Wednesday April 14th 1999
Announcements:

Lecture notes:

Class 39.
Terrestrial planets.
Mercury Venus Earth Mars.

Venus - Soviet "Venera" spacecraft orbited in the late 1970s.  Venus atmosphere is about 100 times as dense as Earth's.
Temperature is about 1000 F.  So most exploration is from radar mappings by the US "Magellan"
mission in the early 90s.

Earth is distinctive because of water.
Requires temperatures not too hot (would evaporate) or not too cold (would freeze)

Mars has carbon dioxide atmosphere.  Only 1/100 as thick as Earth's.  So very cold typically 0 F
"Viking" missions landed in 1976.  Mars used to have water 3 to 4 billion years ago.  But atmosphere
evaporated so the planet cooled.  In 1997 Pathfinder landed

In the beginning Venus Earth Mars had thick water vapor and carbon dioxide atmosphere all similar.  Mars was too small so it lost its atmosphere.  Venus was too hot to ever rain - leaving thick carbon dioxide causing sever greenhouse effect.

But on Earth rains came and oceans dissolved carbon dioxide.
 
 

Planet	Distance from Sun	Mass	Radius	Avg Density
Mercury	.39 AU	.055 Earth masses	.38 Earth Radii	5430 kg/m3
Venus	.72	.82	.95	5250
Earth	1	1	1	5517
Mars	1.52	.11	.53	3930

Earth mass about 6x10²⁴ kg
Earth radius about 6370 km
Avg Density about 5517 kg/m³
 
 

Planet	Orbital Period	Rotation Period
Mercury	.24 years	58.6 days
Venus	.62	243 days
Earth	1	1
Mars	1.88	1.03