Lecture 21: Venus
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Venus
Terrestrial Planets Animation
Venus
•The orbit of Venus is almost circular, with eccentricity e = 0.0068
•The average Sun-Venus distance is 0.72 AU (108,491,000 km)
•Like Mercury, Venus always appears close to the Sun in the sky
Venus
0.72 AU
47o
1 AU
Sun
Earth
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Venus
•Venus is visible for no more than about three hours
•The Earth rotates 360o in 24 hours, or
360 o 15 o
=
24 hr hr
•Since the maximum elongation of Venus is 47o, the maximum
time for the Sun to rise after Venus is
∆t=
47 o
≈ 3 hours
15o / hr
Venus
•The albedo of an object is the fraction of the incident light that is
reflected
™Albedo = 0.1 for Mercury
™Albedo = 0.1 for Moon
™Albedo = 0.4 for Earth
™Albedo = 0.7 for Venus
•Venus is the third brightest object in the sky (Sun, Moon, Venus)
•It is very bright because it is
™Close to the Sun
™Fairly large (about Earth size)
™Highly reflective (large albedo)
Venus
•Where in its orbit does Venus appears brightest as viewed from
Earth?
•There are two competing effects:
™ Venus appears larger
when closer
™ The phase of Venus
changes along its
orbit
•Maximum brightness
occurs at elongation
angle 39o
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Venus
•Since Venus is closer to the Sun than the Earth is, it’s apparent
motion can be retrograde
•Transits occur when Venus passes in front of the solar disk as
viewed from Earth
•This happens about once every 100 years (next one is in 2004)
Venus
(2 hour
increments)
Orbit of Venus
•The semi-major axis of the orbit of Venus is a = 0.72 AU
Venus
Sun
a
•Kepler’s third law relates the semi-major axis to the orbital
period
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Orbit of Venus
•Kepler’s third law relates the semi-major axis a to the orbital
period P
2
⎛ P ⎞ ⎛ a ⎞
⎜⎜
⎟⎟ = ⎜
⎟
⎝ years ⎠ ⎝ AU ⎠
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•Solving for the period P yields
⎛ P ⎞ ⎛ a ⎞
⎜⎜
⎟⎟ = ⎜
⎟
⎝ years ⎠ ⎝ AU ⎠
3/ 2
•Since a = 0.72 AU for Venus, we obtain P = 0.611 Earth years
or P = 225 Earth days
Bulk Properties of Venus
•We have for the radius and mass of Venus
™Rvenus = 6,052 km = 0.95 Rearth
™Mvenus = 4.9 x 1027 g = 0.82 Mearth
•The volume of Venus is therefore given by
Vvenus =
4
π R 3venus
3
•Hence Vvenus = 9.3 x 1026 cm3
Bulk Properties of Venus
•The average density of Venus is therefore
ρ venus =
M venus
4.9 ×10 27 g
=
Vvenus 9.3 ×10 26 cm 3
•We obtain
ρ venus = 5.2 g cm −3
•This is similar to the average density of the Earth’s
•Hence Venus probably contains lot of iron and a large, dense core
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Surface Gravity
•We can compute the surface acceleration on a planet or moon
using Newton’s laws of motion and gravitation:
F = mA =
- GMm
R2
•Solving for the surface acceleration A yields
A=
- GM
R2
Surface Gravity
•Using values for the Earth, Moon, Mercury, and Venus, we
obtain for the surface accelerations
A earth =
A mercury =
- GM earth
= 9.8 m s − 2
2
R earth
GM
A moon = − 2 moon = 1.7 m s − 2
R moon
- GM mercury
R 2mercury
= 3.7 m s − 2
A venus =
- GM venus
= 8.9 m s − 2
R 2venus
•The acceleration on Venus is about 91% of the acceleration on
the Earth
Rotation of Venus
•The surface of Venus is shrouded in thick clouds (in visible light)
•Based on observations of the motions of the cloud tops, the
rotation period of Venus was thought to be about 24 hours
•However, as in the case of Mercury, radar observations of Venus
made in the 1960’s proved surprising…
Mariner 10 (1974)
Galileo (1990)
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Rotation of Venus
•One sidereal day on Venus is takes 243 Earth days
•Unexpectedly, the rotation is retrograde (i.e., clockwise as viewed
from the north celestial pole)
•The time between noons is 117 Earth days = 1 solar day
•The sidereal period of the orbit of Venus is 225 Earth days
Rotation of Venus
sidereal orbital period is 225 Earth days
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Rotation of Venus
Sidereal vs. Synodic Orbital Periods
A: Inferior Conjunction
t=0
Sun
