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[1] Methods used so far to assess the flow velocities of the water commonly assumed to be responsible for forming the major outflow channel systems on Mars have relied widely on various versions of the Manning equation. This has led to problems in allowing for the difference between the accelerations due to gravity on Mars and Earth and for the differences of scale between Martian floods and most river systems on Earth. We reanalyze the problem of estimating water flow velocities in Martian outflow channels using equations based on the Darcy-Weisbach friction factor instead of the Manning n factor. We give simplified formulae appropriate to Mars for the Darcy-Weisbach friction coefficient as a function of bedrock size distribution. For a given channel floor slope and water flood depth, similar mean flow velocities are implied for a wide range of values of the ratio of bed roughness to water depth relevant to Martian outflow channels. Using a recent rederivation of Manning's equation based on turbulence theory, we obtain a new value of 0.0545 s m À1/3 for the Manning n coefficient appropriate to Martian channels and show that previous analyses have generally overestimated (though in some cases underestimated) water flow velocities on Mars by a factor of order two. Combining the consequences of this flow velocity overestimate with likely overestimates of flow depth from assuming bank-full flow, we show that discharges may have been overestimated by a factor of up to 25, leading to corresponding overestimates of subsurface aquifer permeabilities, rates of filling of depressions with water, and grain sizes of sediments on channel floors. Despite the availability of an improved value for the Manning n coefficient for Mars, we strongly recommend that modified forms of the original version of the Manning equation should be replaced by the modern form or, preferably, by the Darcy-Weisbach equation in future work.

We examine the production of magma reservoirs and neutral buoyancy zones (NBZs) on Venus and the implications of their development for the formation and evolution of volcanic landforms. The high atmospheric pressure on Venus reduces volatile exsolution and generally serves to inhibit the formation of NBZs and shallow magma reservoirs. For a range of common terrestrial magma volatile contents, magma ascending and erupting near or below mean planetary radius (MPR) should not stall at shallow magma reservoirs; such eruptions would be characterized by relatively high total volumes and effusion rates. For the same range of volatile contents at 2 km above MPR, about half of the cases result in direct ascent of magma to the surface and half in the production of neutral buoyancy zones. In general, neutral buoyancy zones and shallow magma reservoirs begin to appear as gas content increases and are nominally shallower on Venus than on Earth. For a fixed volatile content, NBZs become deeper with increasing elevation: over the range of elevations treated here (−1 km to +4.4 km) depths differ by a factor of 2–4. These analyses reveal several factors that may help to account for the low height of volcanoes on Venus. Larger primary reservoirs cause the wide dispersal of conduits building edifices. Models of the position of the shallow NBZ reservoir during edifice growth show that for Earth the magma chamber center remains at a constant depth below the growing edifice summit, thus keeping pace with the increasing elevation, while on Venus the chamber center becomes deeper relative to the summit of the growing edifice because of the major change in atmospheric pressure as a function of altitude. Therefore neutral buoyancy zones and magma reservoirs on Venus will remain in the prevolcano substrate longer and in many cases may not emerge into the edifice at all; the lower rate of vertical migration implies that magma reservoirs would tend to stabilize, undergo greater lateral growth, and become larger on Venus than Earth. The proportion of the available magma going into production of the edifice relative to that intruded into the substrate is smaller on Venus than Earth. Large reservoirs would encourage multiple and more widely dispersed source vents and large volumes for individual eruptions. In large reservoirs, positively buoyant materials are likely to be produced from differentiation, substrate remelting, and volatile exsolution. Nonbuoyant materials exsolving volatiles in a shallow reservoir will need higher gas bubble concentrations to produce eruptions than on Earth, and when this gas‐enriched melt emerges at the surface, it is more likely to retain its bubbles than to undergo explosive disruption due to the high surface atmospheric pressure. Therefore there is the potential for the production of a range of erupted lavas that have very high gas bubble concentrations, leading to anomalous, more viscous rheological properties. Inhibition of disruption of volatile‐rich magma for both basaltic and more evolved compositions can ...

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