High in the mesosphere above Earth’s North and South Poles, feathery, glowing clouds occasionally stripe the night sky. Similarly, when spacecraft are launched, a similar noctilucent, or night-shining, cloudy effect occurs as the machine sheds water vapor in its exhaust.
To investigate how these glowing clouds form, astrophysicists used NASA’s “Super Soaker” rocket to make one, reports Michelle Starr for Science Alert. The team is the first to demonstrate noctilucent cloud formation is linked to water vapor cooling. The results were published last month in the Journal of Geophysical Research: Space Physics.
Located above the stratosphere, the mesosphere is very dry and contains the coldest temperatures in Earth's atmosphere at -130 degrees Fahrenheit. In late spring and summer, noctilucent clouds—also known as polar mesospheric clouds (PMCs)—naturally materialize about 47 to 53 miles above Earth’s surface in the dark polar skies when the sun sets below the horizon. The clouds seem to form as water vapor freezes into ice crystals at this height.
To study the phenomenon further, space physicist Richard Collins of the University of Alaska in Fairbanks and his team launched a suborbital rocket filled with water, dubbed the Super Soaker, into the Alaskan sky to try and create an artificial PMC.
"What has attracted a lot of interest in these clouds is their sensitivity – they're occurring just on the edge of viability in the upper atmosphere, where it's incredibly dry and incredibly cold. They're a very sensitive indicator of changes in the upper atmosphere – changes in temperature and/or changes in water vapour," Collins says in a statement.
Since the noctilucent clouds only form in the summer, the researchers launched their rocket in winter on January 26, 2018, when atmospheric conditions made it so a PMC could not naturally occur.
"We wanted to make sure to avoid mixing artificially created and naturally occurring PMCs. That way we could be confident that any PMC we observed was attributable to the Super Soaker experiment," says Irfan Azeem, a space physicist at Astra, LLC, and co-author of the study, in a statement.
The launched rocket carried two canisters filled with 485 pounds of water that were released in an explosion at about 53 miles from Earth's surface, Miles Hatfield writes for NASA. Within 18 seconds after the detonation, a laser radar detected a faint PMC.
To further understand how the PMC formed, researchers took the data they collected from the rocket launch and plugged it into a computer simulation that modeled PMC production. The team found that the only way luminescent clouds could form was from a drop in temperature, Mark Zastrow reports for Astronomy.
In fact, the water released by the rocket may have caused the dip in temperature of about 45 degrees Fahrenheit. This increase in humidity in extreme cold temperatures likely encouraged ice crystals to form, which then reflect the last rays of sunlight to create the cloud’s signature glow, Science Alert reports.