Mercury Has Ice…But Wait, There’s More!

This post was written by Mackenzie White. Mackenzie is in her final semester of her Masters in Geophysics with a focus on Planetary Science at Southern Methodist University. She studies heat flow on the Moon and Mars and enjoys working on new NASA missions. In addition to space rocks, Mackenzie loves skiing, traveling, and running with her dog, Rocky.


Title: New evidence for surface water ice in small-scale cold traps and in three large craters at the north polar region of Mercury from the Mercury Laser Altimeter

Authors: Ariel Deutsch, Gregory Neumann, and James Head

First Author’s Institution: Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, Rhode Island, USA.Status: Published in Geophysical Research Letters, open access

All things considered, Mercury would be a pretty tough place to live. The closest planet to the Sun can experience surface temperatures near 722 K and has a thin exosphere made of particles released from Mercury’s surface. Despite these extreme conditions, over the few last decades the planet has become one of the solar system’s most notable hosts of water ice. 

Radar telescopes first detected the presence of ice at polar regions in the 1990s and in 2011 NASA’s MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging) mission quickly confirmed this finding. Though often exceptionally toasty, Mercury is able to sustain ice at or near the surface due to its axis of rotation keeping polar areas in permanent shadow. 

The study of ice on Mercury has focused on these permanently shadowed regions (PSRs) in and around craters near the poles. The authors of today’s paper show additional locations where ice may exist by looking at new latitudes and crater types, dramatically increasing the potential volume of water ice on the planet. Through studying these water deposits, we gain a deeper understanding of how much ice exists in the solar system and how it makes its way to planetary bodies.

Finding The Ice

The Mercury Laser Altimeter (MLA) instrument onboard MESSENGER measured the round-trip time of laser pulses to travel to the surface and back, providing a value of surface reflectance. Surface reflectance tells us what the surface is made of; anomalies indicate areas that may contain water ice. The accuracy and strength of this signal can vary depending on the angle of measurement, with ideal measurements taken directly overhead. While a previous study was limited to a region bound from 75ºN to 85ºN, today’s authors take advantage of recently calibrated data and new spacecraft orbit inclinations to expand their work up to 90ºN. 

As a general guideline, low-reflectance areas indicate subsurface ice covered by insulating material, while high-reflectance suggests surface water ice. Today’s authors focus on ice exposed on the surface, combining reflectance data, Earth-based radar data, modeled PSR locations, and maximum surface temperature limits. By comparing where each of these parameters indicate the presence of ice, they locate new likely reservoirs of water ice on Mercury. Figure 1 shows an example of this data agreement used to identify an area of surface ice.

Chart, histogram

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Figure 1. Data from an identified ice-containing crater. The authors suggest water ice is exposed where surface reflectance (rs, represented by circles) is >0.3. The green line represents crater topography, the brown line shows biannual maximum surface temperatures (Tmax), and the grey box is the area predicted by thermal modeling to have temperatures low enough to sustain surface water ice. Notably, a group of rs >0.3 intersect a topographic minimum, a low maximum surface temperature, and the model-predicted area.

Small Areas Add Up

Using the new and improved data that shows a steady increase in surface reflectances moving from 85.3ºN to 90ºN, the authors sought to answer the inevitable question: where is the surface ice hiding in this region? 

The study began by looking at relatively familiar territory: large craters. These regions were a good place to start because previous Earth-based radar detection only found exposed water ice in large PSRs (defined by diameters 7-120 km). Though the reflectance measurements were taken at an angle, the authors created an empirically recalibrated map (shown in Figure 2) for the three largest craters near the north pole: Chesterton, Tolkien, and Tryggvadóttir craters. The map revealed these three craters together may contain ice that measures near 3,400 square kilometers. 

While that is certainly a significant volume of ice, the authors pushed the investigation a step further: what happens when you remove large PSRs from the picture? The answer: the reflectance trend remains. While this was revealed earlier in the released recalibrated data, questions of ice location and volume persisted. PSRs smaller than 7 km in diameter with temperatures appropriate for maintaining ice, deemed “microcold traps,” are individually too small for MLA to resolve, leading the study to look for clusters of reflectance enhancements. The authors subsequently identified four clusters in areas below 5 km in diameter named n5, o7, e5, and I7. Figure 2 shows the map of notable surface reflectance for these microcold traps in addition to larger craters.

Diagram

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Figure 2. The calibrated reflectance (rs) map used to identify surface ice from 82.5ºN to 90ºN. Like in Figure 1, the authors suggest water ice is exposed where surface reflectance is >0.3. The map includes large crater ice deposits detected by previous work (Prokofiev, Kandinsky, C, Y, and i5 craters) in addition to their newly identified ice deposits in Chesterton, Tolkien, and Tryggvadóttir craters and microcold traps (n5, o7, e5, and I7).

While the authors only locate four of these regions using MESSENGER, thermal modeling results indicate the existence of substantially more microcold traps that are below the spatial resolution of MLA detection. These potential ice-filled microcold traps suggest there is a lot more ice on Mercury than previously thought. This work now gives us new places to look for evidence of water ice, ultimately contributing to our continuously evolving picture of how and where water exists. 

Featured Image credit: NASA
Edited by Abygail Waggoner

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1 Comment

  1. Very cool!

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