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Jupiter's moon Europa is thought to have an ocean beneath its ice shell and the habitability of the internal ocean depends on the availability of redox gradients. Downward transport of radiolytic materials produced at the surface through the ice shell sets the flux of oxidants into the ocean. Here, we propose that oxidants are transported through the ice shell by the drainage of near‐surface brines formed concurrently with chaotic terrains. We estimate that Europa's porous regolith contains 3.7 × 10¹⁴to 5.6 × 10¹⁸ mol (1.2 × 10¹³ − 1.8 × 10¹⁷ kg) of trapped O₂. Simulations of coupled melt‐migration and eutectic phase behavior show that brines drain before they refreeze, delivering ∼85% of the surface oxidants to the ocean on timescales of 2 × 10⁴ years. From the distribution of chaotic terrains and from Europa's surface age we estimate that brine drainage could deliver O₂  to the ocean at rates of 2.0 × 10⁶to 1.3 × 10¹⁰ mol/yr (0.002−13.2 kg/s).

Cyclical ground deformation, associated seismicity, and elevated degassing are important precursors to explosive eruptions at silicic volcanoes. Regular intervals for elevated activity (6–30 hr) have been observed at volcanoes such as Mount Pinatubo in the Philippines and Soufrière Hills in Montserrat. Here, we explore a hypothesis originally proposed by Michaut et al. (2013,https://doi.org/10.1038/ngeo1928) where porosity waves containing magmatic gas are responsible for the observed periodic behavior. We use two‐phase theory to construct a model where volatile‐rich, bubbly, viscous magma rises and decompresses. We conduct numerical experiments where magma gas waves with various frequencies are imposed at the base of the model volcanic conduit. We numerically verify the results of Michaut et al. (2013,https://doi.org/10.1038/ngeo1928) and then expand on the model by allowing magma viscosity to vary as a function of dissolved water and crystal content. Numerical experiments show that gas exsolution tends to damp the growth of porosity waves during decompression. The instability and resultant growth or decay of gas wave amplitude depends strongly on the gas density gradient and the ratio of the characteristic magma extraction rate to the characteristic magma degassing rate (Damköhler number, Da). We find that slow degassing can lead to a previously unrecognized filtering effect, where low‐frequency gas waves may grow in amplitude. These waves may set the periodicity of the eruptive precursors, such as those observed at Soufrière Hills Volcano. We demonstrate that degassed, crystal‐rich magma is susceptible to the growth of gas waves which may result in the periodic behavior.