By Robert Hazen, Ph.D., George Mason University
We live in an absolute golden age of astronomy, a golden age of planetary exploration. Never before in history has the pace of discovery been so rapid. The red planet, Mars, is by far the most intensively studied of all our solar system neighbors. If any nearby planet harbored life, Mars is the odds-on favorite.
The Red Planet’s Characteristics
When you think of Mars, think of two words: “water” and “life”.
Mars has an elliptical orbit that averages about one and a half times the Earth-Sun distance. As a result, Mars can be as close to Earth as 60 million kilometers, and that’s not very far away by solar system standards. It can also be as far away as 400 million kilometers.
A year that is 686 days long is almost two years, compared to Earth’s one year. A day on Mars is coincidentally very close to 24 hours, and that could be a great advantage to future colonists if we ever colonize Mars. Mars is tilted about 20 degrees on its axis. That tilt causes seasons on Mars very similar to the seasons on Earth.
Mars has two objects in orbit around it—they’re really too small to be called decent-sized moons; Phobos and Demos look sort of like captured asteroids. They’re just tens of kilometers across, but they are moons, nonetheless, by definition.
Mars has surface markings, and these are easily visible from Earth. Mars has very little atmosphere, and we can see those markings prominently with our telescopes; the most obvious are the polar ice caps. These ice caps wax and wane with the 686-day Martian year.
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Water on Mars
The planet also shows brighter and darker patches on the surface; reddish areas that appeared to some 19th-century astronomers as linear features.
We got a much better look at Mars with the Mariner spacecraft missions in the 1960s, and these returned the first close-up pictures of the spectacular Martian surface.
We can observe natural, water-carved valleys; cratered plains; and giant volcanoes. We see extensive systems of what appear to be braided streams and water-worn valleys on the surface of Mars, and suggest that Mars once had abundant water, and perhaps a much thicker atmosphere than today.
So, where did all that water go? It turns out that at less than half of Earth’s gravity, water molecules can escape into space. This is a very slow and gradual process, but it’s an inexorable process. So the water that was once on Mars’s surface has probably just escaped, slowly evaporated away.
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Mars Pathfinder Mission
Viking Lander missions of the 1970s returned pictures of a dry, barren, blasted world—a desert world. The pressure of the carbon dioxide atmosphere is only half a percent of Earth’s atmosphere. Even so, there are intense dust storms that blast the surface with winds over 100 kilometers per hour; they scour the surface repeatedly.
The most spectacular Mars mission so far was the Mars Pathfinder mission, which touched down on July 4, 1997. It featured both a lander and a six-wheeled, 23-pound rover named Sojourner. The mission returned spectacular three-dimensional pictures of the surface. It also had chemical analyses of the rocks, which turned out to be very similar to Earth rocks.
The red color of Mars comes from iron oxides, for example, very common on the Earth’s surface. It also verified that the atmosphere is mostly carbon dioxide, though much less pressure than on Earth’s surface.
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Asteroid Belt beyond Mars
Beyond Mars, we come to the asteroid belt, which is a broadband of rocky debris halfway to Jupiter, just about where you might expect to see another terrestrial planet, by the spacing of the planets. There are perhaps 50,000 objects that are a kilometer or more in diameter. The asteroid belt is just a whole bunch of big rocks out there.
According to one theory, the strong gravitational force of Jupiter prevents these objects from ever clumping together to form a planet. Now, occasionally there are going to be Earth-crossing asteroids. These are asteroids that are in very elliptical orbits that come inside the orbit of Earth and out.
We now see numerous prehistoric impact sites on Earth’s surface, which clearly show that some of these asteroids hit the Earth from time to time. Indeed, we now have strong evidence that an asteroid that was perhaps 20 kilometers in diameter hit the Earth near the modern-day Yucatan Peninsula of Mexico. That was about 65 million years ago. That epic blast resulted in the extinction of the dinosaurs and many, many other groups of animals and plants.
Some scientists, including the American astronomer Carl Sagan, have suggested that these Earth-crossing asteroids pose a serious potential threat, perhaps the most serious threat to the survival of the human race, and that they should be mapped in much greater detail. Furthermore, he recommended we perhaps develop a space presence, a means to deflect such an asteroid if it’s ever shown to be coming towards our planet.
Common Questions about the Red Planet and the Asteroid Belt beyond It
Mars has an elliptical orbit that averages about one and a half times the Earth-Sun distance. As a result, Mars can be as close to Earth as 60 million kilometers.
The Mars Pathfinder mission has been the most spectacular Mars mission so far. It touched down on July 4, 1997. It featured both a lander and a six-wheeled, 23-pound rover named Sojourner. The mission returned spectacular three-dimensional pictures of the surface. It also had chemical analyses of the rocks, which turned out to be very similar to Earth rocks.
According to one theory, the strong gravitational force of Jupiter prevents the asteroids in the asteroid belt from ever clumping together to form a planet.