In the January 2022 issue of Science Advances, Chinese scientists announced further evidence that the moon may contain water ice.
The evidence came from data collected by the Chinese Chang’e 5 lunar lander, which touched down on the moon’s surface on Dec. 1, 2020. That’s good news for astronomers, who care about such seemingly arcane matters. It should be good news for the rest of us as well, as we shall see.
Water is a molecule composed of two atoms of hydrogen bonded to one atom of oxygen.
Many water-seeking spacecraft have orbited or landed on the moon. Scientists designed many of them to detect the hydrogen in water.
There is good reason to do so. Hydrogen loves to bond with oxygen, and the moon has an abundance of oxygen.
Of course, that oxygen doesn’t exist in its gaseous, breathable form. The moon doesn’t have an atmosphere. Instead, oxygen is bound with other elements to form oxides.
On both the moon and Earth, a common oxide is iron oxide, commonly known as rust. On Earth, another is dihydrogen oxide, commonly known as water.
Chang’e 5’s evidence is not conclusive proof for the water’s lunar presence because one atom of hydrogen will also bond with one atom of oxygen, forming a group of substances called hydroxyls. Alcohols in their various forms are examples of hydroxyls.
Chang’e 5’s data may also indicate the presence of hydroxyls, cousins of water, but decidedly not water.
Humans have sent many orbiters and landers to the moon looking for water, but their tests are all designed to find hydrogen. Such is the difficulty — some would say the fatal flaw — of looking for water from 238,855 miles away.
So why not just bring back some rocks from the moon and subject them to detailed laboratory tests? We’ve done it, and it didn’t work.
Apollo 11, the first crewed lunar mission in 1969, and subsequent Apollo missions, brought back rocks that geologists on Earth analyzed for water. The Russians have done the same using robot spacecraft.
Water was consistently detected in small quantities, but the results were tainted.
Rocks from the moon have to be studied in an extreme vacuum to simulate lunar conditions. If a rock is exposed to even a whiff of earthly air, it will absorb water from it.
For that reason, all the Apollo rocks produced suspect results that geologists discarded as evidence.
Why should you care? Water is, after all, the stuff of life, an essential ingredient for its existence on our planet. Biologists say that life was born in the sea. When it moved onto dry land, it carried its water along with it.
Humans are made up mostly of water. An old “Star Trek” episode refers to members of our species as “bags of water,” and that’s about right. Our skins are designed to hold the water in and keep it from spilling back into the environment from whence it came.
Finding water thus becomes critical for our eventual expansion into the cosmos. We can’t carry very much water with us because it’s too danged heavy and bulky to tote around. If we plan to live on the moon, we’ll have to make it from raw materials or mine it from the lunar soil.
How did the water ice get on the moon in the first place? Astronomers offer three possible explanations.
Scenario 1: Water may be the natural result of the processes that make moons and planets.
But there’s a problem. The moon has no blanket of air like Earth, and it has a much lower gravitational pull than our planet. Add the heat from the sun, and water ice turns into vapor and escapes into space.
Much of the water the moon had early in its formation has by now disappeared into the inky void.
However, at the lunar north and south poles, there are places where the sun never shines. According to data gathered by the Lunar Prospector orbiter, most of the moon’s water is located at the lunar poles, where ice is mixed into moon dirt, called the moon’s “regolith” by astronomers.
Half a century’s worth of lunar data seems to replicate the polar findings. However, a set of Earth-based observations lead to an even more startling conclusion.
In 2020, using the Sophia telescope mounted in a Boeing 747 aircraft, astronomers saw signs of water on sunlit areas of the moon.
Scenario 2: Water came in the form of cometary bombardment.
Like all the larger objects in our solar system, the moon is subject to bombardment by comets, which contain a fair amount of water ice. Those comets are zipping along when they hit the moon. The resultant explosions reduce the comets to dust and bury their ice deep in the lunar soil.
In fact, during the period of heavy cometary bombardment four billion years ago, the moon may have had enough water to form an atmosphere and even lakes of liquid water on its surface.
The moon had recently formed from a collision with a planet-sized object and Earth. As debris from the impact coalesced into the moon, it heated to a liquid ball of rock. It was warm enough to melt the cometary. The resulting gases could have formed an atmosphere.
Astronomers like Dirk Schulze-Maku argue that such life-producing conditions could have existed for 500 million years.
Scenario 3: Chang’e 5’s Chinese scientists and other astronomers argue that the sun produces the moon’s hydroxyls and water.
As the sun explodes, it generates a “solar wind,” a constant stream of mostly hydrogen atoms that strikes every object in the solar system. The hydrogen atoms combine with the oxygen in the lunar soil.
Herein lies a problem. The solar wind also prevents oxygen from binding to elements such as iron to form oxides. Besides, where does the abundance of oxygen come from in the first place?
The answer lies in Earth’s powerful magnetic field, which generates a long tail called its magneto trail. Japan’s Kaguya lunar orbiter discovered in 2007 that the magneto trail grabs oxygen from Earth’s atmosphere and transports it from our planet to the moon.
But what about that pesky solar wind that tends to block the formation of oxides?
The moon’s oxygen binds with other elements to form oxides when Earth’s magneto trail partially blocks the solar wind. That condition occurs once a month around the time when we see a full moon on Earth.
Granted, we’re not talking about much water. Based on the 1998 Lunar Surveyor results, NASA scientists estimated 330 million tons of ice spread out over 25,000 square miles of the lunar surface.
A lake made of all the liquid would cover only four square miles and be 35 feet deep. A cubic foot of lunar dirt might yield a half-gallon of water, and that’s enough to sustain a small colony with drinking water.
Based on more recent spacecraft data, the Planetary Society estimates the total as about 600 billion kilograms of water ice, “enough to fill 24,000 Olympic swimming pools.”
That sounds like a lot, but it makes up only .01 percent of the lunar regolith.
It isn’t nearly enough to satisfy the needs of human colonization. The average human on Earth uses about 100 gallons of water a day for drinking, bathing, and eliminating waste products.
The water would have to be mined at the poles from the lunar regolith and then transported, perhaps at long distances, to lunar bases.
A lunar base would have to recycle every drop of the precious liquid, including, ahem, water excreted from the body. Let’s just say that lunar colonists would be drinking the same glass of water over and over again, if you get my drift.
If lunar explorers recycled carefully, they might have enough left over to use for other purposes.
Water is, of course, made up of hydrogen and oxygen. By breaking up the water into its component elements, we can produce oxygen to breathe. Also, hydrogen and oxygen make powerful rocket fuel when recombined. Gigantic tanks filled with liquid hydrogen and oxygen fed the Space Shuttle’s engines.
Separating hydrogen from oxygen requires a considerable amount of energy. Therefore, any lunar base would require a small nuclear reactor or several square miles of solar panels.
Does the moon have enough water to sustain such lunar bases? To find out, NASA is scheduled to land PRIME-1 at the lunar south pole very near a crater called Shackleton.
The mission will drill deep into the lunar regolith looking for water ice.
NASA chose that particular location because a previous mission, the Lunar Reconnaissance Explorer, determined that the bottom of the Shackleton Crater, deep in shadow, may have 22 percent of its surface covered with water ice.
Eventually, lunar bases may become stopping-off points to more distant places like Mars and beyond. And that is reason enough to be excited about finding water on the moon.
Tom Burns is the former director of the Perkins Observatory in Delaware.