How Did We Find Water on the Moon and Where Did It Come From?
The moon is one of the biggest mysteries that astronomers are currently faced with. Though not on the scale as dark matter, dark energy, or early cosmology in terms of scope, it nonetheless has many riddles that have yet to be solved and perhaps may yield surprising science to fields we do not realize. This is because often times the simplest questions have the farest-reaching implications. And the moon has plenty of simple questions yet to be answered. We still are not entirely sure how it formed and what it's full relationship with Earth is. But another mystery that has ties to that formation mystery is where did the water on the moon come from? And is that question related to its formation?
How We Found Out
The whole reason for this discussion starts with Apollo 16. Like previous Apollo missions it brought back lunar samples, but unlike previous missions these ones were rusty upon examination. Scientists at the time including the geologist on Apollo 16 Larry Taylor concluded that the rocks were contaminated by Earth water and that was that, end of story. But a 2003 study found that Apollo 15 and 17 rocks had water in them, bringing the debate back. Evidence from Clementine and the Lunar Prospector probe did offer encouraging hints of water, but no definite findings. Flash forward to October 9, 2009 when the Lunar Crater Observatory and Sensing Satellite (LCROSS) fired a small rocket into the 60 mile-wide Cabeus crater, located near the south pole of the moon. Whatever was in the crater was vaporized by the explosion and a plume of gas and particles was shot into space. LCROSS collected telemetry for four minutes before crashing into that same crater. Upon analysis it showed that up to 5% of the lunar soil was made of water and that the temperatures at the location were near -370o Celsius, helping to secure and preserve the water there by eliminating sublimation effects. Suddenly the Apollo 16 rocks were very interesting - and not a fluke (Grant 59, Barone 14, Kruesi, Zimmerman 50, Arizona).
Oh, if it had only been that easy to put this to bed. But when the Lunar Reconnaissance Orbiter (LRO) (which had been launched with LCROSS) continued to circle the moon and study, it found that while water is on the moon, it is not common. In fact, it found that there is 1 molecule of H20 for every 10,000 particles of lunar soil. This was way less than the concentration found by LCROSS, so what happened? Was the Lunar Exploration Neutron Detector (LEND) instrument sending false readings? (Zimmerman 52)
Maybe it all boils down to how the data was collected, frequently indirectly. Clementine used radio wave which bounced off the moon's surface, then to Earth's Deep Space Network where signal strength was interpreted for signs of water. The Lunar Prospector had a neutron spectrometer which looked at the by-product of cosmic ray collisions, aka neutrons, which lose energy when they hit hydrogen. By measuring the amount that return, scientists could map possible hydrogen beds. In fact, that mission found that concentrations increased the further north/south you went from the equator. However, scientists could not determine that craters were the source during that mission because of a lack of signal resolution. And LEND is built to receive only neutrons form the surface of the moon by having a shield built around the instrument. Some claim the resolution of it was only 12 square meters, which is less than the 900 square centimeters needed to see precise water sources. Others also postulate that just 40% of neutrons get blocked, further damaging any potential findings (Zimmerman 52, 54).
However, another possibility presents itself. What if water levels are higher in craters and lower on the surface? That could explain the differences, but we would need more evidence. In 2009, the Selenological and Engineering Explorer (SELENE) space probe from the Japanese Institute of Space and Astronomical Science examined a lunar crater in detail but found that no H20 ice was present. A year later, the Chandrayaan-1 space probe from India found lunar craters in higher latitudes that reflected radar data consistent with H2O ice or with a rough terrain of a new crater. How can we tell? By comparing the reflection patterns from inside and outside the crater. With water ice, no reflection outside of the crater, which is what Chandrayaan-1 saw. The probe also looked at the Bulliadlus crater, located just 25 degrees of latitude away from the equator, and found that the hydroxyl count was high compared to the area around the crater. This is a signature for magmatic water, another clue to the wet nature of the moon (Zimmerman 53, John Hopkins).
But (surprise!) something might have been wrong with the instrument used by the probe. The Moon Mineralogy Mapper (M3) also happen to find that hydrogen was present everywhere on the surface, even where the sun was shining. That would not be possible for water ice, so what could it be? Tim Livengood, a lunar ice expert from University of Maryland, felt that it pointed to a solar wind source, for that would create hydrogen-bonded molecules after elements impacted at the surface. So, what did this do for the ice situation? With all this evidence and that further LEND findings saw no more ice in several other craters, it looks like LCROSS was simply lucky and happen to hit a local hotspot of water ice. Water is present, but in low concentrations. This view seems boosted when scientists looking at the LRO's Lyman Alpha Mapping Project data found that if a permanently shadowed crater had H20, it was at most 1-2% the mass of the crater, according to a January 7, 2012 article of the Geophysical Research by Randy Gladstone (from the Southwest Research Institute) and his team (Zimmerman 53, Andrews "Shedding").
Further observations with M3 found that certain volcanic features on the moon had water traces in them as well. According to a July 24, 2017 issue of Nature, Ralph Milliken (Brown University) and Shuai Li (University of Hawaii) found evidence that pyroclastic deposits on the moon had traces of water on them. This is interesting because volcanic activity arises from within, implying that the moon's mantle may be more water rich than previously suspected (Klesman "Our")
To uncover the source of the water on the moon, we need to understand where the moon itself came from. The best theory for the formation of the moon is as follows. Over 4 billion years ago, when the solar system was still young, many objects that would become planets were orbiting the sun in various orbits. These protoplanets, or planetesimals, would sometimes collide with one another as the ever-changing gravity of our solar system fluctuated, with the sun and other objects constantly setting off chain-reactions of motion both towards the sun and away. Around this time of mass movement, a Mars-sized planetesimal was pulled in towards the sun and collided with the then-new and somewhat molten Earth. This impact broke off a huge chunk of Earth, and much of the iron from that planetesimal sank into the Earth and settled into its core. That huge section of Earth that broke off and the other, lighter remnants of the planetesimal would eventually cool and become what is known as the moon.
