No sooner have you passed over the volcano, which is too far below to be cause for concern, than the first officer orders you to strap down again. “It’s time to begin our ascent,” he says. “This ought to be a smoother ride than the one on the way down; we will be using rockets all the way.” All the food and drink must be put away; seat backs and tray tables are returned to their upright and locked positions. Then the thunder of the rocket engines drowns out all other sound, and the force of acceleration pushes you back into your seat, harder and harder, with force you have not felt since you blasted off from the space center at the beginning of this voyage.
“You know,” says the first officer, “if Earth had formed a few million kilometers closer to the Sun, it would have turned out like this place. There would have been a little more carbon dioxide (CO2) in the atmosphere.
That would have retained a little more solar heat, which would have increased the CO2level. The result would have been a vicious circle like the one that took place here on Venus. Carbon dioxide makes up almost all
Earth’s crust
Hot spot
Crustal drift
Earth’s mantle Crustal surface
A
Figure 5-7A. On Earth, the crust drifts over a hot spot, as in this rendition of the Hawaiian Island chain.
Crustal surface
B
Venus’ crust
Venus’ mantle Hot spot
Figure 5-7B. On Venus, volcanoes tend to stay put over hot spots for a longer time than on Earth.
the atmosphere of Venus below its clouds. As you know, CO2is called a greenhouse gas because it tends to trap heat. The principle is much the same as that which keeps a greenhouse warm in winter. Short-wave infrared and visible-light rays penetrate the glass or the CO2and heat up the surface. The surface, in turn, radiates long-wave infrared, but the glass or the CO2is opaque to that, so instead of being reradiated into space, the long-wave infrared is absorbed and turned into heat. If this had happened on Earth, the oceans would have boiled away, the oxygen would all have been bound up with carbon, and our planet would be almost as hot and every bit as dry as this place. Of course, we would not care. We would not exist.”
“I’ve heard some people say that a runaway greenhouse disaster could occur on Earth if humans keep generating CO2and cutting down forests without replenishing them,” you say. “But I don’t know what to believe. Is that really true? How critical is the balance? Are we on the verge of tipping it the wrong way?”
“No one knows exactly,” says the first officer. “There’s only one way we will ever find out for sure whether or not the danger is real, and that is for the worst to happen. I would rather not learn the truth that way. Would you?”
The landscape below melts away into a ruddy blur, the cloud ceiling seems to rush down on the craft and swallow it up, and then, having seen enough for one day, you pull down the window blind, close your eyes, and try to imagine yourself strolling through a grassy field or along a windy beach on Earth, a place that you are now beginning to realize is special indeed.
Quiz
Refer to the text if necessary. A good score is 8 correct. Answers are in the back of the book.
1. In one day on Earth, the upper equatorial clouds of Venus (a) travel about one-quarter the way around the planet.
(b) travel about one-half the way around the planet.
(c) travel all the way around the planet.
(d) travel twice around the planet.
2. The “seasons” on Mercury are caused mainly by (a) the tilt of the planet’s axis.
(b) the clouds that cover the planet most of the time.
(c) the greenhouse effect.
(d) the difference between perihelion and aphelion.
3. Excellent conditions for observing Mercury occur when the planet (a) is at inferior conjunction.
(b) is at superior conjunction.
(c) is at opposition.
(d) None of the above
4. When Mercury or Venus is at its greatest elongation either east or west, approximately how much of the surface do we see illuminated?
(a) None of it (b) Half of it
(c) Three-quarters of it (d) All of it
5. Mercury is
(a) smaller than the Moon.
(b) the same size as the Moon.
(c) larger than the Moon but smaller than Earth.
(d) the same size as Earth.
6. Mercury’s core is almost certainly made up primarily of (a) silicate rock.
(b) iron.
(c) volcanic lava.
(d) uranium.
7. High noon on Venus would be just about as bright as (a) the brightest midday on Mercury.
(b) a typical day on the Moon.
(c) a sunny summer afternoon on Earth.
(d) a gloomy winter day on Earth.
8. The greenhouse effect
(a) increases a planet’s surface temperature.
(b) reduces a planet’s surface temperature.
(c) increases the radiation that reaches a planet’s surface from the Sun.
(d) keeps heat energy from reaching a planet’s surface.
9. If either Mercury or Venus were to transit the Sun, to which lunar phase would its appearance most nearly correspond?
(a) Full (b) First quarter
(c) New (d) Last quarter
10. Which gas is the most abundant in the atmosphere beneath the clouds of Mercury?
(a) Oxygen (b) Nitrogen (c) Carbon dioxide
(d) This is an improper question; Mercury has no clouds.
Mars
Mars is the fourth known planet in order outward from the Sun, and is in many ways the most Earthlike. Science-fiction writers have used Mars more than any other extraterrestrial place as the setting for civilizations, outposts, and evolution. It is interesting to suppose that the first Martians will come from Earth. Will you live to see the invasion of Mars by Earthlings?
The Red Planet
Mars is also known as the Red Planet, although its true color varies from rusty orange to gray to white. Some casual Earthbound observers mistake it for a red-giant star. However, because of its significant apparent diame- ter, it does not twinkle as does a star. In ancient mythology, Mars was the god of war. The planet has two moons, named Phobos(Greek for “fear”) and Deimos(Greek for “terror” or “panic”).