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Now the moon is like a ghost town. Nothing moves. Here and there, one exploration vehicle Apollo left (or base of a dusty lunar lander) is to spend time as a silent witness to past human activity. But these days, only some occasional asteroid impacts interrupt the spell of deep stillness that has lasted decades. And such stillness gives scientists an exciting opportunity. Currently, the thin lunar atmosphere is relatively calm. But that will not happen for long. NASA is already planning to send humans back to the Moon. Human activity developed there will raise dust, gases and expelerán rockets also emit other gases into the atmosphere of the moon. Because the atmosphere is so thin, such

disturbances could quickly disturb its natural composition. If scientists ever think to know the lunar atmosphere in a relatively natural state, now is the time to observe. So researchers are building a tube called Explorer and Atmospheric Environment Moondust (in English: Lunar Atmosphere and Dust Environment Explorer or tilted), which will orbit the moon and measure its wispy atmosphere and never been done before. "It's important we understand it in its pristine state, before it has much trouble," says Anthony Colaprete, Ames Research Center, NASA, Moffett Field, California. "It's a very fragile system. You may be very difficult to study once again humans are living and working in the Moon." Right now, you must be thinking: "Wait a minute. I thought I knew that the moon had no atmosphere!" And it's almost right. The "atmosphere" of the Moon is so tenuous that technically is considered as an exosphere, not like an atmosphere. "It's not like the atmosphere in which we might think," says Colaprete. For example, one cubic centimeter of Earth's atmosphere at sea level contains about 100 billion billion molecules. The same volume in the lunar exosphere contains only about 100 molecules. In fact it is so thin that the molecules in the lunar exosphere almost never collide with each other. Instead of constantly bouncing against each other to create a cohesive dough and filled with molecules, as in the atmosphere, the molecules in the lunar exosphere fly unimpeded, as if they were microscopic cannonballs ballistic trajectories are curved. And the strangeness of the exosphere not end there. During the lunar night, the lunar exosphere falls almost entirely to the ground (imagine if our atmosphere to fall overnight!). When sunlight returns, the solar wind raises new particles to replenish the exosphere.Similarly, the intense ultraviolet sunlight strikes the lunar soil particles and makes them lose electrons, thus obtained particles an electrical charge that makes them levitate. Electric fields arise in the environment these charged dust particles making them reach heights of kilometers above the surface, thus forming an important part of the exosphere. The lunar astronauts will live and work in this bizarre environment, so that scientists want to get a better idea of what is the exosphere and its strange behavior. The levitated dust can enter the equipment, space suits and computers, causing damage and shortening the life of appliances. In fact, lunar dust ravaged the spacesuits of the Apollo mission, which arrived almost frayed when the astronauts returned to Earth. Knowing how much dust floats in the exosphere and how it behaves will help engineers designing the next generation lunar devices. After its launch in 2012, spectrometers and detectors LADEE measured dust concentrations of 18 different chemical compounds in the exosphere, including methane and water vapor. These sensors will document how these compounds vary from one place to another and over time. Beyond the inherent scientific value to understanding the chemical composition of the lunar exosphere, knowing how to move the chemicals into the atmosphere can help answer a question of great interest for future human visitors on the Moon: how could Moon take reserves of frozen water? The evidence suggests that the moon could host ice deposits deep in the dark craters at the lunar poles. On the surface of the moon, the fierce sunlight any ice would sublimate rapidly and vapors escape into space. But a deep dark crater, where unimaginable cold combined with the absence of sunlight, could provide a secure means to preserve water ice. A popular idea is that comets brought water ice on the moon in a number of impacts, long ago. But there is one problem: Even if a comet falling into one of those dark polar craters in a moment of luck, the heat of impact evaporate most of the ice. So how then could accumulate significant amounts of ice? The lunar exosphere could help. Suppose a comet crashes into the moon and leaves of H 2 O molecules on the exposed surface. The water could survive essentially jumping to salvation. Water molecules could "jump" off the lunar surface and escaping into the exosphere could then be recaptured by the surface as the exosphere inhale and exhale. Individual water molecules could move in this way until it falls into one of the dark polar craters where ice could accumulate as solid. Data provided by the LADEE could show whether this process of "hops" may be used to explain how the ice of comets may have found their way into the interior of the craters. "We can estimate the probability that the water of the Moon is of cometary origin," says Colaprete