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FAQ - Mars

1. What are the differences in interior composition between Earth and Mars? How different? How similar?

Earth and Mars have fairly similar compositions, with roughly equal amounts of rock and iron in their interiors. Even taking compression into account, Earth's density is still somewhat larger than Mars’, indicating either somewhat more iron or fewer less-dense materials (water?) than Mars. Mars’ surface is largely covered with basalt, a volcanic rock covering much of Earth's surface as well. Earth has large amounts of granite in its crust, which Mars appears to lack. Carbonate (as in limestone) is common on Earth, and has been recently discovered on Mars as well, though how much of it is below the surface is not known. The finding of carbonate implies the past existence of water on Mars. Mars presently lacks oceans, but appears to have a lot of water buried below the surface. Again, how much is not well known.

2. How do we determine what minerals are on Mars?

We have two primary sources of information as to what minerals make up rocks on Mars. The first is that we have identified meteorites from Mars that have landed on the Earth and we can directly study these and the minerals they contain. See and links therein for information on Martian meteorites.

The second way we can determine minerals on other planetary surfaces, like Mars, is through imaging spectroscopy (or spectroscopic remote sensing). Using spectroscopy, we are able to identify specific minerals and map their abundances. When light encounters the surface of a rocky planet, it is absorbed in characteristic ways based on its mineral composition. Scientists have created libraries of spectral signatures of different minerals based upon detailed laboratory studies that simulate the way in which imaging spectrometers analyze planetary surfaces. Then they use these libraries to determine what minerals are present.

See and

3. Are pictures of Mars color enhanced or in true color?

Some are true color – filters match the wavelengths we see with our eyes. Some are false color – filters are used to enhance subtle differences in color.

4. What missions have we sent to Mars?


5. Are most of the Martian rock samples basalt?

Most of the rocks we have studied at Mars landing sites are basaltic in composition. The exceptions to this are the carbonate and high silica rocks at the Spirit landing site and Heat Shield rock at the Opportunity landing site, which is an iron meteorite. In addition, all of the Martian meteorites are mafic or ultramafic in composition.

6. Why is there more of an iron composition to Mars than Earth?

Based on the composition of the Martian rocks we have studied, the mantle of Mars has approximately twice as much iron as Earth’s does. This may be because Earth lost a large part of its mantle in the collision that created the Moon.

7. Why is it assumed that features are formed by water [on Mars] and not some other chemical or materials [fluid] that flows?

Simply put, it makes the most sense! There is water ice at the poles, there is water in the surface rocks, and there are places where water frost can exist on the surface (e.g. the Viking lander sites). Water, as a liquid is by far the most likely fluid to flow on Mars. Also, the shape of the various channels on the surface is more similar to water-carved features than, say, lava (though there are lava channels as well).

8. Water and ice caps on Mars have been mentioned, but do we really know if it is water? Could it be another liquid that freezes and flows?

If you look at likely liquids and the temperatures and pressures on Mars, water is the only possible liquid. Scientists have also measured the light reflected from the surface of Mars. In the spectrum, they see features consistent with water ice and carbon dioxide ice (dry ice). Again, only water can be liquid at any possible temperatures and pressures on the surface of Mars. Finally, we have landed spacecraft on Mars and the minerals that have been seen (as well as frosts) are consistent with water ice.

9. What are megabarchans?

A barchan is a type of sand dune and when they get very large and string together, they form megabarchans. Much of the surface of Mars is covered by fine sand and dust. The regional to global-scale dust storms on Mars (occur by solar heating of the thin atmosphere when Mars is closest to the Sun) produce a great deal of wind and transport material around the surface. Therefore, scientists can find sand dunes on Mars, similar to ones found on Earth.

Megabarchans on Mars (wind from right to left)

10. Has it been determined if the methane gas found on Mars was biological or geological?

The short answer is no. Methane has a short lifetime in the atmopshere due to destruction by UV radiation, but there is evidence that it is being destroyed more quickly than normal. The cause of this is unknown. There are several possible sources for methane: volcanic emissions (however, there is no current active volcanism), chemical reactions, and biological. There seems to be a relationship between water and methane in the atmosphere which would favor a biological source, but this is far from proven.

11. Does Mars have the same hydrological cycle as Earth?

Currently, the cycles are not the same, though the climate of Mars was likely very different in the past. Earth has lakes, rivers, and rain. On Mars, it is currently too cold for liquid water to exist (at least on the surface). So, while there is a some water in the atmosphere and water ice at the poles, there is no rain or liquid water. Mars does have cirrus clouds and at night, some of the water in the atmosphere will turn to morning frost.

12. What happened to the atmosphere of Mars?

There are several theories: 1) catastrophic collision by a large body blew away a significant percentage of the atmosphere, 2) atmosphere has been gradually eroded by solar wind, 3) on-going electromagnetic field and solar wind interaction has gradually stripped away the atmosphere, or 4) all of the above.

13. Is the atmosphere of Mars uniform?

Essentially, yes. However, it is denser in lower areas and thinner in higher areas (as it is on the Earth). The average pressure is about 0.7% that of Earth (at the surface), and ranges from about 1/200 of this on the top of Olympus Mons to about twice this at the bottom of the Hellas Basin. There is also a seasonal variation due to the sublimation (turns from solid to gas) and migration of carbon dioxide ice (frozen carbon dioxide) from one pole to the other.

14. What are fluvial and aeolian features?

Fluvial features are formed by water (or other liquid) and aeolian features are formed by wind. On Mars, fluvial features are gullies, channels, etc. Aeolian features would include dust-filled craters, sand dunes, small ripples, dust devil trails, etc.

15. How much of the ice on Mars is water ice vs. carbon dioxide ice?

There have been some recent estimates that as much as 90% of the ice is water! It has long been known that there are “residual polar caps” in the summer after the dry ice sublimates and migrates to the other pole. Water ice may exist below the surface down to about +/-45° latitude from the poles due to the shape of lobate ejecta surrounding impact craters. However, data from recent Mars missions suggests that there is much more water hidden below the surface. In fact, radar observations of the south pole of Mars imply that there is enough water to cover the planet to a depth of more than ten meters. And that is just at the south pole!

16. How would you draw a water cycle for Mars since liquid water is not presently stable on the surface?

Explanations on the state of water and its life cycle on Mars can be found at:

17. Why are there more crater impacts in the southern hemisphere of Mars?

The simple answer is that the southern hemisphere is older (older, so more time for impacts to occur). This is similar to what we see on the Moon with the lunar highlands and the lower-standing maria. The not so simple answer is (and not everyone agrees) is that the northern lowlands are low due to a giant impact, forming what is now a large basin. Everything else would then be higher than the northern lowlands, with the exception of other deep impact craters/basins such as Hellas. Any craters prior to the giant impact would have been destroyed, erasing the old record.

18. Where can I find images and the locations of where images were taken?

For images from orbit, see:,,, and

For images from the surface (landers, rovers), see: and

19. How do scientists compare the crater size with overall age for a particular region on Mars?

The general approach is to apply a calibration between cratering density and the absolute age that was created for the Moon (and Mars as well). The Moon is the only planetary object for which we have collected samples at specific locations on the surface. That has allowed scientists to connect relative age dating from crater counting to absolute ages from radioactive decay. The application of the calibration to other neighboring objects in space (Mars and Venus as well as the Earth) is based on the assumption that all objects in the inner solar system (from the Sun to as far as the asteroid belt) experienced similar cratering histories.

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