Mars is a planet that shows climate change on a large scale. Although Mars' atmosphere used to be thick enough for water to run on the surface, today that water is either scarce or non-existent. The atmosphere today is also too thin to easily support life as we know it, although life may have existed in the ancient past.
The climate of Mars comes from a variety of factors, including its ice caps, water vapor and dust storms. At times, giant dust storms can blanket the entire planet and last for months, turning the sky hazy and red.
What is Mars' atmosphere made of?
The atmosphere of Mars is about 100 times thinner than Earth's, and it is 95 percent carbon dioxide. Here's a breakdown of its composition, according to a NASA fact sheet:
- Carbon dioxide: 95.32 percent
- Nitrogen: 2.7 percent
- Argon: 1.6 percent
- Oxygen: 0.13 percent
- Carbon monoxide: 0.08 percent
- Also, minor amounts of: water, nitrogen oxide, neon, hydrogen-deuterium-oxygen, krypton and xenon
Climate and weather
Early in its history (particularly in periods older than 3.5 billion years ago) Mars had a thick enough atmosphere for water to run on its surface. Orbital pictures show vast river plains and possible ocean boundaries, while several Mars rovers have found evidence of water-soaked rocks on the surface (such as hematite or clay). However, for reasons that are still poorly understood, the Martian atmosphere thinned.
The leading theory is that Mars' light gravity, coupled with its lack of global magnetic field, left the atmosphere vulnerable to pressure from the solar wind, the constant stream of particles coming from the sun. Over millions of years, the sun's pressure stripped the lighter molecules from the atmosphere, thinning it out. This process is being investigated by NASA's MAVEN (Mars Atmosphere and Volatile Evolution) mission. Other researchers hypothesize that perhaps a giant impact by a small body would have stripped the atmosphere away.
Mars' thin atmosphere and its greater distance from the sun mean that Mars is much colder than Earth. The average temperature is about minus 80 degrees Fahrenheit (minus 60 degrees Celsius), although it can vary from minus 195 F (minus 125 C) near the poles during the winter to as much as a comfortable 70 F (20 C) at midday near the equator.
The atmosphere of Mars is also roughly 100 times thinner than Earth's, but it is still thick enough to support weather, clouds and winds. There is also radiation at its surface, but it shouldn't be enough to stop Mars exploration; analysis by the Curiosity rover found that a single mission to Mars is comparable to the radiation guidelines for astronauts for the European Space Agency, although it does exceed those of NASA.
Giant dust devils routinely kick up the oxidized iron dust that covers Mars' surface. Dust is also a permanent part of the atmosphere, with higher amounts of it in the northern fall and winter, and lower amounts in the northern spring and summer. The dust storms of Mars are the largest in the solar system, capable of blanketing the entire planet and lasting for months. These usually take place in the spring or summer.
One theory as to why dust storms can grow so big on Mars starts with airborne dust particles absorbing sunlight, warming the Martian atmosphere in their vicinity. Warm pockets of air flow toward colder regions, generating winds. Strong winds lift more dust off the ground, which in turn heats the atmosphere, raising more wind and kicking up more dust. A 2015 study further suggested that the momentum of Mars – which is affected by other planets – generates planet-circling dust storms when that momentum is at its greatest during the early part of the dust storm season.
At times, it even snows on Mars. The Martian snowflakes, made of carbon dioxide rather than water, are thought to be very small particles that create a fog effect rather than appearing as falling snow. The north and south polar regions of Mars are capped by ice, much of it made from carbon dioxide, not water.
Today, NASA says seasonal changes are due to the waxing and waning of the carbon dioxide ice caps, dust moving around in the atmosphere, and water vapor moving between the surface and the atmosphere. (Most of the water comes from the north water ice cap, which is exposed and sublimates during the Martian summer when carbon dioxide evaporates off the cap.)
"During winter, the temperatures in the polar regions are cold enough for the CO2 [carbon dioxide] in the atmosphere to condense into ice on the surface. The CO2 then sublimates off the ice cap in the spring and summer, returning to the atmosphere," NASA stated.
"In the northern hemisphere, the CO2 ice cap completely vanishes in the summer, uncovering a large perennial H2O ice cap. During the southern hemisphere summer, a small CO2 covered ice cap survives; this perennial ice cap is offset from the south pole. This cycling of CO2 into and out of ice on the surface changes the atmospheric mass by tens of percent over the course of a Martian year."
As of 2017, several orbital missions monitor long-term climate changes on Mars, including:
- ExoMars Trace Gas Orbiter (European Space Agency or ESA)
- MAVEN (NASA)
- Mars Express (ESA)
- Mars Odyssey (NASA)
- Mars Orbiter Mission or Mangalyaan (Indian Research Space Organization)
- Mars Reconnaissance Orbiter (NASA)
Current surface missions include NASA's Curiosity and Opportunity rovers. Other surface missions are planned in the coming years, including NASA's Mars 2020 and an ExoMars rover from the ESA.
Possibility of life
Mars could have once harbored life. Some conjecture that life might still exist there today. A number of researchers have even speculated that life on Earth might have seeded Mars, or that life on Mars seeded Earth. The Viking landers famously sought life on Mars during the late 1970s, but came up empty. Today, some of those results remain controversial, especially one in which a sample of soil was heated and then checked for organics. While Viking did not find any organics, other researchers have alternate explanations for its failure (such as the instruments not being sensitive to detect life.)
Oceans may have covered the surface of Mars in the past, providing an environment for life to develop. Although the red planet is a cold desert today, researchers suggest that liquid water may be present underground, providing a potential refuge for any life that might still exist there. Several studies have shown that there is abundant water ice beneath the surface.
A feature called recurring slope lineae (RSL) sometimes occur on Martian slopes. In 2015, researchers announced that hydrated salts had been found inside of these features, which suggests that RSLs have a sort of briny water in them – an environment that could be hospitable to some forms of extreme life. However, more studies in 2016 and 2017 shed some doubt on that theory. One study suggested that the water may arise from the Martian atmosphere, while others maintain that the RSLs are instead due to dry sand.
NASA's Curiosity rover is currently seeking habitable environments during its mission on Mars, which began in 2012. NASA's Mars 2020 rover is expected to take that up a notch, which will include caching potential samples with biosignatures for future missions to retrieve. The ESA also plans its own biosignature-hunting rover as a part of the ExoMars mission. The challenge is that these rovers, although powerful, cannot carry the same type of sophisticated laboratory equipment usually used on Earth to find signs of life in old samples of rock. Also, it's difficult even for Earth samples to pass the life test, as geological signatures can mask as life; discoveries of ancient life in Greenland and Quebec, Canada in 2016 and 2017, for example, are controversial.
Additional reporting by Elizabeth Howell, Space.com Contributor.