Sara Santiago, Arizona State University; Jes Therkelsen, Amherst College
Introduction:
Lava flow emplacement is a fundamental geologic process on Mars and other planetary bodies. By studying the surface morphology of lava flows, we gain insight into eruption parameters and ultimately a regionís geothermal history. New images from the Mars Orbiter Camera (MOC) instrument (Malin et al., 1992;1998) on the Mars Global Surveyor spacecraft show relatively young lava flows in the Marte Vallis region of Mars. These flows were recently estimated to be 10 million years old (Hartmann and Berman, 2000), possibly making them the most recent volcanic activity on the planet, and suggesting a more recent volcanically active Mars than earlier believed (Garvin et al., 2000). In this study we attempt to constrain flow rates and velocities for a recent channelized flow in Marte Vallis shown in MOC image SP240703, which was acquired in July, 1998 at a resolution of 18.47 m/pixel. Using Mars Orbiter Laser Altimeter (MOLA) (Zuber et al., 1998; Smith et al., 1998) topographic data, channel dimensions we measured included down-flow gradient, channel width, and channel depth. From these, flow rates and velocities were calculated using the rectangular channel flow model of Gregg and Sakimoto (1998, 2000; Sakimoto and Gregg, in prep.).
Marte Vallis, located between 170-190 ° W longitude and 0-20 ° N latitude, is a region characterized primarily by channels of possible fluvial origin (Tanaka and Scott, 1986). Later volcanic activity, in the form of flood basalts and long lava flows, utilized these channels to transport lava distances as great as five hundred kilometers or more (Plescia, 1990) on slopes <0.01 ° (Gregg and Sakimoto, 2000; Sakimoto and Gregg, in prep). Orcus Patera is to the north of Marte Vallis and older knobby terrain lies to the northwest and southeast. Marte Vallis itself is nestled in the eastern portion of the Cerberus Plains which, in turn, is in the southeast corner of Elysium Planitia (Figs. 1 & 2) (Plescia, 1990; Greeley and Guest, 1987). Our study area is the northeasternmost section of Marte Vallis, to the east of Orcus Patera, where these fluvial discharges and lava flows debouched onto Amazonis Planitia (Plescia, 1990). The flow studied is located in the southern half of MOC image SP240703, which is centered at 19.22 ° N and 174.61 ° W. In the southernmost portion of the MOC image, a distinct dark lava flow and its terminus are clearly visible within the higher-albedo fluvial channel (Figs. 3 & 4).
Results:
When comparing the MOLA topographical profile across the channel with the MOC image, what first appear to be channel levees in the topographic profile are actually knobs and ridges that are cut by the lava flow and visible in the MOC image. The differences in albedo in the MOC image suggest the lava did not reach the top of these features, but rather partway. A regional slope along the channel was foun. Our findings suggest a very shallowly sloping terrain of 0.04 ° ± 0.01 ° . The measured values for width, depth, and slope were used as model inputs. Values were obtained for flow rates and velocities in the center of the flow by changing viscosity between 100 and 1000 Pa-s and keeping density at either 1200 or 2000 (kg/m3) (Table 1). The volume calculation yielded a minimum total volume of 0.65 ± 0.25 km3. Flow rate calculations based on 100 Pa-s viscosity produced duration values of close to 2 x 101 for density of 1200 kg/m3 and 1.3 x 101 for density of 2000 kg/m3.
Conclusion:
Long, relatively fresh Martian lava flows within Marte Vallis, recently revealed in high-resolution MOC images, together with new MOLA topographical give insights into eruption parameters. Accurate channel dimensions, including channel depth, width, and regional slope, can be measured and used as model inputs to attain flow velocities and flow rates. If a flowís terminus is resolvable in MOC images, topographical profiles and high-resolution images can be used to arrive at a flowís surface area, a total volume of flow, and a time duration of eruption. For our flow we found a velocity between 0.04 and 0.4 m/s and a flow rate near 8.4 x 104 m3/s. These results are similar to recent studies done by Kesthelyi et al. (2000) and Gregg & Sakimoto (2000) which strengthens the notion that these flows share many similar characteristics of long, terrestrial-like basaltic flows.