The Sandy Face of Mars

Abstract

t he number of sand, as Archimedes put it, has served as a time-honored metaphor for an inconceivably large quantity of almost anything. But an extreme among extremes is to consider grains of on scale of a planet, and a particularly sandy planet at that. The huge dune fields of Mars unique among extraterrestrial surfaces photographed by planetary spacecraft unlike either ice that blankets many outer-planet moons or bare, solid rock and hardened lava flows typical of Earth's moon, Mercury, Venus and even parts of Mars itself. The Martian dunes also demonstrate sculpting ability of that ruddy world's winds, a phenomenon otherwise seen only on parts of Earth. True, these dunes fail to match dramatic splendor of huge Martian volcano called Olympus Mons (the size of Arizona) or vast canyon network known as Valles Marineris (as long as United States is wide). Yet any proper list of Martian spectaculars must include planet's awesome sand Best known from photos taken in 1976 and 1977 by two Mars-orbiting Viking spacecraft, dunes sweep across planet's barren terrain in varying patterns (SN: 10/30/76, p.276; 11/20/76, p.330). But just how much do they actually hold? Nicholas Lancaster and Ronald Greeley of Arizona State University in Tempe calculate total volume of dunes in north polar region, where they seem most ubiquitous, at 1,158 cubic kilometers (km3). If shaped into a ball, this would form a sphere 13.028 kilometers in diameter (a little more than 8 miles). That may seem tiny by astronomical standards, but it would exceed volume of Deimos, one of Mars' two irregularly shaped moons, for which Thomas C. Duxbury of Jet Propulsion Laboratory in Pasadena, Calif., calculates a volume of about 1,052 km3. Phobos, Mars' larger moon, amounts to about 5,680 km3. Furthermore, Lancaster and Greeley assert in July 10 JOURNAL OF GEOPHYSICAL RESEARCH that their dune accounting should be regarded as a minimum, because they cannot be certain of depth of Martian seas. Nor does their estimate include flat stretches of separating dunes. Dunes dominate about half total area of north polar seas, most of them rippling surface in a band between 760 and 83?N latitude. Around part of that band, from 110? to 260?W longitude, lies what Lancaster and Greeley call the largest continuous area of dunes on Mars, covering about 470,000 km2 -an area size of Nebraska and Colorado combined. According to researchers, four major seas, totaling 680,000 km2-a region slightly smaller than Texas nearly encircle north polar ice cap. Arrayed like threads in a tapestry and scores of kilometers long, Mars' transverse dunes angle across direction of dominant winds, with widely varied distances separating successive ridges or crests. Individual dunes, called barchans, appear elsewhere. Winds have sculpted these crescent-shaped features at intervals ranging from less than 200 meters to as much as a full kilometer apart. Lancaster and Greeley say that if dunes of Mars resemble those of Earth as much as Viking photos suggest, then heights of many Martian dunes can be estimated by assuming an approximately Earth-like ratio between their heights and distances between them. This means that most dunes near Martian north pole rise between 10 and 25 meters, they say. On Earth, scientists note, heightto-spacing ratios of dunes shaped by wind approximate those of features on bottom of sea, shaped by deep ocean currents. This suggests that difference between density of an atmosphere and that of an ocean has little effect on dunes' overall shapes, they conclude. Most desert dune patterns on Earth are very regular, and vary spatially in a systematic fashion, Lancaster and Greeley say. Thus, there seem to be similar relationships between dune height, spacing, and equivalent sediment thickness on Earth and Mars. The origin of Martian dunes remains unknown. Viking photos show dnsLoephotos: sho barha C ~