LESSON 4

THE INNER PLANETS

OBJECTIVES

• Students will be able to describe the characteristics that the inner planets have in common.

• Students will be able to identify the main characteristics that distinguish each of the inner planets.

WHAT DO THE INNER PLANETS HAVE IN COMMON?• The inner planets are small and dense and have

rocky surfaces.• Often called terrestrial planets which means

“Earth”• All have relatively high densities.• Rich in rocky and metallic materials, including

iron and silicon.• Each has a solid surface.• All have atmospheres except Mercury.

DATA

Planet Mercury Venus Earth Mars

Diameter (km)

4,879 12,104 12,756 6,794

Period of rotation

(Earth days)

58.9 244 1.0 1.01

Average distance from sun

0.39 0.72 1.0 1.52

Period of revolution

(Earth days)

88 224.7 365.2 687

Number of moons

0 0 1 2

HOW DO THEY DIFFER?

• Size

• Composition

• Distance from the sun

MERCURY

• Temperature range 430°C to below -170°C• Smallest terrestrial planet• Not much larger than Earth’s moon• Interior made up of dense metal iron

Surface- flat plains and craters; no water and not much atmosphere

Atmosphere- small in mass= weak gravity, gas particles easily escape into space, small amounts of sodium and other gases have been detected around Mercury

Exploring Mercury- most information comes from space probes

Mariner 10 flew by Mercury 3 times in 1974 and 1975Mercury Messenger passed Mercury several times and began orbiting in 2011

VENUS

• Similar in size and mass to Earth, called “Earth’s twin”• Density and internal structure similar to Earth• Thick atmosphere• Unusual pattern of rotation• Hottest surface of any planet

Atmosphere- so thick that it is always cloudy; at the surface you would be quickly crushed by the weight of its atmosphere; pressure is 90 times greater than the pressure of Earth’s atmosphere; you couldn’t breathe on Venus because its atmosphere is mostly carbon dioxide

Rotation- takes about 7.5 Earth months to revolve around the sun and 8 months to rotate once on its axis; rotates from east to west; maybe because a large object struck Venus billions of years ago causing it to change direction or Venus’s thick atmosphere could have somehow altered its rotation

Hot Planet- receives more solar energy than Earth because its closer to the sun; much of the radiation is reflected by Venus’s atmosphere; some radiation reaches the surface and is later given off as heat; carbon dioxide in atmosphere traps heat so well that it has the hottest surface; average surface temperature is 460°CGreenhouse effect- trapping of heat by the atmosphere

Exploring Venus- Venera 7 landed in 1970Magellan probe landed in 1990Venus Express

EARTH

• Only planet in the solar system where you could live easily• Has liquid water• Suitable temperature range• Atmosphere for living things to survive

Water Planet- about 70% of Earth’s surface is covered with water

Temperature- if Earth were a little closer to the sun, it would be so hot that liquid water would evaporate; if it were a little farther away and colder, water would always be solid ice

Atmosphere- has enough gravity to hold on to most gases that make up Earth’s atmosphere; only planet with an atmosphere that is rich in oxygen; like Venus, Earth experiences a greenhouse effect; traps less heat than Venus’s atmosphere; without the atmosphere, Earth would be much colder

MARS

• Called the “red planet”• Reddish color is due to the breakdown of iron-rich

rocks, leaving a rusty dust behind• Too cold for liquid water• Does have water ice now and liquid water in the

past

Atmosphere- more than 95% carbon dioxide; has few clouds but very thin compared to clouds on Earth; temperatures range from -140°C to 20°C

Water and Ice- atmosphere is so thin that any liquid water would quickly turn into a gas; water is located in the planet’s two polar ice caps made entirely of frozen water located under the surface

Volcanoes- some regions have giant volcanoes that are rarely active anymore; Olympus Mons is the largest volcano in the solar system

Mars’s Moons- has two very small moons that are covered with craters

1. Phobos- larger about 22km across2. Deimos- even smaller 13km across

Exploring Mars- recent missions have focused on finding signs of water and possible life on MarsSpirit and Opportunity- found traces of salts and minerals that form in the presence of waterPhoenix mission- took samples of soil and found frozen water near the north polar capMars Express spacecraft- detected methane gas in Mars’s atmosphere