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Athabasca Valles is an outflow channel system located in the Elysium Planitia region of Mars. It emerges full-born from the Cerberus Fossae fissure system, stretches southwest for ∼300 km, and debouches into the Cerberus Palus basin. Whatever its origin, Athabasca Valles is now covered by lava [e.g., 1,2]. Impact crater size-frequency distributions yield a surface age for the lava of 1.5-200 Ma [3]. However, this provides only a lower limit on the timing of channel incision, which could be either coincident with volcanism, if it is the result of lava erosion [4], or up to a few Ga old, if it was carved by earlier fluvial activity [5]. Nonetheless, there is some reason to speculate that the release of floodwaters closely preceded the eruption of low-viscosity lava [6].
Our previous work showed that the emplacement of the Athabasca Valles flood lava (AVFL) was radically different from that of its terrestrial counterparts [2]. Flood-lava provinces on Earth record evidence of long-lived, relatively low-rate flows that thickened via inflation to form broad plateaus [7]. In contrast, the AVFL retains a morphology typical of near-vent lavas over a distance of hundreds of kilometers. Spacecraft data reveal that the AVFL first inundated Athabasca Valles and then, as the eruption waned, drained downstream into Cerberus Palus and beyond, deflating 50-80 m in the proximal reaches of the channel system. Landforms that appeared to be unmodified aqueous erosion features in lower resolution data are shown to be draped by the receding lava in higher resolution images such as those acquired by the HiRISE camera. The total volume of the AVFL is ∼5 x 103 km3 with about a factor of two uncertainty.
Peak lava discharge can be estimated using the methodology detailed in [8]. Even choosing conservative values for viscosity, density and vesicularity, the flow is expected to be turbulent. Turbulent velocities are calculated using the formula v = (g θ H / Cf), which yields a flow rate of ∼5-10 m/s for the AVFL. Using the measured cross-sectional area of 1-2 x 106 m2, the peak flux of lava is estimated to be 5-20 x 106 m3/s. At this rate, the eruption would last 3-12 days, but the average flow rate should be lower than peak discharge. Average rates for terrestrial fissure eruptions are generally an order of magnitude below peak flux [9], which would translate to a duration of 1-4 months for the AVFL. However, observational constraints suggest a short-lived, high-rate flow; therefore, we favor a duration on the order of a few weeks.
[1] Plescia (1990) Icarus 88, 465-490. [2] Jaeger et al. (2007) Science 317, 1709-1711. [3] McEwen et al. (2005) Icarus 176, 351-381. [4] Leverington (2004) JGR 109, 2004JE002311. [5] Werner et al. (2003) JGR 108, 2002JE002020. [6] Head et al. (2003) GRL 30, 2003GL017135. [7] Self et al. (1998) Ann. Rev. Earth. Planet. Sci. 26, 81-110. [8] Keszthelyi et al. (2006) Geol. Soc. Lon. 163, 253-264. [9] Wadge (1981) JVGR 11, 139?168.
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