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Venus vs. Earth


In bulk, Venus and Earth are twin worlds.

nearly the same size

nearly the same density
This similarity leads to an expectation of similar evolutionary histories.

They are made of the same mix of rock and metal, must have had similar
outgassing and impact histories...
2
Venus vs. Earth



In bulk, Venus and Earth are twin worlds

nearly the same size

nearly the same density
This similarity leads to an expectation of similar evolutionary histories.
One look from the outside suggests that they have achieved substantially
different outcomes.
3
Venus: Evidently Not a Twin

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Venus has an atmosphere mostly
composed of Carbon Dioxide (96%
CO2 4% Nitrogen) with a surface
pressure nearly 100 times that of
Earth.
Clouds composed of droplets of
sulfuric acid hide 100% of the
surface 100% of the time.
The surface temperature is 750K
(900 F)
4
Venus: Evidently Not a Twin



Venus has an atmosphere mostly
composed of Carbon Dioxide with a
surface pressure nearly 100 times
that of Earth.
Clouds composed of droplets of
sulfuric acid hide 100% of the
surface 100% of the time.
The surface temperature is 750K
(900 F)
5
Venus: Hotter than Expected

The temperature of a planet is a balance
between the incoming solar radiation and the
outgoing blackbody “glow”, primarily at
infrared wavelengths.

Ignoring subtleties like reflectivity
(albedo) and infrared emission efficiency,
the temperature of a rotating planet is
280K divided by the square root of the
planet's distance from the sun in AU.

The Earth's equilibrium temperature
should be right around freezing. Venus
should be about 330K or 140F.

Accounting for Venus' high reflectivity
this simple calculation suggests a more
Earthlike temperature.

1950's science fiction movies
depicted a tropical Venus.
280K
T=
AU
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7
Venus: The Greenhouse on Steroids

Currently, on Earth, we are concerned about an atmospheric abundance of
Carbon Dioxide of a fraction of a percent.

The Earth's greenhouse is essential for life on Earth.


Accounting for albedo, the Earth's equilibrium temperature would be well below
freezing without it.
Contributors include water vapor, carbon dioxide and methane among others.
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The Greenhouse Effect

Carbon Dioxide is transparent to
visible light.

Light enters the
atmosphere and warms
the surface

The warm surface emits
infrared light

Carbon dioxide traps
infrared

Temperatures increase
until enough infrared leaks
out to balance incoming
sunlight energy.
9
Why the Difference between Earth and Venus??

Liquid water on Earth an lots of it!

Earth was initially a twin to Venus with a crushing
Carbon Dioxide atmosphere, but oceans removed
Earth's CO2 leaving Nitrogen.

Venus being closer to the Sun may



have received less water initially
had a hard time forming oceans due to the higher
temperatures (and the greenhouse effect of the water
vapor itself).
Where is the water today?

Ultraviolet light from the Sun can break up water
molecules

The hydrogen escapes due to its low mass.

“Heavy” hydrogen (deuterium) lingers

On Venus deuterium is 10 times more abundant
than on Earth, suggesting that it indeed has
forever lost an ocean worth of water.
10
Radar Mapping

The Venusian clouds forever hide the surface of Venus from prying eyes at
visible wavelengths

But radio light can penetrate the clouds.
11
Radar Mapping

The Venusian clouds forever hide the surface of Venus from prying eyes at
visible wavelengths


But radio light can penetrate the clouds.
An Earth-based or Venus-orbiting radio transmitter can send out a pulse of
radio light (like a flash picture).

The time delay for the pulse coming back reveals surface topography.

The brightness of the returned signal reveals surface roughness.

A rotating planet combined with the Doppler effect permits a reconstruction
of a detailed picture.
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Venus does not display the clear distinction
between “highlands” and “lowlands” seeing on
the Earth.
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Landers

Spacecraft from Earth have visited the Venusian surface

Given the temperature, survival is brief – a few hours
18
Landers

Spacecraft from Earth have visited the Venusian surface

Given the temperature, survival is brief – a few hours
19
Landers

Spacecraft from Earth have visited the Venusian surface

Given the temperature, survival is brief – a few hours
20
Venusian Surface History

Large (>35 kilometer) impact craters uniformly pepper the surface of Venus.
1000 of them, compared with about 200 on Earth.

Venus' thick atmosphere stops smaller impactors.

The uniformity points to a global resurfacing event ½ billion years ago.
21
Venusian Volcanoes

Volcanoes are common on Venus.

Due to the lack of weather, erosion is slow. The volcanism may look
“recent” simply because of this lack of erosion.

Most impact craters are undisturbed.

Some astronomers argue that Venus is currently geologically dead.
22
The Venusian Surface


Overall, the surface of Venus looks quite young, consistent with a global
resurfacing ½ billion years ago.
At the same time, much of the “action” seems to have occurred at this distant
time, and Venus is likely not quite so active today.
23
The Venusian Surface


Overall, the surface of Venus looks quite young, consistent with a global
resurfacing ½ billion years ago.
At the same time, much of the “action” seems to have occurred at this distant
time, and Venus is likely not quite so active today.
24
One theory:
Earth dissipates its internal heat through
plate tectonics and volcanism at plate
boundaries.
Venus does not have a crust that
accommodates this process. Heat builds up
leading to catastrophic resurfacing at billionyear intervals.