Ancient magnetic fields on the Moon could be protecting precious ice

Jan 27, 2020
Fields in permanently shadowed craters may shield ice from solar wind



moon-shadowed-craters shackleton.jpeg
The Moon’s Shackleton crater, visualized with data from the Lunar Reconnaissance Orbiter, may hold ice because it never sees the Sun. NASA/GODDARD SPACE FLIGHT CENTER

For years, scientists have believed frigid craters at the Moon’s poles hold water ice, which would be both a scientific boon and a potential resource for human missions. Now, researchers have discovered a reason why the ice has persisted on an otherwise bone-dry world: Some polar craters may be protected by ancient magnetic fields.

“It’s really exciting,” says Jim Green, NASA’s former chief scientist, who was not involved with the work. “It makes these areas even more fascinating.”

Hundreds of polar craters are in permanent shadow because of the Moon’s small tilt to the Sun, 1.5° compared with Earth’s 23.4°. The Sun never rises above their rims, keeping temperatures as low as –250°C. In some of the pits, radar instruments on orbiting spacecraft have detected the reflective signature of water ice, perhaps delivered by comet impacts. And in 2018, scientists using an instrument aboard India’s Chandrayaan-1 spacecraft reported measurements showing how molecules of polar ice absorbed infrared light—some of the most definitive evidence yet.

Explaining the ice’s survival has been a challenge, however. Although sunlight doesn’t reach the craters, the solar wind does, and these charged particles can destroy the ice, molecule by molecule, in a process called sputtering. “It’s highly erosive,” says Paul Lucey, a planetary scientist at the University of Hawaii, Manoa, who was also not involved with the work. “The ice would be gone in a few million years.”

Planetary scientist Lon Hood and his colleagues at the University of Arizona now think they know why the ice sticks around. In research presented last week at the Lunar and Planetary Science Conference in Houston, they showed that magnetic anomalies, remnants from the Moon’s ancient past, may be protecting some of these craters. “These anomalies can deflect the solar wind,” Hood says. “We think they could be quite significant in shielding the permanently shadowed regions.”

Researchers have known about the anomalies ever since the Apollo 15 and 16 missions in 1971 and 1972, when astronauts measured regions of unusual magnetic strength on the surface. Some anomalies are now known to be up to hundreds of kilometers across. Although their origin is debated, one possibility is they were created more than 4 billion years ago when the Moon had a magnetic field and iron-rich asteroids crashed into its surface. The resultant molten material may have been permanently magnetized.

Thousands of the anomalies are thought to exist across the lunar surface, but Hood mapped ones at the south pole in detail using data from Japan’s Kaguya spacecraft, which orbited the Moon from 2007 to 2009. He found at least two permanently shadowed craters that were overlapped by these anomalies, the Sverdrup and Shoemaker craters, and there are likely more. Although the remnant fields are thousands of times weaker than Earth’s, they could be sufficient to deflect the solar wind.

Craters with known anomalies could become prime targets for science and exploration. NASA is already planning to visit the south polar region with a rover due for launch next year, called VIPER, and the agency intends to send humans there later this decade as part of its Artemis program. Studying the ice could reveal how it was delivered, which may in turn shed light on how Earth got its water.

More data are needed to confirm the fields’ protective effect. Hood would like to put a solar wind instrument on the surface, which could measure the charged particles that pass the rim of the crater. “You would also need to collect samples and identify what is magnetized,” he says.

Currently no such mission is planned. But given the renewed focus on the Moon for many space agencies, Lucey thinks the mystery of the icy craters needs to be studied. “Why are some places icy and some not icy?” Lucey asks. “These magnetic fields need to be investigated as a possible explanation.”





For decades after the Apollo missions, the Moon was believed to be a dry and hostile place. But over the intervening decades, observations have indicated that its polar regions host water ice.

At the Moon’s southern latitudes, individual water molecules are found in abundances of 100 to 400 parts per million. This is four times larger than what has been measured in lunar soil samples in the lab – further highlighting the gaps in our understanding of water on the Moon.

The University of Colorado Boulder, has attempted to quantify how abundant water ice might be on the Moon. The team used data from NASA’s Lunar Reconnaissance Orbiter (LRO) to investigate “cold traps”: permanently shadowed holes peppered across the Moon’s surface, largely at the poles, which may not have seen the Sun for billions of years. The Moon could harbor thousands of square kilometers of these permanently shadowed holes, ranging in size from a centimetre in diameter to a kilometer across – each with the potential to harbor water ice.

