Introduction

Beginning with the first tentative probes into space in the mid-1960s, the geological exploration of the solar system has revealed a remarkable diversity in the planets and their satellites. Each planetary body displays combinations of surface features that reflect unique geological histories and environments. Yet, when the surfaces of the terrestrial planets and satellites are analyzed in detail, we find that many of them have experienced similar geological processes in their evolution.

The discipline of comparative planetary geology has as its goal the definition of the fundamental processes that have shaped and modified the planets, satellites and other ‘solid surface’ bodies in the solar system. For simplification, we shall refer to all such objects simply as planets. The giant gaseous planets, such as Jupiter and Saturn, are excluded from study because they apparently lack solid surfaces and thus are not appropriate for geological analyses. The goal of planetary geology is achieved by determining the present state of planets, by deriving information of their past state(s) – or geological histories – and by comparing the planets to one another.

Comparative planetary geology has shown that nearly all of the planets have been subjected to major geological processes, including impact cratering, volcanism, tectonism (crustal deformation), and gradation. Gradation involves the weathering, erosion, and deposition of crustal materials through the actions of various agents, such as wind and water. This book deals with wind, or aeolian, processes (Fig. 1.1).

Introduction

It is possible, as we have seen in the last chapter, for a sea of windblown sand to create its own topographical features in the form of sand dunes. In this chapter we discuss the interactions which occur between windblown particles and topographical features such as hills, craters (particularly as related to craters on Mars), vegetation, and other obstructions to the boundary layer. In order to understand these interactions, it is necessary to discuss atmospheric circulation and the effects that obstructions have on wind flow near the ground.

Some of the primary indicators of topographical influences are various types of wind streaks. Mariner 9 and Viking images of Mars show a remarkable array of bright and dark surface markings that result from interactions between the wind and topography (Fig. 1.3(a)). Many of these streaks serve as surface wind direction indicators. The differences in streak geometry and their variability with time have sparked much discussion and controversy on their origin and evolution. We review the streaks of Mars and discuss their possible origins, as well as present some possible terrestrial analogs that may shed light on the atmospheric–topographical interactions involved in streak formation.

Atmospheric motions

General atmospheric circulation results primarily from planetary rotation combined with the variation of incident solar flux with position on the planet (see Chapter 2). On a smaller scale, atmospheric circulation is influenced by topographical features such as mountain ranges, and on Earth is further influenced by the oceans through the exchange of heat and water vapor with the atmosphere.

Ralph Bagnold – an engineer by training, a military man by profession, and in many ways a geologist at heart – melded his interests into an elegant study of aeolian processes that has spanned many decades. In 1941 Bagnold published the first edition of his book, The Physics of Blown Sand and Desert Dunes. Often referred to simply as ‘Bagnold's classic book’, it is indeed a classic in every sense of the word. The fact that nearly every subsequent paper dealing with aeolian processes refers to the Bagnold book bears testimony that the basic principles described by him are essentially correct and have withstood the test of time.

Our book deals with aeolian processes in the planetary context. It is not our intent to ‘replace’ Bagnold's book or the research it represents. We learned that was neither required nor possible early in our own research program! Instead, we have built upon the solid foundation laid by Bagnold, testing the relationships defined by him through different approaches, and extrapolating the results to other planetary environments by attempting to predict how aeolian processes operate on Mars, Venus and, perhaps, Titan, the largest of the saturnian satellites.

We begin with an introduction to aeolian processes and a general overview of aeolian activity on the planets. We then discuss, in Chapter 2, the requirements for aeolian activity – a dynamic planetary atmosphere and a supply of particles capable of being moved by the wind – and describe in Chapter 3 the physical processes involved in particle movement by the wind. In Chapters 4 and 5 we describe wind-eroded and wind-deposited features and landforms.

Introduction

Sand dunes and deserts are intimately linked in the minds of most people and epitomize aeolian processes. The most impressive sand dunes occur in vast sand seas, often called ergs – an arabic word for wind-laid sands. Almost all ergs occur in desert basins, generally in areas downwind from terrain experiencing high rates of deflation. Sand then tends to collect where net transportation is low, as in topographical basins or traps.

Wilson (1971, 1973) surveyed all ergs on Earth that are larger than 12 000 km2 and identified 58 such areas, mostly within regions receiving less than 15 cm precipitation per year. Despite the link between sand dunes and deserts, the two need not occur together. Many deserts lack dunes entirely. And dunes may occur in non-desert areas, such as along coastlines and in river flood plains. Although dunes are aesthetically pleasing, they represent only one form of aeolian deposit.

Bagnold (1941) first described the various processes involved in dune formation and sand migration. His model has been modified slightly and amplified by Hunter (1977) and Kocurek & Dott (1981), who describe three basic processes of deposition by windblown sand: (1) tractional deposition (Bagnold's ‘accretion’ process), in which grains moving by saltation and impact creep come to rest in a sheltered position, (2) grainfall deposition, in which particles settle out of the air, usually in zones of flow separation; Bagnold distinguished (1) from (2) in that, during grainfall, the grains do not move forward when they reach the ground, whereas in tractional deposition, they may bounce along until they find a stable position among other grains, and (3) grainflow deposition, or avalanching, in which grains reach the brink of a dune then avalanche down the slip face (Fig. 5.1).

