A post for general readers:
The recent launch of NASA’s new moon mission, Artemis 1, is mostly intended to demonstrate that NASA’s incredibly expensive new rocket system will actually work and be safe for humans to travel in. But along the way, a little science will be done. The Orion spacecraft at the top of the giant rocket, which will actually make the trip to the Moon and back and will carry astronauts in future missions, has a few scientific instruments of its own. Not surprisingly, though, most are aimed at monitoring the environment that future astronauts will encounter. But meanwhile the mission is delivering ten shoe-box-sized satellites (“CubeSats“) which will carry out various other scientific and/or technological investigations. A number of these involve physics, and a few directly employ particle physics.
The use of particle physics detectors for the purpose of studying the not-so-empty space around the Moon and Earth is no surprise. Near any star like the Sun, what we think of as the vacuum of space (and biologically speaking, it is vacuum: no air and hardly any atoms, making it unsurvivable as well as silent) is actually swarming with subatomic particles. Well, perhaps “swarming” is an overstatement. But nevertheless, if you want to understand the challenges to humans and equipment in the areas beyond the Earth, you’ll inevitably be doing particle physics. That’s what a couple of the CubeSats will be studying, entirely or in part.
What’s more of a surprise is that one of the best ways to find water on the Moon without actually landing on it involves putting particle physics to use. Although the technique is not new, it’s not so obvious or widely known, so I thought I’d draw your attention to it.
The Lunar Polar Hydrogen Mapper (LunaH-Map)
Designed at Arizona State University, the LunaH-Map CubeSat will look for water on the Moon, using a tried and true technique known as “neutron spectroscopy”. The strategy relies from the start on particle physics, taking advantage of the existence of “cosmic rays”, which are (mainly) protons and atomic nuclei traveling at near the speed of light across the universe. These particles are accelerated to extreme speeds by natural particle accelerators found in supernovas and perhaps elsewhere. They may travel for many thousands of years across the galaxy, or even longer from outside our galaxy, before reaching our vicinity. The Sun and its planets and moons are all constantly being peppered by these particles.
On Earth, most cosmic rays strike an atom in the atmosphere before they reach the ground. (The debris from these collisions allowed scientists to discover a number of subatomic particles, such as the positron and the muon, and they play a role in many modern experiments, such as this one.) Since the Moon has no atmosphere to speak of, cosmic rays instead slam straight into the lunar dirt.
What ensues is a “hadronic shower”, a natural particle physics process similar to that found at the Large Hadron Collider, within the “hadron calorimeters” of the ATLAS or CMS experiments. (These portions of the ATLAS and CMS detectors measure the energies of hadrons, particles containing quarks, antiquarks and gluons.) A computer simulation of a hadron shower is shown at left. How does a shower arise?
When a high-energy proton hits an atomic nucleus (typically within a meter of the lunar surface), it breaks the nucleus apart into protons, neutrons and smaller atomic nuclei. Typically some of the remnants now have enough energy themselves to break apart nearby atomic nuclei, whose remnants break apart further nuclei, etc. The result is that the cosmic ray’s large amount of energy is transformed into a shower of protons, neutrons and other nuclear fragments. Whereas the original cosmic ray was moving at nearly the cosmic speed limit (a.k.a. the speed of light, 300,000 kilometers per second), the particles in the shower are typically moving much more slowly, perhaps 10 to 1,000 kilometers per second — still fast by human standards, but far below light speed.
Not surprisingly, since the cosmic ray comes from above, most of the particles in this shower move downward into the lunar soil. They collide with other atomic nuclei and eventually slow to a stop. But there are always a few protons and neutrons that by chance have a collision that knocks them upwards. Consequently, even a downward-directed cosmic ray shower will produce some particles that make their way out of the ground and back into space. Once they get out, there’s nothing to stop them, since there’s no air around the Moon. A spacecraft going by just overhead can hope to detect them as they head out into empty space.
The basic trick of LunaH-Map, which I’ll explain in a moment, is that if the ground that the cosmic ray struck contains hydrogen, any upward-going particles will be slower on average than if there’s no hydrogen there. Most lunar soil has no hydrogen, as determined by various missions to the moon. But lunar soil that contains water ice in it will have plenty of hydrogen, since water is hydrogen and oxygen (H2O). So if you can detect particles of certain speeds as you pass over a certain part of the Moon, you can tell whether there’s water ice embedded in the soil.
