Chapter 2
An Earth-Like World

Much has been learned about the planet Mars since astronomers first began studying it with their telescopes. Some scientific findings proved to be factual, while others were found to be myths. Still, the work of early scientists was invaluable. It stimulated people's curiosity and served as the foundation for centuries of Martian exploration and fascinating discoveries.

Similarities with Earth

Throughout history, many people believed that Mars and Earth were twins. Both are terrestrial, or rocky, planets rather than gaseous. Both have atmospheres and surface features such as towering mountains, deep canyons, and valleys, as well as ice caps at both the northern and southern poles. Also, as Sir William Herschel discovered in the 1700s, both planets are tilted at similar angles: Earth at 23.45 degrees, and Mars at 25.19 degrees. Because of this tilt, certain areas of Mars and Earth are oriented toward the sun at different times of the year; as a result, both planets have four separate seasons.

Another similarity between Mars and Earth is the length of their days. An Earth day lasts for twenty-four hours, because that is how long it takes for Earth to make one complete spin on its axis while it travels around the sun. A Martian day is nearly identical, lasting for twenty-four hours, thirty-nine minutes, and thirty-five seconds. So, someone standing on Mars would see the sun rising in the morning, climbing in the sky during the day, and setting in the evening, just as it appears to do on Earth.

Stark Differences

Yet even though Mars is more like Earth than any other planet in the solar system, the two planets are different in many ways. One dissimilarity is the length of their years. An Earth year lasts for 365 days, which is how long it takes the planet to travel around the sun. Mars is about one and a half times farther from the sun, so its orbit takes longer—687 days—which means the Martian year is nearly twice as long as Earth's. One result of the longer year is that seasons on Mars last nearly twice as long as they do on Earth. Also, Earth's faster orbit means that it is constantly gaining on Mars, and passes the planet about every twenty-six months. These close encounters, known as oppositions, occur when Earth and Mars are closest together. But because of their elliptical orbits, the distance between them is never exactly the same. Typically, they are from 60 million miles to more than 300 million miles apart, but during oppositions they are much closer. For example, on August 27, 2003, Mars was 34,654,500 miles from Earth—closer than it has been in thousands of years.

As for size and gravitational force, Mars and Earth are nothing alike. With a diameter of 4,217 miles, Mars is only about half the size of Earth. The smaller size affects the planet's gravity, which is only about 38 percent as strong as Earth's. So, someone who weighs two hundred pounds on Earth would weigh a mere seventy-six pounds on Mars. Despite the size differences between the two planets, the surface area of Mars is roughly the same size as the land area of Earth. That is because approximately 70 percent of Earth is covered by oceans, and these vast bodies of water do not exist on Mars.

A Rugged, Rocky Planet

Although the terrain on Mars is more like Earth's than any other planet's, it is extremely rugged and desertlike. Much of the Martian surface is covered with thick, powdery soil that has the consistency of flour, and the ground is strewn with jagged rocks and boulders of all shapes and sizes. After viewing close-up photographs of Mars, some people have described the planet as lifeless and desolate, but the late Carl Sagan viewed it in a very different way: "The landscape is stark and red and lovely: boulders thrown out in the creation of a crater somewhere over the horizon, small sand dunes, rocks that have been repeatedly covered and uncovered by drifting dust, plumes of fine-grained material blown about by the winds."¹⁰ The red landscape Sagan referenced is one of Mars's most distinctive features. Because of a high iron oxide (rust) content in the soil, its color ranges from brownish-yellow to deep, dark red. This gives the whole planet a reddish tinge, hence the nickname "red planet."

There are striking differences between the southern and northern regions of Mars. The southern two-thirds of the planet is mostly highlands, and is dominated by huge impact craters that were formed when asteroids and comets struck the planet. Hellas Planitia, the largest and deepest of all the Martian craters, is nearly four miles deep and over twelve hundred miles wide. Heavily cratered land is typically billions of years old, so the southern area is believed to be the most ancient terrain on Mars. Since the northern third of the planet is only lightly cratered, it is thought to be much younger. The area is covered with diverse geological features such as volcanoes, channels, plains, and valleys. Geologists believe the volcanoes are currently dormant, although they do not know what caused the eruptions to stop. There is evidence that some became inactive several billion years ago, while others erupted much more recently. Astronomer William K. Hartmann explains the significance of this: "Some individual flow units could be as young as 10 million years or less. In geological terms, that's so recent that volcanic activity might start up again at any time somewhere on the red planet."¹¹

