Mars is a harsh, cold world. It is much colder than Earth; but then, it is also farther from the sun. The small, barren planet also has a thin atmosphere that is 95 percent carbon dioxide.

Mars's atmosphere is about 100 times thinner than Earth's. Without a "thermal blanket," Mars can't retain any heat energy. On average, the temperature on Mars is about minus 80 degrees Fahrenheit (minus 60 degrees Celsius). In winter, near the poles temperatures can get down to minus 195 degrees F (minus 125 degrees C). A summer day on Mars may get up to 70 degrees F (20 degrees C) near the equator, but at night the temperature can plummet to about minus 100 degrees F (minus 73 C). 

NASA's Mars Curiosity rover measured air temperatures as high as 43 degrees F (6 degrees C) in the afternoon, with temperatures climbing above freezing for a significant number of days.

"That we are seeing temperatures this warm already during the day is a surprise and very interesting," Felipe Gómez, of the Centro de Astrobiologia in Madrid, said in a statement.

Frost forms on the rocks at night, but as dawn approaches and the air gets warmer, the frost turns to vapor, and there is 100 percent humidity until it evaporates. The high humidity could help make Mars more habitable, if the water condenses to form short-term puddles in the early morning hours.

"The conditions on Mars, where the relative humidity is high and the available water vapor is approximately 100 precipitable microns, is the equivalent of the drier parts of the Atacama Desert in Chile," John Rummel, of East Carolina University, told by email.

According to Rummel, the humidity of Mars is tied to temperature fluctuations. At night, relative humidity levels can rise to 80 to 100 percent, with the air sometimes reaching atmospheric saturation. The daytime air is far drier, due to warmer temperatures.

On Earth, some forms of life are able to survive in parched regions by poaching water from the humid air. Among these, lichens dominate, surviving in arid climates without succumbing to the dry spells that frequently occur. Some lichens in super-dry areas have been found to photosynthesize at relative humidity levels as low as 70 percent. Other research has demonstrated that a form of Antarctic lichen can adapt to life under simulated Martian conditions.

"Such short-term wet periods might be long enough and warm enough to allow for Earth organisms to metabolize and even reproduce," Rummel said.

Researchers have calculated that carbon dioxide snow particles on Mars are roughly the size of a human red blood cell. Martian snow is depicted in this artist's rendering as a mist or fog that eventually settles to the surface.
Researchers have calculated that carbon dioxide snow particles on Mars are roughly the size of a human red blood cell. Martian snow is depicted in this artist's rendering as a mist or fog that eventually settles to the surface.
Credit: NASA, Christine Daniloff/MIT News

Like Earth, Mars has four seasons because the planet tilts on its axis. The seasons vary in length because of Mars' eccentric orbit around the sun. In the northern hemisphere, spring is the longest season at seven months. Summer and fall are both about six months long. Winter is only four months long.

During a Martian summer, the polar ice cap, composed mainly of carbon dioxide ice, shrinks and may disappear altogether. When winter comes, the ice cap grows back. There may be some liquid water trapped beneath the carbon dioxide ice sheets, scientists say.  

In the past, Mars may have been warmer and wetter, with an average global temperature of 50 degrees F (10 degrees C).

"We've learned that Mars is a dynamic planet," Michael Meyer, lead scientist for NASA's Mars exploration program, told reporters in 2011. "We've learned that it has a history where it was warm and wet at the same time that life started here on Earth."

Other research suggests that the Red Planet may have once been white, an icy wasteland with an average temperature of minus 54 degrees F (minus 48 degrees C). According to Robin Wordsworth, a researcher at Harvard, a colder scenario is more straightforward to model because Mars only gets 43 percent of the solar energy of Earth, and early Mars was lit by a younger Sun believed to have been 25 percent dimmer than it is today.

"That makes it very likely early Mars was cold and icy," he said in a statement.

And the planet continues to change today. Recent research found that the planet is emerging from an ice age involving shrinking polar caps and growing glaciers at midlatitudes.

"All around the ice cap, there is evidence for a climate change from ice age to interglacial period," planetary scientist Isaac Smith told Smith studied the shrinking ice caps while at the Southwest Research Institute in Colorado.

Additional reporting by Nola Taylor Redd, contributor