This week, Jacob Kloos, a PhD Student here at PVL discusses exciting new research he has just published in the Journal of Geophysical Research, Planets. In his work, Jake found that the famous permanently shadowed regions (PSRs) are surrounded by seasonally shadowed regions (SSRs) which turn out to have important implications for the lunar water exosphere and the amount of water available in different locations at different times of the year - they're not what you would expect! Above, one of the key findings of the research: maps of the lunar poles showing these SSRs.
By Jacob Kloos
Over the past few decades, the north and south polar regions of Earth’s moon have garnered much attention within the field of planetary science. In addition to becoming prime targets for robotic and human exploration, the lunar poles have also been the subject of an increasing number of scientific studies. What makes these areas so intriguing for science and exploration? The answer lies in their unique illumination environments.
Unlike the Earth which rotates on an axis tilted 23.5 degrees from the ecliptic normal, the spin axis of the Moon is tilted only 1.5 degrees, ensuring that the Sun is always near the horizon for an observer at one of the poles. The low axial tilt of the Moon, coupled with its heavily heavily cratered surface, produce complex illumination patterns at high latitudes, giving rise to extremes in both sunlight and shadow: areas that are high in elevation may experience near-continuous sunlight, while some low-lying basins are in permanent shadow. Although no regions on the Moon (or indeed in the solar system) have yet been discovered which can claim the ethereal title of “peaks of eternal light,” some regions, like the rim of Shackleton crater near the South Pole, remain bathed in sunlight for 80-90% of the year. Such areas are attractive sites to send a solar-powered rover.
The permanently shadowed regions (PSRs), which are in many cases directly adjacent to the near-continuously illuminated regions, are not only interesting from an exploration perspective, but also from a scientific perspective. As a direct consequence of not receiving direct sunlight, and because the Moon lacks a substantial atmosphere to sequester and transport heat, permanently shadowed regions are among the coldest places in the solar system, enabling them to trap and store volatiles such as water across geologic periods of time. These volatile deposits constitute a valuable resource for scientific study as they would be well preserved and largely protected from chemical weathering; as such they could provide valuable insight into the delivery of water to the inner solar system - in particular to the Earth-Moon system. As for exploration, water could be extracted in-situ by future explorers, and could provide a source of potable drinking water, breathable air or perhaps even rocket fuel if broken down into its constituent components.
Thus far, our knowledge of the Moon’s polar environment and its volatile content has been transformed by the wealth of high resolution data returned from orbital spacecraft. With neutron spectroscopy data, we now know that hydrogen, which may be in the form of water, is enriched at high latitudes within the upper 1-2 meters of regolith. In addition, numerous independent data sets have detected deposits of water ice within some (though not all) permanently shadowed regions at the poles. Even more exciting, however, is the discovery that the Moon has an active water cycle. Water is produced on/delivered to the Moon from a variety of sources, including solar wind implantation and micrometeorite impacts, and that water is capable of migrating around the surface through a series of ballistic hops. High energy photons from the Sun can break apart molecules that are in flight, however those that survive this destruction mechanism and don’t have enough kinetic energy to achieve escape velocity continue to hop about the surface until they land in a permanently shadowed region. At this point, the molecule is effectively immobilized given the low temperatures, and thus PSRs are referred to as “cold traps” for volatiles.
Evidence of the lunar water cycle comes in the form of orbital data showing diurnal (daily) variations in the surface concentration of water, which has been observed by a number or independent spacecraft. Not only do these observations demonstrate that an active source of water is present, but they also show that water is capable of moving around on the surface, and isn’t simply being incorporated into the crystal structure of the minerals that make up the regolith. It should be noted that differentiating between water (H2O) and hydroxyl (OH) is difficult in the measurements that have been acquired thus far, however it is fair to assume that at least some of the variation observed is due to water given the semi-regular delivery of water that occurs when the Moon passes through micrometeoroid streams. After all, every time there is a meteor shower on the Earth, such as the Geminids in December or the Perseids in August, the Moon is experiencing a similar bombardment of these small yet water-rich cosmic debris particles! Given the energetic impact velocities involved, however, only about 1/3rd of the water from these impactors is redeposited on the Moon while the remaining 2/3rds are lost to space.
