Descriptive Astronomy 101


"Earth's Twin"
"Morning Star"
"Evening Star"
Second planet from the Sun
108.2 million km
0.723 AU

Venus Stats

Revolution = 224.7 days
Rotation = 243 days (retrograde)
Diameter = 12,104 km
- Earth's diameter 12,740 km
Density = 5.3 g/cm3
Axial Tilt = 178°
Mass = 4.87 x 1024 kg
Escape velocity = 10,400 m/sec
Average distance from Sun = 0.723 AU
Orbit inclination = 3.39 degrees
Orbit eccentricity = 0.007
Mean surface temperature = 726 K
Visual geometric albedo = 0.59
Highest point on surface = Maxwell Montes (17 km above mean planetary radius)

Interior Structure

Similar size to Earth
Similar distance from Sun
Suggests similar internal structure
Density = 5.24 g/cm3
Earth density = 5.52 g/cm3

Magnetic Field

Venus lacks a strong magnetic field
Planetary magnetic fields are generated in the liquid portion of the core of a planet.
3 requirements to generate a planetary magnetic field
Rotation of the planet causes the liquid iron in the core to acts as a large electric dynamo - this generates a magnetic field.
Outer Core is liquid and convects due to heating from the Inner Core - this adds to the dynamo effect.
Venus lacks a strong magnetic field
Suggests two possibilities 1) 2)


Surface of Venus is not visible due to planet-wide cloud layer

Composition Venus Earth
CO2 96.4 %       0.033 %
N2 3.5 % 78 %
O2 trace 21 %
H2O trace 0-5 %
Ar trace 0.9 %
CO, H2S, SO2, HCl, HF       trace trace

Atmosphere - CO2

Venus: Why so much?
Earth: Why so little?
Both are believed to have started with very similar atmospheres of CO2, N2, H2O and H2
Originally produced as outgassing
How does this operate?
Both Earth and Venus had abundant CO2
Earth: Why so little today?

Greenhouse Effect

Why is CO2 important?
CO2 is a greenhouse gas - blocks the escape of infrared radiation
A greenhouse keeps temperatures higher inside than outside.
A system in which shortwave radiation is allowed to enter freely and is absorbed, then reradiated as longwave radiation. The longwave radiation is then retained within the system.
Venus: High surface temperatures: 650 - 760 K (710 to 908°F)
Earth: 240 to 320 K (-27 to 116°F)
Why is Venus so hot when compared to the Earth?

Impact Craters

Similar morphology to lunar craters
Only 6 multi-ring basins
Bowl-shaped, central peaks, terraced rims are all found
Ejecta material is different
"Melted or liquid look" to the ejecta
Long irregular flows often extend away from the craters

Impact Craters - Oddities



90% of surface covered by volcanic features
variety of volcanic landforms
indicates a variety of igneous compositions
rhyolitic to basaltic
>100,000 volcanoes may exist on surface


Sheild volcanoes:

Fissure eruptions:

Flood lavas:

Coronae - large, broad upwellings associated with volcanic activity (no similar structures on Earth).


Different than tectonic activity on Earth
Crust is thicker and stronger than Earth's crust.
Reason why:
Why is there little or no water in the crust, on the surface, in the atmosphere?

The thicker, stronger crust and warmer temperatures mean:
Earth's crust deforms and moves easily:

Tectonic Features

No subduction - no clear areas where the crust is sinking into the mantle.
Fracturing and faulting has been observed.
Many of the fractures are associated with ___________________.
Compression has resulted in folding (bending) of the crust and the formation of highland areas.


Very little apparent surface erosional processes, based on views from the surface taken by Venera probes
Primarily limited to:

Planetary Resurfacing

Random distribution of craters
Cratering rate indicates that the surface of Venus is only 300 to 600 million years old.
This and other evidence points to a massive, planetary resurfacing event.
Estimates suggest this resurfacing event took 10 to 100 million years to occur.

Plate Tectonic Resurfacing Model

Interior planetary heat builds up to the point where it overcomes the stronger, thicker crust
Spreading centers develop creating new crust
Subduction zones develop - allowing old crust to sink into the interior
The main features of plate tectonics (ridges, trenches, and transform faults) tend to disappear once plate tectonics has stopped, either because
These features are no longer thermally maintained
These features are topographic lows that become filled in by volcanism.

Additional Reading

Volcanism and Tectonic on Venus by F. Nimmo and D. McKenzie (PDF file)