Lunar Reconnaissance Orbiter Spies Movement of Shadows Near the Moon’s South Pole

At the Moon’s North and South Poles, the Sun is never more than 1.5° above or below the horizon. The resulting pattern of daylight and shadows is unlike anywhere else on the Moon — or the Earth. After zooming in on a small lunar highland area near the South Pole, this visualization recreates the illumination conditions there over a period of two lunar days, equal to two months on Earth. This close to the pole, the Sun doesn’t rise and set. Instead, as the Moon rotates on its axis, the Sun skims the horizon, traveling a full 360 degrees around the terrain. Mountains as far as 75 miles (120 kilometers) away cast shadows across the landscape. With the Sun at such a low angle, it can never reach the floors of some deep craters. Places the Sun never reaches are known as permanently shadowed regions. They are the locations of some of the coldest spots in the solar system, and because of that, they trap volatile chemicals, including water ice, that would immediately sublimate (transform directly from a solid to a gas) in the harsh, airless sunshine that falls in most other places on the Moon.

The Sun appears to travel in a circle at the Earth’s poles, too, but it also travels through a range of altitudes. From spring equinox to summer solstice, for example, the Sun is climbing higher in the sky, reaching an altitude of 23.4°. It only hugs the horizon for a few days around the equinoxes. At the Moon’s poles, the Sun is always near the horizon, and the shadows are perpetually long, sweeping across the surface with the changing solar azimuth.

Research reveals possibly active tectonic system on the moon

Researchers have discovered a system of ridges spread across the nearside of the Moon topped with freshly exposed boulders. The ridges could be evidence of active lunar tectonic processes, the researchers say, possibly the echo of a long-ago impact that nearly tore the Moon apart. "There's this assumption that the Moon is long dead, but we keep finding that that's not the case," said Peter Schultz, a professor in Brown University's Department of Earth, Environmental and Planetary Sciences and co-author of the research, which is published in the journal Geology. "From this paper it appears that the Moon may still be creaking and cracking - potentially in the present day - and we can see the evidence on these ridges." Most of the Moon's surface is covered by regolith, a powdery blanket of ground-up rock created by the constant bombardment of tiny meteorites and other impactors. Areas free of regolith where the Moon's bedrock is exposed are vanishingly rare. But Adomas Valantinas, a graduate student at the University of Bern who led the research while a visiting scholar at Brown, used data from NASA's Lunar Reconnaissance Orbiter (LRO) to spot strange bare spots within and surrounding the lunar maria, the large dark patches on the Moon's nearside. "Exposed blocks on the surface have a relatively short lifetime because the regolith buildup is happening constantly," Schultz said. "So when we see them, there needs to be some explanation for how and why they were exposed in certain locations."

For the study, Valantinas used the LRO's Diviner instrument, which measures the temperature of the lunar surface. Just as concrete-covered cities on Earth retain more heat than the countryside, exposed bedrock and blocky surfaces on the Moon stays warmer through the lunar night than regolith-covered surfaces. Using nighttime observations from Diviner, Valantinas turned up more than 500 patches of exposed bedrock on narrow ridges following a pattern across the lunar nearside maria.

A few ridges topped with exposed bedrock had been seen before, Schultz says. But those ridges were on the edges of ancient lava-filled impact basins and could be explained by continued sagging in response to weight caused by the lava fill.

But this new study discovered that the most active ridges are related to a mysterious system of tectonic features (ridges and faults) on the lunar nearside, unrelated to both lava-filled basins and other young faults that crisscross the highlands.

"The distribution that we found here begs for a different explanation," Schultz said.

Valantinas and Schultz mapped out all of the exposures revealed in the Diviner data and found an interesting correlation. In 2014, NASA's GRAIL mission found a network of ancient cracks in the Moon's crust. Those cracks became channels through which magma flowed to the Moon's surface to form deep intrusions. Valantinas and Schultz showed that the blocky ridges seemed to line up just about perfectly with the deep intrusions revealed by GRAIL.