Using computer simulations, it is possible to model certain aspects of the lunar water cycle, and doing so can provide valuable information about how water molecules move once they are delivered to the surface, how much water is likely to be destroyed or lost to escape during transit as well as patterns in the delivery of water to the permanently shadowed cold traps. Many such simulations have been performed over the years, with the most recent results published on July 4th, 2019 in a paper titled “The temporal and geographic extent of seasonal cold trapping on the Moon”; the authors include myself, John Moores, Jasmeer Sangha, Giang Nguyen and Norbert Schorghofer. As the title suggests, this paper looks at the seasonal component of the lunar water cycle, which had not previously been investigated in lunar volatile studies.
As previously mentioned, the Moon rotates on an axis which is titled ~1.5 degrees with respect to the ecliptic normal. While this obliquity is extremely low by solar system standards (and is low enough to allow for the formation of permanently shadowed regions), it is nonetheless non-zero, meaning that the Moon does in fact experience distinct hemispheric seasons. Over the course of one lunar year, the sub-solar point (the point on a planetary body directly underneath the Sun) will oscillate about the equator by 1.5 degrees. When the sub-solar point is in the northern hemisphere, regions near the north pole will, on average, experience more sunlight given that the Sun is slightly higher above the horizon, while south polar regions will, on average, experience more shadow given that the elevation of the Sun is reduced; the opposite will be true when the sub-solar point is in the southern hemisphere. Thus, throughout the year, the poles experience significant changes in sunlight and shadow, and, because the Moon has a low thermal inertia and lacks an atmosphere, this translates to dramatic changes in seasonal surface temperatures. One important implication of these temperature variations is that it indicates that regions of volatile stability (the cold, shadowed regions) will also fluctuate throughout the year. Our recent paper, in part, sought to quantify and map the seasonal variation in polar shadowed area by using an illumination model.
The image at the top of this article shows the results from our illumination modelling at the north and south pole (latitudes pole-ward of 80 degrees north and south). Here, the white regions correspond to the permanently shadowed regions, while the coloured areas show the locations of “seasonally shadowed regions.” Seasonally shadowed regions (SSRs) are areas of the lunar surface that remain continuously in shadow for at least one lunar day (29.5 Earth days), and are significant as they are expected to have surface temperatures that are low enough to trap migratory water molecules. Some SSRs, such as those near the periphery of PSRs (red areas), remain in shadow for almost the entire lunar year, being illuminated by the Sun for only a short period of time. Others, in contrast, are shadowed for only slightly longer than the typical patch of the lunar surface (which is illuminated and shaded for 50% of the time), and are only briefly able to sequester water. As expected, we found that SSRs reach their maximum expanse at the winter solstice, at which point the SSR area is more than double the PSR area in the respective hemisphere; at the summer solstice, no SSRs are present, and the volatile trapping area reduces only to the PSRs.
To understand the impact of seasonal changes in shadow on the migration and distribution of water, we performed Monte Carlo simulations which model the migration, destruction/escape and cold trapping of water molecules within PSRs and SSRs. The results from these simulations showed that SSRs have a significant influence on the migration of water molecules, and their growth and decay throughout the year gives rise to a seasonal variation in the polar surface concentration of water. Intuitively, one might expect the peak abundance to occur at the winter solstice when the SSRs cover the most ground, however our model shows that the largest concentration should occur near the vernal equinox, or about 2-3 lunar days after the winter solstice. The reason for this has to do with the rate at which SSRs collect water, and the length of time that they are capable of storing that water. When these two factors are taken together, we find that the poles of the Moon are likely to be most enriched with water near the vernal equinox, and deplete with water near the autumnal equinox. PSRs are shown to collect water throughout the year, however they experience seasonal patterns in water deposition which is due to the moderating influence of the SSRs.
Thus far, no spacecraft has been capable of measuring seasonal changes in the concentration of water on the Moon, and therefore our results serve as a type of prediction. As interest in lunar exploration continues to rise, and as robotic missions such as Lunar Flashlight get ready for launch in 2020, much will be learned about the Moon’s polar environment and its volatile content in the coming years, including the significance of seasonally shadowed regions and the seasonality of the lunar water cycle more generally. SSRs may very well prove to be an important part of the story that we some day discover, not only on the Moon, but also on other airless bodies in the solar system with low, but non-zero axial title such as Ceres, where similar processes are likely to take place.
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