Dr. Farrell: The key element there is photoelectrons, or what’s known as the photoelectric effect. Basically, when the sun shines on the day side, the incoming photons release electrons, so the surface – and grains on the surface – emits an electron current. As a consequence, bodies on that side will tend to charge a few volts positive. But on the night side you’ve gotten rid of these photoelectrons, which are really the dominant source on the day side, and now you’re left with very tenuous plasma, both ions and electrons. On the night side, as it turns out, the electron currents dominate by about a factor of 10 over the ion currents, so your surface will tend to charge negative to repel those incoming electrons.
It’s really an issue of how does the surface balance itself so it gets zero current at the surface? These models that we use are very much like spacecraft charging models. You set the boundary conditions – you want zero current on the surface, so what kind of potential do you need to dial in to directly balance the electrons and ions on the surface? If you have too many electrons coming in, which you get in a plasma, then the surface will charge negative. You can dial in a negative voltage, the electrons will be repelled, and then you’ll draw on ions and the current will be balanced at the surface. So it’s almost like this self-regulating property, if you will.
On the night side though, what’s interesting is the plasma is very, very tenuous because the solar wind, in particular, has been blocked out in the lunar day side region. You’re essentially in this plasma void behind the Moon, so there’s a tenuous, kind of warmer plasma. The night side surfaces can charge up very strongly negative – like 100 or 200 volts. During a solar storm, Jasper Halekas at the University of California, Berkeley, reported that Lunar Prospector detected electron beams from the surface emitted at 4 kilovolts, suggesting that the surface was charging up to 4 kilovolts during solar storms. When we talk about this with Exploration folks, even though they’re interested in the dust, they are totally floored that the surface potential varies as much as it does. In fact, from my perspective, the surface potential is really the driver of all things bad, including the dust. It’s almost like, if you put any human electrical system in this environment on the surface and you start cranking around the voltages, you’re going to affect power systems, you’re going to affect dust, you’re going to affect how easy it is for an astronaut or system to dissipate their own charge, particularly on the night side.
NTB: What about the area of the Moon where the two sides meet, that moving line between lunar day and night known as the terminator?
Dr. Farrell: That’s kind of a real important area, particularly if we have a lunar base going to Shackleton Crater very near the South Pole, which is what the NASA lunar architecture people are thinking. We’ll probably be crossing very close – if not crossing directly into – that region. There’s going to be a potential difference, and in particular there’s a lot of debate whether people will actually go into Shackleton or not. It depends on who you talk to. But there are definitely resources believed to be at the bottom of Shackleton that human explorers may want. For example, there’s going to be, maybe, water, or hydrogen, some kind of hydrogen-based molecules. People want to bring that out. So whether they use robotics to excavate, or send in humans to do that, there’s going to be a big charge differential, a voltage differential between the sunlit region up at the top of the crater and the dark region down below, which will not get direct solar wind flow. Within that crater, the surface potential should get strongly negative. If you send in a human system that is powered from sunlit regions, with a power ground near zero volts, the surface all around might be at minus 300 volts, so there’s a big potential difference between that object and the surrounding terrain. Hence, the issue has to really get worked out.