"It's almost a one-to-one correlation," Schultz said. "That makes us think that what we're seeing is an ongoing process driven by things happening in the Moon's interior."

Schultz and Valantinas suggest that the ridges above these ancient intrusions arestill heaving upward. The upward movement breaks the surface and enables regolith to drain into cracks and voids, leaving the blocks exposed. Because bare spots on the Moon get covered over fairly quickly, this cracking must be quite recent, possibly even ongoing today. They refer to what they've found as ANTS, for Active Nearside Tectonic System.

The researchers believe that the ANTS was actually set in motion billions of years ago with a giant impact on the Moon's farside. In previous studies, Schultz and a co-worker proposed this impact, which formed the 1500-mile South Pole Aitken Basin, shattered the interior on the opposite side, the nearside facing the Earth.

Magma then filled these cracks and controlled the pattern of dikes detected in the GRAIL mission. The blocky ridges comprising the ANTS now trace the continuing adjustments along these ancient weaknesses.

"This looks like the ridges responded to something that happened 4.3 billion years ago," Schultz said. "Giant impacts have long lasting effects. The Moon has a long memory. What we're seeing on the surface today is testimony to its long memory and secrets it still holds."

Beresheet Impact Site Spotted

The photo above shows the landing site of the Israeli Beresheet spacecraft on a region of the Moon called Sea of Serenity, or Mare Serenitatis in Latin. On April 11, 2019, SpaceIL, a non-profit organization, attempted to land its spacecraft in this ancient volcanic field on the nearside of the Moon. After a smooth initial descent, Beresheet made a hard landing on the surface. As soon as its orbit placed NASA's Lunar Reconnaissance Orbiter (LRO) over the landing site on April 22, 2019, LRO imaged Beresheet's impact site. The LRO Camera (LROC) consists of three imagers: a seven-color Wide Angle Camera (WAC) and two black-and-white Narrow Angle Cameras (NAC) mounted on the LRO, which has been studying the Moon from orbit for a decade. NAC captured the Beresheet impact photo. LROC took this image from 56 miles (90 kilometers) above the surface. The cameras captured a dark smudge, about 10 meters wide, that indicates the point of impact. The dark tone suggests a surface roughened by the hard landing, which is less reflective than a clean, smooth surface. From so far away, LROC could not detect whether Beresheet formed a surface crater upon impact. It's possible the crater is just too small to show up in photos. Another possibility is that Beresheet formed a small indent instead of a crater, given its low angle of approach (around 8.4 degrees relative to the surface), light mass (compared to a dense meteoroid of the same size), and low velocity (again, relative to a meteoroid of the same size; Beresheet's speed was still faster than most speeding bullets).

The light halo around the smudge could have formed from gas associated with the impact or from fine soil particles blown outward during Beresheet's descent, which smoothed out the soil around the landing site, making it highly reflective.

There are many clues that we're actually looking at a man-made crater instead of a meteoroid-caused one. This is an important consideration, since the Moon, having no atmosphere, is constantly bombarded by space rocks that leave craters.

Most importantly, we knew the coordinates of the landing site within a few miles thanks to radio tracking of Beresheet, and we have 11 "before" images of the area, spanning a decade, and three "after" images. In all of these images, including one taken 16 days before the landing, we saw only one new feature of the size Beresheet would have created.

Existing mathematical models helped us estimate the size and shape of the crater that would have formed if an object of Beresheet's mass and velocity struck the surface. We also referenced craters created by similar-size spacecraft (GRAIL, LADEE, Ranger) that have struck the Moon at about the same speed, and we saw that the white tail stretching from the landing halo towards the south is a shape that's consistent with Beresheet's southward descent trajectory and angle of approach.

For the before image above, we used a photo from December 16, 2016. This is because the lighting conditions that day, based on the angle at which the Sun would have illuminated the Moon at that particular time in its orbit, were the most similar to the April 22 image.

