Mars has fascinated us for years. From the time astronomers first turned their telescopes on the planet, we have imagined life there. Science fiction writers like Edgar Rice Burroughs, Ray Bradbury and H.G. Wells have written about life on and invaders from Mars. We have sent robots to orbit the planet, land on its surface, and sample the rocks and soil. What have we learned about the planet Mars? Was it once like Earth? Is there or was there water on Mars? Is there or was there life on Mars?

In this edition of HowStuffWorks, we will examine the fascinating world of the red planet. We will look at the major geologic features of the planet, the climate, how the planet was formed, what may have happened to it and is there or was there water and/or life on Mars.

Photo courtesy NASA/NSSDC
Viking 1 orbiter view of Mars, with the canyon Valles Marineris in the center

A Little History
As you can see from the image below, Mars has few distinguishing features when viewed from Earth, even with the best telescopes. There are dark and light areas, as well as polar ice caps, but certainly not the clear features that you can see in images from orbiters around Mars. So it was easy for astronomers to make mistakes or embellish their observations. To early astronomers, Mars was a vastly different world than we know today.

Photo courtesy NASA/Space Telescope Science Institute
Photo by David Crisp and WFPC2 Science Team (JPL/CalTech)

View of Mars from the Hubble Space Telescope

In 1877, Giovanni Schiaparelli, an Italian astronomer, was the first person to create a map of Mars. In his map, he showed a system of channels or canali, which has been translated to canals in English. Later, in 1910, Percival Lowell, an American astronomer, made observations of Mars and wrote a book. In his book, Lowell described Mars as a dying planet where the civilizations built an extensive network of canals to distribute water from the polar regions to the center of the planet. Although Lowell's book captured the imagination of the public, it was not seriously considered by the scientific community because his observations were not confirmed. Nevertheless, Lowell's writings sparked generations of science fiction writers. Edgar Rice Burroughs of Tarzan fame wrote several novels about Martian societies, including A Princess of Mars, The Gods of Mars and Warlord of Mars . H.G. Wells wrote The War of the Worlds about invaders from Mars (Orson Wells' radio play of this book caused a national panic in 1938). Ray Bradbury wrote a story about the conflicts between Martians and colonizing humans called The Martian Chronicles. And Hollywood has also fueled the public's fascination with Mars in films like The Angry Red Planet, Invaders from Mars and, more recently, Mission to Mars.

In 1971, the U.S. space probe, Mariner 9, orbited Mars and sent back images of a very different world from that described by Lowell. The images showed Mars as a dry, barren, lifeless world with variable weather that often included massive dust storms that could spread across a majority of the planet.

In 1976, the U.S. space probes Viking 1 and 2 orbited and landed on Mars. Their findings confirmed those of Mariner 9. Mars was indeed a rust-colored, desert planet with rocks and boulders.

Photo courtesy NASA
Artist's conception of the touchdown of the Viking lander

Photo courtesy NASA/JPL
First photograph of the surface of Mars from Viking 1 lander

Photo courtesy NASA/NSSDC
View of Mars from Viking 2 lander

Now, we have extensively mapped the surface of Mars with Mars Global Surveyor, and landed in another area with the Mars Pathfinder mission. NASA has committed to an extensive program of robotic and possibly human exploration of Mars.

The Surface
Mars Facts

  • Surface Gravity = 3.72 m/s2 or 0.38 of Earth's gravity
  • Surface Temperature = -220 to 70 degrees Fahrenheit (average = -81) or -140 to 20 degrees Celsius (average = -63)

  • The surface of Mars can be divided into three major regions:

    • Southern Highlands
    • Northern Plains
      • plains
      • crustal upwarps - Elysium, Tharsis regions
    • Polar Regions
    The Southern Highlands is an extensive region. It has high terrain that is heavily cratered like the moon. The Southern Highlands are thought to be an ancient region, because of the heavy density of craters -- most cratering in the solar system happened in a period long ago.

    Photo courtesy NASA/JPL/Malin Space Science Systems
    Mars Global Surveyor view of Galle crater
    in the Southern Highlands

    The Northern Plains are low-lying regions, much like the maria, or seas, on the moon. The plains show lava flows with small cinder cones -- evidence of volcanoes -- as well as dunes, wind streaks, and major channels and basins similar to dry "river valleys." There is a distinct change in elevation, of several kilometers, between the Southern Highlands and the Northern Plains.

    In the Northern Plains, there are two continent-sized, high regions called crustal upwarps. These upwarps are areas where the molten rock from the interior mantle has pushed or bulged the planet's thin crust up, forming a high plateau. These regions are capped with shield volcanoes, where molten rock from the magma broke through the crust. The smaller region, called Elysium, is in the eastern hemisphere. The largest one is called the Tharsis region and is located in the western hemisphere.

