Dr. Aaron Titus | Department of Physics, High Point University
PHY1050      Astronomy of Stars, Galaxies, and the Cosmos
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Chapter 9

Read Chapter 9 from Discovering Astronomy.


WebAssign -- Chapter 9. Answer homework questions covering Chapter 9.

The terrestrial planets are Mercury, Venus, Earth, and Mars. Although these four planets are similar in many ways, they are also dramatically different worlds.

(Picture of Mercury taken by Mariner 10)


Mercury is the least studied of the terrestrial planets. We have never sent a probe to orbit Mercury. Our only images are from the probe Mariner 10 as it flew by Mercury three times. Mercury looks like Moon, with many impact craters.

One of the most interesting characteristics of Mercury is that it rotates very slowly. As a result, a solar day on Mercury is equal to two of its years. That is, in the time elapsed between noon one day and noon the next day, Mercury makes two revolutions in its orbit about Sun. As a result, it gets very cold on Mercury, as night lasts up to 88 days.

(Picture of Venus taken with UV light by Pioneer, 1979)


Venus is the hottest planet in the solar system. Its atmosphere traps infrared radiation, causing the planet to warm to very high temperatures. These high temperatures lead to very high atmospheric pressure. It's an extreme greenhouse effect.

Venus' atmosphere does not transmit visible light. As a result, we cannot see Venus' surface. Therefore, we use a space probe to orbit Venus and map its surface. The probe sends radio waves to Venus and measures the time elapsed for the radio waves to reflect off of Venus and return to the probe. In this way, it measures distance to the planet's surface and can detect the rises (mountains and ridges) and dips (impact craters).


Earth is the most geologically active, dynamic one of the terristrial planets. Our oceans of water are unique in the solar system. Volcanoes, earthquakes, water, and plate tectonics constantly shape and change Earth's surface.

We study Earth's geology so that we can better understand other planets, and we study geology of other planets so that we can better understand our own.

Of course, the most amazing thing about Earth is the life that it supports.


Of our terrestrial neighbors, Mars is the most well-studied. Right now the rovers Spirit and Opportunity are roaming the Mars' landscape, making measurements, and sending data back to Earth (via a satellite orbiting Mars) via radio waves.

The amazing thing about the Mars Exploration Rover mission is that the rovers were expected to last about 90 days. Well, they are like the Energizer Bunny...still going. After one year, they continue to explore Mars, looking for signs of water and signs of life. Its main mission was to look for evidence of water. Did oceans of water once exist on Mars? If so, then what caused those oceans to evaporate?