Our Moon has yielded some long-held secrets in recent weeks, from possible evidence of ancient life on Venus to NASA’s new discovery of large deposits of water.
Fragments of Ancient Venus Life?
Research by Yale astronomers suggests that our Moon –formed bout 4.51 billion years ago from debris left over after a giant impact between Earth and a Mars-sized body called Theia– may harbor clues that our nearest planetary neighbor, Venus, may have had an Earth-like environment billions of years ago, with water and a thin atmosphere. “Pieces of Venus — perhaps billions of them — are likely to have crashed on the moon,” suggest astronomers Sam Cabot and Greg Laughlin, as asteroids and comets slammed into Venus over the eons dislodging as many as 10 billion rocks and sent them into an orbit that intersected with Earth and Earth’s moon.
Tiny Shadows of Eternal Darkness
A new discovery announced today by NASA revealed that observations by the SOFIA telescope and Lunar Reconnaissance Orbiter showed signs of water in the sun-baked lunar soil, as well as in small, dark craters that go beyond past discoveries of significant deposits of water at the large, permanently shadowed craters at its poles. The new discovery that water may be distributed across the lunar surface, and not limited to cold, shadowed places sets the stage for a manned radiation-hardened moon base –where, as our Blue Planet proves, there is water, there can be life.
“If you can imagine standing on the surface of the moon near one of its poles, you would see shadows all over the place,” said Paul Hayne, assistant professor in the Laboratory of Atmospheric and Space Physics at the University of Colorado, Boulder. “Many of those tiny shadows could be full of ice.” Hayne and his colleagues explored phenomena of hidden pockets of water on the moon called “cold traps”–shadowy regions of the surface that exist in a state of eternal darkness.
Not a Single Ray of Sunlight for Billions of Years
Many have gone without a single ray of sunlight for potentially billions of years, reports Hayne. “And these nooks and crannies may be a lot more numerous than previous data suggest. Drawing on detailed data from NASA’s Lunar Reconnaissance Orbiter, the researchers estimate that the moon could harbor roughly 15,000 square miles of permanent shadows in various shapes and sizes–reservoirs that, according to theory, might also be capable of preserving water via ice.”
To understand cold traps, suggests Hayne, imagine taking a trip to Shackleton Crater near the moon’s south pole.” This humungous impact crater reaches several miles deep and stretches about 13 miles across. Because of the moon’s position in relation to the sun, much of the crater’s interior is permanently in shadow–a complete lack of direct sunlight that causes temperatures inside to hover at around minus 300 degrees Fahrenheit.
“You look down into Shackleton Crater or Shoemaker Crater, you’re looking into this vast, dark inaccessible region,” Hayne said. “It’s very forbidding.”
That forbidding nature, however, Hayne points out, may also be key to these craters’ importance for planned lunar bases. Scientists have long believed that such cold traps could be ideal environments for hosting ice–a valuable resource that is scarce on the moon but is occasionally delivered in large quantities when water-rich comets or asteroids crash down.
“The temperatures are so low in cold traps that ice would behave like a rock,” Hayne observed. “If water gets in there, it’s not going anywhere for a billion years.”
SOFIA Detects Water Molecules at Clavius Crater
“Prior to the SOFIA observations, we knew there was some kind of hydration. But we didn’t know how much, if any, was actually water molecules – like we drink every day – or something more like drain cleaner,” said Casey Honniball, a NASA Postdoctoral Program fellow at the agency’s Goddard Space Flight Center and a researcher at the University of Hawaii at Manoa, and the lead author of the study, Molecular water detected on the sunlit Moon by SOFIA, published in Nature Astronomy, who used infrared instruments onboard SOFIA to study the sunlit lunar surface.
The observations, which spanned a mere 10 minutes, focused on a region at high southern latitudes near the moon’s large crater, Clavius, and they revealed a strong infrared emission at a wavelength of six microns (µm) from the crater and the surrounding landscape. Warmed by the sun, something on the lunar surface was reemitting the absorbed radiation just as molecular water—plain H2O—would.
NASA revealed that SOFIA detected water molecules (H2O) in Clavius Crater (shown below along with an image of NASA’s Stratospheric Observatory for Infrared Astronomy –SOFIA), one of the largest craters visible from Earth, located in the Moon’s southern hemisphere. Previous observations of the Moon’s surface detected some form of hydrogen, but were unable to distinguish between water and its close chemical relative, hydroxyl (OH). Data from this location reveal water in concentrations of 100 to 412 parts per million – roughly equivalent to a 12-ounce bottle of water – trapped in a cubic meter of soil spread across the lunar surface.
H2O on Sunlit Side
“We had indications that H2O – the familiar water we know – might be present on the sunlit side of the Moon,” said Paul Hertz, director of the Astrophysics Division in the Science Mission Directorate at NASA Headquarters in Washington. “Now we know it is there. This discovery challenges our understanding of the lunar surface and raises intriguing questions about resources relevant for deep space exploration.”
As a comparison, the Sahara desert has 100 times the amount of water than what SOFIA detected in the lunar soil. Despite the small amounts, the discovery raises new questions about how water is created and how it persists on the harsh, airless lunar surface.
Under NASA’s Artemis program, the agency is eager to learn all it can about the presence of water on the Moon in advance of sending the first woman and next man to the lunar surface in 2024 and establishing a sustainable human presence there by the end of the decade.
Apollo Missions’ Dry Moon –H2O or OH?
