Bright basal reflectors in radargram from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) of the Martian south polar layered deposits (SPLD) have been interpreted to be evidence of subglacial lakes. However, this interpretation is difficult to reconcile with the low Martian geothermal heat flow and the frigid surface temperature at the south pole. We conduct a comprehensive thermophysical evolution modeling of the SPLD and show that subglacial lakes may only form under exceptional circumstances. Subglacial lakes may form if the SPLD contains more than 60 % dust volumetrically or extremely porous ice (>30 %), which is unlikely. A thick (>100 m) layer of dirty ice (>90% dust) at the base of the SPLD may also enable basal melting, resembling a sludge instead of a lake. Other scenarios enabling subglacial lakes in the SPLD are equally unlikely, such as recent magmatic intrusions at shallow depths.

Brinicles are self-assembling tubular ice membrane structures, centimeters to meters in length, formed by the downward migration of supercooled brine rejected from ice sheets, and found beneath sea ice in the polar regions of Earth.  They provide a plausible setting for geochemical gradients amenable to life at the ice-ocean interface, in some ways analogous to hydrothermal vents at the seafloor-ocean interface. Their occurrence in icy ocean worlds like Europa and Enceladus remains hypothetical. The context of brinicles on Earth includes influences from oceanic flow, which will differ in other worlds, and surficial inputs from the atmosphere that do not exist in oceans with kilometers-thick global coverings of ice formed from the underlying ocean. Thus, it is difficult to project the likely occurrence and role of brinicles based on field observations of their earthly analogues. We discuss brinicles as they are currently understood, including their electrochemical properties in connection with potential habitats at the ice-ocean interface on Europa and Enceladus. We employ a fluid mechanical model (Cardoso and Cartwright, 2017) to assess the properties of brinicles on other worlds and consider their longevity relative to potential brine outflows from the overlying ice. We demonstrate how brinicles may grow by thermal diffusion, and provide simple scaling for their growth and outflow rates. The specifics of the composition and dynamics of both the ice and the ocean in these worlds remain poorly constrained. We demonstrate through calculations using FREZCHEM that sulfate likely fractionates out of accreting ice in Europa and Enceladus, and thus that an exogenous origin of sulfate observed on Europa’s surface need not preclude additional endogenous sulfate in Europa’s ocean. We suggest that, like hydrothermal vents on Earth, brinicles in icy ocean worlds constitute ideal places where ecosystems of organisms might be found.

Titan’s surface icy shell is likely composed of water ice and methane clathrate [1, 2]. Methane clathrate may play a role in Titan’s methane cycle [3–5] affect Titan’s thermal profile [6] , and may affect the habitability of Titan’s ocean. Although the bulk properties of clathrates are similar to those of pure water ice, the thermal conductivity of methane clathrate is about 20% the value for pure water ice [7, 8]. The lower thermal conductivity acts to insulate Titan’s icy shell, changing the thermal profile of Titan. As seismic wave speeds [9, 10] and attenuation [11] are dependent on temperature, any changes to the thermal profile will result in changes to seismic waveforms recorded by seismic instrumentation. Here, we compare the seismic waveforms of model with a 100 km thick pure water ice shell, versus a model with a 10 km clathrate lid over 90 km of pure water ice. Our results have implications for the upcoming Dragonfly mission, which will carry seismic instrumentation as part of its payload [12]. Methods: We use PlanetProfile [13] to create interior structures models of a pure water ice shell and a model with a pure water ice shell with a 10 km clathrate lid. The interior structure models are used as inputs with AxiSEM [14] and Instaseis ([15] to generate seismic waveforms. We interpret the results to quantify the differences in seismic velocities, arrival times of seismic phases, and amplitudes of seismic waveforms at the surface of Titan. Results: The interior structure models show a clathrate lid will reduce the conductive lid thickness by ~ 2/3 compared to the pure water ice shell model. As a result, the clathrate lid model reaches higher temperatures at shallower depths (Figure 1a). The temperature profile affects the seismic velocity (Figure 1b), and the seismic quality factor (Q, Figure 1c) profiles. A clathrate lid creates a steeper negative gradient in seismic velocities and Q. The greatest difference in seismic velocities occurs at the base of the clathrate lid (Figure 2). Because of the change in seismic velocities, the arrival times and observable distances of seismic phases will be different between the two models. Using TauP [16], we calculate the differences for several seismic phases. We find that the change in seismic velocity profile results in a difference of a few seconds at most in arrival times. The range of observable distances will also vary by a few degrees. The small changes might be noticeable on waveforms, but would require high signal to noise ratios, and precise determinations of location and depth of the event. The changes in seismic velocities and Q will also impact the observed ground motion. Using AxiSEM and InstaSEIS, we create a database of seismic waveforms spaced 1 degree in epicentral distance. We compare the same event magnitude and distance between source and seismometer for the two models. For each waveform we calculate the root mean square (RMS) using ground acceleration.