Many unmapped cold traps exist on small spatial scales, substantially augmenting the areas where ice may accumulate. Using theoretical models and refined data from the Lunar Reconnaissance Orbiter, it is estimated that the contribution of shadows on scales from 1 km to 1 cm, is the smallest distance over which cold-trapping could be effective for water ice. Approximately 10–20% of the permanent cold-trap area for water is found to be contained in these micro cold traps, which are the most numerous cold traps on the Moon. Consideration of all spatial scales therefore substantially increases the number of cold traps over previous estimates, for a total area of ~40,000 km2, about 60% of which is in the south. A majority of cold traps for water ice is found at latitudes > 80° because permanent shadows of 80° towards the equator are typically too warm to support ice accumulation. Results suggest that water trapped at the lunar poles may be more widely distributed and accessible as a resource for future missions than previously thought.

The fact that the Moon’s water ice may be distributed across its surface in penny packets seems to support the idea that water was formed when micro-meteorites slammed into the surface, converting hydroxyl into molecular water. This potential reservoir of water has astronomers and space scientists, alike, excited about future Moon missions, as it may influence NASA's growing outline for lunar exploration. This includes making the Moon the primary construction and transportation hub for later missions to the outer planets due to its water, building materials and low gravity.

The lunar surface rock has already been disaggregated by impact into a chaotic upper surface layer called regolith. Regolith is basically ground-up bedrock; impacts of all sizes constantly pummel the surface, breaking, fracturing and grinding up the Moon’s bedrock. Impact both breaks up and creates rock. An impact will destroy a rock both by shock (catastrophic rupture) and through cratering (fragmentation and excavation). The effect of such regolith destruction is to make “soil,” fine-grained rocky material made up of the mineral grains of the bedrock. But impact also creates heat and this heat can weld small fragments into glass-rich aggregate rocks (regolith breccias) as well as quickly cooled fragments of melt that contain mineral inclusions (agglutinates, or glass). In broad terms, impacts destroy and disaggregate more than they create and weld together. Thus, on a given surface, regolith thickness increases with time – older surfaces have thicker regoliths.

The ground up regolith is a readily available building material for construction on the lunar surface. It is an aggregate in the same construction sense as on Earth, but with some significant differences. We could make lime and water from the surface materials of the Moon but it is very time and energy intensive. Thus, we must adapt and modify terrestrial practice to take advantage of the unique nature of lunar materials. It can be sintered* into bricks and blocks, as well as roads and landing pads, using thermal energy (passive solar, concentrated by focusing mirrors) or microwaves that can melt the grains into a hard, durable ceramic.

The use of aggregate materials on the Moon will likely be gradual and incremental. Our initial presence on the Moon will be supported almost entirely by materials and supplies brought from Earth. As we gain facility using lunar resources, we can incorporate more and more local materials into structures. Simple, unmodified bulk soil is an early useful product. It can be used to build berms to protect an outpost from the rocket blast of arriving or departing spacecraft and to cover surface assets for thermal and radiation protection. The next phase will be to pave roads and pads to keep down randomly thrown dust and provide good traction for the multitude of wheeled vehicles supporting the outpost. Fabrication of bricks from regolith will allow us to construct large buildings, initially consisting of open, unpressurized workspaces and garages but ultimately, habitats and laboratories. Making glass by melting regolith can produce building materials of extreme strength and durability; anhydrous glass is stronger than alloy steel at a fraction of the weight.

Eventually, we may be able to export these lunar building materials into space. A major drawback is the gravity well of the Moon – its escape velocity is about 2.38 km/s, smaller than that of the Earth but still substantial. To use large quantities of lunar materials for space construction, we need to develop an inexpensive means to get material off its surface. Fortunately, the small size and no atmosphere of the Moon make this possible by literally throwing stuff off the Moon into space. A “mass driver ” can launch objects off the lunar surface by accelerating them along a rail track using electromagnetic coils that hurl capsulated material into space at specific velocities and directions. We can collect such thrown material at a convenient location, such as one of the libration points. From there, it is a relatively simple matter to send the material to wherever it is needed in cislunar space.

* sinter - [verb] to cause to become a coherent mass by heating without melting.



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