How Does Mars Compare to Earth?

Mars is only about one-half the diameter of Earth, but both planets have roughly the same amount of dry land surface area. 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. Mars and Earth are very different planets when it comes to temperature, size, and atmosphere, but geologic processes on the two planets are surprisingly similar. 

On Mars, we see volcanoes, canyons, and impact basins much like the ones we see on Earth. 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. The table below compares many of Mars' conditions, specifications and features with those on Earth.

This image compares Mars and Earth in their correct relative sizes. Mars (diameter 6790 kilometers) is only slightly more than half the size of Earth (diameter 12750 kilometers). Note the difference in color between the two planets. Almost 70% of Earth's surface is covered by liquid water. In contrast, Mars now has no liquid water on its surface and is covered with bare rock and dust. The four dark circles in the Mars image are the Tharsis shield volcanos (slide #10). Africa is at the center of the Earth image.

From Viking Orbiter Views of Mars, NASA SP-441, p. 14.

This 1988 image from the Lowell Observatory was obtained at the start of spring in the southern hemisphere, so the southern polar cap is prominent. Valles Marineris is the narrow feature protruding to the left of the dark region in the center of the image. Because of the obscuring effects of the Earth's atmosphere, even the best groundbased telescopes usually can resolve features no smaller than about 300 kilometers across when Earth and Mars are closest to one another.

Photo by Leonard Martin.

This image was obtained by the Hubble Space Telescope in 1995 as part of an observing program to monitor seasonal changes in the atmosphere and surface of Mars. The prominent dark feature in the center of the image is called Syrtis Major. The differences in color, which are exaggerated in this computer-enhanced image, are thought to be caused by differences in the deposits of dust and sand covering different regions. Features as small as 50 kilometers are seen in this image. The white region at the top of the image is the north polar cap. On the right, white clouds cover the Elysium volcanos. Hubble Space Telescope image STScI-PR95-17.

This mosaic shows a global view of Mars as seen by NASA's Viking spacecrafts in the late 1970s. The linear structure stretching east-west across the center of the image is Valles Marineris, a very large trough system. The two brown, circular objects on the left side of the image are Pavonis Mons and Ascraeus Mons, two of the large shield volcanos in the Tharsis region (see slides #10 and #11). Most of the volcanic and tectonic activity on Mars in the last 3 billion years has been concentrated in the Tharsis region. Image processing by U.S. Geological Survey.

This shaded relief painting is based on Viking Orbiter images and shows the Valles Marineris trough system with a map of the United States for scale. Valles Marineris is 4000 kilometers long, nearly enough to stretch from New York to California. Valles Marineris reaches a maximum depth of 10 kilometers. The red box outlines the region shown. From Meszaros, 1985, Plate 46.

This image shows the central portion of Valles Marineris. In this region, Valles Marineris actually consists of two troughs that are close together. The linearity of the main segments of Valles Marineris indicates that it is a tectonic feature caused by motions in the mantle of Mars (see slide #14 and Fig. 2a). This is unlike the Grand Canyon on Earth, which was cut by the Colorado River. This image is approximately 475 kilometers across. From Mars Digital Image Map, image processing by Brian Fessler, Lunar and Planetary Institute, Houston.

This image shows the volcano Olympus Mons. With a diameter of more than 600 kilometers (the size of Arizona) and a height of nearly 25 kilometers above the surrounding plains, it is the tallest volcano known to exist in the solar system. When clouds are present, it is sometimes even visible above the clouds (see slide #18). The relative ages of the surface in various parts of Mars can be estimated from the number of impact craters present in a given area, with young regions having fewer craters than old regions. Only two craters are visible here, indicating that Olympus Mons is young, probably the youngest volcanic feature on Mars. By some estimates, the most recent large volcanic eruption at Olympus Mons occurred only 25 million years ago. The oldest activity at Olympus Mons could be much older than this and would have been buried by younger lava flows. The caldera of Olympus Mons is the depression near the top center of the image. The caldera is about 65 × 80 kilometers across (the size of Rhode Island) and occurs near the maximum elevation of the volcano. It formed when magma within the volcano either erupted out of vents on the side of the volcano or temporarily drained deeper into the planet. In either case, the removal of this magma allowed part of the overlying surface to collapse, producing a topographic depression that is termed a caldera. The overlapping series of structures in the Olympus Mons caldera demonstrates that this magma withdrawal occurred a number of different times. Similar calderas are seen on other volcanos both on Mars and on Earth. Viking Orbiter image 641A52.