    Photo courtesy NASA/JPL/Malin Space Science Systems
    Mars Global Surveyor view of the Tharsis region showing the volcanoes (covered by blue-white clouds) and the Valles Marineris canyon (lower right)

    The highest mountain in the solar system is located in the Tharsis Region. This mountain is a shield volcano called Olympus Mons (Mt. Olympus from Greek mythology). Olympus Mons rises 16 miles (25 km) above the surrounding plains and it is 370 miles (600 km) wide at its base. In contrast, the largest volcano on Earth is Mauna Loa in Hawaii, which rises 6 miles (10 km) above the ocean floor and is 140 miles (225 km) wide at its base.

    Photo courtesy NASA/NSSDC
    Color mosaic of Olympus Mons from Viking 1 orbiter

    Photo courtesy NASA/MOLA Science Team
    Topographic view of Olympus Mons

    At the edge of the Tharsis Region is a large system of canyons called Valles Marineris. Valles Marineris is 2,400 miles (4,000 km) long, which is greater than the distance from New York to Los Angeles. The canyons are 420 miles(700 km) wide and 20,000 feet (4.2 miles or 7 km) deep. As you can see, Valles Marineris is much larger than the Grand Canyon. Unlike the Grand Canyon which was formed by water erosion from the Colorado River, Valles Marineris was created by cracking of the crust when the Tharsis bulge formed.

    Photo courtesy NASA /JPL/USGS
    Central region of Valles Marineris
    as seen from Mars Global Surveyor

    The Polar Regions can be seen from the Earth. The polar ice caps are made mostly of frozen carbon dioxide (dry ice) with some water ice. The size of the polar ice caps varies with the season. In the summer, the carbon dioxide from the northern ice cap sublimes, revealing a sheet of water ice below. It is not known whether the southern ice cap has a similar sheet of water ice beneath it. Surrounding the polar ice caps are vast regions of dunes.

    Photo courtesy NASA/NSSDC
    Viking 2 orbiter color mosaic of the south pole of Mars

    Photo courtesy NASA/MOLA Science Team
    Topographic view of Mars north pole

    The Interior
    Let's compare the interior of the Earth with that of Mars. The Earth has a core that has a radius of about 2,100 miles (3,500 km). The core is made of iron and has two parts, a solid inner core and a liquid outer core. Heat is generated from radioactive decay in the core. This heat is lost from the core to the layers above. Convective currents in the liquid outer core along with the rotation of the Earth produce the Earth's magnetic field. In contrast, the core of Mars (shown as red in the figure below) is probably made of a mixture of iron, sulfur, and maybe oxygen.

    Photo courtesy NASA/JPL
    Diagram showing the interior of Mars

    The core probably has a radius between 900 to 1,200 miles (1,500 to 2,000 km). The outer part of the core may be molten, but it is unlikely, because Mars has only a weak magnetic field (less than 0.01 percent of Earth's magnetic field). Although Mars does not have a strong magnetic field now, it might have had a strong one long ago.

    Surrounding Earth's core is a thick layer of soft rock called the mantle. What do we mean by soft? Well, if the core is liquid, then the mantle is a paste like toothpaste. The mantle is less dense than the core (which is why it is above the core), made of iron and magnesium silicates, and about 1,800 miles (3,000 km) thick. The mantle is the source of lava from volcanoes. Like Earth, the mantle of Mars (shown as brown in the figure) is probably made of thick silicates; however, it is much smaller, 800 to 1,100 miles (1,300 -1,800 km) thick. There must have been convective currents in the mantle at one time to account for the formation of the crustal upwarps (e.g. Tharsis region), the Martian volcanoes, and fractures that formed Valles Marineris.

    Surrounding the mantle is the crust. The crust is thin, approximately 15 to 40 miles (25 to 70 km) thick. Unlike the Earth's crust, the crust of Mars is not fractured into tectonic plates. Therefore, Mars has little activity in the way of earthquakes or active volcanoes.

    On Earth, the crust is thin (6 to 60 miles, 10 to 100 km) and fractured into major continental plates. The plates float over the underlying mantle and rub against each other (continental drift). The areas where they rub are filled with cracks or faults such as the San Andreas fault in California. These areas of contact between plates have earthquakes and volcanoes. On Mars, the crust is also thin, but is not broken into plates like the Earth's crust. There is no evidence of active volcanoes or earthquakes, but there must have been volcanic activity at one time because we can observe lava flows (e.g. Northern Plains) from orbit.