The northern hemisphere of Mars is home to the Tharsis Bulge, an expansive volcanic region that rises six miles above the Martian surface and covers an area as large as North America. There are many volcanoes in the Tharsis area, including some so massive that they dwarf any volcanoes found on Earth. Scientists cannot say for sure why Martian volcanoes grow to be so large, but they believe it is because Mars's surface does not consist of separate crustal plates as Earth's does. This phenomenon, known as plate tectonics, means that Earth is constantly on the move, with new crust forming and old crust being swallowed up. Such ongoing movement causes constant changes in Earth's crust and inhibits how large volcanoes can grow. Although plate tectonics may have played a role in shaping Mars very early in the planet's history, scientists can tell that the Martian crust has remained exactly the same for billions of years. California Institute of Technology professor David Stevenson explains: "The lack of plate tectonics on Mars limited the planet's ability to recycle material. Four billion year old rocks are fairly common there."¹² Because Mars is such a stationary planet, its volcanoes have been undisturbed as they grew to towering heights.

The most enormous volcano on Mars, Olympus Mons, is located in the Tharsis region. Olympus Mons is a shield volcano, a type of volcano that was

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formed by smoothly flowing lava rather than by violent eruptions. At 16 miles high and 370 miles across, it is three times higher than Mount Everest, which is the tallest mountain on Earth. Yet despite its immense height, the volcano has a gentle slope and is nearly as flat as a pancake on top.

Another gigantic Tharsis volcano is Alba Patera, which is believed to be the oldest and broadest volcano on Mars. Alba Patera is more than nine hundred miles across and covers a much wider area than Olympus Mons, although it stands only about one-fourth as high. A relatively small Tharsis volcano is Ceraunius Tholus. Even though it is much smaller than most of its neighbors, it is still about the same size as the island of Hawaii.

In addition to volcanoes, another amazing geological feature in the Tharsis area of Mars is a massive canyon system known as Valles Marineris. This geological wonder was formed billions of years ago when pressure within the planet's interior caused the crust to swell and split open, creating immense gashes in the surface. The canyon system is three thousand miles long, five miles deep, and in some places more than four hundred miles wide. If Valles Marineris were on Earth, it would stretch across the entire United States and make the Grand Canyon look like nothing more than a tiny crevice in the ground.

From Crescents to Ripples

Another geologically unique feature on Mars is its sand dunes, which are found in many different areas of the planet. The most common types are crescent-shaped barchan dunes and steep ridges of sand known as transverse dunes. Photographs have also shown Martian dunes in unusual shapes that people have likened to sharks' teeth, fish scales, chocolate candy kisses, or horseshoe crabs. A particularly interesting photo taken by a spacecraft during the summer of 2003 showed what looked like entire fields of fortune cookies made of sand. These odd shapes are created by the direction and strength of the Martian winds, which also influence the size of the dunes. Some are small sand hills, while others stretch more than three hundred feet into the Martian sky. Their color can vary, but most dunes on Mars are dark because of the color of the minerals that make up Martian sand.

The most expansive area of dunes is found in the northern hemisphere of Mars. Sometimes referred to as a "sea of sand," the massive dune field surrounds the north polar cap and covers nearly 250,000 square miles. Photos taken by National Aeronautics and Space Administration (NASA) spacecraft have confirmed that many northern Martian dunes are active, which means they grow, shrink, and move based on the force and direction of the wind. However, a rare grouping of dunes discovered in an area known as the Herschel Basin have rough, deeply grooved surfaces. Unlike dunes that are composed of loose sand, these look as though they are cemented together.

Other sand formations on Mars are categorized as sand ripples rather than dunes. These ripples are usually found in low-lying areas and inside craters, and can reach heights of about twenty feet, which is in stark contrast to those on Earth, where sand ripples are usually no more than a few feet high. Scientists believe that the main reason Martian sand ripples grow so tall is that the gravity on Mars is so weak that they are not as likely to collapse from gravitational pull.

