs” of hot mantle
material?
• Sometimes our terrestrial experience doesn’t help.
– Humbling, but to be expected?
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On the Surface
Surface Composition
• Venera landers returned images and
other data:
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• Soviet Venera 13 and 14 landers measured
the chemical composition of the surface:
Confirmed surface temperature and pressure.
Enough light gets through the clouds to take pictures.
Surface is flat, platy, maybe layered...?
Landscape is desolate and totally inhospitable to life.
– Elemental abundances and inferred mineralogy
is similar to basalt, a common low-Si volcanic
rock found on the Earth, Moon, and Mars.
• Presence of distinct "continents" may imply
the existence of lower-density minerals too.
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Next Lecture...
Summary
• Radar has been used to penetrate the clouds of Venus
and discover the underlying geology of the planet:
and climate:
– Radar bright features are rough.
– Radar dark features are smooth.
•Properties of the atmosphere.
• Abundant evidence for volcanism, tectonism, impact,
and erosion on Venus!
• Entire planet appears to have been resurfaced,
possibly by volcanoes, ~750 million years ago.
• Surface composition appears similar to Earth's
volcanic rocks.
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• Venus atmosphere
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•Greenhouse Effect.
•Climate Implications.
• Reading:
– Chapters 9.5, 9.6, 10.5, 10.6.
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