At opposition, when Mars is closest to us and most easily observed, we see it as full, so the Sun's light strikes the surface almost vertically, casting few shadows and preventing us from seeing any topographic detail, such as craters or mountains. Even through a large telescope Mars appears only as a reddish disk, with some light and dark patches and prominent polar caps. These surface features undergo slow seasonal changes over the course of a Martian year. We saw in Chapter 1 how the inclination of Earth's axis produces similar seasonal changes. (Sec. 1.3) Figure 10.2 shows some of the best images of Mars ever made from Earth (or Earth orbit), along with a photograph taken by one of the U.S. Viking spacecraft en route to the planet.
Figure 10.2 (a) A deep red (800 nm) image of Mars, taken in 1991 at Pic du Midi, an exceptionally clear site in the French Alps. One of the planet's polar caps appears at the top. (b) A visible-light Hubble Space Telescope image of Mars, taken while the planet was near opposition in 1997. (c) A view of Mars taken from a Viking spacecraft during its approach in 1976. Some of the planet's surface features can be seen clearly at a level of detail completely invisible from Earth.
Rotation of Mars
Viewed from Earth, the most obvious Martian surface features are the bright polar caps (see Figure 10.2a). They grow or diminish according to the seasons, almost disappearing at the time of Martian summer. The dark surface features on Mars also change from season to season, although their variability probably has little to do with the melting of the polar ice caps. To the more fanciful observers around the start of the twentieth century, these changes suggested the seasonal growth of vegetation on the planet. It was but a small step from seeing polar ice caps and speculating about teeming vegetation to imagining a planet harboring intelligent life, perhaps not unlike us.
But those speculations and imaginings were not to be confirmed. The caps are mostly frozen carbon dioxide (that is, dry ice), not water ice, as at Earth's North and South Poles. The polar caps do contain water, but it remains permanently frozen, and the dark markings seen in Figures 10.2, once claimed to be part of a network of "canals" dug by Martians for irrigation purposes (see Interlude 10-1), are actually highly cratered and eroded areas around which surface dust occasionally blows. Repeated covering and uncovering of these landmarks gives the impression (from a distance) of surface variability, but it's only the thin dust cover that changes.
The powdery Martian surface dust is borne aloft by strong winds that often reach hurricane proportions (hundreds of kilometers per hour). In fact, when the American Mariner 9 spacecraft went into orbit around Mars in 1971, a planetwide dust storm obscured the entire landscape. Had the craft been on a flyby mission (for a quick look) instead of an orbiting mission (for a longer view), its visit would have been a failure. Fortunately, the storm subsided, enabling the craft to radio home detailed information about the surface.