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ASTRONOMY 161
Introduction to Solar System Astronomy
Class 19
Venus
Friday, February 26
Venus: Basic characteristics
Mass = 4.868×1024 kg (0.815 Earth)
Radius = 6,052 km (0.949 Earth)
Density = 5,204 kg/m³
Sidereal rotation period = -243.0185 d
Albedo = 0.65 (Earth = 0.39)
Average distance from Sun = 0.723 A.U.
Venus: Key Concepts
(1) The surface of Venus is hidden from us by
clouds of sulfuric acid.
(2) The atmosphere of Venus is hot because of
a runaway greenhouse effect.
(3) The surface of Venus shows volcanic
activity but no plate tectonics.
(4) The interior of Venus is similar to that of
the Earth.
Venus: Revolution and Rotation
Orbital period = 225 days
Rotation period = 243 days
Rotation of Venus is retrograde.
On Venus, the time from noon to noon = 117 days
Venus is sometimes called Earth’s twin:
Radius = 95% of Earth’s
Mass = 82% of Earth’s
Surface gravity = 90% of Earth’s
(1) The surface of Venus is hidden from us
by clouds of sulfuric acid.
Venus has 100% cloud
cover.
Its rotation speed can be
found from radar signals
which pierce the clouds
(the same method used
for Mercury, using
Doppler effect).
The clouds of Venus,
about 60 kilometers
above the surface,
are made of
sulfuric acid.
Acid rain evaporates
in the hot air of
Venus before it
reaches the ground.
(2) The atmosphere of Venus is hot
because of a runaway greenhouse effect.
Dense and high-pressure atmosphere:
Air pressure at the surface of Venus = 90 times air
pressure on Earth (2/3 of a ton per square inch).
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96.5% carbon dioxide (CO2)
3.5% nitrogen (N2)
0.015% sulfur dioxide (SO2)
almost no water vapor
CO2 is a greenhouse gas: so Venus has a
GARGANTUAN greenhouse effect.
Surface temperature = 733 Kelvin
(860 degrees Fahrenheit)
Even hotter than Mercury!
Temperatures drop very little during the long night,
thanks to the thick atmospheric blanket.
Escape velocity for Venus = 10.4 km/sec
Venus was once cooler, and may have had seas.
Volcanoes produced large amounts of H2O, CO2, S02
(all are greenhouse gases)
Air temperature rose.
H2O evaporated from seas;
CO2, SO2 released from seas, rocks.
RUNAWAY greenhouse effect!
Venus has swift
E-to-W winds at
high altitudes, but
no circular storms.
Slow rotation means
that the “coriolis
effect” is tiny.
(3) The surface of Venus shows
volcanic activity but no plate tectonics.
Because clouds are
opaque, surface of
Venus has been
mapped using radar,
by the spacecraft
Magellan.
Round-trip travel time
gives us the distance:
d=(t2-t1)/2c
Most of Venus is covered with rolling plains, but there
are two large highland regions:
Ishtar Terra (north) ~ Australia
Aphrodite Terra (equator) ~ Africa
Venus: many volcanoes, few impact craters.
More than 1600 major volcanoes.
Estimated average age of crust = 500 million years
(older than Earth, much younger than Mercury or Moon).
Perspective view of Venusian volcanoes
(Warning: vertical scale is exaggerated by a factor of 10):
About 1000 large impact craters
(more than Earth, much less than Moon).
Perspective view of Venusian craters:
Older craters have been obliterated by lava flows.
Venus
lava
flows:
On the Earth, the cold
rigid lithosphere is
broken into plates:
On Venus, the hot
plastic lithosphere
does not break:
No plate tectonics on
Venus.
Levels of Volcanic Activity
Earth: extremely active
Venus: mildly active
Moon: volcanically dead
Mercury: most likely volcanically dead
We have evidence for convection in the asthenosphere
of Venus. Wrinkled mountains are seen where the
crust is compressed.
Maxwell Mountains – the highest on Venus – consist of
parallel ridges.
More evidence for
convection in the
asthenosphere of
Venus:
Fractures (or
“stretch marks”)
are seen where the
crust is pulled
apart.
Venus in extreme closeup:
Image returned by Venera 13
spacecraft (1982).
Venus Express
First European (ESA)
mission to Venus,
in orbit since May
2006.
First
results
from
Venus
Express:
(4) The interior of Venus is
similar to that of the Earth.
Uncompressed
density of Venus =
uncompressed
density of Earth =
4200 kg/m3.
Venus probably has a
metal core and
rocky mantle, like
the Earth.
Few closing questions:
1) Name the major differences and similarities
between Venus and the Earth.
2) How dense is the atmosphere of Venus?
3) Is there magnetic field on Venus?
4) What is the name of Venus’ moon?
5) Are there volcanoes on Venus?
6) Are there quakes on Venus?