The overall darkening of Mars’ surface in recent decades has significantly raised the Red Planet’s temperature, a possible cause for the substantial shrinkage of the planet’s southern ice cap, observed in the past few years.
The brightness of an object is characterized by its albedo, the percentage of light that’s reflected from its surface. Because dark objects in general absorb more solar radiation and get hotter than light ones do, albedo changes on Earth in arctic regions may be playing a part in climate change (SN: 11/12/05, p. 312: Available to subscribers at Runaway Heat?).
Viking orbiters, which circled Mars in the late 1970s, measured the planet’s albedo at latitudes up to about 60° in both hemispheres, says Paul E. Geissler, a planetary geologist with the U.S. Geological Survey in Flagstaff, Ariz. However, data gathered in 1999 and 2000 by another probe, the Mars Global Surveyor, reveal that albedo for much of that area has changed, he notes.
As much as one-third of the Martian landscape had either darkened or lightened by 10 percent or more between the two sets of observations. Many scientists presume that winds blow light-colored dust from one part of the planet to another, exposing the darker rocks beneath, Geissler notes. On balance, the planet grew darker between the late 1970s and the turn of the millennium.
New climate simulations by Geissler and his colleagues indicate that albedo variations may be driving big changes in the Martian climate. Modeling conditions on Mars using albedo data from the Mars Global Surveyor, the team calculated an average air temperature at the planet’s surface about 0.65°C higher than in comparable simulations using the Viking-era data. In some areas, particularly those over the planet’s southern ice cap, air temperatures might have increased as much as 4°C, the researchers report in the April 5 Nature.
Mars’ southern ice cap is shrinking and has lost billions of tons of carbon dioxide over just the past 4 Martian years, says Geissler. The team’s simulations suggest that the planet’s albedo-induced warming may be responsible for as much as 60 percent of that loss.
Geissler and his colleagues have conducted “a very straightforward study,” says R. John Wilson, an atmospheric scientist with the National Oceanic and Atmospheric Administration in Princeton, N.J. The team’s link between planetary albedo and climate is “nothing surprising,” he says. He adds that some of the team’s other predicted effects—such as slightly stronger winds and an increased frequency of dust devils—are quite subtle and may be difficult to confirm.
The researchers have identified “a plausible mechanism” for the recent shrinkage of Mars’ southern polar ice cap, says Richard Zurek, an atmospheric scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.
The team’s findings don’t point to an external influence, such as an increase in solar radiation, that some climate-change skeptics have suggested may be behind Earth’s recent warming, Geissler says.