7:15 AM | *There is no "weather" on the moon, but there are extreme differences in temperatures*

One of the most universal and famous images in history known as “Earth Rising” (courtesy NASA)

Overview

It’s been 50 years since Apollo 11 landed on the moon, and Neil Armstrong and Buzz Aldrin became the first people to walk on its surface. There were 5 subsequent Apollo missions that successfully landed humans on the moon and brought them safely back to Earth.  What kind of “weather” did these astronauts need to contend with on the moon? The moon has a very thin atmosphere so it cannot trap heat or insulate the surface.  There is no wind there, no clouds, no rain, no snow and no storms, but there is “day and night” and there are extreme differences in temperatures depending on where the sun is shining.

Neil Armstrong and Buzz Aldrin unfurl U.S. flag on Moon and are photographed by automatic camera in the Lunar Module window

Moon’s (very thin) atmosphere

Recent studies confirm that our moon does indeed have an atmosphere consisting of some unusual gases, including sodium and potassium, which are not found in the atmospheres of Earth, Mars or Venus. It's an infinitesimal amount of air when compared to Earth's atmosphere. At sea level on Earth, we breathe in an atmosphere where each cubic centimeter contains 10,000,000,000,000,000,000 molecules; by comparison the lunar atmosphere has less than 1,000,000 molecules in the same volume. That still sounds like a lot, but it is what we consider to be a very good vacuum on Earth. In fact, the density of the atmosphere at the moon's surface is comparable to the density of the outermost fringes of Earth's atmosphere where the International Space Station orbits.

There are some clues about what makes up the moon’s atmosphere. The Apollo 17 mission deployed an instrument called the Lunar Atmospheric Composition Experiment (LACE) on the moon's surface. It detected small amounts of a number of atoms and molecules including helium, argon, and possibly neon, ammonia, methane and carbon dioxide. From here on Earth, researchers using special telescopes that block light from the moon's surface have been able to make images of the glow from sodium and potassium atoms in the moon's atmosphere as they are energized by the sun.

This is the scene on television witnessed by millions on Earth as Neil Armstrong descends the Lunar Module ladder just prior to becoming the first human being to set foot on the Moon.

We think that there are several sources for gases in the moon's atmosphere. These include high energy photons and solar wind particles knocking atoms from the lunar surface, chemical reactions between the solar wind and lunar surface material, evaporation of surface material, material released from the impacts of comets and meteoroids, and out-gassing from the moon's interior. But which of these sources and processes are important on the moon? We still don't know (more info: https://www.nasa.gov/mission_pages/LADEE/news/lunar-atmosphere.html).

One of the critical differences between the atmospheres of Earth and the moon is how atmospheric molecules move. Here in the dense atmosphere at the surface of Earth, the molecules' motion is dominated by collisions between the molecules. However the moon's atmosphere is so thin, atoms and molecules almost never collide. Instead, they are free to follow arcing paths determined by the energy they received from the processes described above and by the gravitational pull of the moon. The technical name for this type of thin, collision-free atmosphere that extends all the way down to the ground is a "surface boundary exosphere

Extreme temperature differences

The moon rotates on its axis in about 27 days. Daytime on one side of the moon lasts about 13 and a half days, followed by 13 and a half nights of darkness. When sunlight hits the moon's surface, the temperature can reach 260 degrees Fahrenheit (127 degrees Celsius). When the sun goes down, temperatures can dip to minus 280 F (minus 173 C). Temperatures change all across the moon, as both the near and far side experience sunlight every lunar year, or terrestrial month, due to lunar rotation. The moon tilts on its axis about 1.54 degrees — much less than Earth's 23.44 degrees. This means the moon does not have seasons like Earth does. However, because of the tilt, there are places at the lunar poles that never see daylight.

The Diviner instrument on NASA's Lunar Reconnaissance Orbiter measured temperatures of minus 396 F (minus 238 C) in craters at the southern pole of the moon and minus 413 F (minus 247 C) in a crater at the northern pole. These super-cold brightness temperatures are, according to NASA, similar to those found on Pluto by NASA's New Horizon.  The temperature range on Pluto was found to be a comparable minus 400 to minus 360 F (minus 240 to minus 217 C). Astronauts on the moon were protected from the extreme temperatures by their spacesuits. The suits had several layers of insulating material covered by a highly reflective outer layer. The suits also had internal heaters and cooling systems.

Lots of impact craters are visible on the moon’s surface as a result of a very thin atmosphere.

In addition to the extreme differences in temperatures, the very thin atmosphere also means the moon’s surface is unprotected from cosmic rays, meteorites and solar winds. The surface of the Moon features a huge number of impact craters from comets and asteroids that have collided with the surface over time. Because the Moon lacks an atmosphere and has no weather, these craters remain well preserved. The very thin atmosphere means no sound can be heard on the moon and the sky always appears black. 

Finally, with respect to water/ice on the moon, in 2009, NASA sent a rocket into the Cabeus crater, near the moon’s south pole. The crash kicked up a plume of dust that rose almost ten miles above the crater’s rim, while the NASA satellite’s instruments recorded observations. Grains of water ice were part of the cloud, confirming that water exists in permanently shadowed regions near the poles of the moon.  Researchers think there was a time period when Earth and the moon were being bombarded by comets and asteroids, which contain a lot of water ice, and this might have delivered our own oceans and lakes as well as the moon’s icy deposits. However, a new analysis of moon rocks brought home from the Apollo 15 and 17 missions in the 1970s makes scientists think there was water in the moon’s interior that was brought to the surface during volcanic eruptions. This might also have happened on Earth.

Meteorologist Paul Dorian
Perspecta, Inc.
perspectaweather.com