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Planets inThe Solar System
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Mercury cannot be imaged well from Earth; best pictures are from Mariner 10
Surface Features on Mercury
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Mercury was long thought to be tidally locked to the Sun; measurements in 1965 showed this to be false.
Rather, Mercury’s day and year are in a 3:2 resonance; Mercury rotates three times while going around the Sun twice.
Rotation Rates
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Mercury is less heavily cratered than the Moon
Some distinctive features:Scarp (cliff), several hundred kilometers long and up to 3 km high
The Surface of Mercury
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Mercury much less well understood:
• Formed about 4.6 billion years ago
• Melted due to bombardment, cooled slowly
• Shrank, crumpling crust
Evolutionary History of the Moon and Mercury
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Mercury is much denser than the Moon and has a weak magnetic field—not well understood!
Mercury Interior
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Moon Mercury Earth
Radius 1700 km 1440 km 6380 km
Mass 7.3 × 1022 kg 3.3 × 1023 kg 6.0 × 1024 kg
Density 3300 kg/m3 5400 kg/m3 5500 kg/m3
Escape Speed
2.4 km/s 4.3 km/s 11.2 km/s
Physical Properties
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Venus
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• Venus is much brighter than Mercury, and can be farther from the Sun
• Called morning or evening star, as it is still “tied” to Sun
• Brightest object in the sky, after Sun and Moon
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• Radius: 6000 km
• Mass: 4.9 x 1024 kg
• Density: 5200 kg/m3
• Rotation period: 243 days, retrograde
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Slow, retrograde rotation of Venus results in large difference between solar day (117 Earth days) and sidereal day (243 Earth days); both are large compared to the Venus year (225 Earth days)
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Dense atmosphere and thick clouds make surface impossible to see
Surface temperature is about 730 K—hotter than Mercury!
Long-Distance Observations of Venus
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The Surface of VenusSurface mosaics of Venus:
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Photographs of the surface, from the Venera landers:
The Surface of Venus
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Venus is the victim of a runaway greenhouse effect—just kept getting hotter and hotter as infrared radiation is reabsorbed
The Atmosphere of Venus
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Earth
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• Mantle
• Two-part core
• Thin crust
• Hydrosphere (oceans)
• Atmosphere
• Magnetosphere
Overall Structure of Planet Earth
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Mantle is much less dense than core
Mantle is rocky; core is metallic—iron and nickel
Outer core is liquid; inner core is solid, due to pressure
Volcanic lava comes from mantle, allows analysis of composition
Earth’s Interior
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Earth’s upper mantle, near a plate boundary; this is a subduction zone, where one plate slides below another
Surface Activity
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Plate motion is driven by convectionSurface Activity
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If we follow the continental drift backwards, the continents merge into one, called Pangaea
Surface Activity
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The Sun has less effect because it is farther away, but it does modify the lunar tides
The Tides
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Mars
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Radius: 3400 km
Moons: Deimos, Phobos
Mass: 6.4 x 1023 kg
Density: 3900 kg/m3
Length of day: 24.6 hours
Physical Properties of Mars
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From Earth, can see polar ice caps that grow and shrink with the seasons
Much better pictures from Mars missions, close-up
Long-Distance Observations of Mars
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Current thinking: Open water (rivers, lakes) once existed on Mars
Water on Mars
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Jupiter
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Three views of Jupiter: From a small telescope on Earth; from the Hubble Space Telescope; and from the Cassini spacecraft
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• Mass: 1.9 × 1027 kg (twice as much as all other planets put together)
• Radius: 71,500 km (112 times Earth’s)
• Density: 1300 kg/m3—cannot be rocky or metallic as inner planets are
• Rotation rate: Problematic, as Jupiter has no solid surface; different parts of atmosphere rotate at different rates
• From magnetic field, rotation period is 9 hr, 55 min
Orbital and Physical Properties
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Major visible features:
Bands of clouds; Great Red Spot
The Atmosphere of Jupiter
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Jupiter radiates more energy than it receives from the Sun:
• Core is still cooling off from heating during gravitational compression
Could Jupiter have been a star?
• No; it is far too cool and too small for that. It would need to be about 80 times more massive to be even a very faint star.
Internal Structure
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No direct information is available about Jupiter’s interior, but its main components, hydrogen and helium, are quite well understood. The central portion is a rocky core.
Internal Structure
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The Moons of JupiterJupiter with Io and Europa. Note the relative sizes!
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Interiors of the Galilean moons:
The Moons of Jupiter
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Saturn’s AtmosphereThis true-color image shows the delicate coloration of the cloud patterns on Saturn
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Mass: 5.7 × 1026 kg
Radius: 60,000 km
Density: 700 kg/m3—less than water!
Rotation: Rapid and differential, enough to flatten Saturn considerably
Rings: Very prominent; wide but extremely thin
Orbital and Physical Properties
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Orbital and Physical PropertiesView of rings from Earth changes as Saturn orbits the Sun
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Interior structure similar to Jupiter’s
Saturn’s Interior
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The Moons of SaturnThe Huygens spacecraft has landed on Titan and is returning images directly from the surface
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Uranus
Image by Voyager 2 at a distance of 1 million km
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Neptune was discovered in 1846, after analysis of Uranus’s orbit indicated its presence
Details of Neptune cannot be made out from Earth either; arrows again point to moons:
The Discovery of Neptune
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Neptune
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Uranus and Neptune are very similar
Orbital and Physical Properties
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Uranus Neptune
Mass 14.5 x Earth 17.1 x Earth
Radius 4.0 x Earth 3.9 x Earth
Density 1300 kg/m3 1600 kg/m3
Orbital and Physical Properties