Otterbein College Department of Physics and Astronomy

THE TERRESTRIAL PLANETS

Mercury, Venus, Earth, Mars

Physical features we'll discuss:

rotation and revolution
size, density, inner structure
atmosphere
surface features
surface temperature
magnetic field

How are these features related, and what can we deduce from them?

THE VIEW FROM EARTH

Mercury and Venus are never far from the sun in the sky (Venus is the morning star and the evening star).

Mercury is small and hard to see. Venus is covered with clouds. Even Mars is usually hard to see clearly from Earth

SPACE PROBES (since the 60's)

Mercury: Mariner (1973)

Venus: Mariner, Pioneer (atmospheric probes) (1962-1978); Venera (landers, 1970), Zond, Vega (1961-1984), Magellan (1989)

Mars: Mariner (1964-1972), Viking landers (1976), Mars Observer (lost, 1993), Mars (1960-1973) Mars Pathfinder, Mars Global Surveyor (1997)

MASS

Mars: deduce from orbits of small moons, Phobos and Deimos

Venus and Mercury: must deduce from gravitational effect on other planets, or from trajectories of spacecraft

DENSITIES

Mercury: size of moon, density of earth; must have large metallic core

Venus: very similar to Earth in size and density

Mars: smaller than Earth, similar in density to Moon; must have small core

	MERCURY	VENUS	EARTH	MARS
REVOLUTION (year) (Kepler's third law)	89 days	225 days	1 year	~ 2 years
ROTATION (day) (determined by Doppler shift for Mercury and Venus)	59 days (synchronous 3:2)	^-243 days (retrograde)	1 day	24.6 hours
AXIAL TILTS	~0°	~180°	23.5°	25.2°
TEMPERATURES	100 - 700 K (^-280 °F - 800 °F)	750 K (890 °F)	210 - 320 K (^-80 °F - 120 °F)	150 - 250 K (^-190 °F - ^-10 °F)

ATMOSPHERES

Mercury: none

Mars: thin, mostly CO₂; 1/150 pressure of Earth's

Venus: thick, mostly CO₂, heavy cloud cover (sulfuric acid); 90 times pressure of Earth's

Temperatures are largely governed by the atmosphere. Venus's surprisingly high temperatures are caused by a runaway greenhouse effect

EVOLUTION OF THE ATMOSPHERES

Primordial atmospheres of hydrogen and helium, like the Jovian planets, since escaped. Replaced by heavier gases leaking out of the interior of the planet.

On Earth, the CO₂ was originally dissolved in the oceans (and later bound up by biological organisms).

On Venus, high temperature evaporated the water, so all the CO₂ was forced into the atmosphere, increasing the greenhouse effect

The water on Mars is probably still there, but frozen.

Mercury, like the Moon, is too small to hold an appreciable atmosphere.

SURFACE FEATURES

Mercury:

craters
scarps (due to shrinkage of planet?)
"weird terrain"

Venus:

two large "continents" make up 8% of the surface
volcanism, coronae (incipient convection? but no evidence of plate tectonics)
perhaps still geologically active

Mars:

volcanic plains, like maria
cratered highlands
Tharsis bulge and "grand canyon"
Evidence of water in times past:
runoff channels (dry river beds), perhaps 4 billion years old;
outflow channels near equator; evidence of catastrophic melting about 3 billion years ago
"splosh" craters look as though there was frozen water (permafrost) that was melted by meteorite impact

INTERNAL STRUCTURE AND MAGNETIC FIELDS

Mercury: large core, less rocky material. Core probably solid. Possesses magnetic field.

Venus: no magnetic field.

Mars: small magnetic field, discovered in 1997 by Mars Global Surveyor.

To have a magnetic field, a planet probably must have a liquid metallic core that is being stirred by planetary rotation. Venus rotates too slowly, and the core of the Moon is solid. (In this theory, the magnetic fields of Mars and Mercury were "frozen in" when their cores solidified)

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