Introduction

Many images of Mars show sand dunes and wind-related surface features that are clear evidence of atmospheric processes. These remarkable pictures have sparked new interest in understanding the complex phenomena associated with windblown particles, which in turn has led to increased knowledge and understanding of aeolian processes on Earth. In this chapter we discuss some of the basic physics of particle motion, particularly in regard to the effects of widely different atmospheric densities on the terrestrial planets. Most of these phenomena, such as the characteristics which determine onset of motion and the transport rate of material in motion, are not yet completely understood because of their physical complexity and the difficulty of observation.

Much of the early research on the basic physics of windblown sand was performed by R. A. Bagnold. As an officer in the British Army, he led expeditions across the sand seas of Egypt in the 1930s. His observations provided the foundation for understanding sand motion and the formation of sand dunes and ripples. Later, in England, he built a wind tunnel in order to make quantitative measurements of threshold wind speeds and mass transport rates. His findings are reported in his delightfully lucid cornerstone book (Bagnold, 1941) and in many papers.

A parallel effort on the problems of soil erosion has been carried out by the US Department of Agriculture at a wind tunnel laboratory in Manhattan, Kansas. Most of the understanding of the physics of soil movement by wind, especially as related to agricultural applications, was derived by W. S. Chepil who reported his work in a large number of papers during a 25-year period beginning about 1940.

Introduction

Dust. It gets in our eyes, our shoes, even in our lungs, sometimes causing disease to those who breath it. Dust storms cause visibility problems on highways, resulting in many accidents and deaths each year. And soil erosion is a major worldwide problem where surfaces are disrupted through cultivation, overgrazing, mining, construction, vehicular traffic, or other activities which disturb the surface or destroy vegetation cover.

Dust storms are common on both Earth and Mars. Although many storms on Earth reach sizes that can be seen on photographs taken from orbit, they do not become global in extent, as some do on Mars. Nonetheless, the great North American dust storm of 12 November 1933, covered a region greater than the combined areas of France, Italy, and Hungary (Goudie, 1978), and for the people involved in the dust storm, it might just as well have been global.

The dust storm of November 1933 is an example of a storm caused by an area of extreme high pressure and strong barometric pressure gradient. It covered the region southward from the Canadian border, stretching from Montana to Lake Superior, and to the western Ohio and lower Missouri River valleys (Hovde, 1934; Miller, 1934). That year a severe drought was experienced over the northern and central Great Plains, and crop and pasture grass failures left the soil exposed to the wind. Maximum recorded wind velocities on 12 November were 25 m/sec at Bismarck, North Dakota, and at Davenport, Iowa.

Introduction

Anyone who has experienced a sand storm is acutely aware of the effectiveness of windblown sand as an agent of abrasion. Every year countless cars and trucks are caught in sand storms where, in exposures sometimes as short as a few minutes, windows can be frosted and paint can be stripped to bare metal. Multiply this high rate of abrasion over geological time and some appreciation can be gained of the effectiveness of wind erosion (Fig. 1.5).

Wind erosion takes place either through abrasion, the wearing away of relatively solid rock or indurated sediment, or through deflation, in which loose particles, such as sand, are blown away. Wind erosion commonly occurs through windblown particles that act as abradants – the agents of wind erosion.

In this chapter we first discuss the mechanics of wind abrasion on the microscopic scale and review the formation of ventifacts. We then present results from various laboratory simulations and estimate rates of wind abrasion on Earth and Mars. In the last section we discuss large wind-eroded landforms.

Aeolian abrasion of rocks and minerals

Studies of wind abrasion have generally been carried out either by materials scientists and engineers who are concerned with problems such as the abrasion of turbine blades by airborne particles, or by geologists who are concerned with the erosion of rock materials. Engineering studies typically are well documented and quantitative, but do not usually involve materials appropriate for geological applications. And, although geological studies involve analyses of rock abrasion, most of these studies are qualitative or, at best, only partly quantitative for terrestrial environments; thus the results are not appropriate for extrapolations to other planets.

My last lecture was devoted mostly to Eddington's contributions to theoretical astrophysics and to justifying Russell's assessment of him as the most distinguished representative of astrophysics of his time. In this lecture, I shall turn to Eddington as an expositor and an exponent of the general theory of relativity, to the part he played in the Greenwich-Cambridge expeditions to observe the solar eclipse of May 29, 1919 with the express purpose of verifying Einstein's prediction of the deflection of light by a gravitational field, and to his efforts, extending over sixteen years, in cosmology and – quoting his own description – in ‘unifying quantum theory and relativity theory’. But in contrast to my last lecture, I am afraid that this lecture will not altogether be a happy one.

I shall begin with the happier side.

After founding the principles of the special theory of relativity in 1905, Einstein's principal preoccupation in the ten following years was to bring the Newtonian theory of gravitation into conformity with those same principles and, in particular, with the requirement that no signal be propagated with a velocity exceeding that of light. After many false starts, Einstein achieved his goal in a spectacular series of short communications to the Berlin Academy of Sciences during the summer and the autumn of 1915. Because of the war, the news of Einstein's success would not have crossed the English Channel (not to mention the Atlantic Ocean) had it not been for the neutrality of the Netherlands and Einstein's personal friendship with Lorentz, Ehrenfest, and deSitter.