We typically don't think about what's taking place on different moons in the Solar System (and beyond), but they actually have some amazing phenomena to offer. Today I'll be focusing on the four moons in the Solar System that are known to have collisional atmospheres. There are about ten total moons that are known to have atmospheres within our Solar System (which has over 170 total moons!), but today we'll be focusing on four of them.They are: Titan, Triton, Io, and Callisto. Buckle up, it's gonna be a long ride.
We'll begin our journey with Titan.
Titan is Saturn's largest moon and the second largest in the Solar System (Ganymede is the largest). It is the only moon in the Solar System with clouds and a dense atmosphere which resembles that of a planet. Scientists even theorize that the conditions on Titan are similar to Earth's early years. However, since Earth is closer to the Sun, it has always been warmer with a more habitable climate. According to NASA, in many respects,Titan is actually one of the most Earth-like worlds that have been found to date. Titan's atmosphere extends about 370 miles high (600 kilometers), which means it is a lot taller than Earth's atmosphere. Due to it's tall atmosphere, Titan was actually believed to be the largest moon in the Solar System for a long time. It wasn't until about 1980 when Voyager was close enough to Titan to discover that Ganymede dwarfed this moon. Titan's atmosphere is active and complex. It is composed mainly of nitrogen (95%) and methane (5%). There is also a presence of organic molecules that contain carbon and hydrogen, which often include oxygen and other similar elements that are found in Earth's atmosphere which are essential for life. There is a mystery which surround Titan's atmosphere. Since sunlight breaks down methane, scientists believe that there is another source which is replenishing what is lost, but it is not yet known. One theory is volcanic activity, but this has not been confirmed yet.
Titan's atmosphere may also escape in a similar way that Earth's does. Cassini detected polar winds that draw methane and nitrogen (charged from interactions with light) along Saturn's magnetic field and beyond the atmosphere. Something similar is also believed to happen on Earth with our own magnetic field.
There is also an abundance of methane lakes on Titan's surface, which are mainly concentrated near its South Pole. This is something special about Titan: methane is able to exist in a liquid state, with lakes and seas covering the moon. There is also a hydrological cycle (I know the name is confusing), but it's similar to what occurs on Earth with water. In fact, currently it's the only other place that we know of where this takes place. Since methane exists as a liquid on Titan, it is able to form clouds, which will cause methane rain. Clouds of methane ice and cyanide gas float over the surface of the moon. (Pretty cool, right?) It's just like Earth in most aspects, just with something that we're unfamiliar with in a liquid state.
Next up, Triton.
Triton is the largest of Neptune's 13 moons. It is the only large moon in the Solar System that has a retrograde orbit (orbits in the opposite direction of its planet's rotation). The surface of Triton is colder than any other planet or moon in our Solar System! In fact, it is so cold on Triton that EVERYTHING freezes, even gases. We're surprised that Triton even HAS an atmosphere. Now, although Triton has an atmosphere, it is VERY, VERY thin. It is so thin that the air pressure on Earth is 50,000 times higher than on the surface of Triton! Most of the air on Triton is nitrogen. Since everything freezes on Triton, nitrogen is even found on the surface in the form of frost. Some of this nitrogen frost warms up just a little, which allows it to be able to evaporate and form the little atmosphere that Triton has.
There are also several ice geysers near its South Pole, which shoot nitrogen, dust, and chemicals which contain methane high into its atmosphere. Triton has clouds of nitrogen ice particles in the lower parts of its atmosphere. There is also a layer of haze comprised of hydrocarbons, which forms when sunlight hits the methane molecules in Triton's atmosphere.
There also seem to be darker areas on the ground near the geysers on Triton, and scientists hypothesize that winds blew the darker materials from the geysers in that direction. Triton's atmosphere also has layers like Earth's: a troposphere, a thermosphere, and an exosphere. Its atmosphere extends about 500 miles or 800 kilometers above the surface.
Now we move on to Io.
Io is one of the four Galilean moons of Jupiter. It orbits closer to the cloud tops of Jupiter than our moon does to the Earth. It is strongly subjected to Jupiter's powerful gravitational field and highly intense radiation belts.
Io has a ton of volcanic activity, the most in the Solar System. It has lava flows, erupting with plumes of sulfurous material, and a changing atmosphere filled with noxious gases. Io has a thin atmosphere that does not remain bound to it for very long. As a small moon, Io does not have substantial gravity. It is estimated that one TON of material needs to be ejected into Io's atmosphere every SECOND in order to replenish the gases that escape from its atmosphere, but it is not yet known whether this material comes from volcanic gas or maybe even the evaporation of ices on the surface.
In terms of "weather" on Io, there are no clouds or lightning. However, once the particles from the atmosphere get into the magnetosphere, they create a donut-shaped cloud of material around the moon.
Lastly, we have Callisto.
Callisto is Jupiter's second largest moon and the third largest moon in our Solar System. Comparatively, it's about the same size as Mercury. It used to be thought of as "boring" and "dull" by scientists (ouch), but this is only because it seemed like this crater covered world didn't have much to offer. It did not have active volcanoes or shifting tectonic plates. However, things changed in the 1990s when NASA's Galileo spacecraft revealed that Callisto may have been hiding a beautiful secret the entire time: a salty ocean beneath its surface. That created promise for this moon to possibly harbor life.
In terms of its atmosphere, Callisto has a very tenuous atmosphere primarily composed of carbon dioxide. Since it has such a thin atmosphere, it is susceptible to being lost, and must be replenished. It could possibly be replenished by a slow sublimation of carbon dioxide ice from Callisto's icy crust.
Oxygen has not yet been directly observed in Callisto's atmosphere, but it is theorized that it is present in a much greater concentration than carbon dioxide, which again leads to the possibility of habitability. However, since there is no geologic activity, Callisto is considered a "dead world."
That marks the end of our journey, but there are many other moons that I could have discussed. Perhaps I will make a second part to this and highlight Ganymede or Europa, maybe even Enceladus as well. That's besides the point though. I just wanted to share how cool some of the moons in our Solar System are. Wait till we learn about the moons of exoplanets, they could be even better (and potentially habitable), but that's a question for another day (and also for James Webb *cough* *cough*).