Imaging Foundations with Richard Wright. September 16, August 16, July 15, Beyond the Printed Page. Constant Contact Use. There are many fascinating places, large and small, that we might like to visit, but humans could not survive on any without a great deal of artificial assistance. And the red planet is so dry that it has not had any rain for billions of years.
The outer layers of the jovian planets are neither warm enough nor solid enough for human habitation. Any bases we build in the systems of the giant planets may well have to be in space or one of their moons—none of which is particularly hospitable to a luxury hotel with a swimming pool and palm trees.
Perhaps we will find warmer havens deep inside the clouds of Jupiter or in the ocean under the frozen ice of its moon Europa. All of this suggests that we had better take good care of Earth because it is the only site where life as we know it could survive. Recent human activity may be reducing the habitability of our planet by adding pollutants to the atmosphere, especially the potent greenhouse gas carbon dioxide. Human civilization is changing our planet dramatically, and these changes are not necessarily for the better.
In a solar system that seems unready to receive us, making Earth less hospitable to life may be a grave mistake. The crusts of all of the terrestrial planets, as well as of the larger moons, have been modified over their histories by both internal and external forces.
Externally, each has been battered by a slow rain of projectiles from space, leaving their surfaces pockmarked by impact craters of all sizes see Figure 3 in Overview of Our Planetary System. We have good evidence that this bombardment was far greater in the early history of the solar system, but it certainly continues to this day, even if at a lower rate.
The collision of more than 20 large pieces of Comet Shoemaker—Levy 9 with Jupiter in the summer of see Figure 3 is one dramatic example of this process. Figure 3: Comet Shoemaker—Levy 9. The comet was approximately million kilometers from Earth, heading on a collision course with Jupiter.
Weaver STScl , E. Smith STScl. Figure 4: Jupiter with Huge Dust Clouds. The Hubble Space Telescope took this sequence of images of Jupiter in summer , when fragments of Comet Shoemaker—Levy 9 collided with the giant planet. Here we see the site hit by fragment G, from five minutes to five days after impact.
Several of the dust clouds generated by the collisions became larger than Earth. Hammel, NASA. During the time all the planets have been subject to such impacts, internal forces on the terrestrial planets have buckled and twisted their crusts, built up mountain ranges, erupted as volcanoes, and generally reshaped the surfaces in what we call geological activity.
Among the terrestrial planets, Earth and Venus have experienced the most geological activity over their histories, although some of the moons in the outer solar system are also surprisingly active. These rotate rapidly around the planet, but there seems to be less turbulence and fewer storms on Saturn than on Jupiter. One interesting phenomena that has been observed in the storms on Saturn is the presence of thunder and lightning see video, below.
The planet likely has a small rocky and metallic core. Cassini scientists waited years for the right conditions to produce the first movie that shows lightning on another planet — Saturn. In , the Dutch astronomer Christian Huygens realized that the features were rings Figure below.
The rings do not touch the planet. The Voyager 1 and 2 spacecraft in and sent back detailed pictures of Saturn, its rings, and some of its moons. There are several gaps in the rings that scientists think have originated because 1 the material was cleared out by the gravitational pull within the rings or 2 by the gravitational forces of Saturn and of moons outside the rings. The spokes appear seasonally and their origin is as yet unknown.
Titan is even larger than the planet Mercury. Nitrogen is dominant and methane is the second most abundant gas. Titan may have a layer of liquid water and ammonia under a layer of surface ice. Although conditions are similar enough to those of early Earth for scientists to speculate that extremely primitive life may exist on Titan, the extreme cold and lack of carbon dioxide make it unlikely Figure below. From Earth, Uranus is so faint that it was unnoticed by ancient observers.
William Herschel first discovered the planet in Although Uranus is very large, it is extremely far away, about 2. Light from the Sun takes about 2 hours and 40 minutes to reach Uranus. Uranus orbits the Sun once about every 84 Earth years.
Uranus has a mass about 14 times the mass of Earth, but it is much less dense than Earth. Like Jupiter and Saturn, Uranus is composed mainly of hydrogen and helium, with an outer gas layer that gives way to liquid on the inside. Uranus has a higher percentage of icy materials, such as water, ammonia NH 3 , and methane CH 4 , than Jupiter and Saturn. When sunlight reflects off Uranus, clouds of methane filter out red light, giving the planet a blue-green color.
There are bands of clouds in the atmosphere of Uranus, but they are hard to see in normal light, so the planet looks like a plain blue ball. Most of the planets in the solar system rotate on their axes in the same direction that they move around the Sun. Uranus, though, is tilted on its side so its axis is almost parallel to its orbit.
In other words, it rotates like a top that was turned so that it was spinning parallel to the floor. Scientists think that Uranus was probably knocked over by a collision with another planet-sized object billions of years ago. Uranus has a faint system of rings Figure below.
This image from the Hubble Space Telescope shows the faint rings of Uranus. The planet is tilted on its side, so the rings are nearly vertical. Rings are composed of countless small pieces of rock and ice, each orbiting its planet like a tiny moon. The rings look flat because the particles all orbit in essentially the same plane. The rings are located closer to the planets than any of their moderately sized or large moons, but the inner edge of the rings is still well above the planet's cloud tops.
Why are the jovian planets so different from the terrestrial planets? We can trace almost all the differences to the formation of the solar system. The frost line marked an important dividing point in the solar nebula. Within the frost line, temperatures were too high for hydrogen ices to form. The only solid particles were made of metal and rock. Beyond the frost line, where hydrogen compounds could condense, the solid particles included ices as well as metal and rock.
While terrestrial planets accreted from planetesimals made of rocks and metals, they ended up too small to capture significant amounts of the abundant hydrogen and helium gas in the solar nebula. The jovian planets, however, formed farther from the Sun where ices and rocks were plentiful.
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