Mars is also smaller than Earth. Its diameter at the equator is 6,794 kilometres, while Earth’s is 12,756 kilometres. If you measure Mars’s diameter from pole to pole, it is 6,752 km, compared to Earth’s 12,720 km. These slight differences mean that both planets are not quite perfectly spherical, and are slightly oblate. This slight flattening is due to the rotation of each planet on its axis.
Mass & Gravity
Mars is an Earth-like planet in many ways, but it does vary in size and gravitational pull. From spacecraft and telescope observations, planetary scientists know that it smaller and less massive than Earth. Its mass is 0.107 Earth masses and its gravity is about 62 percent less than Earth’s gravitational tug. That means you would weigh less on Mars than you do on Earth.
The length of a Mars day is slightly longer than an Earth day. The Red Planet takes 24 hours and 40 minutes to turn once on its axis. By comparison, Earth’s takes 23 hours and 56 minutes.
Mars’s year is also longer than Earth’s. That’s because it takes 687 days to make one trip around the Sun, as opposed to Earth’s 365.25-day-long year. The longer year is due to Mars’s distance from the Sun, and the planet has the second most eccentric orbit of any body in the solar system. At its most distant (at aphelion), Mars lies 249.23 million kilometres from the Sun. At perihelion (its closest approach), Mars lies only 206.62 million km away. On average, Mars is about 227.9 million km from the Sun. For comparison, we often say Earth is about 150 million km away from the Sun. In its orbit, our planet travels out to 152.1 million kilometres from the Sun and comes as close as 147 million km.
Atmosphere & Temperature
The Martian atmosphere is the very definition of “thin air”. Its atmospheric pressure is about a hundred times less than Earth’s. Future Mars explorers will not be able to breathe Martian air because it’s almost completely carbon dioxide with a few traces of nitrogen, oxygen, and water vapor. As if that wasn’t bad enough, the temperatures on Mars never get much warmer than 20 C at noon on the equator. It’s more likely to be below around most of the time, with wintertime measurements as low as -153 C in polar wintertime.
Two missions to Mars — the MAVEN and MOM spacecraft — are studying the atmosphere to find clues to why Mars’s atmosphere changed. Evidence suggests it was much warmer, wetter and thicker in the past. At some point, the atmosphere began to escape to space, and the Mars surface water began to disappear. Measurements of gases in the atmosphere may help tell the story of Mars’s climate change.
Mars is known as a rocky, “terrestrial” planet, which makes it very similar to Earth. Recent data from the Mars orbiting missions show that the Red Planet has a solid iron core, which helps generate the planet’s weak magnetic field. Other modern studies of the planet suggest that the crust of Mars may have some simple plates riding atop a solid mantle. Long-ago plate motions are thought to be what began the formation of the Valles Marineris canyon system.
Clearly there has been volcanic activity in the past, eruptions that built Olympus Mons and the other volcanoes that rise up from the rusty red surface. Scientists do not know if there is an active volcanism deep beneath the surface. Constant volcanic activity from the three largest volcanoes on Mars built up a huge region called the Tharsis Bulge (or Tharsis Rise). This region has roughly the same mass as the dwarf planet Ceres, and it may well have affected the may well have affected the planet’s rotation rate. It has also been implicated in Mars’s plate tectonics and possibly also changes in its climate.
Mars shows us a dry, dusty, rocky surface. The southern half of the planet is much more rugged, with many more craters and highlands. The northern half of Mars has more smooth basins and what appear to be dry lakebeds and sinuous riverbeds. This difference in surface characteristics is called the Martian dichotomy. Impact craters account for part of the dichotomy, and planetary scientists suspect that long-gone oceans and lakes explain the smoothness of some areas, while the motion of long-melted glaciers may have carved out interesting terrain in other places. There are ice caps at both poles that grow and shrink with the change of seasons.