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Planetary-scale waves are thought to play a role in powering the yet-unexplained atmospheric superrotation of Venus. Puzzlingly, while Kelvin, Rossby and stationary waves manifest at the upper clouds (65--70 km), no planetary-scale waves or stationary patterns have been reported in the intervening level of the lower clouds (48--55 km), although the latter are probably Lee waves. Using observations by the Akatsuki orbiter and ground-based telescopes, we show that the lower clouds follow a regular cycle punctuated between 30$^{\circ}$N--40$^{\circ}$S by a sharp discontinuity or disruption with potential implications to Venus's general circulation and thermal structure. This disruption exhibits a westward rotation period of $\sim$4.9 days faster than winds at this level ($\sim$6-day period), alters clouds' properties and aerosols, and remains coherent during weeks. Past observations reveal its recurrent nature since at least 1983, and numerical simulations show that a nonlinear Kelvin wave reproduces many of its properties., Comment: 21 pages, 10 figures, 2 animated figures and 2 tables

Climate-interior coupled evolution is investigated for Venus by merging a partial-melting/parameterized mantle convection model with a gray radiative-convective atmospheric model. A positive feedback process can operate by the release of water to the atmosphere via mantle melting, leading to an increase in atmospheric opacity and the radiative temperature gradient. The resulting amplification of the greenhouse surface temperature raises the mantle temperature leading to an increase in the partial-melting rate. Using thin-lid convection, a coupled model for Venus running over an interval of 3 to 1 Ga shows a significant increase in surface temperature, partial-melting extent, and extrusive magma flux compared to a model where there is no communication between greenhouse-modulated surface temperature and partial-melting in the interior. Coupled and uncoupled models that transition (with variable timing) to stagnant-lid convection either shut down partial melting via lithospheric thickening, or evolve to large amounts of melt due to increased interior temperatures.

The range of surface ages on Venus and the rate of recent volcanism are two major questions that can be addressed through an analysis of Magellan images. We report on a 3-D Monte Carlo resurfacing model that seeks to explain the resurfacing history of Venus by simulating the evolving surface under the competing processes of impact cratering and volcanism. By incorporating the observed size distribution of volcanic forms on Venus, and the assumptions of constant volcanic and impactor fluxes, we seek a solution that is consistent with the observed impact crater population. In the Monte Carlo simulations, a volcanic flux of 0.37 km3/yr, operating for 550 m.y. on initially craterless surfaces, yields surfaces that are statistically similar to Venus'. It is shown that the statistics of the impact cratering record are inconsistent with an equilibrium resurfacing model.

[1] Widespread, massive layered sediments, studied by surface rovers and Mars-orbiting spectrometers, are found to be rich in sulfates. No similarly massive carbonates have been detected. We present the results of coupled atmospheric and geochemical calculations of the formation of the sulfate-rich Meridiani sediments, and offer an explanation for why extensive layered carbonates are not found there or elsewhere on Mars. Large scale volcanism from the build-up of Tharsis during the late Noachian would have injected large amounts of SO2 into the atmosphere. Efficient photochemical conversion of SO2 to H2SO4 would have caused widespread sulfuric acid/water clouds, similar to those seen on Venus today. Precipitation from these clouds and acidification of surface water would have sustained a thick, warm CO2 atmosphere via carbonate inhibition. Such an atmosphere could have been subjected to loss to space via impact erosion and sputtering. Once atmospheric SO2 gas production dropped and waters become more alkaline, the remaining CO2 in the atmosphere collapsed to form poorly consolidated carbonate patinas on rock surfaces and in open fractures.

[1] We report on laboratory experiments in which we allowed an SNC-analog mineral mixture to react with water under a simulated Mars atmosphere containing the added gases SO2, HCl and NO2. The addition of acidic gases was designed to simulate the atmospheric effects of periods of increased volcanic activity on Mars. The S/Cl ratio in our experimental brines was about 200, compared with an average value of approximately 5 measured in martian fines at the Mars Pathfinder and Viking 1 and 2 landing sites. This suggests that if salts were produced under an atmosphere higher in acidic volatiles than at present, they would have produced evaporite deposits higher in S than is presently found in the martian fines. Erosion of these high-sulfur evaporites and mixing with eroded mafic basalts could have produced the globally homogenous, salt-rich mobile fines that are present on Mars today.