Ch. 10: Earthlike Planets: Venus and Mars

Terrestrial Planets’ Spin Axis and Rotation Rate Terrestrial Planets’ Spin Axis and Rotation Rate. Venus and Mercury have no tilt; Mars is tilted like Earth.

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.

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.

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

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.

Mars Globe, showing some major features

Mars atmosphere The atmosphere is almost all carbon dioxide. Specifically, it is 96% carbon dioxide (CO2), 1.9% argon (Ar), and 1.9% nitrogen (N2). There are trace amounts of O2, CO, H2O and CH4. The pressure varies but is less than 1% of the pressure on the surface of Earth. Variation in pressure is due to the condensation of carbon dioxide on the polar ice caps. Dust is also an important feature of the atmosphere. The MAVEN spacecraft is studying the atmosphere.

Martian sunset – we see a blue sky, due to scattering, with little absorption of blue

Dust storms can cover the entire planet.

Mars Ice Caps The ice caps are mostly frozen water but contain large amounts of carbon dioxide on a seasonal basis. The carbon dioxide condenses directly out of the atmosphere, similar to frost, and removes about a quarter of the atmosphere’s gases each winter season. The southern ice cap has continuous carbon dioxide ice due to being at higher altitude. The next slide shows the northern ice cap.

Southern pole elevation map

Dry ice (CO2) sublimation on the polar ice cap causes odd features.

A Mars map shows the continent-sized features and five landing sites of US spacecraft (Phoenix landed on the polar cap and is not shown)

Martian Volcano, called Olympus Mons, with 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.

Wind-blown dust and sand is a major feature of the surface

Theories of the origin of the Terrestrials The “rocky” planets all formed in a warmer environment near the Sun in the early solar nebula. Current theories are based on computer models, which show that the inner planets formed from a band of planetesimals that may have been influenced by a Jupiter which was closer to the Sun and then moved away. For an article about this, if you are curious, see http://www.skyandtelescope.com/astronomy-news/our-new-improved-solar-system/ For a video showing how the Late Heavy Bombardment could have happened, see http://www.skyandtelescope.com/sky-and-telescope-magazine/beyond-the-printed-page/video-evolution-of-the-solar-system/