As the solar wind flows over natural obstructions on the moon, it may charge polar lunar craters to hundreds of volts, according to new calculations by Nasa’s Lunar Science Institute team.
Polar lunar craters are of interest because of resources, including water ice, which exist there.
The moon’s orientation to the sun keeps the bottoms of polar craters in permanent shadow, allowing temperatures there to plunge below minus 400 degrees Fahrenheit, cold enough to store volatile material such as water for billions of years.
“However, our research suggests that, in addition to the wicked cold, explorers and robots at the bottoms of polar lunar craters may have to contend with a complex electrical environment as well which can affect surface chemistry, static discharge and dust cling,” said William Farrell, of Nasa’s Goddard Space Flight Center.
It seems that the moon has changed dramatically in recent years, and has a dynamic environment.
Solar wind inflow into craters can erode the surface, which affects recently discovered water molecules. Static discharge could short out sensitive equipment, while the sticky and extremely abrasive lunar dust could wear out spacesuits and may be hazardous if tracked inside spacecraft and inhaled over long periods.
The solar wind is a thin gas of electrically charged components of atoms – negatively charged electrons and positively charged ions – that is constantly blowing from the surface of the sun into space.
Since the moon is only slightly tilted compared with the sun, the solar wind flows almost horizontally over the lunar surface at the poles and along the region where day transitions to night, called the terminator.
The Nasa scientists created computer simulations to discover what happens when the solar wind flows over the rims of polar craters.
They discovered that, in some ways, the solar wind behaves like wind on Earth – flowing into deep polar valleys and crater floors.
But unlike wind on Earth, the dual electron-ion composition of the solar wind may create an unusual electric charge on the side of the mountain or crater wall – that is, on the inside of the rim directly below the solar wind flow.
Since electrons are more than 1,000 times lighter than ions, the lighter electrons in the solar wind rush into a lunar crater or valley ahead of the heavy ions, creating a negatively charged region inside the crater.
The ions eventually catch up, but rain into the crater at consistently lower concentrations than that of the electrons. This imbalance in the crater makes the inside walls and floor acquire a negative electric charge.
It turns out that the Apollo astronauts in the orbiting command module saw faint rays on the lunar horizon during sunrise that might have been scattered light from electrically lofted dust.