Meteor Crater in Arizona	Mars: Optical Image 1
Mercury: Optical Image From Mariner 10	Lunar Craters
Venus: Crater Cunitz	Footprint on the Moon

1. On Venus, craters of less than 2km across are almost non-existent, while on Mercury, Mars and the Moon, large and very small craters can be seen. Why is this the case?

2. When we think about cratering, we usually think of big rocks falling onto a planet, but even tiny pebbles can make craters. Near the footprint in the above picture, you can see many small "pock-marks." These marks are the craters made by microscopic particles that hit the surface moving at speeds of several kilometers per second. Marks like these can not be found on the Earth's surface. Why not?

3. Craters accumulate over time. This means that the number of craters you see gives a rough idea of the age of the surface. Using this method, sort the surfaces of Venus, Earth, Mercury, Mars and the Moon by age. How can you reconcile these vastly different ages with the idea that all the planets formed at the same time?

Shield volcanoes on Venus - The Pancake Domes	Shield volcanoes on Earth - The Galapagos Islands

1. Give two pieces of evidence that Venusian lava does not flow far before cooling.
2. What might cause the difference between the two planets?

Mars Channel Features	Imprint of flowing water in the desert of the Republic of South Yemen

1. What are the similarities between these surface features? What does this suggest about the possiblity of water on Mars?
2. Scientists who disagree with the hypothesis that the Martian channels were caused by flowing water often point out that it is not clear where the water comes from, or where it goes. In the Yemen image, the water flows from the top of the image in a number of tributaries which combine to form the river which flows off the bottom of the image. This implies that "down" in the image is downhill on Earth. Examine the Mars Channels. Which direction is downhill? How do you know?
3. Sketch the apparent flow of water through the Martian Channels. Identify the "sources" (places where the water comes from), and the "sinks" (places where the water goes). Are you convinced that these channels were created by flowing water such as is found on Earth? Can you think of another explanation for geological formations like these?

Part 4: Giant Planets

Look at these images of the outer Gas Giant Planets.

Jupiter	Saturn with Storm
Uranus	Great Dark Spot on Neptune

1. The different compositions of the outer planets give them their different colors. Jupiter and Saturn appear yellowish and reddish because of the polysulfides and phosphorus in their atmospheres. Uranus and Neptune have more methane and ammonia in their atmospheres and so they appear bluish. Notice that while Jupiter, Saturn and Neptune have much structure in their atmospheres, Uranus looks very smooth with no obvious features. What does this tell us about the interior of Uranus compared to the other giant planets?
2. The bands and stripes on Jupiter, Saturn and Neptune are common to all planets with atmospheres. On the Earth, these bands are trade winds (westward surface winds), and jet streams (eastward high-altitude winds). On the Earth, and Venus, the energy that drives the winds comes from the Sun, which heats the ground and the lower atmosphere, starting the process of convection. However, Jupiter, Saturn and Uranus have no "ground", and are much too far from the Sun for their winds to be "solar-powered". Where does the energy to drive these winds come from?