Climate is the average weather—long-term properties of the atmosphere like temperature, wind, cloudiness, and precipitation, and properties of the surface like snow, glaciers, rivers, and oceans. Earth has a wide range of climates, but the range among the planets is much greater. Studying the climates of other planets helps us understand the basic physical processes in a larger context. One learns which factors are important in setting the climate and how they interact.

Earth is the only planet with water in all three phases—solid, liquid, and gas. Mars has plenty of water, but it’s almost all locked up...

Until the beginning of the space age, Venus⁵ was considered Earth’s sister planet. In terms of size, mass, and distance from the Sun, it is the most Earth-like planet, and people assumed it had an Earth-like climate—a humid atmosphere, liquid water, and warm temperatures beneath its clouds, which were supposed to be made of condensed water. This benign picture came apart in the 1960s when radio telescopes¹⁴ peering through the clouds measured brightness temperatures close to 700 K. Also in the 1960s, the angular distribution of reflected sunlight—the existence of a rainbow in the clouds—revealed that they...

To this point we have invoked the greenhouse effect in fairly general terms, but it is time to discuss the modern-day climate of Venus in greater detail. The details matter if we want to compare Venus to Earth to see how likely a runaway greenhouse is for our planet. The essence of the atmospheric greenhouse is that the gas is more transparent to sunlight than it is to infrared radiation. The sunlight that reaches the surface is absorbed and turned into heat, but the infrared radiation can’t carry it up to the levels where it is radiated to space—the...

The fascination with Mars⁶ has partly to do with the possibility that life could have evolved there. Searching for evidence of liquid water, past and present, is therefore a major objective. Liquid water depends on the climate, so understanding Mars’s climate, past and present, is also a major objective. As on Earth, climate change is recorded in Mars’s sediments and ice deposits. The current climate is there for us to observe with telescopes, orbiters, and landers. There is much we don’t understand about planetary climates, and studying Mars will help us understand climates in general.

Mars is currently a cold,...

Mars has seasonal cycles, as does Earth. Its obliquity is 25° and the Earth’s is 23°. At the poles of each, frost accumulates during the fall and winter and evaporates during the spring and summer; and right at the poles, the frost lasts throughout the year. In these respects the seasons on Mars are like the seasons on Earth, but there are differences. First, on Mars, there are two kinds of frost—water and CO₂. Second, the frosts never melt—both substances go directly from the solid phase to the vapor phase and back without an intermediate liquid phase. Third,...

Titan⁹ is a moon of Saturn that acts like a terrestrial planet. It’s fairly successful, too, but it’s had to use different raw materials to do it. The basic problem is the low temperature. Saturn and Titan are ten times farther from the Sun than Earth is, so the energy in sunlight is just 1% of the terrestrial value. Thus Titan’s surface temperature is −179 °C, or 94 K, which means water is useless for making rain and rivers. Instead, Titan uses a substance that exists naturally on Earth only in vapor form—methane, what we call natural gas—as...

Jupiter⁷ is not only the largest planet, but it is also the one whose composition most resembles that of the Sun. A fundamental assumption about the objects in the solar system is that they all formed from the same material with the same proportions of elements. During its first 10⁵ years, the solar system was a melting pot in which material from stars with different chemical compositions was blended together. The high temperatures resulting from the release of enormous amounts of gravitational potential energy aided the melting process. Evidence for this view comes from comparing the elemental abundances on the...

A personal anecdote illustrates the amazing properties of Jupiter’s weather. I was a member of the imaging team that had to prepare for the encounter of Voyager 1 with Jupiter on March 5, 1979. We had to decide where to point the camera during closest approach in order to photograph the most interesting features at the highest possible resolution. At closest approach, the field of view of the camera would be much too small to photograph the whole planet, so we had to choose the interesting features and predict where they would be. The engineers wanted our predictions four weeks...

If Venus and Earth are sister planets, then Saturn⁸ and Jupiter⁷ should be brothers. The ancient Romans said they were father and son, and that is equally appropriate. They share many basic characteristics, but their climates are distinctly different. They both have multiple jet streams and banded cloud patterns. Saturn’s weather is normally very calm, but every 20–30 years a giant storm erupts.¹⁰¹ These storms last for a few months and then disappear. In contrast, Jupiter’s giant storms endure without change for decades or centuries. Saturn’s winds are stronger than Jupiter’s although Saturn is twice as far from the...

Uranus is the planet that spins on its side (fig. 10.1). The rotation axis of the planet is tipped at an angle of 98° with respect to the orbit axis. In other words, the obliquity is 98°. The only other planet with an obliquity greater than 90° is Venus, whose obliquity is 177°. Venus might have evolved gradually into its current spin state. It is close enough to the Sun that tides could have a big effect on its rotation. The extreme length of the day at Venus—117 Earth days—is evidence that the spin of Venus has evolved....

The first big lesson from exploring the planets is humility: No matter how much you know from studying the Earth, you will be surprised. Your predictions about what you will find will often be wrong since those predictions are extrapolations of processes and environments found on Earth. Since the Earth doesn’t exhibit the same range of possibilities that the planets do, the predictions require both knowledge and imagination. Either our knowledge of Earth science has been inadequate or our ability to extrapolate it to other planets has been inadequate because our collective imagination did not prepare us for what we...