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9. Terrestrial Planets
Terrestrial Planets
Mercury, Venus, Earth, Mars. Close to Sun. Closely spaced. Small radius. Small
mass. Rock and metal. Solid surface. High density. Slower rotation. Weak magnetic field. Few moons.
No rings.
Terrestrial Planets, as seen from Earth
When viewed by the unaided eye, all planets are just dots
(though Mars is reddish). Mercury and Venus are always seen close to the Sun, with Venus called the
“morning star” or “evening star.” With a telescope, Mercury and Venus show phases. Every two years,
we get good views of Mars (when we pass it on our inside orbit).
Mercury: General Characteristics
Size: one third (38%) Earth’s diameter (4800 km). Distance From
Sun: 0.4 AU. Density: similar to Earth’s. Surface: impact craters, maria; no surface activity. Atmosphere:
none. Temperature: hot days, cold nights. No weather. Gravity: one third (38%) Earth’s gravity. Axial
Tilt: none. Orbit: 3:2 spin-orbit resonance.
Mercury: Surface Features
Dominant impact craters show old surface, with little activity. Scarps (long
cliff lines) are believed caused by shifting crust or by cooling. Maria (smooth, dark, uncratered basins)
are presumably formed like on the Moon.
Caloris Basin and Weird Terrain
The Caloris Basin (1300 km, 800 mile) is Mercury’s largest impact
site. Directly opposite the Caloris Basin the surface is jumbled and broken, called Weird Terrain.
Formation of Caloris Basin and Weird Terrain
The impact that formed the Caloris Basin sent seismic
waves through Mercury, which intersected on the far side, creating the Weird Terrain.
Kelvin Temperature Scale
Astronomers use the Kelvin temperature scale, rather than Celsius or
Fahrenheit. Absolute zero is the lowest temperature possible, which is zero Kelvin, 0 K. Human body
(and Earth) is 300 K. Surface of Sun (and center of Earth) is 6000 K.
Atmosphere and Temperature on Mercury
Mercury has no atmosphere, because it’s too small (low
mass), and it’s too hot. Temperature ranges from daytime 600 K (620 °F) to nighttime 100 K (-280
°F). Carbon dioxide could freeze at night, and lead could melt during the day. But, a thin layer of ice
apparently exists at the poles.
Mercury’s Spin-Orbit Resonance
Mercury spins three times for every two orbits of the Sun. This 3
to 2 spin-orbit resonance is similar to the 1 to 1 spin-orbit resonance of the Moon to the Earth. Also
called tidal locking, and synchronous rotation.
Venus: General Characteristics
Size: nearly (95%) the size of the Earth. Distance From Sun: 0.7
AU. Density: similar to Earth’s. Surface: volcanoes, lava plains, some impact craters. Atmosphere: very
high pressure (90 times Earth’s), nearly all (96%) carbon dioxide. Temperature: hot day and night,
everywhere, 750 K. Gravity: nearly (90%) the same as Earth. Spin: very slow (-243 days), backwards
Venus: Surface Features
Venus has been mapped by radar, showing a dominance of volcanic
features: high volcanoes (Maxwell Mons), lava domes, and lava flows. Scarce cratering confirms an
active geology and a young surface, less than 500 million years old.
Venus: Surface Features
Pancake Domes, Lava Flow, Surface Rocks, Impact Craters
Venus: Atmosphere and Weather
Composition: carbon dioxide (96%), nitrogen (3.5%). Pressure: 90
times Earth’s pressure, equivalent to 1300 pounds per square inch. Surface Temperature: 750K
(890°F), everywhere, day and night—hot enough to melt tin, lead, and zinc—the hottest planet in the
Solar System. Weather: little surface wind, but strong high-altitude winds with sulfuric acid clouds and
rain
Why is Venus so Hot and so Dry?
Carbon dioxide traps infrared energy emitted from the surface.
Heat drives carbon dioxide out of the original water, and out of the rocks, increasing carbon dioxide
levels. This runaway greenhouse effect raises the temperature so high that water “boils” away and is lost
to space.
Venus Spins Backwards
This can be thought of as a backward spin, or as an axial tip of 180°. Only
reasonable explanation: A collision tipped the axis and nearly stopped the rotation.
