Will future astronauts fill their water bottles with moon water? Maybe so after astronomers used the airborne SOFIA telescope to make an amazing discovery.
NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA) is a special Boeing 747 aircraft modified to carry 106-inch reflecting telescope up into the stratosphere above 99 percent of the water vapor in Earth's atmosphere. Water vapor blocks infrared light (light emitted by warm objects like your body or oven). From this high perch, astronomers can study objects such as the Milky Way's supermassive black hole, comets and star-forming regions in a whole new light.
Also, because SOFIA can fly anywhere in the world, including over the middle of the ocean, scientists can observe a variety of celestial events that are only visible from specific locations. For instance, its predecessor, the Kuiper Airborne Observatory, discovered Uranus's rings when the planet passed in front of a star in 1977 — an event only visible over the Indian Ocean.
This week a team of astronomers confirmed the presence of water on the sunlit surface of the moon from observations made with SOFIA. We used to think of the moon as bone dry. But in recent years spacecraft have found water ice within permanently shadowed craters at the lunar poles. Here, bitter cold and deep shade preserve it for eons.
Unfortunately, it hasn't been easy to tell water apart from its close chemical relative, hydroxyl (written as OH) using remote sensing from orbiting spacecraft. Hydroxyl is composed of a single hydrogen and oxygen atom; H2O has two hydrogens and one oxygen.
“Prior to the SOFIA observations, we knew there was some kind of hydration,” said Casey Honniball, the lead author of the results. “But we didn’t know how much, if any, was actually water molecules – like we drink every day – or something more like drain cleaner.”
Unlike some previous efforts, SOFIA definitively detected the specific wavelength of light absorbed by water in sunlight reflecting off the moon. Based on the signal's strength, H2O exists in concentrations of 100 to 412 parts per million, roughly equal to 12 ounces of water spread across a cubic meter (a cube about 3 feet on a side) of soil. For comparison, the Sahara desert has 100 times the amount of water than what SOFIA found.
What got me excited is where SOFIA found the water — inside Clavius, one of the near-side's largest and most iconic craters. Located in the moon’s southern hemisphere Clavius is 145 miles (231 km) across and easily visible in a small telescope just south of another prominent lunar crater, Tycho. And as good fortune would have it, both are in view the next few nights, Oct. 27-30.
How did it get there, and how does it persist under the harsh, airless conditions on the moon? Water in any form should just evaporate away. Key to its delivery and preservation may be tiny grains of comet and asteroid dust called micrometeorites. With no atmosphere to stop them, micrometeorites rain down on the moon continuously. Some carry small amounts of water that they would deposit on impact.
Or a two-step process may be at play, where the solar wind — the stream of protons and electrons that wafts from the sun — implants hydrogen atoms on the lunar surface which then chemically react with oxygen-bearing minerals to make hydroxyl. Heat released from micrometeorite impacts might convert the OH to water. No one's sure how the water is stored. It could be in beadlike structures that form in the heat generated from the tiny impacts or simply hidden between grains of lunar soil.
Water is a precious resource especially if you're planning on returning to the moon. And we are! Under the Artemis program NASA hopes to land the first woman and the next man on the moon in 2024. Hauling the heavy stuff there could be avoided if we could extract it on the spot instead.
Sometimes scientific discovery is serendipitous. Normally, SOFIA astronomers wouldn't target a big, bright object like the moon for observation, but in August 2018 they wanted to test the camera's ability to do it anyway. That's when the team made their discovery. You can read more about it by downloading the freely available paper from Nature Astronomy. SOFIA’s follow-up flights will look for water in more sunny places and during different lunar phases to better understand how extensive it is and how it's produced and stored.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.