Planets inThe Solar System

Mercury cannot be imaged well from Earth; best pictures are from Mariner 10

Surface Features on Mercury

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

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

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

Mercury is much denser than the Moon and has a weak magnetic field—not well understood!

Mercury Interior

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

Venus

• 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

• Radius: 6000 km

• Mass: 4.9 x 1024 kg

• Density: 5200 kg/m3

• Rotation period: 243 days, retrograde

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)

Dense atmosphere and thick clouds make surface impossible to see

Surface temperature is about 730 K—hotter than Mercury!

Long-Distance Observations of Venus

The Surface of VenusSurface mosaics of Venus:

Photographs of the surface, from the Venera landers:

The Surface of Venus

Venus is the victim of a runaway greenhouse effect—just kept getting hotter and hotter as infrared radiation is reabsorbed

The Atmosphere of Venus

Earth

• Mantle

• Two-part core

• Thin crust

• Hydrosphere (oceans)

• Atmosphere

• Magnetosphere

Overall Structure of Planet Earth

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

Earth’s upper mantle, near a plate boundary; this is a subduction zone, where one plate slides below another

Surface Activity

Plate motion is driven by convectionSurface Activity

If we follow the continental drift backwards, the continents merge into one, called Pangaea

Surface Activity

The Sun has less effect because it is farther away, but it does modify the lunar tides

The Tides

Mars

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

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

Current thinking: Open water (rivers, lakes) once existed on Mars

Water on Mars

Jupiter

Three views of Jupiter: From a small telescope on Earth; from the Hubble Space Telescope; and from the Cassini spacecraft

• 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

Major visible features:

Bands of clouds; Great Red Spot

The Atmosphere of Jupiter

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

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

The Moons of JupiterJupiter with Io and Europa. Note the relative sizes!

Interiors of the Galilean moons:

The Moons of Jupiter

Saturn’s AtmosphereThis true-color image shows the delicate coloration of the cloud patterns on Saturn

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

Orbital and Physical PropertiesView of rings from Earth changes as Saturn orbits the Sun

Interior structure similar to Jupiter’s

Saturn’s Interior

The Moons of SaturnThe Huygens spacecraft has landed on Titan and is returning images directly from the surface

Uranus

Image by Voyager 2 at a distance of 1 million km

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

Neptune

Uranus and Neptune are very similar

Orbital and Physical Properties

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