The Moon’s Atmosphere
Does the Moon have an atmosphere? Yes, but just barely, and it’s very different from the atmosphere we know on Earth. When people talk about the Moon’s atmosphere, they are often referring to its main layer: the lunar exosphere.


comparing Atmospheres
Atmospheric Layers
(Images not to scale)
Getting to Know the Lunar Exosphere
The lunar atmosphere is mostly an exosphere, which itself is mostly empty space. If you were to visit the Moon and walk around on its surface, you might think it had no atmosphere at all. Molecules around the Moon are so few and far between that they travel long distances without running into anything, even each other. Earth’s atmosphere, on the other hand, is dense and collisional: air molecules are bumping into each other (and interfering with each other’s movements) all the time.
Since the Moon’s atmosphere is not collisional, we can study each chemical component independently. For example, some scientists work to understand how water forms and changes on the Moon by investigating the behavior of hydrogen atoms in the exosphere. Chemical components of the lunar exosphere have their own ‘cycles’ of movement between the ground and the space nearby, and distinct migration patterns from place to place on the Moon’s surface.

What is the Moon’s Atmosphere Made Of?
The lunar exosphere is almost entirely made up of helium, neon, and argon, with trace amounts of other elements. Hydrogen molecules make up a tiny fraction of the lunar exosphere, but are a topic of serious scientific interest, partly because hydrogen is one of the chemical ingredients in water.
Sodium is another scarce but heavily-studied element in the lunar exosphere. Sodium’s chemical signature is relatively easy to detect from far away, making it an appealing target for observers who use Earth-based telescopes or analyze data from spacecraft in lunar orbit.

Most lunar exosphere science relies on data collected at a distance, but humans have also set up experiments on the ground. When the crews of Apollo 12, 14, and 15 lifted off from the Moon, they left behind devices designed to measure the amount of gas near the lunar surface. These initial experiments confirmed the presence of an extremely sparse atmosphere. Then Apollo 17 astronauts deployed a spectrometer called LACE (the Lunar Atmospheric Composition Experiment) to identify the atmosphere’s chemical makeup.
Decades later, new spacecraft brought updated technology into lunar orbit for follow-up observations. THEMIS-ARTEMIS began as a five-satellite Earth-focused mission. When its original assignment ended in 2008, two of its five probes were redirected to locations near the Moon, where they now measure charged particles swept away from the lunar exosphere by solar wind. NASA’s Lunar Reconnaissance Orbiter (LRO), an SUV-sized spacecraft with seven science instruments on board, has been sending data home from lunar orbit since 2009. In 2013-2014, the Lunar Environment and Dust Explorer (LADEE for short) joined LRO for a six-month orbital mission, gathering detailed information about the lunar exosphere’s composition and structure – and giving new insight into how these change over time.
The Moon’s Changing Environment
In contrast to Earth’s thick layers of air, the Moon’s exosphere is a bit like the haze you get when you whack a dirty rug with a broom: it includes a sampling of particles knocked free by impact. But instead of a rug, these particles come from the lunar surface. Instead of flying through the air, they’re ejected into near-emptiness. And instead of a broom, the Moon is pummeled by space debris. Micrometeorites and other impactors temporarily kick up dust particles and even individual atoms from the Moon’s surface. All the while, the solar wind delivers protons and other materials from the Sun. As some particles settle to the ground or escape into space, new impacts send others aloft.

Human and robotic activity change the lunar exosphere, too. Exhaust from a single lunar lander can briefly dominate the space nearby. Since spacecraft exhaust includes water and other chemical ingredients that also occur naturally on the Moon, studies of the lunar environment must account for exploration-related changes. Scientists are already running simulations to anticipate the atmospheric effects of future Moon landings.
Writer: Caela Barry, NASA’s Goddard Space Flight Center
Science Advisors: Dana Hurley and Parvathy Prem, Johns Hopkins University Applied Physics Laboratory; Bill Farrell, NASA’s Goddard Space Flight Center