Mars, The Red Planet

FAS Astronomers Blog, Volume 33, Number 7.

Other than the Earth, the best-known planet in our Solar System is probably Mars. However, it isn’t a very large planet – only around ½ the width of the Earth. It also isn’t a great view through a small telescope – very little detail is evident. If you’re lucky, you might see one of the polar ice caps, but usually it is just a small fuzzy red ball.

But Mars is reasonably close, and it has a thin atmosphere with a rocky surface. This is a planet we can explore with our orbiters and landers. For more on the exploration of Mars, see Journey to a Red Planet and Exploring Mars, Past and Present.

Mars is the fourth planet from the Sun. It is 1½ times farther from the Sun than the Earth at an average distance of 142 million miles. Unlike the Earth, it has a slightly elliptical orbit (5 ½ times that of the Earth), which takes it as close as 128 million miles from the Sun (perihelion) and as far as 155 million miles from the Sun (aphelion).

Mars takes 687 days (just under two years) to complete one orbit about the Sun. Therefore, the Earth and Mars line up in what we call opposition around every two years. This is the optimal time to send spacecraft to Mars. In 2003, Mars had one of its closest oppositions when it was less than 35 million miles from the Earth. This was the closest approach in 60,000 years. Mars won’t be this close to the Earth again until 2287 (mark it on your calendar). Mars had a more distant opposition in 2012 when it was over 62 million miles away. Mars also lines up on the other side of the Sun every two years or so. This is called a conjunction when Mars is over 100 million miles away. See Planetary Opposition and Conjunction for more about all this. 

Mars is a bit small as planets go. It is around half the width of the Earth at 4,220 miles and is only 10% as massive. A day on Mars is just slightly longer than a day here on the Earth at around 24 ½ hours. Mars is similar to the Earth with a rocky surface, and it does have season due to its 25.2^o tilt in its axis. However, unlike the Earth, which has a very stable tilt, the tilt of Mars has varied considerably over the past few billion years.

Unlike the Earth, which has one large moon, Mars has only two very small moons (Phobos and Deimos). Both were discovered by Asaph Hall in 1877 and are thought to be captured asteroids. Hall named them after the sons of the Greek god Ares (Mars). Phobos is the god of fear and Deimos the god of dread.

Phobos is the largest and closest to the Martian surface. Its orbit is so fast that it beats Mars’ rotation and rises in the west and sets in the east. Someday it will crash into the surface. Deimos is smaller and much higher. It will someday escape the gravitational pull of Mars and return to being just another asteroid orbiting the Sun. 

Today, Mars is cold and dry with a very thin atmosphere of carbon dioxide. The surface temperature ranges from a chilly -225 degrees up to the mid 60s. However, landers and rovers have been exploring the surface of Mars for many years. They’ve discovered that Mars, unlike current conditions, was once a wetter and warmer planet. There is ample evidence that liquid water flowed over its surface and much of that water is now trapped under the surface as water ice.

You may have noticed that Mars is red. Yes, it is called the red planet and is named after the Roman god 

of war. But why is it red? Mars has iron material in its soil that has oxidized (we call it rust) giving the surface and atmosphere a red (or rusty) color. However, where did the rust come from? Mars is a very dry world with little or no water (except maybe frozen under its surface.)

For a long time, scientists thought the rust on Mars was something called hematite, which can form in dry conditions – much like seen on Mars today. Just recently a February 2025 study suggests that Martian rust is more Ferrihydrite, which is formed with water. This leads to the conclusion that Mars’ rust is very old and a remnant from a time when Mars was much warmer and wetter than it is today.

The Martian surface has several interesting features. In general, the northern hemisphere is much lower in altitude and smoother than the southern hemisphere. 

When the first spacecraft (Mariner 4) passed close by Mars in 1965, it took 21 grainy black and white photos. At the time, scientists were hoping to see a planet much like the Earth – although colder and drier. Instead, the images showed craters. This view of the red planet was very moon like rather than Earth like. Fortunately, as other spacecraft were set to Mars, we discovered the planet was much more interesting than initially thought. 

Mars does have craters, and the largest is Hellas Basin in the southern hemisphere. It is around 1,400 miles wide and is thought to have formed from the impact of a large asteroid some 4 billion years ago during the Late Heavy Bombardment. It also represents the lowest elevation on the planet. 

Ancient volcanoes can be found on Mars. Three of these volcanoes are lined up in in what is known as the Tharsis region. Just to the northwest is the largest volcano in the Solar System, Olympus Mons. It is so massive that it would cover the state of Arizona. These volcanos grew much larger than terrestrial volcanos because Mars lacks the tectonic plates found here on the Earth. The Earth’s crust moves spreading volcanos out along a line. We see this with the islands of Hawaii. The crust on Mars just sits there allowing volcanos to build up on top of themselves over a long period of time. 

There is a huge crack in the surface of Mars. It was discovered by the Mariner 9 spacecraft in 1971 and named Valles Marineris after the spacecraft. At 2,500 to 3,000 miles long, it would stretch across the entire United States and the Grand Canyon would fit into one of its side channels. 

Mars does have polar ice caps – similar to what we see on the Earth. The thin surface layer is composed of carbon dioxide ice that sublimates (turns from a solid to a gas) during the spring and summer. This exposes a permanent layer of water ice. The top layer reforms later during the Martian fall and winter. 

Mars does NOT have a face on its surface. You may have seen this photo taken by one of the Viking orbiters back in the 1970s. 

After all the great images of huge mountains and vast canyons, this is the one photo that many people remember. It was taken by the Viking 1 orbiter when it focused on the Cydonia region. The folks at JPL referred to it as resembling a “human head”. Oops. This photo soon became known as the “Face on Mars”. Many thought this so called “face” was an artificial object – maybe even built by aliens. So, controversies raged. It would be 20 years before we really found out what the face was. And it was … a big rock. Sorry, no aliens. 

So, everyone is happy now … there is no face on Mars.