    The Atmosphere
    The atmosphere of Mars is different from Earth in many ways:

    Mars Facts

  • Average Density = 3.94 g/cm 3
  • Orbital Speed and Period = 14.4 miles/s (24 km/s) and 686 days (1.88 years)
  • Rotational Period = 24.6 hours or 1.02 days (Martian day is called a Sol)
  • Tilt of Axis = 25 degrees
  • Magnitude = -2.0
  • Escape Velocity = 3 miles/s or 5 km/s

    • It is composed mostly of carbon dioxide (95.3 percent compared to less than 1 percent on Earth).
    • It has much less nitrogen (2.7 percent compared to 78 percent on Earth).
    • It has very little oxygen (0.13 percent compared to 21 percent on Earth).
    • It has about 1/1000 as much water vapor (0.03 percent).
    • It exerts only 7 millibars of pressure (Earth's atmospheric pressure is 1,000 millibars).
    • Its average surface temperature is -81 degrees Fahrenheit (-63 degrees Celsius).
    Because the "air" on Mars is so thin, it holds little heat. So most of the heat comes from the ground when it absorbs solar radiation. Also, the thin air is responsible for the wide, daily changes in temperature (almost 100 degrees Fahrenheit or 60 degrees Celsius). Martian atmospheric pressure changes with the seasons. During the Martian summer, carbon dioxide sublimes from the polar ice caps into the atmosphere, thereby increasing the pressure by about 2 millibars. During the Martian winter, carbon dioxide re-freezes and falls from the atmosphere, thereby causing the pressure to decrease again. Finally, because the Martian atmospheric pressure is so low and the average temperature is so cold, liquid water cannot exist; under these conditions, water would either freeze or evaporate into the atmosphere.

    Photo courtesy NASA
    A layer of thin water ice, as seen from Viking 2 lander

    Martian Weather
    The weather on Mars is pretty much the same each day. There are light winds from one direction in the morning and then from the reverse direction in the evening. Clouds of water ice can be found at altitudes of 12 to 18 miles (20 to 30 km) and clouds of carbon dioxide form at approximately 30 miles (50 km). Because Mars is so dry and cold, there is never any rain. Therefore, Mars is a desert much like Antarctica on Earth.

    Photo courtesy NASA/JPL
    Martian clouds, as seen from Mars Pathfinder

    Photo courtesy NASA/JPL/Malin Space Sciences
    Clouds over the Tharsis region,
    as seen from Mars Global Surveyor

    During the spring and early summer, the sun heats up the atmosphere enough to cause small convection currents. These currents stir up dust into the air. The dust absorbs more sunlight and heats up the atmosphere further, thereby causing more dust to lift into the air. As this cycle continues, a dust storm develops. Because the atmosphere is so thin, great speeds (60 to 120 mph or 100 to 200 kph) are required to stir up the dust. These dust storms spread across large regions of the planet and can last for months. Dust storms are also thought to be responsible for the variable dark regions on Mars that are seen from ground-based telescopes, which were mistaken for canals and vegetation by Percival Lowell.

    Photo courtesy NASA/JPL/Malin Space Sciences
    Storm over the Martian North Pole,
    as seen from Mars Global Surveyor

    These dust storms are also a major source of erosion on the Martian surface, as seen below in the image of the Herschel Dunes.

    Photo courtesy NASA/JPL/ Malin Space Sciences
    Photograph of Herschel Dunes from Mars Global Surveyor

    The Origins
    Mars Facts

  • Average distance from sun = ~137 million miles (228 million km)
  • Diameter at Equator = 4,070 miles (6,790 km)
  • Mass = 6.42 x 1023 kg (0.1 Earth masses)

  • To understand the origin of Mars and its surface features, we need to answer the following questions:

    • Why are the Southern Highlands higher than the Northern Plains?
    • Why are the Southern Highlands more densely cratered than the Northern Plains?
    • How did the crustal uplifts occur in the Tharsis and Elysium regions?
    • Why are these regions filled with volcanoes?
    • How did Valles Marineris form?
    • How did channels and sediments form?
    Remember that no human geologist has been to Mars. The information that we have to answer these questions comes from images of Mars taken from orbit, some information from the Viking and Pathfinder probes, and comparisons with other planets in the solar system (Mercury, Venus, Earth, Earth's moon). So, the current theory goes something like this:
    1. Mars formed from clumping or accretion of small objects in the early solar system.
    2. There was a period of intense bombardment from meteors.
    3. The hot mantle pushed through and lifted portions of the crust .
    4. There was one or more periods of intense volcanic activity and lava flows.
    5. The planet cooled and the atmosphere thinned.

    Mars Forms
    Mars formed by the clumping, or accretion, of small objects in the early solar system, which took about 100,000 years. Mars grew and developed a larger gravity field, which attracted more bodies. These bodies would fall into Mars, impact, and generate heat. The continued accretion of impacting material and the heat generated caused the material to sort out into a core, mantle, and crust (the crust may have formed and crystallized from a cooling mantle surface). Gases released from the cooling formed a primitive atmosphere.

