Imagine what a lunat sunrise might look like... Grey mountains rising above grey plains pockmarked with craters of all sizes with the shadows slowly shortening as, after two weeks of darkness, the Sun rises sedately over the horizon. Wow...
The Moon spins on its axis once a month (compared to 24 hours for the Earth) so both sunset and sunrise last a very long time. The line dividing day from night on any planet or moon is known as the "terminator" and on the Moon this line moves very slowly. (In fact, in an episode of Futurama, Fry and Leela are stuck out on the lunar surface and try to out run the terminator, can you work out if that is possible?) Despite the fact that the Moon is too small to hold on to an atmosphere, there may still be weather on our nearest neighbour. Recently, NASA reported that there may be a dust storm which runs along the entire length of the lunar terminator from the North pole down to the South.
The evidence for this comes from an experiment left on the Moon by the astronauts of Apollo 17 back in 1972. The lunar surface has been pummeled by meteorites over billions of years, breaking up the surface material and producing vast amounts of dust. The Lunar Ejecta and Meteorites (LEAM) experiment was designed "to look for dust kicked up by small meteoroids hitting the moon's surface" according to Timothy Stubbs at the Goddard Space Flight Centre. The LEAM experiment was designed to investigate how much dust is stirred up by impacts (which still occur now, just less frequently than they did in the early days of the solar system). The detectors only operated for 620 hours of lunar darkness and 150 hours of lunar daytime, so data are limited, but scientists are now busy re-analysing it to look for evidence of this dust storm.
Lunar rays sketched by Apollo 17 astronauts CREDIT: NASA
According to Gary Olhoeft, professor of geophysics at the Colorado School of Mines, what the data show is more particles coming from the east or west than any other direction, moving with slower speeds than would be expected if the dust had been stirred up by an impact. The current theory is that the day side has an overall positive charge while the night side has a net negative charge and the resulting electric field causes any dust which also has a slight electric charge to jump across the dividing line. This charged dust could also explain the lunar fountains sketched by the Apollo 17 astronauts which are similar to crepuscular rays caused by the atmosphere on the Earth.