Lunar and Planetary Institute

Explore! Mars Inside and Out! - Background

About Mars

A Little About Mars
The Martian day, the time it takes Mars to spin once on its axis, is 24 hours and 40 minutes long, very similar in length to Earth's day. Its year is almost twice as long as Earth's, however. It takes Mars 687 Earth days to orbit the Sun. That path around the Sun is slightly more elliptical than Earth's, and the Sun is not exactly in the center of its orbital path.

Like Earth, Mars is tilted on its axis. This tilt, combined the elliptical orbit, contributes to seasons on Mars. Because Mars is closer to the Sun during its southern hemisphere summer, the summer in that hemisphere is warmer than the northern hemisphere summer.

Surface temperatures are cold — a warm summer day might reach 0°C (32°F), and winter at the poles can be as cold as –125 °C (–193°F), and its atmosphere is very thin. The atmospheric pressure at the surface of the planet is about 1/100th of that of Earth's. Mars' atmosphere is mostly carbon dioxide (95%), nitrogen (3%), and argon (2%), with trace amounts of other gases, like oxygen (0.15%). Earth's atmosphere is mostly nitrogen (77%) and oxygen (21%). The thin Mars' atmosphere offers little protection from dangerous incoming radiation, and, unlike Earth, Mars does not have an ozone layer to protect the surface from solar ultraviolet radiation.

Mars has massive dust storms — storms that can cover the entire planet! Wind speeds can reach 100 km/hour (62 miles/hour), stirring up the fine red dust. The dust gives the Martian sky a pinkish-tinge.

Image of dust storm on Mars.
A dust storm obscures the surface features on Mars
Image Courtesy of: NASA/JPL/Malin Space Science Systems

The Martian atmosphere contains much less water vapor than Earth, making clouds a rarity on Mars. There is no liquid water present at the surface. There may be frozen water in the ground, and Mars has ice caps in its polar regions that are a combination of carbon dioxide and water ice.

Image of Northern Ice Cap.
Northern ice cap of Mars. The polar cap is about 1100 kilometers (700 miles) across.
Image Courtesy of NASA

Mars is about half the size of Earth. Because it has less mass, it has a smaller gravitational attraction. Surface gravity on Mars is less than 40% of Earth's. If you weighed 100 pounds on Earth, you would weigh 38 pounds on Mars.

How did Mars Form?
Mars formed at the same time our solar system formed, 4.6 billion years ago. Our solar system began when a cloud of dust and hydrogen and helium gases drifting in our galaxy started to condense and contract under its own gravity, forming a wide, flat, rotating disk. Most of the material collected in the center, condensing into a sphere of gas that eventually became our Sun. The remainder of the cloud formed a wide disk, swirling around the Sun, called the solar nebula. The rocky, terrestrial planets — Mercury, Venus, Earth, and Mars — all formed in the inner, hotter part of our Solar System, where metals and silicates were concentrated. Much of the gas and ice in the solar system could not exist as solids at the high temperatures in this region. So it was from the heavier materials that the rocky inner planets were made. The dust and particles collided, merged, and broke apart. Through the process of "accretion," these tiny particles formed larger and larger bodies, eventually becoming so massive that their gravity began to assist by attracting more and larger aggregates of material, speeding up the process of accretion. Because of this, the largest bodies grew the fastest, sweeping up material in their paths, and eventually becoming Mars and the other planets.

How has Mars Changed Through Time?
Mars has undergone several changes during its 4.6 billion year history — both inside and out!

Getting Organized Inside. Like all planets, Mars became hot as it formed, from the constant pummeling by impactors as it accreted, and from the radioactive decay of elements. Very soon after it formed, its interior melted or partially melted, and the materials making up Mars reorganized. The denser elements — iron and iron sulfide — separated from the more silicate-rich materials and sank to the interior, forming Mars' core. The silicate-enriched layer surrounding the core formed Mars' mantle. The least dense silicate materials formed the crust, perhaps crystallizing from a magma ocean that enveloped the planet. For the first few hundred million years, Mars probably had a magnetic field, generated by convection (fluid flow) in the molten core. As Mars cooled, the magnetic field died.

Image of Earth, Moon and Mars inner and outer cores.
Image showing cross-sections of the Earth, Moon, and Mars.
The crust in each case is a very thin outer layer.

