Even though daytime temperatures in the tropics of Mars can be about –20°C, a summer afternoon there might feel about the same as an average winter day in southern England or Minneapolis. That’s because there’s virtually no wind chill on the Red Planet, according to a new study—the first to give an accurate sense of what it might feel like to spend a day walking about on our celestial neighbor.
“I hadn’t really thought about this before, but I’m not surprised,” says Maurice Bluestein, a biomedical engineer and wind chill expert recently retired from Indiana University–Purdue University Indianapolis. The new findings, he says, “will be useful, as people planning to colonize Mars need to know what they’re getting themselves into.”
There’s no doubt that Mars is cold: Planet-wide, the average temperature is about –63°C, compared with Earth’s more hospitable 14°C. Even in the martian tropics, nighttime temperatures can drop to near –90°C, says Randall Osczevski, an environmental physicist who recently retired from Defence Research and Development Canada in Toronto. Yet thermometer readings on Mars are highly misleading to earthlings, he notes. That’s because people typically base their notions of comfort in cold, windy conditions on their personal experiences—all of which have taken place within our planet’s much thicker atmosphere.
At Mars’s surface, atmospheric pressure is less than 1% that of air pressure on Earth at sea level. That’s about the same pressure as Earth’s atmosphere at an altitude of 32 kilometers—or about 2.5 times the cruising heights of jet aircraft, Osczevski says. But air that thin doesn’t do a good job of carrying heat away, even when the winds are blowing at 100 kilometers per hour (as they sometimes do in the Red Planet’s global dust storms). In other words, he notes, on Mars the wind chill—the added cooling effect generated by air sweeping heat away from a body warmer than its environment—is almost nonexistent.
So Osczevski came up with the idea of converting martian temperatures into something more understandable. This new parameter—the Earth equivalent temperature—is the air temperature on Earth, in the shade and in still air, that gives the same heat loss and surface temperature as the frigid yet insubstantial winds of Mars.
Results from such conversions, reported in the current issue of the Bulletin of the American Meteorological Society, are surprising. In winter, the average temperature in southern England is about 4.4°C. Considering the average wind speed is about 23 kilometers per hour, that brings the wind chill temperature down to about 0°C. Coincidentally, according to Osczevski’s estimates, the Earth equivalent temperature for the average summertime afternoon high on Mars where NASA’s Pathfinder landed in 1997 is 1°C (which stems from combining the site’s average afternoon high of –14°C with a typical wind speed of 14 km/hr).
Similarly, Mars’s global average temperature of –16°C, in wind-free conditions, is only 1°C colder than the average winter wind chill temperature of Minneapolis, Minnesota (which during that season combines an average temperature of –8.1°C with an average wind speed of 16.5 km/hr).
“In this aspect, at least, Mars doesn’t seem to be as environmentally harsh as people generally expect,” says Robert Zubrin, an aerospace engineer and founder of the Mars Society, a Lakewood, Colorado–based organization that promotes the human exploration and settlement of the Red Planet.
Although wind chill on Mars won’t be as tough a problem as previously presumed, heat loss will still be a challenge, Osczevski says. For example, a Mars explorer exposed to 15 km/hr winds in –40°C would lose only 60% as much heat as an Arctic explorer in the same conditions. Only 30% of that overall heat loss would be carried away by Mars’s thin air (a form of convection); most of the remainder would be lost through radiation. Even in an environment with no atmosphere, hot objects radiate energy into cold surroundings, Osczevski says. That relative shift in the mode of heat loss means that engineers will need to design space suits that keep heat from radiating away—possibly by using antireflective coatings.
Still, even space-suited people will lose heat through boots and gloves when they stand in the shade or collect samples from there, Osczevski notes: “Picking up a rock from the shade will be like picking up a lump of dry ice.”