A visualization of the Moon’s Clavius crater. Credit: NASA’s Scientific Visualization Studio. Visualizer Ernie Wright (USRA), Technical support Laurence Schuler (ADNET Systems, Inc.), Ian Jones (ADNET Systems, Inc.)
Previous Missions Did Not Add Significant Amounts, But Future Landers May Contaminate Lunar Water
Though the Moon is mostly dry as a desert, a 2009 NASA probe confirmed that there is water frozen in permanently shadowed craters. Which begs some questions. Where did this water come from? What lessons does it offer us on the Moon’s — and Earth’s — formation and history? But the rocket exhaust of our lunar landers also produces water. Scientists at the Johns Hopkins Applied Physics Laboratory (APL) and their colleagues elsewhere used PSC’s flagship Bridges-2 supercomputer to simulate whether past and future lunar expeditions will contaminate this lunar water, complicating our studies of this reservoir.
WHY IT’S IMPORTANT
In the 1950s and ’60s, astronomers realized that the Moon had craters with permanent shadows, particularly near its poles. And that led to a crazy thought: Could this super dry satellite actually have water in its shaded places?
Water on the sunlit surface of the Moon would become gaseous very quickly, and then be ripped apart by ultraviolet light. But water in those permanent shadows? It could still be there, frozen in place for billions of years.
In 2009, NASA’s Lunar Crater Observation and Sensing Satellite (LCROSS) probe intentionally crashed into a shaded lunar crater. This impact kicked up debris that an accompanying orbiter was able to confirm, for the first time, contained water. But this led to another question. NASA had sent six manned missions to the Moon, and there have been a number of other, robotic landers. All of them landed — and the Apollo missions had taken off again — using rocket fuel that, when burned, produces water.
“We know that there is some water [on the Moon], but we’re not really sure how it’s distributed, how much there is. And then, you know, the big mysteries of where did it come from, how old is it, are still wide open. And that’s one of the reasons why there’s a lot of scientific interest in trying different orbital techniques that could map that water better from orbit. There’s also a lot of interest, of course, in landing near the poles, and trying to understand that water better.”
— Parvathy Prem, Johns Hopkins APL
So was the water measured in lunar craters really from the Moon? Or had the landers produced enough water vapor to collect and freeze in the shadows? Just as importantly, if there is true Moon water in those craters, are the missions contaminating it with enough Earth-derived water that scientists won’t be able to use it to study the Moon’s formation and history? Even if past missions haven’t contaminated lunar water, will the much larger landers now being planned, some of which will land much closer to those craters, contaminate it?
Parvathy Prem, a planetary scientist at Johns Hopkins University, wanted to work out just how much water was being generated by human exploration, and whether enough of it could get to the shaded craters to significantly change the composition of the water there. To simulate the movement of water from rocket nozzles to the Moon’s shadows, she and colleagues at APL and at the Space Science Institute and DeepSpace Technologies Inc., and NASA Goddard Space Flight Center earned allocations for a number of supercomputers, including Hopkins’ own Rockfish as well as those in the NSF-funded ACCESS program, such as the Texas Advanced Computing Center’s Stampede2. Most recently, her ACCESS work has used PSC’s flagship Bridges-2 system.
Beginning on the near side of the Moon, with the Apollo sites marked, this visualization quickly moves to the South Pole and zooms in to show how the changing illumination conditions there over an entire year keeps the bottoms of some craters in permanent shade. Credit: NASA’s Scientific Visualization Studio. Visualizer Ernie Wright (USRA), Scientist Noah Petro (NASA/GSFC), Technical support Laurence Schuler (ADNET Systems, Inc.), Ian Jones (ADNET Systems, Inc.)
HOW PSC HELPED
To figure out the impact of fuel-derived water on the Moon’s water reserves, Prem would have to simulate water molecules’ long journey. This would start in rocket nozzles and end in shaded craters thousands of miles away. Initially, most of these water molecules would be exposed to sunlight. They’d stay gaseous, and many of them would be broken apart by the Sun’s intense ultraviolet rays, which of course aren’t weakened by an atmosphere like on Earth.
The simulation would need to include tens to hundreds of millions of H2O molecules interacting with sunlight, the mostly dry surface of the Moon, and each other as they hopped to find the shadows. While not exactly a large simulation by modern standards, it was by no means small. The task was beyond any standard computer’s capabilities, requiring serious numbers of processors.
“I’m not exaggerating here. This work would not have been possible without Bridges-2 … These simulations are kind of complicated … they’re not of the same scale as say, climate simulations, or some biomedical simulations [but] I always feel that these simulations have an outsized scientific impact relative to the computational resources they need.”
— Parvathy Prem, Johns Hopkins APL
Offering tens of thousands of powerful computational cores, Bridges-2 provided the power Prem needed to track the movement and chemical interactions of those water molecules on their long journey.
Led by Dr. Bill Farrell (Space Science Institute/DeepSpace Technologies Inc.), the team compared simulation results to the amount of real-life water measured so far in lunar craters. These are broad estimates, ranging from 2 to 60 metric tons in the craters of the Moon’s south pole. The good news is that, in Prem’s simulations, the Apollo lunar modules contributed at most 0.36 metric tons to the Moon’s reservoir. The bad news is that the SpaceX Starship landers, in some cases, can deliver more than 10 metric tons of water to the permanently shadowed regions. This would make it difficult if not impossible for astronomers to identify where the lunar ice exposed at the surface of permanently shadowed regions came from and how it developed, unless they can disentangle or minimize the effects of landers. The team reported their results in The Planetary Science Journal in May 2024.
Prem’s work has made understanding both lunar water reserves and the effects of human exploration more urgent. She and her colleagues have suggested measurements and instruments that could be sent in the near term to measure the possible effects of polar landers on the Moon’s water. They’re also looking to better understand their own simulations, exploring their limitations and how they can be made even more accurate.