Mars is a cold, dry, desert landscape of sand and rocks (Figure 1). Many land features on the present-day surface of Mars, such as volcanoes, canyons, and valleys, make it look very similar to Earth, but humans could not survive in the present environment on Mars. The average surface temperature is -63 degrees C (-81 degrees F), and nighttime temperatures on Mars can plunge to -110 degrees C (-170 degrees F). By comparison, the lowest temperature ever recorded on Earth was -89 degrees C (-130 degrees F) at Vostok, Antarctica in July, 1983. One reason Mars is much colder than Earth is its distance from the Sun. On average, Mars orbits the Sun at a distance of about 228 million km (about 142 million miles). This is about 1.5 times farther from the Sun than the Earth. Mars’ greater distance from the Sun also gives it a longer orbital period, or year. One year on Mars is 686 Earth days (Figure 2). Although one Mars year is almost twice as long as an Earth year, Mars’ rotational period, or day, is only 40 minutes longer than the Earth’s.
Figure 1. This image from the Mars Pathfinder images shows a Martian scene not too different from desert scenes on Earth. Credit: NASA/JPL
Figure 2. Comparison of physical properties of Earth and Mars. Graphic graciously provided by the University of Washington, Kay Dewar and Jim Tillman. Mars Image Credit: NASA and Phil James
My Martian Can Jump Higher Than Your Earthling
Mars has a diameter of about 6,800 km (~4,200 miles). This is about half the diameter of Earth (12,700 km; ~7,900 miles) Figure 2 compares the surface areas of Earth and Mars. As a consequence, gravity on Mars is only about 38% of Earth's (Figure 2). So, if you weighed 100 pounds on Earth, you would only weigh about 38 pounds on Mars. And if you can jump one meter (3.25 feet) high on Earth, you would be able to jump 2.64 meters (almost 9 feet) high on Mars. The lower gravity on Mars could prove beneficial to future astronauts, as it would permit them to easily walk around the surface wearing large spacesuits and carrying heavy backpacks.
Figure 3. If you could form a ball out of the Earth’s continents, the ball would be about the same size as Mars! Credit: NASA/JPL/Corby Waste
Mars’ Wobbling Wackiness
Mars and Earth share a similar tilt - Mars at its present 25 degrees, and Earth at a fairly constant 23.5 degrees (Figure 2). During the past ten million years, Earth's axial tilt has only varied between about 22 and 24.5 degrees, because our relatively large Moon helps maintain a stable tilt. But Mars, which has two tiny moons, has experienced more extreme changes in its axial tilt - between 13 and 40 degrees over timescales of about 10 to 20 million years. At times when Mars' axial tilt is high, the summer polar cap points directly towards the sun, which allows the entire polar ice cap to melt or sublime (change from a solid to a gas). These fluctuations in tilt lead many scientists to believe that extreme changes in climate and seasons have occurred throughout Mars' history.
Surface Features and Geology
Mars is only about one-half the diameter of Earth, but both planets have roughly the same amount of dry land surface area (Figure 3). This is because over two-thirds of the Earth's surface is covered by oceans, whereas the present surface of Mars has no liquid water.
Although Mars and Earth are very different planets when it comes to temperature, size, and atmosphere, geologic processes on the two planets are surprisingly similar. On Mars, we see volcanoes, channels, and impact basins much like the ones we see on Earth. Because of this similarity, scientists can study certain geologic features and processes on Earth in order to learn about the same or similar features on Mars. These comparisons are called analogs. Because we don't yet have the capability to send humans to do fieldwork on Mars, these Mars/Earth analogs enable scientists to make inferences about features they see on Mars by combining field studies on Earth with satellite imagery of the Martian surface, as well as imagery obtained by landers.
Because of the similar geologic processes, many of the same physical land features we see on Earth also exist on Mars. But the sheer size of some landforms on Mars dwarfs that of similar features on Earth. For example, Olympus Mons (Figure 4) is not only the largest volcano on Mars, it's the largest known volcano in the solar system. Towering at 26 km (16 miles) high, Olympus Mons is about four times as tall as Mauna Loa, the largest volcano on Earth, and about three times as high as Mount Everest (Figure 5). Mars also boasts a canyon system that would extend all the way across the United States, if on Earth (Figure 6). Valles Marineris (Figure 7), sometimes called the Grand Canyon of Mars, is 7 km (4.3 miles) deep and 4000 km (2500 miles) long, which is actually ten times longer and about four times deeper than the Grand Canyon.
Figure 4. In terms of area, Olympus Mons is similar in size to the state of Arizona! Credit: NASA
Figure 5. Olympus Mons dwarfs Earth’s two largest mountains, Mauna Kea and Mount Everest.
Figure 6. Valles Marineris would stretch across the continental United States, and beyond. Credit: Dr. Allan Treiman/LPI
Figure 7. Valles Marineris is a large scar on the surface of Mars formed by the stretching of Mars’ crust. Valles Marineris is named after the spacecraft that discovered it, Mariner 9. Image courtesy of NASA/JPL-Caltech
The surface of Mars is covered with impact craters (Figure 8), bowl-shaped depressions that are created when an asteroid or comet collides with a planetary surface. Scientists estimate that on Mars, there are more than 43,000 impact craters with diameters greater than 5 kilometers (3 miles). In contrast, only about 120 impact craters have been identified on the surface of the Earth. Why don't we see a similar number of impact craters on both Earth and Mars? For one thing, the Earth's surface is still geologically active. Because of ongoing volcanic eruptions and plate tectonics, many land surfaces are still fairly young. In addition, erosional processes happen much faster on Earth than on Mars, largely because Earth has flowing water on its surface and rain. Most impact craters on Earth have been "erased" from view by erosional processes and geologic activity. The best preserved impact crater on Earth is Barringer (Meteor) Crater (Figure 9) in northern Arizona. Barringer Crater is 1.2 km (0.75 miles) in diameter and is thought to be about 50,000 years old.
