PLANETS 2. The Rocky Worlds

Let's begin to explore the Sun's kingdom, close to home, by looking at the inner, rocky worlds.

Mercury – the Sun-baked Mini-Planet




The closest planet to the Sun is sunbaked Mercury. Much of what we know about this strange, rocky world came from the spacecraft Mariner 10, which flew by it three times in 1974 and 1975.


Mercury is a tiny planet, only 3,033 miles (4,880 kilometers) in diameter. It has no atmosphere and no water. Because of its closeness to the Sun, Mercury gets extremely hot during the day – as much as 800°F (427°C)! At night, it cools down to a very chilly -350°F (-212°C). These extremes make Mercury the planet with the widest temperature range of any in the Solar System.


Mercury's day – the time it takes to spin once around on its axis – is equal to 59 Earth days. Its year – the time it takes to go once around the Sun – is equal to 88 Earth days. As a result, Mercury manages to fit just 3 days into every 2 of its years! During each of these long Mercurian days, the Sun appears 3 times bigger and 10 times brighter in the sky than it does on Earth.


Mariner 10's cameras showed that Mercury has a surface very much like that of the Moon. It is covered by craters of all sizes, and has at least one great plain – the Caloris Basin – that is surrounded by rings of mountains. Most of these scars on Mercury's surface were probably caused long ago by meteors and asteroids that crashed into it. Because there is no weather on Mercury to wear down the craters and mountains, they have remained more or less unchanged for billions of years.


Like all the rocky worlds, Mercury is made of a heavy iron core, surrounded by a mantle and a crust of lighter rocks. Mercury's iron core must be quite large because the planet is heavy for its small size.


Venus – the Cloud-Covered Oven




Venus, the next planet out from the Sun, was for a long time an even bigger mystery than Mercury. Although Venus is our closest neighbor in space apart from the Moon, its surface is always hidden beneath a thick layer of bright, fleecy clouds. Thanks to a number of space probes, however, we have peeked under the clouds of Venus and learned some of its secrets.


Venus has been called Earth's "sister" planet. In fact, the two worlds are almost the same size. Venus is 7,523 miles (12,104 kilometers) in diameter, Earth 7,928 miles (12,756 kilometers). They are probably very much alike inside, too. Still, these sister planets are different from each other in important ways.


Venus takes longer to spin on its axis than it does to go around the Sun. In other words, its "day" (lasting 243 Earth days) is longer than its "year" (lasting 225 Earth days). Stranger still, Venus spins backwards. If you could stand on Venus's surface and see through its thick clouds, you would see the Sun rise in the west and set in the east more than 3½ of our years later!


In other ways, too, Venus is very different from our planet. Its atmosphere weighs 90 times more than Earth's. Standing on Venus, you would have as much weight over your head as a diver would half a mile under the ocean!


The main gas in Venus's atmosphere is carbon dioxide. This gas works like the glass in a greenhouse. It traps heat from the Sun and causes the temperature at the surface of Venus to become very high. Venus is, in fact, hotter than Mercury, even though – at 67 million miles (108 million kilometers) – it's almost twice as far from the Sun. Venus's surface temperature, both day and night, stays above 850°F (more than 450°C).


Once Earth's atmosphere was probably much like Venus's is today. Billions of years ago, volcanoes on both planets released large amounts of carbon dioxide into the atmosphere. But on Earth, most of the carbon dioxide was later taken up by plants. The plants produced oxygen, which made the life we know today possible.


Now, today, through neglect, the Earth's people are making its atmosphere more like Venus's again. We are releasing carbon dioxide into the air by burning coal and oil. At the same time, we are cutting down huge areas of forest that once turned carbon dioxide into life-giving oxygen. Will we learn to change our ways before Earth, too, becomes an enormous, hot greenhouse?


Venus certainly has some unwelcome surprises for the space traveler. Not only is it very hot, with a crushingly heavy atmosphere, but its surface seems to be a barren wasteland. Pictures sent back from the surface show a gloomy desert of tumbled rocks. Radar measurements, taken from above the clouds, show wider areas of lifeless craters, valleys, and plains.


