Ice in Micro Cold Traps on Mercury: Implications for Age and Origin

Abstract

Evidence in radar, reflectance, and visible imagery indicates that surface and subsurface water ice is present inside permanently shadowed regions in the north polar region of Mercury. The origin of this ice and the time at which it was delivered to the planet are both unknown. Finding the smallest, most easily eroded ice deposits on Mercury can help answer these questions. Here we present evidence for volatiles trapped in cold traps of scales ∼ 1-10 m. We consider two possible delivery methods for these deposits: a gradual, slow accumulation by micrometeorites or solar wind implantation and an episodic deposition, either primordial or by a recent comet impact. We conclude that the mechanism that best explains the presence of volatiles in these micro cold traps is a comet impact that most likely occurred in the last ∼ 100 Ma.Plain Language Summary Craters near the north pole of Mercury, one of the warmest planets in the solar system, cast persistent shadows that are so cold they can trap water ice for billions of years. Although many observations show that ice is present inside these polar cold traps, its age and the way it was delivered to the planet are not well known. Here we present evidence for the presence of ice deposits on scales of 1-10 m, the smallest discovered so far, near the north pole of Mercury. Due to the relative shallowness of these micro cold traps, an episodic deposition, such as a comet impact, is a more probable delivery mechanism than an ongoing slow accumulation by micrometeorites. Using previously estimated surface erosion rates, we find that the age of this ice is most likely lower than 100 million years.Lately, it was hypothesized that ice may persist in micro cold traps that form in PSRs cast by craters and random small-scale topographic features (Hayne & Aharonson, 2015). In this case, the term micro does not mean Key Points: • Using MLA data and a thermal illumination model, we find evidence for ice trapped in micro cold traps of scales < 10 m and thickness < 1 m • Surface micro cold traps occupy 1%-2% of the polar region, compared to ∼ 7% occupied by the larger cold traps • A recent comet impact is more likely to explain our findings than continuous delivery by, for example, solar wind implantation or micrometeorites

Impact Gardening as a Constraint on the Age, Source, and Evolution of Ice on Mercury and the Moon

“…Thermal modeling by Rubanenko et al () hypothesized the presence of “microcold trap” surface ice deposits existing in submeter regions of permanent shadow that may only be a few decimeters thick. The relative efficiency of gardening in ice makes these small unprotected deposits particularly vulnerable and we predict that if they exist, micro cold trap ice deposits would be thoroughly gardened over 10 Myr timescales (Figure c).…”

Equilibrium Temperatures and Directional Emissivity of Sunlit Airless Surfaces With Applications to the Moon

“…at the same scale, making them good statistical analogs for thermal models (e.g., Davidsson et al, 2015;Rubanenko & Aharonson, 2017;Rubanenko et al, 2018). We constrain the impact of this assumption on our results in section 3.1.…”

“…In all of the following derivations we assume surface slopes are distributed Gaussian. Even though the slope distribution of realistic surfaces is often non‐Gaussian (see Figure 1b and Rosenburg et al, 2011), the temperature distribution of Gaussian surfaces at some scale closely follows that of the realistic lunar topography at the same scale, making them good statistical analogs for thermal models (e.g., Davidsson et al, 2015; Rubanenko & Aharonson, 2017; Rubanenko et al, 2018). We constrain the impact of this assumption on our results in section 3.1.…”

Examining the Potential Contribution of the Hokusai Impact to Water Ice on Mercury

“…Estimates of Mercury's water‐ice inventory range from ~10 16 to 10 18 g (Deutsch et al, ; Eke et al, ; Lawrence et al, ; Moses et al, ; Susorney et al, ). Recent studies have suggested that additional water‐ice reserves may exist in small‐scale cold traps distributed across rough, intercrater terrain in the polar regions (Deutsch et al, ; Neumann et al, ; Rubanenko et al, ).…”

“…Estimates of Mercury's water-ice inventory range from~10 16 to 10 18 g (Deutsch et al, 2018;Eke et al, 2017;Lawrence et al, 2013;Moses et al, 1999;Susorney et al, 2017). Recent studies have suggested that additional water-ice reserves may exist in small-scale cold traps distributed across rough, intercrater terrain in the polar regions Neumann et al, 2017;Rubanenko et al, 2018). enhancements has been found at the lunar poles (e.g., Colaprete et al, 2010;Fisher et al, 2017;Hayne et al, 2015;Schultz et al, 2010), the inferred concentrations of water ice are significantly lower than, and distribution of water-ice patchier than, those at Mercury (e.g., Lawrence, 2017).…”