Mars: General Characteristics
Size: one half (53%) Earth’s diameter (6800 km). Distance From
Sun: 1.5 AU. Density: somewhat less than Earth’s. Surface: deserts, canyons, volcanic peaks, polar caps
of water-ice and frozen carbon dioxide. Atmosphere: very low pressure (0.7% of Earth’s), almost all
(97%) carbon dioxide; sand storms. Temperature: much colder than Earth. Gravity: one third (38%)
Earth’s gravity. Axial Tilt: 24° (produces seasons). Moons: two small, captured asteroids
Mars: Surface Features
The obvious reddish color of the Martian soil is due to iron oxide (rust).
Geologic features include: gravel plains, sand dunes, chasms, volcanic mountains, and the polar caps.
Dry river beds, ancient dry lakes, and water-carved canyons are clear evidence of past water movement
(plus a much thicker atmosphere and previous higher temperatures).
Mars: Surface Features
Sand Dunes, Dust Devil, Twin Peaks by Mars Pathfinder, Water-Ice Ground
Frost, Dry River, Impact Crater, with Sand Dunes, Mars Sojourner, Polar Caps, Martian “Islands”,
Olympus Mons
Mars: Atmosphere
Composition: carbon dioxide (95%), nitrogen (2.7%). Pressure: 0.7% of Earth’s
pressure, equivalent to Earth’s pressure at an altitude of 45 miles. Surface Temperature: average is 220
K (-67°F), but varies with season, time of day, and latitude. Weather: convection-based, Coriolis-driven
winds blow (similar to Earth), but with planet-wide seasonal gales and dust storms (related to polar cap
thaw and freeze)
Seasons on Mars
Martian seasons are caused by both the axial tilt (like the Earth) and by the
varying distance to the Sun (unlike the Earth). During north-hemisphere winter, the polar cap grows as
carbon dioxide freezes out of the atmosphere, while the south polar cap shrinks. Air migration from one
pole to the other causes planetary gales and sand storms.
How did Mars lose its atmosphere?
Past flowing water shows a prior thick atmosphere. Mars is a bit
smaller than the Earth, making the atmosphere slightly thinner. Mars is a bit further from the Sun than
Earth, making it slightly colder. Cold temperatures allowed carbon dioxide to dissolve into the water.
Less carbon dioxide makes it even colder, triggering a reverse greenhouse effect. Water remains
underground as permafrost.
Factors Related to Martian Atmosphere Loss: size, distance from Sun, magnetic field, solar radiation,
impacts, and reverse greenhouse effect
Earth’s Carbon Cycle
Quantities are giga (billion) tons of stored carbon.
Biologic Effects on Earth’s Atmosphere
Over billions of years, plant life has removed carbon dioxide
from our atmosphere, replacing it with oxygen. Decomposition of plant and animal life has stored carbon
in the form of sediments (fossil fuels and carbonate rocks). This results in an atmosphere depleted of
carbon dioxide, with mild greenhouse warming.
Life on Mars?
1890’s—Percival Lowell sketches Martian “canals.” 1965—Mariner flyby reveals a
barren, cratered Mars. 1976—Viking Martian landers test soil (inconclusively) for life. 1996—Researchers
announce possible life-indicators in Martian meteorite ALH84001. 1999—Analysis of Martian meteorite,
Nakhla, indicates possible biological evidence
Planetary Exploration: Mercury
1974-5, NASA’s Mariner 10 made first flyby. Messenger spacecraft
achieved orbit March 17, 2011.
Planetary Exploration: Venus
Ten Soviet Venera probes landed on Venus in the 1970s and 1980s.
NASA’s Magellan orbiter radar-mapped the surface in the 1990s.
Planetary Exploration: Mars
1965, Mariner 4 completed first
Martian flyby. 1976, Viking landers
tested soil (inconclusively) for life. 2004, Mars Exploratory Rovers, Spirit (stationary) and Opportunity
(mobile) began multi-year mobile geologic studies. 2011, Mars Science Laboratory rover is due to
launch.
Planetary Exploration: Mars
2012 Curiosity Rover lands on Mars, run by atomic power cell.
Questions to be Answered:
•
What are the general characteristics of the terrestrial planets (as compared to the Jovian
planets)? [review]
•
How do the terrestrial planets appear, as viewed from the Earth?
•
What are the surface features of each terrestrial planet, and how were they formed?
•
What is the composition and probable evolution of each planetary atmosphere?
•
What is special about Mercury’s spin and orbit, and how did this occur?
•
How might Venus have acquired its unusual spin?
•
Might Mars have supported ancient life?
•
What are the major differences between the terrestrial planets?
•
How does mass and orbital radius affect planetary evolution?
•
How does the development of life affect planetary evolution?
•
What is some of the basic history of planetary exploration?