The South Polar Brine Debate

In the last few years, several key papers including some published in Nature Astronomy (e.g., [Lauro et al., 2021]) and Nature Communication (e.g., [Lauro et al., 2022]) have interpreted bright basal reflectors in radargram from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) of the Martian south polar layered deposits (SPLD) to be evidence of subglacial lakes. Naturally, the presence of a potential subglacial lake on present day Mars has several outstanding implications for our understanding of Martian geological history and the present day astrobiological potential of the red planet. However, this interpretation of the radar data is difficult to reconcile with the low Martian geothermal heat flow and the frigid surface temperature at the south pole. And even though several recent papers have provided alternative interpretations of the bright radar reflections, including CO2 – H2O layer boundaries [Lalich et al., 2022], saline ice [Bierson et al., 2021], or smectites [Smith et al., 2021], the debate has persisted with Lauro et al. (2022) most recently claiming the presence of multiple subglacial lakes underneath the SPLD.

While subglacial lakes are common in various areas of Earth [Livingstone et al., 2022], the main factor that hinders the formation of subglacial lakes on Mars is the frigid temperature at the south pole (<165 K) [Sori and Bramson, 2019] and expected low geothermal heat flow on present-day Mars [Broquet et al., 2021; Ojha et al., 2021; Ojha et al., 2019; Parro et al., 2017; Plesa et al., 2018]. Therefore, augmentation of the basal temperature by exceptionally high heat flow from a recent magmatic intrusion would be necessary to melt pure water ice at the Martian south pole [Sori and Bramson, 2019]. Alternatively, if one were to suppose that the base of the SPLD is composed of saline ice, then the upper limit on the temperature required for melting can be relaxed to a much lower value. In Lauro et al. (2022), the authors posit that the presence of pore-space within the SPLD and other impurities like dust and CO2 within the SPLD can notably affect the basal temperature enabling the formation of subglacial lakes underneath the SPLD. However, to date, no comprehensive thermal modeling exercise has been conducted to ascertain the exact conditions within the SPLD that could enable basal melting. 

With liquid water (pure or briny) at the base of the SPLD as an a priori condition, we recently conducted a comprehensive thermophysical evolution modeling exercise to ascertain the conditions that can enable basal melting at the south pole of present-day Mars (link).

For example, the figure below shows the temperature at the base of the Martian southern polar cap under some of the most lenient scenarios. We vary the surface temperature (Ts), volume of dust within the ice (Vdust), and the thermal conductivity of the dust (kdust). Regions within the white contours are areas where the basal temperature exceeds 200 K and the black rectangle shows the putative location of the subglacial liquid water in the south pole. 

An alternative scenario that has been proposed is the the subglacial liquid water is sustained by a recent magmatic intrusion. In this paper, we also explore if an intrusion at depth can enable basal melting. The short answer is yes; however, there is no evidence for a recent magmatic intrusion in the south pole of Mars. The movie below shows the augmentation of the basal temperature by a 5 km wide intrusion at relatively shallow depth.  

Furthermore, intrusions cool over geologic timescale. Any intrusion sustained lakes, if they exist, will re-freeze over a few million years. 

In summary, we show that basal melting is very unlikely to be an ongoing process in the Martian south polar cap. Given the difficulty in forming and sustaining subglacial lakes on Mars, we suggest that alternative interpretations of the bright radar reflections are more likely to be accurate and merit consideration.

Habitability of Exoplanets

The habitability potential of planets in the circumstellar habitable zone of M-dwarf stars are of great interest to many in the exoplanet and planetary science community. Many recently discovered potentially habitable planets like LHS 1140 b, Proxima Centauri B, and the planets of the Trappist system orbit around M-dwarf stars. However, the surface habitability of these planets is complicated by the relatively high fraction of stellar luminosity emitted at UV and shorter wavelengths by M dwarfs and the propensity of these stars for flaring. Another challenge is the prediction by atmospheric models that these planets may exist in an eyeball-like climate state, with liquid water present at the substellar point and the rest of the planet mostly frozen.

