Looking for Pure Ice in Moon’s Craters

Moon surface

Estimating the amount of ice or water deposited at the lunar subsurface has been one of the most important fields of research since the start of the 21st century. Water is an essential element for keeping up the life and functioning of future lunar explorers and astronauts, as well as fuel for spacecrafts and rockets etc. Also, the information on the water allocation can give an insight to the new information about the rise of our Solar System, as well as information about the asteroids and heavenly objects that encountered our Moon – Earth system and brought ice or water material from the farthest corners of our Solar system. There is a possibility that not only a heavenly object could have put some traces of water from other places in the Solar system, but also solar wind brought some hydrogen from the early phases of the Sun. There is a theory that the water on the Moon comes from the internal core and had erupted during the volcanic period on the Moon.

The water can be stored on the Moon for a long period. Even though there is an absence of atmosphere, there are the regions near the poles that had never seen sunlight. On the Earth, it is not the same when it comes to poles, because Earth’s orbit is tilted, which provides us with seasons, so each pole experiences half of the year of daylight when it is polar summer and the same lack of sunlight during polar night.

On the Moon, the tilt is less than 2° so there is not so much of a seasonal change there, and the shadows on the Moon are rather permanent. Polar craters, like Sylvester N, never experience sunlight, and the temperature there reaches its cold extreme, and any water or other substance that happens to be there has no way out because of the low temperature and can stay there for a million years.

Lots of different technologies and sensors were used to detect the ice at the poles of the Moon, as well as neutron scattering, radio detector, reflectance & transmittance spectroscopy at different wavelengths. However, none of these methods brought enough results to estimate the amount of water on the Moon, the way it is allocated, and its depth. One of the potentially most informative method is radio detector, as it can discern big and pure storages of water ice. Though it is informative and valuable, it is easy to confuse the water ice with rocks that are up to one meter in size, as both of them have the same radar backscatter.

There is a new approach promoted by LROC scientists. They use long-exposure NAC images that allow to check the areas where the radar backscatter is increased in order to identify the likelihood of ice (not the rocks) being there. Not so much sunlight is reflected from the craters that can reach permanently shadowed regions, so long-exposure NAC can sense this light if the exposure time to make an image is longer than typical. Though the long-exposure pictures might be useful to search for rocks in permanently shadowed regions, having an idea that if there are many rocks, it is highly possible that rocks are the reason for the radar backscatter and not the ice.

The images from the LROC gathered data not only from permanently shadowed regions at the poles, but also equatorial regions. The information from the equatorial craters has been known for a long time. There is no water ice possible there since the temperature is extremely high. The resolution of pictures in the equatorial craters is higher, which allows it to discern the types of rocks present there. The resolution of the polar pictures is much lower because of the lack of light and long-exposure. However, the pictures with lower resolution can provide more information on the area there, as while the picture takes some time to expose, the spacecraft is moving, showing more area on the picture.

NAC images also proved that craters where the radar backscatter is higher have more resolvable types of rocks, whether it is an equator or poles area. So, the information from the radar that was considered to indicate pure ice can actually be resolvable rocks deposits. Also, from other missions, we learned that, though there might be not so much pure ice water deposits in the pole craters, the water on the Moon is present in the soil or is mixed with the soil. However, the final insight of this investigation of polar craters will be provided after we get good quality pictures from the Shadow CAM tool. Only after that we will be able to get the full picture of the situation with ice on the Moon, its allocation, depth, and amount.