Within our solar system, Mars is the only planet with a thin atmosphere composed primarily of carbon dioxide (CO2).
The Carbon Dioxide Planets
Mars and Venus are the only planets in our solar system with atmospheres composed primarily of CO2. The atmosphere of Venus, which is 90 times heavier than that of Earth, exerts the kind of pressure found 3,000 feet below the surface of our oceans. Mars, on the other hand, has an atmospheric pressure less than one percent that of Earth at sea level. To find atmosphere that thin on Earth, you would have to travel more than 120,000 feet into space, far above our tallest mountains.
Centuries of Speculation
As one of the closest planets to Earth, Mars has been studied extensively by telescope. In the late 1700s, scientists William Herschel and Johann Schröter theorized the existence of an atmosphere on Mars. They based their observation of what appeared to be clouds moving across the surface of the planet. In 1947 at an observatory in Texas, Gerard Peter Kuiper positively identified CO2 in the Martian atmosphere. In 1976, the exact composition of the atmosphere on Mars was firmly established from measurements taken by NASA's Viking landers, the first probes from Earth to land intact on Mars.
The atmosphere of Mars is, on average, 95 percent CO2, 3 percent nitrogen and 2 percent argon. (Traces of oxygen, H2O, methane and other gases are also present.) However, the amount of CO2 in the Martian atmosphere fluctuates according to the season. The Martian north and south poles are in continual darkness during each hemisphere's winter. During the winter, temperatures drop below the freezing point of CO2; as much as 25 percent of atmospheric CO2 is deposited at the poles as dry ice. In the summer, when the poles are exposed to sunlight, the solid CO2 changes into a gas and enters the atmosphere.
Evidence of Martian water, sedimentary deposits, dried flood plains and water erosion on rocks suggests the planet once had a thicker atmosphere. Possible causes for the disappearance of the atmosphere include: erosion of the atmosphere by solar wind, low Martian gravity (which enables gas molecules to escape the planet) and a meteor collision that tore away a large percentage of the atmosphere in a single, massive event.
Magnetic Fields and Solar Winds
The sun releases a flowing stream of charged particles called the solar wind. The solar wind can strip molecules from the atmosphere of planets as it passes. Earth is surrounded by a magnetic field, which protects our atmosphere by deflecting most of the solar wind particles. Four billion years ago, Mars had a magnetic field similar to that of Earth. However, a series of asteroid impacts may have knocked out the planet’s magnetic field, leaving its atmosphere vulnerable to the stripping effect of the solar wind.
Data gathered by the NASA Mars Global Surveyor spacecraft in 1998 suggest the solar wind might be tearing away large chunks of atmospheric gas on a daily basis, rather than through a process of slow erosion. "It helps explain why Mars has so little air," says David Brain of the University of California at Berkeley.