No cryosphere-confined aquifer below InSight on Mars

Abstract

The seismometer deployed by the InSight lander measured the seismic velocity of the Martian crust. We use a rock physics model to interpret those velocities and constrain hydrogeological properties. The seismic velocity of the upper ~10 km is too low to be ice-saturated. Hence there is no cryosphere that confines deeper aquifers. An increase in seismic velocity at depths of ~10 km could be explained by a few volume percent of mineral cement (1-5%) in the pores and may document the past or present depth of aquifers. Plain Language Summary Large amounts of water may be stored in the Martian crust and episodically released to flood the surface. Where this water exists, and even why, is uncertain. The seismometer on the InSight lander measured the speed of seismic waves in the Martian crust. The presence of ice and water affects seismic velocity. We argue that the measurements preclude a layer of ice-filled crust that confines liquid water in an aquifer. Key points • We interpret the seismic wave velocity of the Martian crust measured by InSight. • We quantity the effects of ice and water on seismic velocity using a rock physics model. • Measurements preclude a layer of ice-filled upper crust that confines liquid water in an aquifer. 1. Introduction 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 Eastern Elysium Planitia within the past 10s of millions of years (Burr et al., 2002; Voight and Hamilton, 2017). 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 (Clifford and Parker, 2001). 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.

A Minimally Cemented Shallow Crust Beneath InSight

“…This is indicated by the observation that the granular and fractured media models predict velocities that are too high for fully ice‐saturated sediments and basalt. Manga and Wright (2021) drew a similar conclusion for the upper 8 km of crust because their modeled V s for an ice‐saturated basalt was high compared to measured V s . It is unlikely that we misinterpreted a basalt layer for an ice‐saturated sediment layer; the predicted V p for the Amazonian and/or Hesperian basalt layer matches, but V s is overpredicted by at least 0.5–2.3 km/s (Figures 1 and 2).…”

“…Other indirect methods exploit the sensitivity of geomechanical properties to cements, which influence geophysical properties such as seismic velocity, electrical conductivity, and gravity. For example, Manga and Wright (2021) used seismic velocities interpreted with rock physics models for fractured rocks to infer that there is likely no ice‐saturated cryosphere in the 0–7.5 km depth range beneath the InSight landing site, though they suggested that some mineral cement could be present at greater depths.…”

Constraints on the martian crust away from the InSight landing site

“… 17 ) favored a hypothesis involving the removal of pore space by viscous deformation at depth. Because porosity can significantly reduce both the P- and S-wave speeds 34 , 35 and the closure of pore space is expected to take place over only a few kilometers 36 , the removal of pore space would generate a density (or seismic velocity) discontinuity. On the Moon, impact cratering has also been suggested as the cause for low seismic velocities or crustal porosity 37 – 39 .…”

InSight Constraints on the Global Character of the Martian Crust

“…The addition of porosity to a rock can dramatically reduce its seismic velocity (e.g., Heap, 2019) and a stepwise change in crustal porosity could be the origin of one, or perhaps both, of the intracrustal seismic discontinuities beneath the InSight landing site. One manner to reduce (or possibly remove) porosity, was proposed by Manga and Wright (2021). In their model, the precipitation of minerals in a liquid‐water aquifer could have filled some of the pre‐existing pore space.…”