Mars from the Inside Out

After years of intensive study, scientists have gained a wealth of knowledge about Mars. There are still many unknowns, though, and one of them is the exact composition of the planet's interior structure. Scientists cannot analyze the Martian interior in the same way they analyze Earth's: by reviewing seismological data, which is the measurement of earthquake activity. Because that sort of information is not yet available about Mars, researchers must analyze statistics about the planet's size, mass, and gravity; compare this data with information that is known about Earth; and form opinions based on what they have learned. So far, studying the Martian interior has not been an exact science, and it has resulted in conclusions that are based on a combination of facts and scientific inferences.

Scientists believe that Mars, like Earth, has an interior that consists of three layers: an inner shell (the core), a middle shell (the mantle), and an outermost layer (the crust). Although there is little information available about the composition of the Martian crust, scientists do have some theories about its size. They believe it varies from about nine miles thick to eighty miles thick in the area of the Tharsis Bulge. Based on this information, even at its thinnest point Mars's crust is thicker than Earth's crust, which ranges from four to twenty-five miles thick. The Martian crust is also believed to be more rigid than Earth's.

The presence of volcanoes and the evidence of solidified lava formations in many areas of Mars are strong indications that there is a mantle beneath the planet's crust. As extreme pressure built up in the mantle, the molten rock forced its way up through

the crust, forming such surface features as the Tharsis Bulge, volcanoes, and deep valleys like Valles Marineris. No one knows the exact composition of Mars's mantle, but geologists believe it is similar to Earth's: a dense, molten-hot layer of semisolid rock that is about the consistency of melted plastic.

Of all the elements that make up the interior of Mars, scientists are most certain about the planet's core. That is because spacecraft have taken measurements of Mars's moment of inertia, a gauge of how a planet's rotation on its axis is influenced by the distribution of mass inside it. Based on these findings, scientists have concluded that there is something dense in the center and something less dense in the outer layers, and that the Martian core has a radius of between 930 and 1,300 miles. They cannot say for sure whether the core is liquid or solid, but unlike Earth, which has a magnetic field created by the constant motion of its liquid outer core, Mars has little or no magnetic field. That is a likely sign that its core is solid or partly solid.

Harsh Atmosphere and Climate

Much more is known about the atmosphere of Mars than about the planet's interior. Spacecraft have been able to take precise measurements, which have shown that the atmospheres of Mars and Earth are radically different. In fact, if any humans set foot on the red planet, they would not be able to breathe. One reason is that the Martian atmosphere is composed almost entirely of carbon dioxide (CO₂), a colorless, odorless gas that, when inhaled in large, concentrated amounts, is deadly to humans. On Mars, the atmospheric CO₂ level is more than 95 percent, whereas on Earth it is less than 1 percent. The remaining gases in the Martian atmosphere include small percentages of nitrogen and argon and mere traces of oxygen and water vapor. In comparison, the abundant gases in Earth's atmosphere are nitrogen (78 percent) and oxygen (21 percent).

The atmosphere on Mars is also extremely thin—thinner than the atmosphere nineteen miles above Earth—so it cannot filter out the ultraviolet radiation from the sun that is deadly to living matter. Research scientist Paul Withers describes another effect of the thin Martian atmosphere: "It weighs about one percent of what Earth's nitrogen-oxygen atmosphere weighs. So atmospheric pressure on Mars is much lower than on Earth. If you blew up a balloon on Earth and released it on Mars, it would explode immediately because there would be hardly any atmosphere pushing back against it."¹³

Because of the great distance from Mars to the sun, the planet's climate is extremely cold, and there are vast differences between surface temperatures and atmospheric temperatures. For instance, daytime temperatures near the Martian equator can reach 60 or 70 degrees Fahrenheit at the surface level, but just a few feet higher the temperature is only about 15 degrees. Average temperatures at the poles can dip to less than –200 degrees. The planet's frigid climate is also affected by the shape of its orbit around the sun, which is much more elliptical than Earth's. This causes a wide variation in the amount of sunlight that reaches the surface. For instance, when Mars is closest to the sun (a period known as perihelion), the planet receives 40 percent more sunlight than during

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aphelion, when it is farthest from the sun. Thus, seasons on Mars are more extreme than seasons on Earth.

The Martian climate is also influenced by the composition of its atmosphere: Because the atmospere is so thin, it cannot hold heat from the sun. This, combined with the fact that Mars has no oceans to store and distribute solar heat, causes temperatures to fluctuate wildly. They rise quickly when the Martian surface heats up and then plummet when the sun has gone down and its heat escapes back into space.