The Great Dichotomy — or Why There is Such a Big Difference Between the North and South. In short, scientists don't know, but they are investigating it! A quick look at elevations on Mars shows that the northern hemisphere is relatively low or deep and the southern hemisphere is high. The crust in the southern hemisphere is about 25 kilometers (15 miles) thicker than in the northern hemisphere, and this causes the southern highlands to be about 4 kilometers (2.5 miles) higher in elevation than the northern lowlands. This evidently happened in the first few hundred million years of Martian history. The interesting problem is understanding why the crust is thicker in the south than it is in the north. Some scientists suggest that the northern hemisphere low is a depression created either by one giant asteroid impact or by several big impacts. Others suggest that motions inside of the mantle of Mars, known as convection, may have concentrated crust into the southern hemisphere. More information, gathered by spacecraft and by astronaut explorers, will be needed to solve this mystery.

Image of Mars Orbiter Laser Altimeter (MOLA) map showing elevations of the Martian Surface.
Mars Orbiter Laser Altimeter (MOLA) map showing elevations of the Martian Surface.
The northern hemisphere is low, while the southern hemisphere is high.

Making an Impact. Like all terrestrial planets, Mars has been significantly cratered. Our early solar system was a messy place; asteroids and comets large and small abounded. These space rocks struck the planetary surfaces, adding more heat, breaking up the outer planetary layers, and creating large bowl shaped depressions of shattered rock. With time, much of this debris was accreted into the planets, leaving their orbital paths relatively clear. By about 3.8 billion years ago, the period of intense bombardment came to a close, and impacts — though continuing even today - became smaller and less frequent.

Scientists expect impacts to strike all parts of planetary surfaces equally. The Moon and Mercury are cratered across their surfaces. If large area on a planet is not cratered, scientists interpret that this surface is younger than the more cratered areas; it has not had time to accumulate a large number of impacts. In other words, something has happened to that surface to erode or fill up the craters (or, in Earth's case, to regenerate the crust all together). The hemispheres of Mars are very different. The rugged Southern Highlands record the long history of impact events. The Northern Lowlands also are heavily cratered, but somethng has covered or buried most of them. leaving the surface smooth. Scientists are unsure exactly why the north and south are so different. Perhaps the craters have been filled by lava flows or covered by sediment, or eroded by flowing water. This is one of the big mysteries about the Martian surface.

For more information on impacts and craters, visit Impact Cratering.

Image of Mars Orbiter Lase Altimeter (MOLA) maps shows a distinction between lowlands and highlands.
Mars Orbiter Laser Altimeter (MOLA) maps show a distinction between lowlands and highlands.
The northern lowlands have overall elevations about four kilometers (a few miles) lower
than the cratered uplands of the southern hemisphere. The northern lowlands also are smoother; the craters in this region have been buried by more recent geologic activity.

Image and caption courtesy of Planetary Science Research Discoveries.

Volcanism in a Big Way. Early Mars was volcanically active, spewing lava across its surface, and water and carbon dioxide into its atmosphere. Much of this early history, recorded in the older Southern Highlands, is obscured by impact craters.

From about 3.5 billion years until more recently, the volcanic — and tectonic — activity has been concentrated around the Tharsis region near the equator. Tharsis is a huge bulge in the crust, capped by prominent volcanos. Some scientists suggest the bulge overlies a region of hotter than normal mantle. The high temperatures allowed the development of numerous large volcanos. These volcanos thickened the crust, causing Tharsis to be higher than other parts of Mars. The crust was pulled apart to form the immense canyon of Valles Marineris more than 3 billion years ago.

Image of Mars surface showing the prominent Valles Marineris.
Surface of Mars showing the prominent Valles Marineris. Over 3000 km long and up to
8 km deep, Valles Marineris would stretch from Los Angeles,
California to Washington, D.C. if it occurred on Earth.
Image courtesy of NASA and the U.S. Geological Survey.

Image of Mars Express orbiter.
Mars Express orbiter image, showing a canyon in eastern Valles Marineris.
Image courtesy of ESA / DLR / FU Berlin (G. Neukum)

Olympus Mons, the tallest volcano in our solar system, began forming about 1 billion years ago. Scientists do not really know how young the most recent volcanic activity is on Mars. Certainly, some volcanos have erupted in the last 100 million years. Although that sounds like a long time, it is within the last 2% of our solar system's history! Some lava flows are so fresh and have so few craters, that they may be less than 100,000 years old. It is likely that some martian volcanos will erupt again in the future.

Olympus Mons, and the other volcanos of the Tharsis region, are 100 times more massive than volcanos on Earth! These volcanos are so large because Mars' outer layer does not move relative to the mantle underneath — the surface of Mars is stationary. The volcanos remain over chambers of molten rock and grow as lava flow after lava flow after lava flow pours out of the interior, each adding to the volcano.

For more information on volcanism, please visit Volcanism.

