planetary geomorphology

planetary geomorphology Geomorphology is the science of Earth's surface forms, but its concern with landforms is not limited to the Earth alone. A science is a way of reasoning or enquiry, and enquiry into the landforms of one planet, Earth, is readily enriched by reasoning about the landforms of other planets. It was in this spirit that Galileo Galilei pointed his 3.8-cm diameter telescope at the Moon in 1609 to observe the curious circular depressions on its surface. By 1665, Robert Hooke, a contemporary of Sir Isaac Newton, was performing the first geomorphological experiments, comparing the lunar depressions (1) to the cooled surface crust of boiled gypsum, and (2) the impact of musket balls into a clay–water target material. Reasoning by analogy. Hooke realized that alternative hypotheses for lunar craters might involve (1) internal heat melting (and bubbling) surface crusts, or (2) impacts by particles from space. By the time of Grove Karl Gilbert's classic study of lunar craters in 1893, both these geomorphological hypotheses remained viable, either as (1) volcanism, or (2) impact cratering. Gilbert also propelled balls of clay and metal into various target materials. The similarity in morphology to the lunar craters he observed through the Naval Observatory telescope provided compelling evidence for an origin by impact cratering. Gilbert even studied a possible Earth analogue to this process at Coon's Bluff in northern Arizona. Although Gilbert eventually concluded that Coon's Bluff was volcanic, the example of his reasoning process was taken up by later geologists, notably Eugene Shoemaker, who unequivocally established its impact origin in the 1960s. The site is now known as Meteor Crater.

Scientifically, the most interesting aspect of planetary geomorphology is the discovery of new landscapes, particularly as space missions image planetary surfaces by means of remote sensing systems with increasing spatial and spectral resolution. For example, the surface of Venus has been revealed only since the mid-1980s through Russian and American orbiting radar spacecraft. Despite its geophysical similarity to the Earth, in size, density, and general composition, Venus has a very different surface. It lacks plate-tectonic landforms, and is instead dominated by volcanic plains, locally punctuated by upland plateaux that may form by processes of mantle upwelling. Long channels of probable volcanic origin occur on the plains. The longest of these extends for 6800 km, longer than Earth's longest river.

Venus is similar to Earth in its lack of very densely cratered ancient terrains. During the first few hundred million years of Solar System's history, the number and sizes of impactors were much greater than during the past three to four billion years. Remarkably, Mercury, the Moon, Mars, and most satellites of the gaseous outer planets preserve the ancient cratered landscapes that date back to the earliest history of the Solar System. Thus, impact craters are the most ubiquitous of planetary landforms.

On Mars, the ancient heavily cratered terrains are locally dissected by erosional valleys, as shown in Fig. 1. This shows that Mars, like the Earth, once had an active hydrological cycle that ultimately led to running water on its surface. Mars also shows a great variety of aeolian, glacial, periglacial, volcanic, and hillslope landforms. It clearly has had a complex history involving episodes of active erosion as well as tectonism and volcanism.

Future plans for planetary exploration by the United States, Japan, and the European Union should provide continuing opportunities for planetary geomorphology. Missions are envisaged to provide new data on Mercury, Venus, Mars, and various satellites of the outer Solar System. As with Galileo's first telescopic observations, these explorations will stimulate discussion on the origins of the newly revealed landscapes. Geomorphological studies of these landscapes will employ digital processing of remote sensing images (Fig. 1), computer simulation of geodynamics, and morphometry, and regional mapping. Fieldwork on terrestrial analogues, like Meteor Crater, remains important. Moreover, plans for human exploration may well lead to extraterrestrial fieldwork. Nor are the prospects for planetary geomorphology exhausted by the limits of the Solar System. The existence of extrasolar planets has been confirmed, and space telescopes are envisaged to image their surfaces. The legacy of Galileo, Hooke, and Gilbert will live on in future reasoning by planetary geomorphologists.

Victor R. Baker

Bibliography

Cattermole, P. (1995) Earth and other planets: geology and space research. Oxford University Press.
Greeley, R. (1993) Planetary landscapes. Chapman and Hall, London.