Lightning, too, is common on Venus. Lightning strikes on the ground and the steady glow of lightning in the sky will greet any future visitors. Higher up, the yellowish cloud tops are filled with sulfuric acid, a dangerous chemical. To travel safely through the burning acid and lightning, spacecraft need to be well protected.


Venus would be the last place in the Solar System to visit for a holiday!


Earth – the Planet of Life




Perhaps, instead, we should stay home on Earth – the third planet from the Sun. It is the most beautiful and comfortable planet in the Sun's kingdom. More importantly, it is the only world on which we are certain that there is life.


At a distance of 93 million miles (150 million kilometers), Earth receives just the right amount of heat and light from the Sun. Unlike Mercury or Venus, it doesn't bake. And unlike the outer planets, it isn't a frozen ball of icy gases and rock. Apart from its hot deserts and chilly poles, Earth is a good place for living things.


How did life first develop on our planet? About 4 billion years ago, Earth probably had a choking atmosphere of carbon dioxide and other unpleasant gases. Its surface was bombarded by deadly ultraviolet rays from the Sun. But then a wonderful thing happened.


Because the temperature was just right, water vapor in the Earth's early atmosphere formed thick clouds. Soon it began to rain everywhere. It rained so hard, and for so long, that huge oceans were formed.


Over millions of years, these oceans became the birthplace of the first living things on Earth. Tiny plants appeared, shielded by a thin layer of sea water from the Sun's killing ultraviolet rays.


Slowly, the first plants turned much of the carbon dioxide in the atmosphere into oxygen. This is the gas that humans, and all other animals, breathe. At the same time, high in the atmosphere, ultraviolet rays turned some of the oxygen into ozone. That change was important because ozone blocks out ultraviolet. A layer of ozone was formed around the Earth that protected all life on its surface from the Sun's harmful rays. In the millions of years that followed, life spread out from the oceans to the land and developed into the countless forms we know today.


Mars – the Red World of Mystery




Only one other planet in the Solar System seems to be anything like Earth. This is the Red Planet, Mars, the outermost of the rocky worlds. We have learned a great deal about Mars from space probes that have been sent to look at it close up. In 1976, Viking 1 and 2 became the first spacecraft to carry instruments onto its surface.


Mars is a smaller planet than Earth, just 4,218 miles (6,787 kilometers) in diameter. At a distance of 142 million miles (228 million kilometers) from the Sun, it is also quite a bit cooler. Mars has two, tiny, potato-shaped moons – Phobos and Deimos – that are probably captured asteroids.


On the Martian surface, the cameras of spacecraft have shown an amazing mixture of craters and plains, giant canyons, and monstrous volcanoes. Mars's biggest canyon – Valles Marineris – dwarfs our own Grand Canyon in Arizona. Its greatest volcano – Olympus Mons – towers three times higher than Mount Everest.


Most unexpected of all, the Mariner 9 and Viking probes found dried-up river beds on Mars. These show that the Red Planet was once a lot wetter than it is today. Scientists think that most of Mars's water is now locked up in layers of frost deep underground. In the past, though, it may have been brought to the surface during periods of great volcanic eruptions.


Did life develop in the waters of Mars long ago, as it did on Earth? If so, are there living things on Mars today?


We still don't know for sure. The Viking landers found nothing definite in their search for life, and the Red Planet as a whole seems to be an unfriendly place. Its surface – reddened by iron oxide (rust) – is bone dry. Its atmosphere of carbon dioxide is 100 times thinner than Earth's. Its weather sometimes produces 130-mile-per-hour winds that blow dust storms over most of the planet. And its temperatures are usually well below zero.


On the other hand, it's possible that simple living things – germs, for instance – might survive in certain places on Mars. Frozen surface water has been found in the Martian polar caps. There may even be water droplets under the soil of the deeper valleys. And in summer, parts of Mars can warm up to 80°F (26°C).


Perhaps to settle the question of life on Mars – and other mysteries of the red Planet – we shall have to go there ourselves. Mars may well be the next target for the astronauts and cosmonauts of our world.