MARS: The North Polar Sand Sea and related wind patterns

Abstract

Viking Orbiter 2 images of the north polar region reveal an enormous sand sea (erg) covering an area of >5 x 105 km 2 around the perennial ice cap. All dunes are either transverse or barchan. The various dune morphologies and modification of primary dune types reflect a wind regime having more than one wind direction. In the summer, two major wind directions prevail: (1) off-pole winds that become easterly due to coriolis forces and (2) on-pole winds that become westerly. During the winter and/or spring, only the on-pole winds exist. Strong winds (>75 m/s) are required for sand accumulation to form the thick transverse dunes. The strongest winds in the north polar region are thought to exist during summer over the transverse dune field between 110øW and 220øW ß this area is a relatively •varm belt (temperature >230 K) between two ice zones (temperature <220 K). A frequent cyclogenetic process in this area may cause sand storms. The dunes seem to be currently active. The lack of well-developed longitudinal dunes implies that the dune field is young. The relationship of the present dune field to the perennial ice indicates that the dunes began to form after the formation of the present ice cap. Introduction One of the many discoveries to come from the Viking mission was the existence of vast dune fields in the north polar region of Mars [Cutts et al., 1976]. The circumpolar dune field is the only major sand sea (erg) discovered thus far on Mars, comprising an area of about 7 x 105 to 8 x 105 km 2, which is greater than Rub A1 Khali in Arabia, the largest active erg on Earth (5 x 105 km 2' Wilson [1973]). Most of the dunes occur between 110øW and 220øW and between 77øN and 83øN, although scattered dunes are found between 240øW and 90øW. The field corresponds to the dark north polar collar that was observed to follow the recession of the north polar ice cap by Mariner 9 (Figure 1). From Mariner 9 results, the collar was interpreted to be a wind eroded region [Leovy et al., 1973; Sagan et al., 1973; Burk, 1976]. In addition to the sand dunes the north polar region of Mars includes several other geological units, including various plains units, layered deposits, and perennial ice [Cutts et al., 1976; Scott and Carr, 1978; Squyres, 1978]. The plains units in the north polar region are smooth, may be mantled, and have very few craters, suggesting a young age. Their classification as cratered plains by previous investigators based on poor resolution Mariner 9 images [Scott and Carr, 1978] is rather a misnomer. The perennial ice layer is composed of H20 plus dust [Kieffer et al., 1976] and is exposed to the sun shortly after summer solstice [Soderblom et al., 1973].

Chasma Australe, Mars: Structural Framework for a Catastrophic Outflow Origin

“…in Hesperian times, the martian lithosphere was divided into two mobile plates. The (Arvidson et al 1979, Breed et al 1979, French and Gierasch 1979, Tsoar et al 1979, Thomas 1981, 1982, Ward and Doyle 1983, Christensen 1988, Plaut et al 1988, Thomas and Weitz 1989, Lancaster and Greeley 1990, Greeley et al 1992, Edgett and Christensen 1994, and Thomas and Gierasch 1995 emphasized the importance of the wind and the wind deposits: some of them (e.g., Cutts 1973a,b, Howard 1978, or Howard et al 1982 advanced hy potheses favoring the erosion by wind of the polar reentrants.…”

Dark material in the polar layered deposits and dunes on Mars

“…The Martian polar layered deposits appear to record climate variations over at least the last 10 to 100 million years [Murray et al, 1972;Cutts, 1973;Cutts et al, 1976Cutts et al, , 1979 Dark dunes nearly surround the north polar layered deposits on Mars [Tsoar et al, 1979;Dial, 1984], forming a "sand sea" or "erg." By analogy with aeolian dunes on Earth, the dark dunes on Mars are believed to have been formed by saltation or traction of sand-sized particles in response to Martian winds.…”

Chasma Boreale, Mars: Topographic characterization from Mars Orbiter Laser Altimeter data and implications for mechanisms of formation

“…Streamlined deposits are associated with the smaller chasmata (Figures 4a and 8) and the mesa beyond the mouth of Chasma Boreale (Figure 4f ). While eolian processes may streamline deposits, wind direction maps produced by Tsoar et al [1979] do not show directions consistent with the particular shape of these deposits. Winds have modified these deposits, however, as there are dunes on top of them, and they may have even been deposited by wind.…”

“…Therefore, in relation to point 2 above, evidence of subglacial drainage may not necessarily be straightforward, or even observable. Analysis of past fluvial activity in the Martian north polar cap is further complicated by the fact that, unless this event occurred quite recently, subsequent eolian activity (ubiquitous in the north polar region [e.g., Tsoar et al, 1979;Greeley et al, 1992]) could partially or even wholly mask any remaining evidence of the event. Dunes, in particular, could cover the area, as they have been known to emanate from arcuate scarps in the layered terrain [Thomas and Weitz, 1989].…”

“…[49] Fourth, according to analysis of the dune forms in Viking images, there are two wind directions during the summer (easterly off-pole and westerly on pole), and during the winter and spring, there are only on-pole winds [Tsoar et al, 1979]. It is possible that if katabatic winds operated with such efficiency on the north polar cap, then they might disrupt this balance of seasonal winds which has shaped the Olympia sand sea, and they might even remove the expansive ergs.…”