On July 20, 1976, Viking 1 made the first successful landing on Mars in Chryse Planitia, a plains covered basin in the northern hemisphere. Viking orbiter pictures reveal more surface detail of the area and show the basin to be more complex than was seen on Mariner 9 images. The plains consist of areas with smooth and relatively uniform surfaces with prominent lunarlike mare ridges, mesas and plateaus, surfaces that appear to be ‘etched,’ fields of knobs, low shields that may be volcanic, and vast areas that have been subjected to channel‐forming processes. At least two sets of channels, originating from distant sources, terminate in Chryse Planitia. The four major units of the basin are basal hilly and cratered terrain, plateau material which can be divided into upper and lower units, lower smooth plains, and upper smooth plains. There is no evidence for the origin of the basin. Deposition of the plateau‐forming material to the east followed the period of bombardment, and since that time the history of the basin includes eruptions of flood lavas, channel formation, and deposition with at least two channel‐forming events. The later history of the basin includes possible local volcanic events, etching, and aeolian activity.
Small‐scale surface features on lava flows provide information on the eruption parameters and the volcanic history of the flows in which they occur. High‐resolution images of four sites on Mars reveal lava flows with a different type or combination of features including pressure ridges, tumuli, festoon ridges, and ring ridges. Pressure ridges, tumuli, and pressure plateaus are spaced irregularly and tend to be oriented parallel to flow. They are formed by inflation of small flow lobes and are indicative of compound flows emplaced by sporadic eruptions with low effusion rates. Festoon ridges are oriented transverse to flow direction, mimic the flow margin, and have a regular spacing. Using a model by Fink and Fletcher (1978) and Fink (1980 a) interior viscosity values of approximately 108 Pa s were calculated for festoon ridges tens to hundreds of meters apart and a few meters high on two basalt flows in Iceland and a flow on Mars. This viscosity is comparable to results from similar studies of more silicic flows and suggests that high viscosities are necessary to form large festoon ridges. By analogy to the Icelandic flows, Martian flows containing festoon ridges are considered to be large, single unit basalt flows emplaced through flood‐style eruptions. High viscosity of the basaltic flows may result from cooling, a high crystal content, or low effusion temperatures. Disruption of the surface of some of these large sheet flows possibly by lava‐water interaction is suggested by the presence of ring ridges and rugged terrain consisting of mesa‐like mounds and channel‐like troughs.
The Lunae Planum-Chryse Planitia region provides the opportunity to study a sequence of channeling events and to determine their temporal and genetic relationships to plains units in the northern hemisphere of Mars. Two sets of small channels and four major channel systems can be divided into four periods of channeling by superposition and contact relationships to the plains. All of the channels are considered to have formed by water erosion. The first two channeling events occurred early in the history of this area and formed small, narrow channels within the old rugged terrain. These channel events were separated by deposition of a mantle unit. The small channels probably formed by runoff of surface water or by a sapping process. These channels preceded the emplacement of vast volcanic plains in both Lunae Planum and Chryse Planitia. Channels postdating the plains are Vedra, Maumee, Bahram, and Maja valles; the first three of these deposited a sedimentary unit on the western slope of Chryse Planitia that was eroded by Maja Vallis. These large-scale channels were probably formed predominantly by catastrophic floods and may represent two periods of water release from Juventae Chasma. The origin of Bahram Vallis remains uncertain. Geomorphology Regional SettingLunae Planum is an elevated plains in the northern hemisphere of Mars (Figure 1), characterized by relatively low albedo and a ridged texture. Immediately east ofLunae Planum but separated from it by a zone of rugged terrain is Chryse Planitia, a large asymmetric basin of smooth plains. The total relief from the center of the plateau to the lowest part of the basin, one of the lowest points on Mars, is about 8 km. A series of channels including small unnamed channels and Vedra, Maumee, Bahram, and Maja valles extend from Lunae Planum into the basin. To the north, Chryse Planitia grades into Acidalia Planitia, part of the extensive northern plains. A scarp separates Lunae Planum from the Tharsis volcanic plains to the west. To the south the area ends at a series of troughs and canyons which are the western end of the equatorial Vallis Marineris system. One channel originating in this area extends along the base of the western scarp and then turns eastward, forming Kasei Vallis, the northern boundary of Lunae Planum. Smooth Plains Lunae Planum was described by Milton [1974] as an area of predominantly smooth plains with scattered craters and lunadike wrinkle ridges. The smooth plains in Chryse Planitia were distinguished from these by their featureless appearance and relatively smaller crater population [Milton, 1974; Wilhelms, 1976]. Higher-resolution Viking images have revealed a diverse surface in Chryse Planitia with wrinkle ridges, areas modified by an erosional process, channeling, and fields of knobs [Greeley'et al., 1977]. On the plateau the increased image resolution reveals a mottled surface with numerous distinct wrinkle ridges, high-albedo areas with a 7994
The ability of the wind to move particles and the flux of windblown sand are both dependent on the topographic roughness of the surface, as measured by the aerodynamic roughness (zo). For most surfaces, topographic roughness controls many of the characteristics of the radar return, and the magnitude of the radar backscatter can be regarded as a measure of the surface roughness at or near the wavelength scale. Radar backscatter data may therefore be useful in obtaining a value of aerodynamic roughness which can be used to assess aeolian sediment transport via remote sensing. In this study, calibrated LHH, CHH and KuVV radar data were used to derive characteristic backscatter coefficients (σo) for three lava flow units, an alluvial fan, and a playa surface. Preliminary analyses show that values of σo and zo both increase with topographic roughness and that there is a good correlation between the two coefficients. This correlation suggests that it may be possible to assess aerodynamic roughness directly from radar data.
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