So why is this theory so important in our talk of the source of moon water? One of the ideas is that the water that was on Earth at that time would have been scattered after the impact. Some of that water would have landed on the moon. There is both supporting and negative evidence for this theory. When we look at certaim isotopes, or variants of elements with more neutrons, we see that some ratios of the hydrogen match up with their counterparts in Earth's oceans. But many point out that such an impact that would help transfer water would surely vaporize it. None would have survived to fall back to the moon. But when we look at moon rocks we do see high-levels of water trapped in them.
And then things get weird. Alberto Saal (from Brown University) was taking a closer look at some of those rocks, but different ones from Apollo 16 found at different areas of the moon (specifically, the aforementioned Apollo 15 and 17 rocks). When examining olivine crystals (which form in volcanic materials), hydrogen was spotted. He found that the levels of water in the rock were highest in the center of the rock! This would suggest that the water was trapped inside the rock while it was still in molten form. Magma did get to surface as the moon cooled and its surface cracked, supporting the theory. But until comparisons of water levels are made with other samples of lunar rocks from different locations, no conclusions can be made (Grant 60, Kruesi).
Comets and Asteroids
Another intriguing possibility is debris striking the moon, like comets or asteroids, contained water and deposited it there upon impact. Early in the solar system objects were still settling down and comets would have collided with moon frequently. Upon impact, the material would settle into craters but only those near the poles would be in shadow and cold (-400 degrees Fahrenheit) for a long enough time to remain frozen and intact. Anything else would have sublimated under the constant radiation bombarding the surface. LCROSS seems to have found evidence that supports this model of water distribution, with carbon dioxide, hydrogen sulfide, and methane found in the same plume as the before-mentioned rocket strike. Those chemicals are also found in comets (Grant 60, Williams).
Another theory is an alternative (or possibly in conjunction) with this viewpoint. About 4 billion years ago, a period in the solar system known as the Late Heavy Bombardment Period took place. Much of the inner solar system met up with comets and asteroids that had for some reason been expelled from the outer solar system and directed inward. Many impacts occurred, and Earth was spared from a large portion of it because of the moon taking the brunt of it. Earth has had time and erosion on its side and most evidence for the Bombardment has been lost, but the moon still bears all the scars of the event. So if enough of the debris that hit the moon was water-based, then that could have been a source of water for both the moon and Earth. The main problem with all of this is that those ratios of hydrogen in the moon water do not match those of other known comets.
A possible theory that takes the best from the preceding ones involves the constant particle flow that leaves the Sun all the time: the solar wind. This is a mix of photons and high-energy particles that leave the Sun as it continues to fuse elements together and expels other particles as a result. When the solar wind strikes objects, it can sometimes alter them at the molecular level by imparting energy and matter at just the right levels. So if the solar wind were to hit the moon with enough of a concentration, it could alter some of the material on the surface of the moon into some forms of water, if it was present on the surface either from the Late Bombardment Period or from the Planetesimal Impact.
As mentioned earlier, evidence for this theory has been found by the Chandrayaan-1, Deep Impact (while en transit), Cassini (also while en transit) and Lunar Prospector probes. They have found small but traceable amounts of water all over the surface based on reflected IR readings and those levels fluctuate along with the level of sunlight that the surface receives at the time. Water is created and destroyed on a daily basis, with the hydrogen ions from the solar wind hitting the surface and breaking chemical bonds. Molecular oxygen is one of those chemicals and gets broken up, is released, mixes with the hydrogen, and causes water to form (Grant 60, Barone 14).
Unfortunately, most of the water on the moon resides in the polar regions, where little to no sunlight is ever seen and some of the lowest temperatures ever recorded are. No way the solar wind could get there and make enough of a change. So, like most mysteries that exist in astronomy, this one is far from over. And that's the best part.
Andrews, Bill. "Shedding Light on the Moon's Shadows." Astronomy May 2012: 23. Print.
Arizona, University of. "It's cold and wet at the Moon's south pole." Astronomy.com. Kalmbach Publishing Co., 22 Oct. 2010. Web. 13 Sept. 2018.
Barone, Jennifer. “The Moon Makes a Splash." Discover Dec. 2009: 14. Print.
Grant, Andrew. "New Moon." Discover May 2010: 59, 60. Print.
Klesman, Allison. "Our Moon's Mantle is Wetter Than We Thought." Astronomy Nov. 2017. Print. 12.
Kruesi, Liz. "Identifying the Moon's Water." Astronomy Sept. 2013: 15. Print.
John Hopkins. "Scientists Detect Magmatic Water on the Moon's Surface." Astronomy.com. Kalmbach Publishing Co., 28 Aug. 2013. Web. 16 Oct. 2017.
Williams, Matt. "Scientists Identify the Source of the Moon's Water." universetoday.com. University Today, 01 Jun. 2016. Web. 17 Sept. 2018.
Zimmerman, Robert. "How Much Water Is On the Moon." Astronomy Jan. 2014: 50, 52-54. Print.
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