SOFIA’s results build on years of previous research examining the presence of water on the Moon. When the Apollo astronauts first returned from the Moon in 1969, it was thought to be completely dry. Orbital and impactor missions over the past 20 years, such as NASA’s Lunar Crater Observation and Sensing Satellite, confirmed ice in permanently shadowed craters around the Moon’s poles. Meanwhile, several spacecraft – including the Cassini mission and Deep Impact comet mission, as well as the Indian Space Research Organization’s Chandrayaan-1 mission – and NASA’s ground-based Infrared Telescope Facility, looked broadly across the lunar surface and found evidence of hydration in sunnier regions. Yet those missions were unable to definitively distinguish the form in which it was present – either H2O or OH.
Scientists using NASA’s telescope on an airplane, the Stratospheric Observatory for Infrared Astronomy, discovered water on a sunlit surface of the Moon for the first time. SOFIA is a modified Boeing 747SP aircraft that allows astronomers to study the solar system and beyond in ways that are not possible with ground-based telescopes. Molecular water, H2O, was found in Clavius Crater, one of the largest craters visible from Earth in the Moon’s southern hemisphere. This discovery indicates that water may be distributed across the lunar surface, and not limited to cold, shadowed places.
SOFIA’s Infrared Zooms in on Clavius Crater
SOFIA offered a new means of looking at the Moon. Flying at altitudes of up to 45,000 feet, this modified Boeing 747SP jetliner with a 106-inch diameter telescope reaches above 99% of the water vapor in Earth’s atmosphere to get a clearer view of the infrared universe. Using its Faint Object infraRed CAmera for the SOFIA Telescope (FORCAST), SOFIA was able to pick up the specific wavelength unique to water molecules, at 6.1 microns, and discovered a relatively surprising concentration in sunny Clavius Crater.
“Without a thick atmosphere, water on the sunlit lunar surface should just be lost to space,” said Honniball, who is now a postdoctoral fellow at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Yet somehow we’re seeing it. Something is generating the water, and something must be trapping it there.”
Several Forces at Play
Several forces could be at play in the delivery or creation of this water. Micrometeorites raining down on the lunar surface, carrying small amounts of water, could deposit the water on the lunar surface upon impact. Another possibility is there could be a two-step process whereby the Sun’s solar wind delivers hydrogen to the lunar surface and causes a chemical reaction with oxygen-bearing minerals in the soil to create hydroxyl. Meanwhile, radiation from the bombardment of micrometeorites could be transforming that hydroxyl into water.
How the water then gets stored – making it possible to accumulate – also raises some intriguing questions. The water could be trapped into tiny beadlike structures in the soil that form out of the high heat created by micrometeorite impacts. Another possibility is that the water could be hidden between grains of lunar soil and sheltered from the sunlight – potentially making it a bit more accessible than water trapped in beadlike structures.
“Accidental Discovery” –A Test Observation
For a mission designed to look at distant, dim objects such as black holes, star clusters, and galaxies, SOFIA’s spotlight on Earth’s nearest and brightest neighbor was a departure from business as usual. The telescope operators typically use a guide camera to track stars, keeping the telescope locked steadily on its observing target. But the Moon is so close and bright that it fills the guide camera’s entire field of view. With no stars visible, it was unclear if the telescope could reliably track the Moon. To determine this, in August 2018, the operators decided to try a test observation.
“It was, in fact, the first time SOFIA has looked at the Moon, and we weren’t even completely sure if we would get reliable data, but questions about the Moon’s water compelled us to try,” said Naseem Rangwala, SOFIA’s project scientist at NASA’s Ames Research Center in California’s Silicon Valley. “It’s incredible that this discovery came out of what was essentially a test, and now that we know we can do this, we’re planning more flights to do more observations.”
SOFIA’s follow-up flights will look for water in additional sunlit locations and during different lunar phases to learn more about how the water is produced, stored, and moved across the Moon. The data will add to the work of future Moon missions, such as NASA’s Volatiles Investigating Polar Exploration Rover (VIPER), to create the first water resource maps of the Moon for future human space exploration.
In the same issue of Nature Astronomy, scientists have published a paper using theoretical models and NASA’s Lunar Reconnaissance Orbiter data, pointing out that water could be trapped in small shadows, where temperatures stay below freezing, across more of the Moon than currently expected. The results can be found here.
“Water is a valuable resource, for both scientific purposes and for use by our explorers,” said Jacob Bleacher, chief exploration scientist for NASA’s Human Exploration and Operations Mission Directorate. “If we can use the resources at the Moon, then we can carry less water and more equipment to help enable new scientific discoveries.”
Image at top of page: a multi-temporal illumination map of the lunar south pole, Shackleton crater (19 km diameter) is in the center, the south pole is located approximately at 9 o’clock on its rim. The map was created from images from the camera aboard the Lunar Reconnaissance Orbiter. The South Pole is also a good target for a future human landing because robotically, it’s the most thoroughly investigated region on the Moon. (NASA/GSFC/Arizona State University).
“We know the South Pole region contain water ice and may be rich in other resources based on our observations from orbit, but, otherwise, it’s a completely unexplored world,” said Steven Clarke, deputy associate administrator of the Science Mission Directorate at NASA Headquarters in Washington. “The South Pole is far from the Apollo landing sites clustered around the equator, so it will offer us a new challenge and a new environment to explore as we build our capabilities to travel farther into space.”