When the Mariner 9 spacecraft arrived at Mars in 1971, it recorded a planet-wide storm that completely enveloped the planet in red iron oxide dust. In subsequent years, scientists have come to realize that dust storms, far from being infrequent, are a regular feature of the Martian climate. Few are large enough to cover the entire planet, but regional storms that cover wide areas, local storms and dust devils are common.
A blanket of iron oxide dust covers the surface of Mars, and its rusty red color gives the planet is characteristic hue. When sunshine warms the thin carbon dioxide atmosphere, winds lift this layer of dust and redistribute it. With an atmospheric density of 0.02 kilograms per cubic meter (0.0012 pounds per cubic foot) -- compared with 1.217 kilograms per cubic meter (0.076 pounds per cubic foot) for Earth -- the winds can lift only the smallest particles. As a result, the dust in a Martian storm resembles cigarette smoke. It reflects 20 to 25 percent of the sunlight that hits it, thus making the clouds it forms bright in comparison to the Martian terrain.
Many of the dust storms on Mars begin in Hellas Basin, which is a large crater in the southern hemisphere. With a depth of 6 kilometers (3.7 miles) and a diameter of 2,000 kilometers (1,243 miles), it is one of the largest impact craters in the solar system. Dust collects in the bottom of the crater, where the temperature is 10 degrees warmer than that of most of the Martian surface, which experiences an average temperature of minus 50 degrees Celsius (minus 58 degrees Fahrenheit). The temperature difference drives winds that carry the dust out of the crater and distribute it around the hemisphere. When conditions are right, the winds can combine with other factors to create a global dust storm.
The Great Storm of 2001
In 2001, the Hubble Space telescope and the Mars Global Surveyor spacecraft observed a dust storm begin in the northern hemisphere in June, which was early spring in that hemisphere. Subsequent events, including winds and temperature differentials at the Hellas Basin, resulted in a storm that engulfed the planet and lasted for about three months. The atmospheric temperature of Mars averages minus 60 degrees Celsius (minus 80 degrees Fahrenheit). During the storm, the temperature of the upper atmosphere increased by 45 degrees Celsius (80 degrees Fahrenheit). The temperature at the surface of the planet dropped, however, because it was shrouded in darkness under the clouds of dust.
Effects on Mars Rovers
The dust and winds on Mars have the potential to create problems for Earth-based rovers exploring the surface. Winds can blow at speeds of up to 160 kilometers per hour (99.4 miles per hour), but because the atmosphere is so thin, that feels like a breeze of 16 kilometers per hour (9.94 miles per hour) to the rovers. These winds can deposit dust on the panels of solar-powered equipment, however, and render it inoperable. Despite this potential, two of the solar-powered NASA rovers, Spirit and Opportunity, exceeded their mission lifespans. Both were expected to last 90 days, but Spirit lasted about 1,900 days, and after more than 3,000 days, Opportunity was still operational as of March 2013.
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