Because LRO was beyond the horizon during Beresheet's descent and landing, it couldn't capture a photo until its orbit brought it nearby 11 days later. LRO passes over the lunar poles with each revolution. Meanwhile, the Moon rotates on its axis below the spacecraft, allowing LRO to pass over every part of the Moon twice a month (once during lunar night and once during lunar day). LROC may take more images of the landing site when it passes the same area again on May 19.

Efforts are ongoing to bounce laser pulses from the Lunar Orbiter Laser Altimeter, also on board LRO, to measure the return from the Laser Retroreflector Array of small corner cube mirrors. This instrument was provided by NASA's Goddard Space Flight Center and was installed on the top deck of the Beresheet spacecraft. Attempts are ongoing to examine if the retroreflector may have survived the impact.

SwRI scientist helps characterize water on lunar surface

A Southwest Research Institute scientist with expertise in how water reacts with lunar soil contributed to a new study that indicates water and/or hydroxyl may be more prevalent on the Moon's surface than previously thought."Water on the Moon is of intense interest for many reasons," said SwRI's Dr. Michael Poston, a coauthor of the paper, "Widespread Distribution of OH/ H2O on the Lunar Surface Inferred from Spectral Data," published in Nature Geoscience online. Water has been the focus of many lunar missions, largely because it is a critical resource for a Moon habitat. "When you split water molecules, you end up with oxygen and hydrogen, critical components for breathable air and rocket fuel. Hydroxyl (OH) is a more reactive relative to water and not as attractive as water in terms of supporting a lunar station." Up until the last decade or so, scientists thought negligible amounts of water were present on the Moon, perhaps existing mainly as ice in the permanently shaded craters near the poles. However, in 2009, NASA's Moon Minerology Mapper spectrometer onboard India's Chandrayaan-1 spacecraft and two other NASA flight spectrometers detected signatures interpreted as water in sunlight reflected from the Moon's surface.

These data, along with measurements from NASA's Lunar Reconnaissance Orbiter (LRO), also indicated the amount of water on the surface could vary diurnally ? a cycle based on the time of day ? and be more common at higher latitudes. One of these measurements came from the Lyman Alpha Mapping Project (LAMP), built and managed by SwRI.

"This research is a great example of how multiple instruments and investigations allow you to do more than what you could do with one instrument alone," added the paper's lead author Dr. Josh Bandfield, of the Space Science Institute in Boulder, Colorado. "As new data about the temperature environment on the Moon emerged, it became apparent that we needed to recalibrate our spectral datasets."

The Moon produces a mixture of reflected and emitted light. It glows in the infrared when heated by the Sun, and its surface reflects infrared light from the Sun. Using detailed surface temperature maps from the Diviner Lunar Radiometer Experiment on LRO and thermophysics modeling, Bandfield could subtract the right amount of Moon glow from the reflected sunlight to better characterize the inferred measurements of water.

Poston has conducted extensive experiments with water and lunar samples collected by the Apollo missions that revealed the amount of energy needed to remove water molecules from lunar rock. This helps scientists understand how tightly water is bound to surface materials.

Based on the team's results, it appears that OH/H2O is present on lunar surfaces under much more wide-ranging conditions than previously understood. The OH/H2O, either in a steady state or actually static, does not appear to be daily migrating about the lunar surface in significant quantities. This limits the amount of OH/H2O that would land in dark polar craters and be trapped there for millennia.

"The next step is to determine whether it's water, hydroxyl, or a mixture of the two - and where it came from," Poston said. "Is it from external sources, delivered by comet or asteroid impacts? Is it from internal processes on the Moon itself, such as ancient volcanism? Or could it be an ongoing process of the solar wind reacting with lunar materials to create OH or HH2O?"

"Some of these scientific problems are very, very difficult, and it's only by drawing on multiple resources from different missions that we are able to hone in on an answer," said LRO project scientist John Keller of NASA's Goddard Space Flight Center in Greenbelt, Maryland.