Quantifying the volume and distribution of Mars' subsurface lithologies, mineral cements, and liquid water are critical to unraveling the planet's geologic evolution (Carr & Head, 2003Di Achille & Hynek, 2010;Scheller et al., 2021). Mars' crust comprises igneous and sedimentary rocks that are lithified and fractured to varying degrees (Golombek et al., 2018;Pan et al., 2020;Tanaka et al., 2014). Two open questions are (a) what is the depth where pores close entirely within the Martian crust and (b) what percentage of existing pores in the Martian crust host liquid water or ice, or water incorporated into mineral cements.

Large volumes of water are hypothesized to have carved and passed through the Martian outflow channels (e.g., Baker, 2001). Because these channels originate from discrete sources, a groundwater origin is typically invoked (e.g., Head et al., 2003). Given the large discharges needed to create the observed landforms, in some cases a couple orders of magnitude greater than the largest catastrophic floods on Earth (Baker, 1982), large and permeable aquifers would be needed (e.g., Carr, 1979;Manga, 2004). While most of the outflow channels are Hesperian (e.g., Tanaka, 1997), their formation continued through the Amazonian (e.g., Rodriguez et al., 2015). Some of the youngest channels originated from fissures in Athabasca Valles, Eastern Elysium Planitia within the past 10s of millions of years (Burr et al., 2002;Voigt & Hamilton, 2018). The subsurface of Mars thus appears to have hosted and episodically released large volumes of water over most of Martian history. Hence, detecting the presence and quantifying the volume of subsurface water and ice would help constrain the water budget and cycle from the Noachian to present (e.g., Clifford and Parker, 2001), the amount of water lost to space (e.g., Jakosky, 2021), the fate of possible oceans (e.g., Citron et al., 2018), and the amount of water sequestered in minerals (e.g., Scheller et al., 2021).To discharge water at the surface, aquifers must have sufficient pressure for water to reach the surface. One way to achieve hydraulic heads greater than hydrostatic and hence enable surface discharge is to confine aquifers beneath an overlying ice-saturated crust or cryosphere (e.g., Andrews-Hanna and Phillips, 2007;Carr, 1996;Harrison & Grimm, 2004). As Mars cools and this cryosphere thickens, hydraulic heads will increase and may also create the pressure needed to fracture the crust (Wang et al., 2006). The MARSIS radar system has identified reflections interpreted as lakes confined under Mars' southern ice cap (Lauro et al., 2021;Orosei et al., 2018), though this interpretation is contested (Ojha et al., 2021) and would require recent magmatism (Sori & Bramson, 2019). Aquifers in the crust, if they exist, would be at depths of several kilometers (Clifford et al., 2010), deeper than the MARSIS and SHARAD radar systems can penetrate in volcanic terrain (e.g., Abotalib & Heggy, 2019). MARSIS has not identified a deep reflector indicative of an aquifer below the outflow channels in Athabasca Valles, for example (Clifford et al., 2010). Other geophysical data, such as seismic shear wave velocity, Vs, may be useful because Vs is sensitive to physical properties of the subsurface and probes greater depths.Our objective is to interpret Vs measured by the InSight mission in Elysium Planitia. We use rock physics models to compute effective medium properties and to help distinguish between porous basalt filled with gas, liquid water, ice, or mineral cement. We focus on two observations. First, Vs within Mars' upper

Lakes Enriquillo and Azuei, the two largest lakes in Hispaniola and in the Caribbean, have risen 10 and 5 m respectively within the last 8 years. Higher lake levels have submerged towns, road systems, agricultural lands and utilities, and have threatened to submerge the major overland highway that connects the Dominican Republic and Haiti. In this study, we use CHIRP seismic data, satellite imagery, and regional meteorological data to quantify and assess controls on the recent lake level rises. Although data are limited, the analyses indicate that the lakes' water level changes may be attributed to a combination of increased rainfall and natural or man-made changes to the hydraulic connectivity of the various water bodies within the drainage basin. We show that a weak correlation exists between changes in Lake Enriquillo's and Azuei's water levels and precipitation rates (0.2 and 0.08 respectively, 1984-2012) and that both lakes experience periods of anti-correlation where, for example, water level drops at Lake Azuei (~20 masl) coincide with water level rises at Lake Enriquillo (41 mbsl). From these observations, we propose that the lakes experience intermittent periods of hydraulic connectivity along reactivated or newly developed stratigraphic-controlled sub-surface transport pathways. We also note that moderately small (

-The extent to which persistent, rather than transient, fissures (wide planar voids) can exist along upper crustal faults is important in assessing fault permeability to mineral and hydrocarbonbearing fluids. Variscan (late Carboniferous) faults cutting Dinantian (Lower Carboniferous) limestones on the Gower peninsula, South Wales, host clear evidence for fissures up to several metres wide. Evidence includes dendritic hematite growth and elongate calcite growth into open voids, spar ball and cockade breccia formation, laminated sediment infill and void-collapse breccias. Detailed mapping reveals cross-cutting geometries and brecciation of earlier fissure fills, showing that fissures were formed during, rather than after, active faulting. Fissures therefore probably formed by geometric mismatch between displaced fault walls, rather than by solution widening along inactive faults.