Figure 8. The layered appearance of the inner walls and floor of this Martian crater show that the erosion and deposition of sediment has played a significant role in the evolution of the Martian surface. Credit: NASA/JPL/MSSS
Figure 9. Barringer (Meteor) Crater in northern Arizona was formed about 50,000 years ago by an impact from a 50 meter-wide (162 ft-wide) nickel-iron meteorite. Credit: Lunar and Planetary Institute
Volcanoes and canyons are not the only familiar surface features seen on Mars. Long winding channels (Figure 10) that resemble dried river beds on Earth can be seen snaking across the surface. Dried-up networks of tributaries (Figure 11) and river deltas (Figure 12) hint at the long-term existence of liquid water. Gullies, usually formed by liquid water on Earth, are found on the walls of craters and other slopes (Figure 13). All of these features suggest Mars, at some point in the past, must have been warm enough to support liquid water. Not only that, the water had to stick around for a very long time to carve out the river channels, tributaries and to form the deltas.
Figure 10. Viking orbiter image of a long, sinuous channel, most likely formed by flowing water. Credit: NASA
Figure 11. Network of streams on the Martian surface likely filled with liquid water long ago. The smaller streams at the upper-left merged together into a single, larger stream at the bottom-right. Credit: NASA
Figure 12. Ancient river delta? This geologic feature resembles river deltas seen on Earth. This image, captured by the Mars Global Surveyor, is evidence for liquid water existing on the surface of Mars for a long time. Credit: NASA/JPL/MSSS
Figure 13. Gullies typically occur year round in areas on the Martian surface where temperatures are well below freezing. Nearly all of them form on slopes that face away from the Sun. Credit: NASA/JPL/MSSS
Mars has a very thin atmosphere composed mostly of carbon dioxide (95%), while Earth's atmosphere is mostly nitrogen (77%) and oxygen (21%). The average atmospheric pressure on Mars is less than 1% of Earth's, and no human could survive there without a pressurized spacesuit. Mars' atmosphere also lacks a protective ozone layer, which allows much of the Sun's dangerous ultraviolet radiation to reach the planet's surface.
Mars appears red when viewed from space (Figure 14) due to the presence of red hematite (a type of iron oxide, similar to rust) in the rocks and soil. And if you looked up at the sky from the surface of Mars, you wouldn't see a blue or cloudy gray sky like you would from Earth. Instead, you would see a bright, pinkish-color sky (Figure 15). This is due to the fine, red dust carried by the Martian winds. Because of Mars' thin atmosphere, these winds can blow up to 100 km (62 miles) per hour, sometimes stirring up the thin, Martian dust and creating global dust storms that engulf the entire planet.
Figure 14. The Hubble Space Telescope captured this image of Mars in 1997. The northern polar cap is visible at the top of Mars and clouds can be seen at the bottom of the planet. Credit: NASA/JPL/Caltech/David Crisp and the WFPC2 Science Team
Figure 15. The Imager for Mars Pathfinder captured this image of a Martian sunset, revealing the pinkish hue of Mars’ sky. Credit: NASA/JPL
Satellite imagery of Mars has revealed that huge, swirling dust storms periodically cover nearly the entire planet, and measurements taken during the Viking Lander missions in the 1970s showed that wind gusts of up to 95 kilometers (60 miles) per hour occur during these dust storms. In addition, the low gravity conditions on Mars enable sand and dust grains to jump as high as one meter (3.25 feet) off the ground and then travel 3 to 10 meters (9 to 30 feet) downwind. These observations and measurements tell scientists that sand and dust are routinely transported across the Martian surface in massive amounts. When Mariner 9 arrived at Mars in 1971, the first images to arrive back at Mission Control revealed a huge dust storm that covered the entire surface of Mars (Figure 16). It was nearly a month before the dust settled and scientists were able to begin mapping the planet. Then in June 2001, the Hubble Space Telescope detected a dust storm brewing in Hellas Basin, a huge impact crater in Mars' southern hemisphere. Within a day, the dust storm enshrouded the entire planet (Figure 17) and was so large that amateur astronomers could see it from Earth with their telescopes. Although dust storms occur on Earth, they don't come close to reaching the size of dust storms on Mars. First, because Mars is a global desert, it offers plenty of fuel for feeding and sustaining dust storms. Second, because dust absorbs sunlight, it can substantially heat Mars' dry, thin atmosphere, which then increases winds. Earth's atmosphere, however, contains water vapor, which helps control atmospheric temperatures. During the 2001 dust storm, the global air temperature on Mars was about 30 degrees C (86 degrees F) warmer than before the storm began.
Figure 16. The Mariner 9 spacecraft took this image of a global dust storm on Mars on its approach to the Red Planet in 1971. Credit: NASA
Figure 17. The Hubble Space Telescope captured these before and after images of Mars after a dust storm enveloped the planet in 2001. Credit: NASA/J. Bell (Cornell), Mike Wolff (SSI) and the Hubble Heritage Team (STScI/AURA)