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# Anatomy of Mars Help

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By — McGraw-Hill Professional
Updated on Sep 16, 2011

## Introduction to the Anatomy of Mars

The Red Planet is about 53 percent the diameter of Earth, roughly 6,800 kilometers (4,200 miles). This would put Mars neatly between Earth and the Moon in size if the three orbs could be lined up next to one another (Fig. 6-2). If you were to stand on Mars and look toward the horizon, you would see a strangely foreshortened vista. Similarly, standing on top of one of the highest mountains, you would be able to perceive the curvature of the planet.

Figure 6-2. Mars is a bit more than half the diameter of Earth.

## The Interior

The smaller size of Mars, combined with a density somewhat lower than that of Earth, produces a less intense gravitational field than the one we know. Your weight on Mars would be 37 percent of your weight on Earth. If you weigh 160 pounds here, you would weigh 59 pounds there. You would be able to throw a baseball much farther on Mars than you can on Earth. While golf-loving astronauts might not be able to drive a ball as far on Mars as they did on the Moon, they would do better than they can on Earth. A golf course on Mars would have to be much larger than one on Earth, and to make things more interesting, there would be no shortage of boulders and sand traps.

There is evidence that the crust of Mars is thicker than that of Earth. There is also evidence that the Martian mantle historically has been less active than that of Earth.

The surface area of a sphere is proportional to the square of the diameter, whereas the volume is proportional to the cube of the diameter. Mars, being approximately half the diameter of Earth, has one-quarter the surface area but only one-eighth the volume. This means that the surface-area-to-volume ratio of Mars is twice as great as that of Earth, causing Mars to cool off faster after its formation along with the rest of the Solar System. All these factors have combined to create a world where the crust does not move very much.

On Earth, crustal plates float around over the mantle, so volcanoes move gradually away from the hot spots underneath. On Mars, however, little or no such movement has occurred, so some volcanoes have built themselves up to enormous proportions. The crowning glory of the Martian volcanoes is Olympus Mons (Mount Olympus), which is 24 kilometers (15 miles) tall and measures 600 kilometers (370 miles) in diameter at its base.

## The Surface

Mars is pitted in some places with impact craters. In fact, when Mariner 4 took the first close-up photographs of Mars in 1965, coming within 9,800 kilometers (6,100 miles) of the surface, craters seemed to dominate the landscape. This led astronomers to believe that Mars might be as desolate as our own Moon. It was up to later missions to demonstrate otherwise. An entire planet cannot be characterized by looking at only one spot. Suppose an alien civilization were to send a probe past Earth and happened to obtain photographs of only the Sahara Desert?

The southern hemisphere of Mars consists of highlands, and this is where most of the impact craters are found. The northern hemisphere, in contrast, is several kilometers lower in elevation and appears flooded over by the lava from volcanic eruptions. Various parts of the surface have a dusky gray, almost green appearance. This greenish cast was seen by the first people who looked at Mars through “spy glasses.” The dark regions, along with illusory straight dark lines that seemed to lead from them toward the polar ice caps, led some respected scientists to believe that Mars must be home to an intelligent civilization.

Numerous probes followed Mariner 4 , and with each new set of photographs and data, the hope for finding life on Mars diminished. Many scientists believe there is an abundance of water locked up on Mars in the form of permafrost beneath the surface. Some water ice also appears in the polar caps. In addition to water ice, during the polar winters there is frozen carbon dioxide (dry ice) in the polar caps, particularly the southern cap, which endures a longer winter.

Dried-up riverbeds are the most interesting features seen on the surface of Mars. After careful analysis, most geologists have agreed that these marks could only have been made by running water. One theory is that Mars was at one time covered to some extent by a shallow sea or perhaps by large glaciers that melted during a series of volcanic eruptions.

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