Planets in
The Solar System
Surface Features on Mercury
Mercury cannot be imaged well from Earth; best
pictures are from Mariner 10
Rotation Rates
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.
The Surface of Mercury
Mercury is less heavily cratered than the Moon
Some distinctive
features:
Scarp (cliff), several
hundred kilometers
long and up to 3 km
high
Evolutionary History of the Moon
and Mercury
Mercury much less well understood:
• Formed about 4.6 billion years ago
• Melted due to bombardment, cooled slowly
• Shrank, crumpling crust
Mercury Interior
Mercury is much denser than the Moon and
has a weak magnetic field—not well
understood!
Physical Properties
Radius
Mass
Moon
Mercury
Earth
1700 km
1440 km
6380 km
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
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)
Long-Distance Observations of
Venus
Dense atmosphere and
thick clouds make
surface impossible to
see
Surface temperature is
about 730 K—hotter
than Mercury!
The Surface of Venus
Surface mosaics of Venus:
The Surface of Venus
Photographs of the surface, from the Venera
landers:
The Atmosphere of Venus
Venus is the victim of a runaway greenhouse
effect—just kept getting hotter and hotter as
infrared radiation is reabsorbed
Earth
Overall Structure of Planet Earth
• Mantle
• Two-part core
• Thin crust
• Hydrosphere
(oceans)
• Atmosphere
• Magnetosphere
Earth’s Interior
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
Surface Activity
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 convection
Surface Activity
If we follow the continental drift backwards, the
continents merge into one, called Pangaea
The Tides
The Sun has less
effect because it
is farther away,
but it does
modify the lunar
tides
Mars
Physical Properties of Mars
Radius: 3400 km
Moons: Deimos, Phobos
Mass: 6.4 x 1023 kg
Density: 3900 kg/m3
Length of day: 24.6 hours
Long-Distance Observations of Mars
From Earth, can see polar ice caps that grow
and shrink with the seasons
Much better pictures from Mars missions,
close-up
Water on Mars
Current thinking: Open water (rivers, lakes)
once existed on Mars
Jupiter
Three views of Jupiter: From a small telescope
on Earth; from the Hubble Space Telescope;
and from the Cassini spacecraft
Orbital and Physical Properties
• 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
The Atmosphere of Jupiter
Major visible features:
Bands of clouds; Great Red Spot
Internal Structure
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.
The Moons of Jupiter
Jupiter with Io and Europa. Note the relative
sizes!
The Moons of Jupiter
Interiors of the
Galilean moons:
Saturn’s Atmosphere
This true-color image shows the delicate
coloration of the cloud patterns on Saturn
Orbital and Physical Properties
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
View of rings from Earth changes as Saturn
orbits the Sun
Saturn’s Interior
Interior structure similar to Jupiter’s
The Moons of Saturn
The 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
The Discovery of Neptune
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:
Neptune
Orbital and Physical Properties
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