A Gloomy Mars Warms Up

To determine the extent of surface changes on Mars, scientists took images from the Viking (launched 1975) and Mars Global Surveyor (1996) satellite missions and mapped them into a climate model developed at NASA Ames Research Center. They discovered that a wind-whipped, dusty surface has a measurable effect on the amount of sunlight that is reflected by the planet. The results of this research show that an increase in darkened surface areas may account for a one degree Fahrenheit rise in the surface air temperature of the planet.

“We know that warmer temperatures and increased wind strengths are near the darkened areas where less sunlight is reflected by the surface, and cooler temperatures and weakened winds generally correspond to brightened areas” explained Lori Fenton, the experiment’s principal investigator at NASA Ames Research Center, in California's Silicon Valley. “What we don’t understand is how these changes in the planet’s brightness affect the martian climate.”

“Albedo” is the technical term for a planet’s ability to reflect sunlight. According to scientists, variations in the planet’s albedo are generally attributed to changes in distribution of dust on the surface.

Research indicates that as the dark areas on Mars expand and darken over time, its albedo decreases, and its surface air temperature rise. Large surface areas, some almost twice the size of the continent of Africa (over 34 million square miles), have been observed to darken or brighten by 10 percent or more, which speaks to the magnitude of these global surface changes.

“The coupling of these processes with albedo changes could produce a surface-atmosphere feedback loop, in which surface albedo reductions, or darkened areas, enhance the windiness and dust devil formation that produce surface changes. Increased heating near the surface leads to greater atmospheric instability,” explained Fenton.

In other words, scientists think that when surface areas darken and expand, relatively more energy from the sun is being absorbed by the surface, which causes temperatures to rise near the surface. This, in turn, produces a less stable atmosphere generating more turbulent eddies and whirling dust devils. The more dust that is redistributed to bright surfaces, the more surfaces darken and expand, which causes more sunlight to be absorbed, increased temperatures, and less stable the atmosphere, say scientists.

“In particular, the slight increase in surface air temperature is similar to climate variations seen on Earth, even though the processes involved are significantly different,” added Fenton.

Moreover, scientists suspect that a change in global surface albedo could influence the formation of dust storms on both local and global scales. They report that the surface brightening and atmospheric cooling following the 2001 global dust storm may affect the timing of future large dust storms. Research demonstrates that surface areas may brighten depending on the way dust settles, which would suppress winds and ‘dust devil’ formation, the two mechanisms potentially responsible for dust storm initiation.

“Although the events that trigger dust storms have yet to be understood, this work demonstrates that one contributing factor may be a decrease in surface albedo. Martian climate indicators, such as global dust storm occurrences, polar energy balance, and annual global-mean air temperature, are dependent on many interrelated and poorly understood processes. By investigating solely the effects of changes in surface albedo (from two very different Mars years), we have shown that albedo interacts with, and could in part drive, other climate-influencing processes on Mars,” said Fenton.

These and related studies appeared in Nature, 2007.