•The synodic orbital period is longer than the sidereal orbital
period
•This is reversed in the outer planets
Venus
•Note that the synodic orbital period of 584 Earth days is almost
exactly 5 times longer than the solar day on Venus:
synodic period 584
=
= 4.99
solar day
117
•This may represent a spin-orbit resonance between Venus and
the Earth, but the truth is unclear
•We are not sure why the spin of Venus is slow and retrograde
•This may be due to a massive impact in the distant past of
Venus
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Venus
•This may be due to a massive impact in the distant past of
Venus
•Telescopic observations of Venus reveal:
™Lots of CO2 in clouds
™Temperature of 240 K at cloud tops
Venus
•This led astronomers to believe that the surface temperature of
Venus was probably only slightly higher than that of the Earth
•Radio observations of the blackbody (Planck) spectrum radiated
by the surface of Venus displayed a Wien spectrum with
temperature T = 600 K
•Venus has been explored by 20 different spacecraft, both
American and Soviet
™Mariner 2, 5, 10 (radar mapping from orbit)
™Venera 4 – 12 (landed on surface)
•The earliest Venera spacecraft were crushed by the high
pressure in the atmosphere before reaching the surface!
Venus
•Atmospheric composition:
™Clouds are mostly sulfuric acid
™“Air” is mostly CO2
™Carbon Dioxide: 96.5%
™Nitrogen: 3.5%
™Trace gases: < 0.01%
™Practically zero water
™Almost no oxygen
•Why is the composition of the atmosphere of Venus so different
from Earth’s?
•The primary atmosphere of Venus (mostly hydrogen) was lost
into space due to the high temperature and low mass of Venus
Venus Express Mission
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Runaway Greenhouse Effect
•When CO2 and H2O were originally out-gassed from volcanoes
to form the secondary atmosphere, the temperature was too
high for the them to be absorbed by rocks
•Precipitation (rain) never formed, therefore no oceans or lakes
were produced
•Instead, a heavy “blanket” of greenhouse gases surrounded the
planet:
Runaway Greenhouse Effect
•Due to the greenhouse-gas blanket, the equilibrium
temperature was very high: T = 1,500 K
•The water vapor rose to very high altitudes, where it was split
apart by solar UV radiation:
•The hydrogen escaped into space, and the free oxygen reacted
with sulfur and carbon to form CO2 and SO2
Runaway Greenhouse Effect
•The planet ended up hot and dry as a result
•This is an irreversible process, called the “Runaway Greenhouse
Effect”
•Hopefully this will never happen to the Earth, but who knows
for sure?
•The future for Earth probably depends on how well we can
manage the production of greenhouse gases
•Already, we see strong evidence for global warming caused by
man’s pollution…
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Surface of Venus
•Radar observations from orbit indicate
that Venus is mostly smooth
•There are only 2 or 3 continent-sized
features comparable in elevation to
Earth’s continents
•Many craters are observed
•Most of the craters are volcanic in origin,
but some are due to asteroid impacts
Venus Topography Animation
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Surface of Venus
•Some of the craters are lava-filled due to violent impacts and
volcanic activity
• Over 85% of the surface is covered with volcanic rock
•There is no evidence for “seafloor spreading” and no plate
tectonic activity, although we don’t understand why
•Volcanism “resurfaces” the planet every few million years
•This process fills in craters
Surface of Venus
•Volcanism is probably
ongoing
•Radio flares suggest
lightning discharges
occurring near active
volcanoes
•Fluctuations are
observed in the
concentration of SO2
above clouds
•However, no erupting
volcanoes have ever
been seen!
Rotating Venus Movie (Magellan data)
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Interior of Venus
•Venus is probably
differentiated, but
has only one plate
•The crust is siliconrich, like the Earth’s
•The core of Venus is
probably molten, like
the Earth’s
•Venus has no
magnetic field,
probably due to the
very slow rotation
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