1. The Inner Planets
Lesson 4

2. 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.

3. 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.

4. 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
1.52
Period of revolution (Earth days) 88
224.7
365.2
687
Number of moons 1 2

5. How Do They Differ?
Size
Composition
Distance from the sun

6. 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
430C = 806F
-170C = -274F
Closest to the sun
Craters formed early in the history of the solar system
Due to no water and little atmosphere, craters have not worn away over time

7. 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 1975
Mercury Messenger passed Mercury several times and began orbiting in 2011

8. 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

9. 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
Clouds made up mostly of droplets of sulfuric acid
Day is longer than its year
Rotation opposite of most planets and moons

10. Magellan probe landed in 1990 Venus Express
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°C Greenhouse effect- trapping of heat by the atmosphere
Exploring Venus-
Venera 7 landed in 1970
Magellan probe landed in 1990
Venus Express
460C = 860F
Venera 7- first probe to land on Venus, survived a few minutes because of high temp and pressure
Magellan carried radar instruments which works through clouds so it was able to map nearly the entire surface; confirmed Venus is covered with rock and surface has more than 10,000 volcanoes; lava flows formed plains
Venus express has helped scientists understand how Venus’s clouds form and change

11. 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

12. 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
Temperature- called having “Goldilocks” conditions, not too hot and not too cold
Oxygen makes up about 20 % of Earth’s atmosphere, rest is nitrogen and small amounts of argon, carbon dioxide and water vapor

13. 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

14. 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
Could walk around on Mars but you would have to wear an airtight suit and carry your own oxygen
140C = -220F
20C = 68F

15. Mars’s Moons- has two very small moons that are covered with craters
1. Phobos- larger about 22km across
2. Deimos- even smaller 13km across
Exploring Mars- recent missions have focused on finding signs of water and possible life on Mars
Spirit and Opportunity- found traces of salts and minerals that form in the presence of water
Phoenix mission- took samples of soil and found frozen water near the north polar cap
Mars Express spacecraft- detected methane gas in Mars’s atmosphere