To explain all this, I need to tell you why particle speeds are sensitive to the presence of hydrogen, and which types of particles are best to measure.
Hydrogen and its Effect on Speed
The effect of hydrogen on particle speed involves something you probably understand intuitively, even if you were not taught it in a first-year physics class. It’s illustrated in the figure below.
- If you bounce a ping-pong ball off a heavy, stationary rock, the rock won’t budge, and the ping-pong ball will bounce off it in a new direction but without losing any of its speed.
- If you bounce a ping-pong ball off a second, stationary ping-pong ball, both ping-pong balls will be moving after the collision, and the speed of both balls will be less than the initial speed of the first ball.
(For those who’ve had a little physics: these facts are required by conservation of energy and momentum. In the first case the rock absorbs almost none of the ping-pong ball’s kinetic energy, so the ball retains what it had before, as for a tennis ball bouncing off a wall; in the second case, the first ping-pong ball loses a significant fraction of its kinetic energy to the second one.)
Imagine, then, a proton or a neutron that emerges from the shower of particles that follows a cosmic ray impact. Much slower than the original cosmic ray, it is still moving at many kilometers per second. What happens as it repeatedly strikes atoms in the soil?
Protons and neutrons have about the same mass. Typical atomic nuclei in the Moon, such as oxygen or silicon, contain more than ten protons and neutrons, and so, with much larger masses than a single proton or neutron, they act like a heavy rock. Our speedy neutron or proton will bounce off such a nucleus without slowing down. (A minor detail: It may not always simply bounce; other things may happen which can slow it down somewhat, but not enough to affect what I’m about to tell you.)
But because hydrogen’s nucleus is itself just a single proton, the collision of a proton or neutron with a hydrogen nucleus is like the collision of two ping-pong balls — two objects of equal or nearly equal mass. The result: the one that’s moving will lose on average half its energy, or about 30% of its speed, relative to the lunar surface. If there’s a lot of hydrogen in the soil, then this process may happen repeatedly to most protons and neutrons in the cosmic ray shower. And so protons and neutrons emerging from soil rich in hydrogen are on average much slower compared to those emerging from soil that’s poor in hydrogen.
The LunaH-MAP cubesat, like many spacecraft before it, is looking for this effect on neutrons. Why on neutrons and not on protons? Because there are protons everywhere around the Moon, streaming in from the Sun and from elsewhere. Neutrons, by contrast, only can exist on their own (as opposed to inside a stable atomic nucleus) for about 15 minutes. Consequently, any neutrons from the Sun or other distant source won’t make it to the Moon. So any neutrons near the Moon, even some distance overhead, are much more likely to have come from the Moon than from anywhere else.
LunaH-MAP comes quite close to the Moon’s surface (just a few kilometers above it), which allows it to examine the Moon in considerable detail. All it does, as it flies over the surface, is count how many neutrons it encounters. What’s crucial is that it is only sensitive to neutrons of moderate to high speed, and it can’t detect the slow ones (slower than about 7 kilometers per second.) Above most regions of the Moon, where the heat of the Sun quickly vaporizes any water ice and releases it to space, the spacecraft will find many neutrons of moderate speed, dislodged by cosmic rays and leaking out from the surface. But in craters near the poles, where there are regions mostly or always in shadow, ice deposited by comets still remains; and there, as the spacecraft passes overhead, many of the neutrons will have been slowed down so much that LunaH-MAP can’t detect them. (This figure shows how dramatic this can be; if the first meter of dirt were just 3% ice, the number of moderate-speed neutrons would drop in half!) In this way, by counting the number of neutrons observed as it flies through the dark lunar sky, LunaH-MAP can distinguish hydrogen-poor soil below it from hydrogen-rich soil.
That’s how, without ever landing there, LunaH-MAP can give us a detailed map as to where hydrogen, and likely water ice, is to be found on the Moon. Similar techniques can and have been used on planets such as Mars and asteroids such as Ceres. Pretty cool, right? Just another great example of how seemingly exotic and esoteric discoveries of one century — cosmic rays were first observed in 1911, and the neutron was identified in 1932 — turn out to be essential tools in the next.