Ch. 9 and 10: the other Terrestrial Planets We survey the four terrestrials first, then look at details for Mercury, Venus, and Mars.

Terrestrial Planets’ Spin Axis and Rotation Rate

Terrestrial Planets’ Spin Mercury’s sidereal rotation is tidally locked to 2/3 of an orbit. Venus rotates very slowly backwards compared to the other Planets, so that it is rotating clockwise (others are CCW). Mercury and Venus both have almost no axial tilt, Earth and Mars both have similar axial tilts and rotation rates.

This tidal locking is different from that of the Mercury’s Rotation is tidally locked to the Rotation period. The solar day on Mercury is 176 days long, which is two Mercury-years! This tidal locking is different from that of the Moon revolving around Earth because Mercury’s orbit is elliptical and not nearly round like the Moon’s.

Atmospheres and Temperatures Mercury has no atmosphere, just like our moon. Because there is no atmosphere to trap heat, the night side of Mercury gets very cold (100 K or -280oF). The day side of Mercury gets up to 700 K (or 800oF). The atmosphere of Venus is made up of carbon dioxide, with clouds of sulfuric acid. The atmosphere is some 90 times denser than Earth’s. The Greenhouse effect causes the surface temperature of Venus to be close to 730K (or 860oF) day or night. Mars has a very thin atmosphere (less than 1% of Earth’s) of mainly carbon dioxide. The surface temperature is approximately 50 K (50oC) lower than Earth’s.

scarps or cliffs like giant cracks in its surface. Mercury’s Surface Mercury’s surface has a large number of these scarps or cliffs like giant cracks in its surface. Mercury never had plate tectonics like the Earth. When the crust of Mercury cooled it shrank causing the crust to crack.

Mercury’s Caloris Basin, due to a very large impact

Because of Venus’s dense cloud cover most of what Venus, Up Close Because of Venus’s dense cloud cover most of what we know about Venus’s surface and rotation comes from using radar. There have been only a few spacecraft to land on Venus, but each survived for only a short time. (This is an ultraviolet photo.)

Venus Radar Map taken by the Pioneer Venus spacecraft

Earth Radar Map, shown with scale similar to the Venus map

Venus map made by the Magellan spacecraft In 1995 the Magellan spacecraft was able to make a much more detailed radar map of Venus. Possibly active shield volcanoes, craters, and volcanic structures called coronae were seen by Magellan.

Venus’s Surface Features including some volcanic cones and “coronae”

A Venus Corona – a collapsed volcanic cone

Impact Craters on Venus – as seen with radar from above

Venus surface, taken with a camera on the Soviet “Venera” probe (“Venera” just means “Venus” in Russian).

Atmospheres and Temperatures Mercury has no atmosphere, just like our moon. Because there is no atmosphere to trap heat, the night side of Mercury gets very cold (100 K or -280oF). The day side of Mercury gets up to 700 K (or 800oF). The atmosphere of Venus is made up of carbon dioxide, with clouds of sulfuric acid. The atmosphere is some 90 times denser than Earth’s. The Greenhouse effect causes the surface temperature of Venus to be close to 730K (or 860oF) day or night. This is hot enough to melt lead or tin. Mars has a very thin atmosphere (less than 1% of Earth’s) of mainly carbon dioxide. The surface temperature is approximately 50 K (50oC) lower than Earth’s.

Venus’s Atmosphere has an extreme Greenhouse effect

A Mars map shows the continent-sized features and five landing sites of some early US spacecraft

A Martian Volcano, called Olympus Mons has clouds around it Mars has the largest volcanoes in the Solar System, but all are dormant. They are taller than any Earth volcano because the low gravity on Mars allows them to grow tall.

Martian Ocean

Terrestrial Planet Interiors Mercury’s mantle is solid (not semi-molten like the Earth’s) and its iron core may have a solid center. There should be a liquid iron layer which causes the magnetism of Mercury. Not much is known about the interior of Venus – but it is thought to be like a young Earth. (so we don’t have a figure for it above) Mars is mostly solid and no longer geologically active in any way.

Earth’s Magnetosphere is due to it’s magnetism. The Earth’s magnetic field is formed because of the Earth’s fairly rapid rotation and because the Earth has a molten iron outer core which can flow and can have electric currents in it. There is no Lunar magnetic field so it has little iron.

Terrestrial Planet Magnetic Fields Mercury has a very weak magnetic field, partly due to its slow rotation, even though it may have liquid iron in the core. Venus has no measurable magnetic field, probably due to its very slow rotation rate. Mars has a magnetic field weaker than Mercury, meaning that its core is either not liquid or not metallic.