GAM 2018 Blog
- Published: Monday, April 09 2018 09:00
By Joshua Bandfield
In the past decade, there have been a lot of water-on-the-Moon discoveries reported in scientific journals and making their way into the popular news. With new and highly capable spacecraft missions, we have been seeing the Moon in a new light (literally! – given the expansive wavelength coverage of the spacecraft measurements). We are also revisiting samples returned from the Apollo and Luna missions using extraordinarily detailed lab measurements to glean new insights about the Moon. What's remarkable is that as we approach the half-century mark after Apollo, we are still making fundamental discoveries about the Moon, and often using samples collected decades ago.
For years it seemed like we “discovered” water on Mars about once every six months or so, but recently it has been the Moon's turn under this particular spotlight. The reason for all the excitement is that water is so fundamental to many processes that we are so keenly interested in. How did the water get there? That's what we're trying to sort out and there are a couple of possibilities.
One possibility is that the water has always been there. Most of us had thought the Moon was relatively dry, with most water driven off in the aftermath of the giant impact with Earth that is thought to have formed the Moon. However, scientists have recently measured lunar samples with highly sensitive instruments, finding tiny amounts of water trapped in crystals. This indicates the Moon's crust and mantle started out with more water than previously thought. Water plays a big role in how rocks melt and crystallize and, if it is concentrated enough, can also make dramatic explosive volcanoes much more common.
Much higher abundances of ice have been found at the lunar poles, deep within craters where the sun never makes it above the crater rim. These regions are continuously cold (–200°C !), keeping water and other compositions, such as sulfur and organic molecules, stable. We think these deposits have been slowly accumulating throughout much of the history of our solar system. That's one of the reasons they are so interesting; these cold craters serve as a sort of witness plate experiment that has been running for billions of years. How does the water get there? Well...it's complicated, but one way to do it is via comets and asteroids, which vaporize when they impact the Moon. Most of the water escapes to space, but small amounts land on these cold surfaces and are trapped.
Finally, spacecraft measurements show evidence for minuscule amounts of water covering vast regions of the Moon. This water can form from the solar wind, which contains charged hydrogen atoms. When the hydrogen collides with the lunar surface, it breaks and damages the crystals that make up lunar soil and rocks, composed mainly of silicon, oxygen, iron, calcium, and magnesium. Any oxygen that is broken free will quickly find hydrogen to combine with, forming water (and other compositions, such as OH – hydroxyl).
So, depending on where and how we measure it, water on the Moon can tell us very different stories. In one case, it is perhaps a vestige of the Moon's violent origin. In another, it may be a record of comets and asteroids over billions of years. And lastly, it can be forming today from material streaming off our sun and colliding with the lunar surface. It is remarkable how something as seemingly simple as water can point to such a rich and varied set of processes that have shaped the Moon throughout its history.
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Joshua Bandfield is a Senior Research Scientist for Space Science Institute and investigates the compositions and landscapes of planetary bodies throughout the solar system. He uses infrared measurements and images returned from orbiting spacecraft and landers to determine the properties of planetary surfaces. He has been involved with spacecraft planning, operations, and science activities for NASA spacecraft since 1995. Josh is a geologist by training and likes rocks. |
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