Astro 1 Section 1 Professor Brandt Wednesday April 14th 1999 Announcements: Lecture notes: Class 39. Terrestrial planets. Mercury Venus Earth Mars. Venus - Soviet &quot;Venera&quot; 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 &quot;Magellan&quot; 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 <a href="/keyword/carbon-dioxide/" >carbon dioxide</a> atmosphere. Only 1/100 as thick as Earth's. So very cold typically 0 F &quot;Viking&quot; 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 <a href="/keyword/carbon-dioxide/" >carbon dioxide</a> atmosphere all similar. Mars was too small so it lost its atmosphere. Venus was too hot to ever rain - leaving thick <a href="/keyword/carbon-dioxide/" >carbon dioxide</a> causing sever greenhouse effect. But on Earth rains came and oceans dissolved <a href="/keyword/carbon-dioxide/" >carbon dioxide</a> . Planet Venus Earth Mars Distance from Sun Mass .72 1 1.52 .82 1 .11 Radius .95 1 .53 Avg Density 5250 5517 3930 Mercury .39 AU .055 Earth masses .38 Earth Radii 5430 kg/m3 Earth mass about 6x1024 kg Earth radius about 6370 km Avg Density about 5517 kg/m3 Planet Mercury Venus Earth Mars Orbital Period .24 years .62 1 1.88 Rotation Period 58.6 days 243 days 1 1.03