The Planets

Mercury

Very similar to Earth’s moon in several ways:

• Small; no atmosphere

• lowlands flooded by ancient lava flows

• heavily cratered surfaces

Most of our knowledge based on measurements by Mariner 10 spacecraft (1974 - 1975)

View from Earth

The Interior of MercuryLarge, metallic core.

Over 60% denser than Earth’s moonMagnetic field only ~ 0.5 % of Earth’s magnetic field.

Difficult to explain at present:

Liquid metallic core should produce larger magnetic field.

Solid core should produce weaker field.

Venus and Mars

Two most similar planets to Earth:

• Similar in size and mass • Atmosphere• Similar interior structure• Same part of the solar system

Yet, no life possible on either one of them.

The Surface of VenusEarly radar images already revealed mountains, plains, craters.

Venera 13 photograph of surface of Venus:

Venus’ atmosphere has

thick clouds.

Venus is made up of a lot of carbon dioxide. This gas traps heat from the sun and since Venus is so close to the sun, this causes Venus to be incredibly HOT!

Venus rotates backwards from all the other planets.

Craters on VenusNearly 1000 impact craters on Venus’s surface:

Surface not very old.

No water on the surface; thick, dense atmosphere

No erosion

Craters appear sharp and fresh

Mars• Diameter ≈ 1/2 Earth’s diameter

• Very thin atmosphere, mostly CO2

• Rotation period = 24 h, 40 min.

• Axis tilted against orbital plane by 25o, similar to Earth’s inclination (23.5o)

• Seasons similar to Earth •Due to the axial tilt similar to that of Earth. Growth and shrinking of polar ice cap Crust not broken into tectonic plates

• Volcanic activity (including highest volcano in the solar system)

JupiterLargest and most massive planet in the solar system:

Contains almost 3/4 of all planetary matter in the solar system.

Explored in detail by several space probes:

Pioneer 10, Pioneer 11, Voyager 1, Voyager 2,

Galileo

Most striking features visible from Earth: Multi-colored cloud belts

Visual image

Infrared false-color image

Jupiter’s Rotation

Jupiter is the most rapidly rotating planet in the solar system:

Rotation period slightly less than 10 hr.

Centrifugal forces stretch Jupiter into a markedly oblate shape.

SaturnMass: ~ 1/3 of mass of Jupiter

Radius: ~ 16 % smaller than Jupiter

Av. density: 0.69 g/cm3 Would float in water!

Rotates about as fast as Jupiter, but is twice as oblate No large core of heavy elements.

Mostly hydrogen and helium; liquid hydrogen core.

Saturn radiates ~ 1.8 times the energy received from the sun.

Probably heated by liquid helium droplets falling towards center.

Composition of Saturn’s Rings

Rings are composed of ice particles

moving at large velocities around Saturn, but small

relative velocities (all moving in the same

direction).

The Atmosphere of UranusLike other gas giants: No surface. Rotates on its side. Gradual transition from gas phase to fluid interior.

Mostly H; 15 % He, a few % Methane, ammonia and water vapor.

Optical view from Earth: Blue color due

to methane, absorbing longer wavelengths

Cloud structures only visible after artificial computer enhancement of optical images

taken from Voyager spacecraft.

NeptuneDiscovered in 1846 at position predicted from gravitational disturbances on Uranus’s orbit by J. C. Adams and U. J. Leverrier.

Blue-green color from methane in the atmosphere

4 times Earth’s diameter; 4 % smaller than Uranus

The Moons of NeptuneTwo moons (Triton and Nereid) visible from

Earth; 6 more discovered by Voyager 2Unusual orbits:

Triton: Only satellite in the solar system orbiting clockwise, i.e. “backward”.

Nereid: Highly eccentric orbit; very long orbital period (359.4 d).

Pluto as a PlanetVirtually no surface features

visible from Earth.

~ 65 % of size of Earth’s Moon.

Highly elliptical orbit; coming occasionally closer to the sun

than Neptune.

Orbit highly inclined (17o) against other planets’ orbits

Neptune and Pluto will never collide.

Surface covered with nitrogen ice; traces of frozen methane and carbon monoxide.

Daytime temperature (50 K) enough to vaporize some N and CO to form a very tenuous atmosphere.

MeteoritesDistinguish between:

Meteoroid = small body in space

Meteor = meteoroid colliding with Earth and producing a visible light trace in the sky

Meteorite = meteor that survives the plunge through the atmosphere to strike the ground...

.

• Sizes from microscopic dust to a few centimeters.

• About 2 meteorites large enough to produce visible impacts strike the Earth every day.

• Statistically, one meteorite is expected to strike a building somewhere on Earth every 16 months.

• Typically impact onto the atmosphere with 10 – 30 km/s (≈ 30 times faster than a rifle bullet).

Meteorite Impacts on EarthOver 150 impact craters found on Earth.

Famous example: Barringer

Crater near Flagstaff, AZ:

Formed ~ 50,000 years ago by a meteorite of ~ 80 – 100 m diameter

Impact Craters on EarthBarringer Crater: ~ 1.2 km diameter; 200 m deep

• Impact of a large body formed a crater ~ 180 – 300 km in diameter in the Yucatán peninsula, ~ 65 million years ago.

• Drastic influence on climate on Earth; possibly responsible for extinction of dinosaurs.

Much larger impact features exist on Earth:

Analysis of Meteorites3 broad categories:

• Iron meteorites • Stony meteorites

• Stony-Iron meteorites

The Asteroid Belt

Sizes and shapes of the largest asteroids, compared to the moon

Small, irregular objects, mostly in the apparent gap between the orbits of Mars and Jupiter.

Thousands of asteroids with accurately determined orbits known today.

The Geology of Comet NucleiComet nuclei contain ices of water, carbon dioxide, methane, ammonia, etc.:

Materials that should have condensed from the outer solar nebula.

Those compounds sublime (transition from solid directly to gas phase) as comets approach the sun.

Densities of comet nuclei: ~ 0.1 – 0.25 g/cm3

Not solid ice balls, but fluffy material with significant amounts of empty space.