A Clever Way to Map the Moon’s Surface—Using Shadows

As early as 2025, NASA’s astronauts will be back on the moon. It will be the first return since the 1970s, and the first time humans will explore the moon’s south polar region. What they find there could change the course of lunar exploration.

They will be investigating areas inside deep craters where the sun never rises above the surrounding walls. In these permanently shadowed regions, frigid temperatures may have persisted long enough to have trapped water, frozen below the surface. Such ice could potentially be used as drinking water and as a source of fuel, helping future explorers spend longer periods on the lunar surface.

But before any of this can happen, NASA needs to select a safe landing site with navigable routes to these potential water deposits. It has drawn up a short list of places to touch down, using high-resolution models of the lunar surface. Now, there is a new tool that could help determine which is best.

Researchers have developed an additional, novel way of creating 3D maps of the moon’s surface that could offer increased assurance of the actual terrain that explorers and rovers will encounter. The approach is rooted in a technique that has been used for approximately 50 years: using shadows to reveal the topography of the moon’s surface, such as changes in elevation within craters or the steepness of slopes. “It’s natural for our eyes to see the shapes and forms of objects when we look at shadows,” says Iris Fernandes, a geophysicist at the Niels Bohr Institute at the University of Copenhagen and lead author of the study detailing the new technique.

This system of terrain modeling essentially does the same but uses multiple shadowed images of an area, data on the incoming angle of the light in each satellite image, and elevation data to build a 3D model of what’s casting the shadows in those pictures. For example, shadowed images of a crater taken at different times, when sunlight hits the terrain at different angles, can be used to work out, for instance, that the crater’s wall must have a 20-degree incline to produce the shadows observed. Traditionally, to use this shadow technique, some assumptions need to be made about what the terrain looks like.

Then an initial rough elevation model is created using the technique and repeatedly improved until it matches the shadowed images to an acceptable degree of accuracy. “This trial and error can take a long time,” says Fernandes. In their new method, Fernandes and her colleague Klaus Mosegaard worked around this by solving an equation that relates the angles of incoming sunlight and the shape of the terrain.

This is the first time that anyone has produced a topographic model using this equation. The result is that the new approach doesn’t require any prior assumptions about the terrain, and produces high-resolution terrain maps in one try, making it faster than existing methods. This is a big advantage when building terrain models for multiple areas.

The team tested their approach on an area centered in the Mare Ingenii, a region on the far side of the moon. They fed the algorithm the angles of incoming sunlight from photographs containing shadows taken by NASA’s Lunar Reconnaissance Orbiter (LRO)—a satellite that continuously circles the moon, capturing information—along with elevation data collected by its laser altimeter. The resulting high-resolution terrain model matched the shadowed photographs to a high degree of accuracy, and vastly improved the elevation resolution.

The elevation data gathered by the LRO’s laser altimeter has a resolution of 60 meters per pixel; the new method’s final terrain model had a resolution of 0. 9 meters per pixel. This meant that craters with diameters as small as three meters became identifiable.

“It’s a different approach for understanding the topography of the moon that could help prepare for future human and robotic exploration,” says Noah Petro, a planetary geologist at NASA’s Goddard Space Flight Center who wasn’t involved in the research. The LRO has been orbiting the moon since 2009, collecting data that has been used to create a digital terrain model that covers 98 percent of the moon’s surface. This is the base map that any higher-resolution terrain models, such as the one from the new study, are placed on.

Together, such high-resolution maps are the foundation for planning trips to the surface. Landing sites need to be flat with no boulders. Travel routes to and from craters ideally shouldn’t be steep, so that they can be navigated by rovers.

High-resolution maps of the lunar landscape can be used to model light conditions too. Predicting when and where to expect shadows and sunlight is crucial for planning upcoming missions, says Paul Hayne, a planetary scientist at the University of Colorado Boulder’s Laboratory for Atmospheric and Space Physics. Potential landing sites will need to receive solar radiation for at least part of the day to recharge instruments and rovers.

Sunlit areas directly adjacent to craters could also be useful, because exploring shadowed regions may take time, meaning rovers might need to be recharged as soon as they exit a crater. A more detailed understanding of the terrain can also help NASA decide which permanently shadowed regions to target when searching for water ice. For example, the steepness of crater walls can provide insight into how long ago the crater formed and whether the shadows and temperatures could have persisted for long enough for water ice to be present.

“We often need highly accurate terrain models to turn a snapshot into a time history, to find the cold-traps where ice might be stable for long periods,” says Hayne. And on top of all this, the new imaging approach should also help with navigation. Rovers need to be able to travel along precisely calculated routes.

Onboard motion detectors can help rovers navigate, but sensor and estimation errors can add up over large distances, causing vehicles to drift off course. One way to overcome this is to have rovers use onboard cameras to create high-resolution terrain models on their own, and then pinpoint their location relative to known features and adjust their path accordingly, says Martin Schuster, a robotics scientist at the German Aerospace Center’s Institute of Robotics and Mechatronics. “Matching local terrain models to externally created high-resolution models, like the one produced in the new study, can help rovers localize,” he says.

If the resolution of previously created terrain maps is too low, staying on path can be more difficult. The moon is a quarter of a million miles from Earth. Getting there is difficult, and if astronauts experience unexpected issues on the surface, they will be limited in how they can respond.

Anticipating what terrain features explorers and rovers will encounter is therefore extremely important—and could even be lifesaving. Finding the best, most accurate ways to map the moon’s surface is an integral part of mission preparation. “We want to use all available data to tell us everything we can about the places we want to explore,” says Petro.

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