Volatile Weather

The extreme temperature fluctuations on Mars influence the circulation of its atmosphere, which can result in chaotic and sudden weather changes. The Space Telescope Science Institute describes how radical these weather changes can be:

If you think the weather on Earth is unpredictable, try living on Mars. One week, the sky is pink and cloudless, filled with windblown dust raised from the rusty Martian surface. By Martian standards, it's warm, about minus 40 degrees Fahrenheit. Then, in a matter of days, the dust is swept from the atmosphere, temperatures plummet 40 degrees, and brilliant water ice clouds appear against a dark blue sky. Dramatic weather changes like these may not seem very different from a batch of severe thunderstorms passing through your home town, but for Mars these changes can sweep over the entire planet every week.¹⁴

The "windblown dust" refers to Martian dust storms, which are far more severe than any storms on Earth. Strong winds, reaching speeds of up to a hundred miles per hour, churn up dust and sweep it high into the atmosphere, where it grows into massive dust clouds that can cover the skies with a thick reddish haze. As the dust absorbs energy from the sun, it heats the atmosphere, causing temperatures to rise as much as fifty degrees.

The biggest Martian dust storm that scientists have ever observed occurred in June 2001, and it completely blanketed the planet for three months. According to NASA scientist Phil Christensen, the storm began as a small dust cloud inside an impact crater in the southern hemisphere of Mars. It then "exploded" and started growing fast. At its peak, the dust storm made it impossible for scientists to see the Martian surface, but they still felt fortunate to have observed it. Christensen explains why: "Atmospheric scientists have been waiting for a beautiful storm like this. The data we're collecting are marvelous, and I suspect there will be a rush of papers in the months ahead answering some of the questions we have about these events."¹⁵

Clouds are another common weather-related phenomenon on Mars. Even though the Martian atmosphere contains barely any trace of water vapor, there is enough to cause ice crystals to form when the temperature is at its coldest. When these ice crystals combine with dust particles suspended in the atmosphere, clouds form. Depending on where they form and the color of the soil in the area, clouds may be white, yellow, brown, or pinkish-red, and they resemble waves, plumes, streaks, or puffs of cotton. When clouds form in low areas such as valleys, canyons, and craters, they hover close to the ground in the form of fog or haze. Clouds typically appear in the Martian sky in the early morning and then disappear as the sun warms the planet during the day.

A Waterless World

Clouds hovering above the Martian landscape are one of the rare sources of water on the red planet. The only other place where (frozen) water is known to exist is in the northern and southern polar caps. For years, scientists believed the polar caps were made of frozen carbon dioxide (commonly known as dry ice) rather than water ice. However, findings collected by sophisticated laser instruments proved that theory to be incorrect. Scientists now know that both polar caps are made of thick mounds of water ice, which are covered with thinner blankets of fine dry-ice crystals. The CO₂ covering is considered a seasonal cap because it continuously grows and shrinks based on the season. For instance, during the southern hemisphere's summer (which begins in late September), temperatures become warmer, which causes the dry ice on the southern polar cap to "melt" or vaporize, sending CO₂ back into the atmosphere. During the same period, it is winter on the opposite side of Mars. The resulting cold temperatures cause atmospheric CO₂ to freeze, which increases the size of the seasonal cap and causes the northern polar cap to expand.

Because the Martian atmospheric pressure is so low and the temperatures are so extreme, it is not possible for liquid water to exist on the planet today. If any water managed to make its way to the surface, it would immediately boil away, or vaporize. However, scientists believe that was not always the case. Photographs of Mars have clearly shown deep gullies, channels, long twisting valleys, and other surface indentations that are likely the remains of ancient riverbeds, ponds, lakes, or even vast oceans. After analyzing evidence gathered by spacecraft, scientists are now certain that water was once abundant on Mars. Many think water may still exist, but probably in the form of springs or permafrost (permanently frozen ground) hidden beneath the planet's surface.

Even though Mars is barren and desertlike today, the knowledge that water once ran freely across the planet opens up an intriguing possibility: that some form of life once existed there. Robert Zubrin explains why he is one of the scientists who subscribes to that theory:

All creatures great and small surviving in extreme environments have one thing in common: their environment includes a source of water, however meager. The fact that Mars shows a remarkable amount of evidence of both surface and subsurface water in its distant past argues for the possibility of life in the past or perhaps even now in an unexpected "Garden of Eden."¹⁶