How and Why Martian Geology is So Different to that of Earth

Vic Gostin

Visiting Fellow, University of Adelaide

--- Abstract - Profile ---

Abstract: With only half the earth’s diameter and much less internal radiogenic heat, the crustal evolution of Mars has been vastly different to our own. Since its formation and primary melting, the Martian lithosphere (brittle crust) has thickened to such a degree that it has remained rigid over most of its history. Thus about two-thirds of its surface is extremely old and heavily cratered, like that of our Moon. Initial planetary differentiation created a small core, and later, an overheated mantle. These hidden processes probably led to mantle overturn, forming the major depression of the northern hemisphere, and then to the Tharsis bulge with its accompanying shield volcanoes.

Evidence of crustal extension or stretching is displayed in numerous closely spaced parallel faults. The long-term stability of such a stress field is indicated by the limited occurrence of cross-cutting younger fault sets resulting from changes in the stress directions. Crustal extension has also played a role in the formation of the enormous interconnected canyon system of Valles Marineris, but its full history is complex and yet to be unravelled.

Unlike our Earth, Mars has no plate tectonics. There has been no compression, thrusting, or overturning of sequences; no strike-slip faulting, or major deformation and elevation of mountain ranges. There are no regional metamorphic rocks on the Martian surface (eg. slate, shist, gneiss, quartzite, marble). Where layering is visible, it is usually horizontal and not deformed. There are no deep "ocean" trenches, volcanic island arcs, or oceanic greenstone belts. Without recycling of the "continental" crust, there are no granites or acid volcanics. Clearly, Mars is a small planet deprived of much of Earth’s great geological variety!

Ancient flood basalts cover vast areas, while younger shield volcanoes grew to much greater heights than those on Earth. Water ice is not only present at the poles but forms substantial permafrost areas. Glaciers played an important role in the past, while wind erosion and resulting sedimentation continue to be active. Liquid water may be present subsurface, but only intermittently has it flowed over the Martian landscape. Thus while water-carved channels exist, and deltas grew into large lakes, the presence of unweathered olivines and pyroxenes indicates that there was little surface moisture required for much chemical weathering. The discovery of sulphates and haematite shows some aqueous chemical activity, but where are the bugs? It is possible that Mars is now a "dead planet", and only continued exploration will reveal its ancient secrets.

PROFILE:  Dr Victor Gostin is a graduate of Melbourne University, a Ph.D. from ANU, Canberra, and a Visiting Fellow at the University of Adelaide. He has been actively interested in geology and astronomy since his high school days and has lectured in geology at Adelaide University for 31 years. Dr Gostin has wide research interests including sedimentology, environmental geology, planetary geology (especially of Mars), meteorites and meteorite impacts. In 1985 he identified a unique layer in the ancient rocks of the Flinders Ranges formed by a giant meteorite impact splatter. This extensive layer was derived from Australia’s largest meteorite impact at Lake Acraman (Gawler Ranges), and this exciting discovery turned his attention to the study of meteorites, the effects of giant impacts, and to planetary geology. As a result he has been honoured by having an asteroid named after him. In recent years, Dr Gostin compiled a book dealing with Australian environmental geoscience and took part in the 2001 Jarntimarra expedition with Mars Society Australia.