This Viking Orbiter image shows Uranius Tholus, one of the smaller volcanos in the Tharsis region of Mars. It is only 60 kilometers across and 3 kilometers higher than the surrounding plains. In comparison with Olympus Mons (slide #11), the greater number of impact craters near Uranius Tholus implies that it is substantially older than Olympus Mons. One such crater in the top center of the image has been flooded by lava from the surrounding plains. Because this crater must have formed after the volcano but before the plains, the plains must be younger than the volcano. (This is an example of using superposition relationships to determine the relative age of a series of features by determining which features lie on top of other features.) This area is believed to be more than 3 billion years old. Viking Orbiter image 516A23.

This mosaic shows a series of sinuous lava flows in the Elysium region of Mars. The lava flowed from south to north (toward the top of the image). Individual flows are estimated to have average thicknesses of 60 meters. The flows are probably basalts, similar to lava flows in Hawai'i. The region shown is about 90 kilometers across. Mosaic of Viking Orbiter images 651A08-651A12. Image processing by Brian Fessler, Lunar and Planetary Institute.

This image shows a computer simulation of processes in the interior of Mars that could have produced the Tharsis region. The color variations are variations in temperature. Hot regions are red and cold regions are blue and green, with the difference between the hot and cold regions being as much as 1000°C. Because of thermal expansion, hot rock has a lower density than cold rock. These differences in density cause the hot material to rise toward the surface and the cold material to sink into the interior, creating a large-scale circulation known as mantle convection. This type of mantle flow produces plate tectonics on Earth. The hot, rising material tends to push the surface of the planet up, and the cold, sinking material tends to pull the surface down. These motions contribute to the overall topography of the planet. This deformation of the planet's surface is shown in gray along the outer surface of the planet in this image. The amount of deformation is highly exaggerated to make it visible here. The actual uplift in Tharsis is estimated to be about 8 kilometers at its center. The volcano heights cited earlier are elevations above this regional uplift. This uplift also stretches the crust, forming features such as graben and Valles Marineris . In addition, the hot, rising material may melt as it approaches the surface, producing the observed volcanic activity. Computer simulation by Walter S. Kiefer, Lunar and Planetary Institute, and Louise H. Kellogg, University of California; computer graphics by Amanda Kubala, Lunar and Planetary Institute.

This is a mosaic of Viking images of most of the Argyre impact basin, which is approximately 1100 kilometers in diameter. The basin is at the upper left and is surrounded by a ring of rough debris that was ejected from the basin. Argyre formed when an asteroid or comet roughly 50 kilometers across impacted Mars. Large basins such as Argyre are believed to have formed very early in the history of Mars, about 4 billion years ago. Viking Orbiter mosaic P-17022, viewed looking east.

At one time, astronomers believed the surface of Mars was crisscrossed by canal systems. This in turn gave rise to speculation that Mars was very much like Earth, capable of supporting life and home to a native civilization. But as human satellites and rovers began to conduct flybys and surveys of the planet, this vision of Mars quickly dissolved, replaced by one in which the Red Planet was a cold, desiccated and lifeless world. However, over the past few decades, scientists have come to learn a great deal about the history of Mars that has altered this view as well. We now know that though Mars may currently be very cold, very dry, and very inhospitable, this wasn’t always the case. What’s more, we have come to see that even in its current form, Mars and Earth actually have a lot in common. Between the two planets, there are similarities in size, inclination, structure, composition, and even the presence of water on their surfaces. That being said, they also have a lot of key differences that would make living on Mars, a growing preoccupation among many humans, a significant challenge.