Micro Cold Traps on the Moon
When our new paper, Micro cold traps on the Moon was published in Nature Astronomy alongside another lunar water-related paper, we had little inkling of the fanfare NASA would unleash. Following a well-publicized press conference on October 26th, the journal’s embargo was lifted – what followed was a veritable media frenzy. I gave roughly two dozen interviews over the course of three days, as did several of my colleagues.
Given the scientific and public interest, I wanted to write about our major findings. This post focuses on the importance of micro cold traps for understanding ice on the Moon and what it means for planetary science and exploration. Although I’ll touch on some connections to the SOFIA observations, I’ll focus primarily on the billions of tiny potential ice reservoirs revealed by our study.
Why is water on the Moon important?
“Follow the water” has recently been NASA’s mantra for exploring the solar system, and with good reason: Life on Earth depends upon its vast holdings of the precious liquid. Yet we know little about where it came from. Comparisons with other rocky planetary bodies could lend insights into the origins of Earth’s water, and its subsequent evolution through time. The Moon is especially important in this regard, because it likely formed from a massive collision with the primordial Earth. Signatures of this violent event remain imprinted in the Moon’s chemical makeup. Sophisticated laboratory analyses have also revealed small amounts of endemic water in the samples returned by the Apollo astronauts. But this is likely ancient water left over from the Moon’s formation roughly 4.5 billion years ago. What about all the water delivered since then?
For more than half a century scientists have suspected that water might exist in abundance at the lunar poles[1]. This intriguing idea was based on the fact that the Moon’s spin axis (i.e., the pole) is oriented almost exactly perpendicular to the direction to the Sun[2]. This ‘special’ configuration can produce shadows that never see the light of day.
Imagine that you are standing at the north pole of the Moon. Over the course of a lunar day (about 29 and a half Earth days, the synodic month), you would see the Sun revolving 360° around the horizon. Now imagine that you stroll into a large crater. During the month, the Sun still revolves around the horizon, but now it is hidden behind the walls of the crater – you’re in perpetual shadow. In fact, the Moon’s spin axis has been stable for so long – probably at least a couple billion years – that we refer to these areas of terra incognita as permanently shadowed regions (PSRs).
Early simulations of water on the Moon suggested that the PSRs could be filled with ice. This result depended sensitively on both the abundance of PSRs and the supply rate of water to the Moon. If the destruction of water (primarily by ultraviolet radiation) outstrips the rate of delivery by comets, meteorites, and the solar wind, then the PSRs would be relatively parched. On the other hand, an imbalance in favor of the supply could lead to massive ice deposits. Although widely viewed with skepticism at the time, radar observations of the planet Mercury revealed exactly the type of ice deposit predicted by the models. However, similar measurements of the Moon were more ambiguous.
What did we know about water on the Moon before this study?
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What are "micro cold traps"?
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How much water could be there?
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What are the next steps?
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