Lecture 21: Venus, Summaries of Law

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Lecture 21: Venus

Venus

Terrestrial Planets Animation

Venus

•The orbit of Venus is almost circular, with eccentricitye = 0.

•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

0.72 AU

1 AU

Venus

Sun (^) Earth

47 o

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 isa = 0.72 AU

a

Sun

Venus

•Kepler’s third law relates the semi-major axis to the orbital period

Orbit of Venus

•Kepler’s third law relates the semi-major axisa to the orbital

periodP

(^23)

years AU

⎟ ⎠

⎞ ⎜ ⎝

⎛ ⎟⎟^ = ⎠

⎞ ⎜

⎜ ⎝

⎛ (^) P a

•Sincea = 0.72 AU for Venus, we obtainP = 0.611 Earth years

orP = 225 Earth days

•Solving for the periodP yields

3 / 2

years AU

⎟ ⎠

⎞ ⎜ ⎝

⎛ ⎟⎟^ = ⎠

⎞ ⎜⎜ ⎝

⎛ P a

•We have for the radius and mass of Venus ™R (^) venus = 6,052 km = 0.95 Rearth ™Mvenus = 4.9 x 10^27 g = 0.82 Mearth •The volume of Venus is therefore given by

venus Rvenus 3

4 V = π

Bulk Properties of Venus

•Hence Vvenus = 9.3 x 10^26 cm^3

•The average density of Venus is therefore

•We obtain

venus

venus

venus

9. 3 10 cm

10. 9 10 g

V

M

×

× ρ = =

venus 5.^2 gcm

ρ =

•This is similar to the average density of the Earth’s •Hence Venus probably contains lot of iron and a large, dense core

Bulk Properties 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

Rotation of Venus

sidereal orbital period is 225 Earth days

Rotation of Venus

•The synodic orbital period is longer than the sidereal orbital period •This is reversed in the outer planets

Sidereal vs. Synodic Orbital Periods

A: Inferior Conjunction

Sun

t = 0

Venus

•Note that the synodic orbital period of 584 Earth days is almost exactly 5 times longer than the solar day on Venus:

4. 99 117

584 = = solar day

synodicperiod

•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

Runaway Greenhouse Effect

•When CO 2 and H 2 O 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 CO 2 and SO 2

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…

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 SO (^2) above clouds

•However, no erupting volcanoes have ever been seen!

Rotating Venus Movie (Magellan data)

Interior of Venus

•Venus is probably differentiated, but has only one plate

•The crust is silicon- rich, 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