In this ongoing work, we use numerical models to investigate the feasibility of basal melting on various exo-Earths. In terrestrial glacial studies, the term "basal melting" is used to describe any situation where the local geothermal heat flux and any frictional heat produced by glacial sliding are sufficient to raise the temperature at the base of an ice sheet to its melting point. Basal melting is responsible for the formation of terrestrial subglacial liquid water lakes, maintaining habitable environments during Snowball Earth events, and may be responsible for the formation of fluvial features during the faint young sun period on Mars, as well as for producing the much-debated possible subglacial lake in the south polar region of Mars today. 

We find that basal melting is entirely feasibly on exo-Earths and may play an equally important role in the habitability of exoplanets as it does for the terrestrial planets of our solar system. If even a handful of potentially habitable exo-Earths discovered so far were to contain thick (> few kms) hydrospheres, we show that basal melting may be occurring on those bodies, even with heat flow a factor of three lower than that observed on the Earth's Moon. At such interface, water-rock interactions may provide a variety of chemicals and energy that could play a role in the origin and sustenance of putative life forms at the ocean floor, akin to those found at hydrothermal vents on Earth. Another notable result from this work is that in some cases, two liquid water oceans may exist on exo-Earths, a shallow liquid-water ocean due to the reduced melting point of pressurized ice Ih and basal melt ocean at the ice-crust interface. In either case, these subglacial lakes, often in contact with the planet's crust and shielded from the high energy radiation of their parent star by thick ice layers, may provide habitable conditions for an extended period.

Update from 09-23-2023: I was recently on the Planetary Society podcast discussing this work. Hear more about this work here.

Habitability of Mars

The faint young sun paradox on Mars

Stellar evolution models indicate that the Sun’s luminosity was approximately 30% lower 4 Ga ago. Mars receives only 43% of the solar flux incident on Earth, thus, provided the orbit of Mars has not changed significantly in the last 4-billion years, the climate of early Mars should have been extremely cold. However, much morphological and compositional evidence on Mars suggests abundant surface water sustained by subsurface hydrology during the Noachian eon [4.1 – 3.7 Ga ago]. The resulting faint young Sun paradox, between climate models that struggle to elevate surface temperature above 273 K and geological evidence suggesting the presence of abundant liquid water during the Noachian, is an outstanding question in Mars science, with major implications for early Martian climate, hydrology, and habitability.

We recently used GRS and gravity data along with numerical models to propose that geothermal basal melting of thick ice sheets may help resolve that paradox (link).

Amagmatic Hydrothermal Systems on Mars

Long-lived hydrothermal systems are prime targets for astrobiological exploration on Mars. Unlike magmatic or impact settings, radiogenic hydrothermal systems can survive for >100 million years because of the Ga half-lives of key radioactive elements (e.g., U, Th, and K), but remain unknown on Mars. In this work, we used geochemistry, gravity, topography data, and numerical models to find potential radiogenic hydrothermal systems on Mars. We showed that the Eridania region, which once contained a vast inland sea, possibly exceeding the combined volume of all other Martian surface water, could have readily hosted a radiogenic hydrothermal system (link).

Gamma-Ray Spectroscopy

GRS Analysis

GRS mapping provides elemental geochemistry of the near-surface regolith of Mars, by measuring the spectrum of gamma photons emitted from the Martian surface; characteristic spectral peaks from specific nuclear reactions allow the quantification of most major rock-forming elements, along with select minor and trace elements (Al, Ca, Cl, Fe, H, K, S, Si, Th). Peak area above background can be used to infer mass fraction (wt%, ppm) of each element over an area of the planet’s surface, leading to chemical abundance maps such as shown here. I like to combine GRS maps with other geophysical datasets to understand various geological processes.

Gravity

Gravity Modeling

I like to use gravity data to understand the surface and interior processes on terrestrial planets. We previously used gravity data from Mars to constrain the composition of the polar caps (link) and large sedimentary deposits like the Medusae Fossae Formation (link).

ISRU

in-situ resource utilization

In-situ resource utilization (ISRU) is the practice of collection, processing, storing, and use of materials found or manufactured on other astronomical objects like Mars. Information about the structure, depth, and purity of shallow ice is also required to plan possible ISRU on future Mars missions. We have previously discovered extensive deposits of water ice that can be >100 meters thick in the Martian shallow subsurface at mid-latitudes (link).