II.

Observation from Earth

Mars appears as a fairly bright, red, starlike object in Earth’s night sky. Because of the relative movements of Earth and Mars around the Sun, Mars appears to move backward in the sky for a short time around opposition, which is the time when the two planets are closest. As Mars and Earth orbit the Sun, the distance between them varies from about 56 million km (about 35 million mi) at their closest approaches to about 375 million km (about 233 million mi) when the planets are on opposite sides of the Sun. This change in distance causes the apparent size of Mars to vary by more than a factor of 5 and its brightness to vary by a factor of 25. Because the orbit of Mars is elliptical and not circular, Earth and Mars approach each other more closely during some orbits than others. For example, in late August 2003 Earth and Mars passed closer to each other than at any time since 1924. The two planets will not get that close again until the year 2287.

When Mars is viewed through a telescope, it looks like a reddish-orange disk. When Mars is close to Earth, an observer with a telescope can usually see white ice caps at the north and south poles of Mars. These polar caps grow and shrink throughout the Martian year, just as the polar caps of Earth do. The darker areas of Mars’s surface may look greenish to the telescope observer, but this is an optical illusion caused by the contrast in color between the dark patches and the redder, brighter areas. Scientists believe that the dark areas are regions of relatively unweathered dark rocks and sand, while the bright areas are regions with deposits of dusty, fine-grained oxidized iron minerals. Scientists now believe that the “canals” people observed on Mars during the 19th century are actually another optical illusion, caused by the mind’s tendency to draw connections between irregular patches in a fuzzy image.

The Hubble Space Telescope (HST) provides the clearest Earth-based views of Mars, and astronomers use it to study the composition of the surface and to monitor the weather on the planet. HST has provided detailed images of local and global dust storms, enormous spiral-shaped water ice cloud systems, and changes in the bright and dark surface markings that have occurred since the first detailed images were taken during the 1970s. The telescope also has enabled spectroscopic measurements that provide comprehensive information on atmospheric chemistry and on the nature and variability of ices and minerals on the surface. Using HST images and other data, astronomers have determined that the atmosphere of Mars is generally cooler and clearer when the planet is farther from the Sun and warmer and dustier when it is closer. There also appear to be longer-term trends in the Martian climate, but as is the case for Earth’s climate, scientists are only now beginning to untangle the complexities required to understand and perhaps one day even predict climate changes on Mars. Orbiting spacecraft around Mars furnish constant data about the planet. However, they orbit so close to the planet and are in a fixed orientation relative to the Sun that they cannot see features in the early morning or late afternoon parts of the Martian day. As a result, astronomers still need telescopes like the HST to study Mars, particularly its early morning and late afternoon cloud formations.