Dr. William (Bill) Farrell, Scientist, Lunar Exploration Program
- Created: Thursday, 01 May 2008
NTB: You’ve mentioned the electrical fields. Do you understand yet how the dust develops its electrostatic charge?
Dr. Farrell: That is an interesting question. The short answer is “yes;” the long answer is “not really.” The short answer is, on the Moon, the dust can get charged a number of different ways. Since the dust is sitting in a plasma, it actually charges just by being in equilibrium with the plasma. The plasma is an ionized gas. On the dust surface, the dust will try to do everything it can to maintain perfect current balance, so it will charge up, basically, to fend off electron currents – plasma electron currents – which tend to be stronger than ion currents. This is true in the night-side region.
In the day-side region they’ll charge up positive, in some sense to draw back the photoelectrons that have been emitted from its surface. So in some sense the grains are pre-charged. But any kind of rubbing, like, for example, an astronaut walking along or a rover driving over it, will add extra charge – actually a lot of extra charge – to what’s known as “triboelectric effect.” When we talk about it more formally and in the field of solid state, it’s called “contact electrification.” And we’re familiar with this. In the wintertime, when you scrape over a carpet, you charge up. You’ve got the same kind of thing going on as the astronauts walking across the lunar surface with this loose dust. It will charge up because it’s come in contact with an object.
So you actually have a couple of different sources of charging on the Moon. The question is, which one is dominant? The other question is, all of this is in an electrical environment that itself is variable because the surface electric fields will vary, depending on solar conditions, particularly during a solar storm that gives off a lot of energetic electrons. The surface can get strongly negatively charged, particularly in the night-side region up near the terminators. If that’s the case, now your dust is going to feel more charged because it becomes more charged during solar storms.
So there are a lot of different inputs. There’s a natural input, which is actually a function of what is known as space weather. Then there’s also the anthropogenic input – which is the way we refer to it – where humans and rovers drive over it, or excavate it in the case of digging for something, which people might want to do in a polar base.
NTB: You’ve noted that the electrostatic charge forms differently on the day side of the Moon than it does on the dark side of the Moon. Can you explain why, or how, that works?