Image of Olympus Mons and volcanoson the Tharsis bulge.
Olympus Mons (upper left) and volcanos on the Tharsis bulge. The white features are clouds.
Warm air (containing water vapor) rises up the volcano slopes and cools at higher altitude.
The water vapor freezes to form clouds of ice crystals.
Image courtesy of NASA/JPL/MSSS

Moving Plates. There is no evidence that plate tectonics — the movement of rigid plates (lithosphere) on a mobile upper mantle (asthenosphere) — is occurring now on Mars. Mars lacks the pattern of features, such as chains of volcanos, long ridges, or folded mountains, that would be expected if plate tectonics were occurring or recent. All evidence is that Mars has had a stationary outer layer for at least the last 3 billion years, when the Tharsis region began to form. Recently, some scientists have speculated that Mars had plate tectonics in its early history, based on magnetic patterns in the Southern Highlands recorded from orbiting spacecraft. This is another area for scientific exploration.

For more information on tectonics and plate tectonics, visit Tectonism.

Losing the Atmosphere.Early Mars probably had a thicker atmosphere with more carbon dioxide and water vapor, provided by vigorous volcanic activity. Mars' magnetic field shielded the surface from the charged particles of the solar wind and dangerous cosmic radiation. This Mars was warmer and wetter. The higher atmospheric pressure permitted flowing water at the surface. By about 3.5 billion years ago, Mars had entered its cooler, drier phase. As its interior cooled, the gases and water vapor from the volcanism gradually dwindled and the magnetic field disappeared. The unprotected atmosphere was worn away by the solar wind and the Martian surface was bathed in radiation.

Image of thin atmosphere enveloping in Mars.
This image looks obliquely at part of the southern hemisphere of Mars.
The numerous circular structures are impact craters, indicating that this is an old part
of the martian surface. The thin martian atmosphere can be seen as a layer
between the rock surface of the planet and the black of space.
Image courtesy of NASA.

Image of the cold, dry Mars of Today.
An image of the cold, dry Mars of today, taken by Mars Exploration Rover, Spirit.
Image courtesy of NASA, JPL, and Cornell

Disappearing Water. Early Mars was wetter and warmer. Several lines of scientific evidence support this. Images obtained by Mars orbiters have revealed that the ancient Southern Highlands are covered by dendritic drainage patterns — networks of stream channels, or "valley networks" that erode into the highland craters. While there are some differences, these features are generally similar to gently meandering river channels on Earth. The valley networks on Mars are interpreted to have formed at a slow rate, and thus they require a time in Martian history when flowing liquid water was stable at or near the surface of the planet. Chemical measurements made from orbit reveal the presence of clay associated with some of these channels; the formation of clay requires that water was present at some time. Additional evidence for liquid water was found by the Mars Exploration Rovers. They documented structures in the rocks that are created by flowing water, and minerals formed in salty, acidic water. One meteorite from Mars contain mineral deposits — carbonate — that may have precipitated when the rocks were soaked in water.

Image of stream drainage across the Southern Highlands of Mars.
Stream drainage across the Southern Highlands of Mars. The streams erode the edges of
some of the older, larger craters. This pattern is similar to stream drainage patterns — dendritic drainage — seen on Earth that is caused by flowing water.
The image is 200 kilometers across (125 miles)
Viking Orbiter image 606A56
Image courtesy of NASA.

Features that look like the Mississippi River Delta (minus the water) are found on
Mars' surface, suggesting that water flowed across the surface for a long period of time,
gradually creating a delta in a body of water. The feature is 11 kilometers wide (7 miles)
and 13 kilometers from top to bottom (8 miles) in the image.
Image courtesy of NASA/JPL/Malin Space Science Systems

Rover images of layers in the rocks at the Martian surface.
Rover images of layers in the rocks at the Martian surface. The thin layers are interpreted to be sediment deposited by flowing water. The "blueberries" are small, BB-sized deposits of hematite. Hematite is a mineral that typically, though not always, forms in water.
Image courtesy of NASA.

Some scientists have calculated that Mars may have had a global layer of water that was 120 meters thick. Imagine Mars with an ocean at its northern hemisphere, and streams flowing across the landscape, draining into it.

Image of an artistic rendering of what an ancient ocean might have looked on Mars.
An artistic rendering of what an ancient ocean might have looked like on Mars.
Image copyright Michael Carroll, all rights reserved.

About 3.5 billion years ago, things changed. Mars became cooler and drier, related to changes in its atmosphere. The thin atmosphere and low air pressure no longer permitted liquid water to exist at the surface. Under these conditions, water turns directly from ice into gas — it sublimes — when it is exposed and warmed at the surface. As Mars cooled and the conditions became unstable for liquid water to exist at the surface, the water may have been sequestered underground, either as a liquid or as ice. Occasional warm periods in Mars' history resulted in melting of the subsurface ice and gigantic floods. The floods are recorded by outflow channels that feed into the Northern Lowlands. These features are much more chaotic than the orderly drainage patterns of the Southern Highlands. Outflow channels, similar in features to braided streams on Earth, form from catastrophic floods of water. Multiple wide channels "braid" together, transporting gigantic blocks of the substrate.