    Planetary Bombardment
    As Mars formed, it was heavily bombarded by meteors in the inner solar system. These bombardments formed craters and multi-ring basins all over the planet. Similar impacts occurred on Earth and the moon forming craters at this same time. On Earth, the craters were eroded by wind and water, but the moon's craters are still visible.

    Image courtesy NASA
    Bombardment of Mars in the early solar system

    Some geologists think that a huge impact occurred that thinned the crust of the northern hemisphere. This would account for why the Southern Highlands are higher than the Northern Plains.

    Crustal Uplift in the Tharsis and Elysium Regions
    Now imagine Mars as a soft-boiled egg, and the inside is hot as the shell cools. If the shell is weak in spots, the egg will crack and the cooked yolk will bulge out. A similar occurrence happened with the Tharsis region. The hot mantle bulged out, thereby pushing up the crust and cracking or fracturing the surrounding lava plains (forming Valles Marineris). In some spots, the mantle pushed through the surface of the crust, thereby forming the many volcanoes seen in this region (e.g. Olympus Mons). This crustal uplift may have occurred more than once in the history of Mars.

    Volcanoes Erupt
    During this period, there were widespread volcanic eruptions. Lava flowed from these volcanoes and filled the low-lying basins. Eruptions released gas that contributed to a thick atmosphere. Volcanoes also release heat from the planet's inside, so the thick atmosphere and heat could support liquid water. Therefore, there might have been rain, flooding, and erosion. The erosion would produce sedimentary rocks in the basins and plains and form channels in the rock. There may have been more than one period of widespread volcanic eruptions in Mars's history, but eventually volcanic activity decreased.

    The Planet Cools, the Atmosphere Thins
    The bulges that caused the crustal uplifts and the widespread volcanic activity released vast amounts of heat from the inside of Mars. Since Mars is not as large as the Earth, it cooled much faster. As Mars cooled, the surface temperature began to drop. Water and carbon dioxide from the atmosphere began to freeze and fall to the surface in vast amounts. This freezing removed large amounts of gas from the atmosphere causing it to thin. In addition, any surface water may have frozen into the ground forming permafrost layers. Intermittent and decreasing volcanic eruptions would release more heat that would melt more water ice and cause flooding. The flooding would erode channels and carry more material down to the surrounding plains.

    Mars has been relatively inactive. It has cooled substantially, even down to the core, so there is little convective motion inside the planet to produce any substantial magnetic field.

    While this is the current theory about the origin of Mars, it needs more information to test it. Rocks from different regions must be sampled and studied. Remember that our probes have only landed in three sites on the entire planet. So, roving probes, such as Mars Pathfinder's Sojourner, will play an important part in exploring the red planet, gathering the necessary data.

    Courtesy NASA / JPL / Malin Space Science.
    Mars Pathfinder rover Sojourner sampling a Martian rock

    Water on Mars?
    The question of whether there is water on Mars is a major one, for the several reasons:

    • Human Mars explorers will need a source of water.
    • Any life on Mars, past or present, would need water.
    • The possible terraforming and settling of Mars will require water.
    There is evidence that water has flowed on Mars, and may still be there. We have detected water in the atmosphere and under the polar ice caps. From orbit, we have seen geological features that look like river valleys and water erosion. On Earth, rain flowing down a volcanic ash layer formed the alcove, channel and apron you see below. Similar features have been seen on the Martian gullies.

    Photo courtesy NASA/JPL/ Malin Space Science
    Martian gullies (left) and Earth gullies (right)

    Furthermore, because of the extreme cold of the Martian surface, there may be large deposits of water in the form of permafrost under the Martian surface. We see permafrost in the polar latitudes on Earth, so the same could be true for Mars. Further robotic exploration of Mars could provide those answers.

    Life on Mars?
    This has been the most important question since the time of Percival Lowell. The Viking landers carried out tests for life processes. They incubated soil samples in nutrient soups and looked for the release of gases such as carbon dioxide, methane and oxygen that might result from the activities of bacteria. The results of these and other experiments seemed to show that the Martian soil was chemically active, but not biologically active. Although it may seem unlikely that life could exist in the dry, cold deserts of Mars, biologists have recently found bacteria living in similar environments on Earth, such as Antarctica.

    When scientists examined a Martian meteorite found in Antarctica, they saw what looked like fossilized bacteria in the rock. And although this finding has been questioned, it has renewed interest in the search for life on Mars.

    Photo courtesy NASA
    Scanning electron microscope image of Martian meteorite ALH84001, showing possible fossilized bacteria (center)

    The final answer to this question, as well as answers to other Martian mysteries, may require humans to explore the red planet in person.

    Photo courtesy NASA
    Artist's rendering of a human Mars expedition

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