Astronomy 10: Lecture 9

Lecture 9: The Terrestrial Planets

Reading Assignment: Arny: Chapter 8, Cosmos Chapter 5

Summary of the Terrestrial Planets

The terrestrial planets are close to the Sun, few moons, small, dense, rocky out parts and iron cores

Planet dSun
P (yr)
P (days)
moons symbol
Mercury 0.4 0.24 0.38 0.055 5.4 59 0.0° 0
Venus 0.7 0.62 0.95 0.82 5.2 243 177° 0
Earth 1.0 1.0 1.0 1.0 5.5 1.0 23.5° 1
Mars 1.5 1.88 0.53 0.11 3.9 1.03 25.2° 2

M = 6.0 x 1024 kg
R = 6.371 x 103 km
1 AU = 1.5 x 108 km

Mercury: The Messenger

Planet #1 from the Sun; 0 moons.

Difficult to study: always near the Sun in the sky. Sets or rises within 1 to 2 hours of the Sun -> blurred images since light is traveling through lots of atmosphere.

The rotation rate was finally measured with Radar. "Day and Night" cycle on Mercury = 176 days (88 consecutive days each of sunlight and darkness). The planet spins 3 times for every 2 orbits. Tidal friction brings the planet into synchronous rotation, but elliptical orbit keeps it from being 1:1. The orbital period is 88 days. Because of the orbit it is possible for the Sun to appear to move West to East for a brief period in the orbit.

Almost no atmosphere. Planet not massive enough to retain atmosphere, and too close to the Sun which heats gases to enough energy so that it can escape from the surface. Without an atmosphere to circulate heat around the surface the temperatures are very extreme. Daytime, T = 430°C. Night, T = -170°C

Mariner 10 (1974/5): flew past Mercury 3 times. Photos reveal the surface to be heavily cratered like the Moon. There are few if any uncratered Maria such as those on the Moon, however. There is no evidence for plate tectonics. Seems to be a geologically inactive world today. Not massive enough to generate sufficient heat in core, too small to retain any heat produced.

Interior structure inferred from density and mass. Mass measured by encounter of Mariner spacecraft with the planet. Recall that with Kepler's 3rd law one can measure the mass of planet by the orbital properties of a satellite.

Venus: Goddess of Love

Planet #2 from the Sun; 0 moons.

Shrouded in highly reflective clouds: bright "evening (or morning) star". Also relatively close to the Sun in the sky. Clouds are composed of sulfuric acid.

Thick Atmosphere: (90 times Earth's surface pressure!)
96% CO2, < 4% N2 (Earth: 79% N2, 20% O2)
(Earth's CO2 is trapped in rocks and oceans)

Temperature of surface: 480°C! Hottest planet, due to runaway greenhouse effect.:

If there were an increase in the CO2 (or other greenhouse gases) in Earth's atmosphere, the surface would heat up -> release more CO2 into atmosphere -> runaway greenhouse

In 1990-1993 NASA's Magellan spacecraft orbited Venus taking detailed radar maps of the surface. 100m resolution! The spacecraft saw craters, volcanos, large plains, valleys. 2 large "continents" 2-3 km above plain, but crust consists of only 1 plate (thick crust). Planet is still geologically active!

Russian spacecraft, Venera, landed on Venus and was destroyed promptly by the corrosive atmosphere.

Has long retrograde spin period. Perhaps the result of a great collision with a large planetesimal in its early formation.

Mars: God of War

Planet #4, 2 tiny, irregular moons.

Red appearance due to rust (iron oxide). Surface visible through thin atmosphere. Clouds of ice particles visible in atmosphere. Mars has polar ice caps (mostly CO2) that grow and shrink with the seasons.

Thin atmosphere (1% Earth's): 90% CO2. Windy: major dust storms occur. Atmosphere thicker in the past -> more greenhouse effect -> warm enough for liquid water to exist on the surface. Some great catastrophe may have blasted away the atmosphere some 2-3 Billion years ago, or perhaps the atmosphere was slowly lost because the planet's surface gravity isn't strong enough to hold onto the thicker atmosphere.

T = -130°C to 30°C : generally cold.

Spacecraft (1960s - 1970s): saw craters, volcanos, canyons. No plate tectonics, planets seems geologically inactive. Amount of cratering indicates that erosion has not been at work for several billion years. Volcanic activity (Olympus Mons) may have continued up until 250 Million years ago, however. Volcanic activity rather light and does not help replenish the atmosphere.

No planet-wide magnetic field. So likley no molten rock in interior.

Ancient river beds! water was abundant long ago. Now it is believed to be frozen in a permafrost layer and in the polar ice caps.

Mars Pathfinder (1997) and Mars Global Surveyor: flood plains, dried up lake beds, morning fog in valleys, etc.

Two moons: Phobos and Deimos (Fear and Panic). Both irregular in shape because they are not massive enough to be self-gravitating. They are likley captured asteroids. They show some cratering. Phobos has cracks indicating that it was once struck so violently as to nearly break it in two.

Life on Mars: Lowell's canals not real. 1976 Viking landers' tests were inconclusive. 1996: a meteorite from Mars is reported to have microscopic tube-like structures that look like fossilized tiny bacteria + several lines of chemical evidence. Very controversial.

Extraordinary Claims Require Extraordinary Evidence

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