Image of outflow channels cut by flood waters in Ares Vallis.
Outflow channels cut by flood waters in Ares Vallis. The blocky "chunks" in the broad
channel at the bottom of the image are displaced blocks of material pulled from
the walls of the channel as the water rushed along.
Image courtesy of ESA/DLR/FU Berlin (G. Neukum)

Mosaic of images from NASA's Viking Mission
Tear-drop shaped islands formed as flood waters rushed through this area.
The circular depressions are impact craters. The region shown is 475 kilometers
(295 miles across). Mosaic of images from NASA's Viking mission.
Image courtesy of NASA.

Recent images of gullies on the slopes of Martian craters, compared to older images of the same crater, show a new flow of material down the crater slopes. Some scientists interpret this flow to suggest that water occasionally flows on the surface of Mars today. Ice below the surface may melt and carry material down slope, before the water evaporates or refreezes. The cause of these features continues to be debated by scientists. However, some scientists suggest that these gullies are created by the flow of dry sand, with no water present at all. Another martian mystery!

Gullies on the wall of Newton crater on Mars.
This image shows several gullies on the wall of Newton crater on Mars. Some scientists
believe that the gullies are evidence of the recent flow of liquid water at
the surface of Mars. The image is 3 kilometers(2 miles across).
NASA Mars Global Surveyor image. Image courtesy of NASA.

Where is the water now? Much of Mars' water is underground, either as a liquid or as ice. Subsurface water is common on Earth, too! Much of our drinking water comes from "groundwater." NASA's Mars Reconnaissance Orbiter and the European Space Agency's Mars Express have instruments aboard designed to detect evidence of subsurface water on Mars. Stay tuned for those mission results!

And do not forget the polar ice caps! Mars' northern and southern ice caps contain water ice, as well as carbon-dioxide ice — like the dry ice you can get in supermarkets. Mars' northern ice cap is mostly water ice.

Image of water and carbon-dioxide ice ('dry ice') occur in the Southern Polar Ice Cap.
Water and carbon-dioxide ice ("dry ice") occur in the Southern Polar Ice Cap.
Image courtesy of NASA.

Image of residual water ice in Vastitas Borealis Crater.
Residual water ice in Vastitas Borealis Crater.
Image courtesy of ESA/DLR/FU Berlin (G. Neukum).

Might There be Life on Mars?
All life as we know it requires liquid water, hence the strong interest in finding evidence of past liquid water on Mars, and understanding the history of this water. There is strong scientific evidence that liquid water once occurred on the surface of Mars, so it is possible that life could have become established. The first evidence for life on Earth, in the form of fossil bacteria, occurs about 3.5 billion years ago — the time that the Martian environment was changing from warmer and wetter to colder and drier. Microbial life on Earth probably existed before this time period, possibly becoming established after the period of intense asteroid bombardment was over, but there is no record of it.  In short, it took life up to a billion years to become established on Earth, and this may be a reasonable timeline for Mars, as well.

Given this start, and using Earth as a model, conditions on much of Mars would have been suitable for life for about a half billion years, before the climate deteriorated. However, the features recording flooding events suggest that there were occasional warmer and wetter periods, and there may have been refuges for life, such as moist areas near warm volcanic regions. Given the harsh conditions, and lack of evidence, it is improbable that life evolved into complex multi-cellular forms, like it did on Earth between one billion years and 500 million years ago.

There is no evidence that life exists on Mars right now — but finding life — or evidence of past life — is challenging when you are examining an entire planet! You need to be in the right place. Scientists will continue to work to identify where the conditions might be right for life, as we understand it, on Mars.

In the 1990's NASA scientists announced the presence of organic molecules, mineral features that could have been formed by biological activity, and possible microscopic fossils of primitive, bacteria-like organisms in a Martian meteorite. They interpreted the features to have formed on Mars more than 3.6 billion years ago, and to be evidence that life existed on Mars. The results have been hotly debated in the scientific community. Many scientists believe the structures could have been formed by chemical processes, rather than biologic; such chemically formed features are known to exist. Others suggest that the organic signature is contamination from Earth. The debate continues, with the balance tipped toward the side that contends the features are not evidence of life. Debate is part of a healthy scientific process — and it has served an additional purpose — it has helped scientists better identify the "signals of life" and bring more tools to the identification.


Last updated
October 19, 2009

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