To better understand how mesospheric clouds work, researchers from the NASA decided to recreate one. Rich in lessons, the results of the experiment were recently published in the scientific journal JGR space physics.
Also called noctilucent clouds or noctiluques, the polar mesospheric clouds are probably the most exotic in Earth’s atmosphere. As their name suggests, they occur in the mesosphere at high latitudes at an altitude of around 80 kilometers. Under their dreamlike air, these formations appear mainly in summer, the season when the upper atmosphere reaches its lowest temperature. Sometimes they even get lost in mid-latitudes. However, the spectacle is fleeting. Indeed, they only become visible when the sun passes below the horizon and its rays illuminate them from below.
A mesospheric cloud in the polar night
Although these clouds are known to be composed of fine ice crystals, the mechanisms leading to the condensation of water vapor at these extreme altitudes remain poorly understood. In addition, their occurrence seems to be increasing in recent decades, which raises the question of the link with global warming. ” They are a very sensitive indicator of changes in the upper atmosphere : temperature changes and / or water vapor changes Says Richard Collins, physicist and researcher at the University of Alaska.
To find out more, researchers from the NASA carried out an experiment called Super Soaker aiming to recreate one of these clouds. To do this, scientists launched a suborbital rocket into the upper mesosphere above Alaskan lands. When reaching the desired altitude, the launcher then released a small amount of water. The test took place on January 26, 2018, at a period deliberately unfavorable to the formation of noctilucent clouds. ” We wanted to make sure not to mix artificial and natural clouds », Explains Irfan Azeem, co-author of the study. ” This way we could be sure that any mesospheric cloud observed was attributable to the Super Soaker experiment. “.
The amazing role of water vapor
The least we can say is that the project has been instructive. Thus, eighteen seconds after the release of the 220 kilograms of water contained in the reservoir, the radars detected the appearance of a mesospheric cloud. By calculating the changes required for the observed training to take place, the authors notably found that‘a sudden cooling must have occurred.
” We don’t have direct measurements of the cloud temperature, but we can infer the temperature change via what we think is necessary for the cloud to form “, Notes Richard Collins, lead author of the paper. ” With the amount of water present, the only way we could get a cloud formation was to say that in the cloud, there had been a drop in temperature of about 25 ° C “. In other words, water vapor directly participates in the formation of mesospheric clouds.
” These results indicate that the injected water not only acts as a reservoir for the production of mesospheric clouds, but also actively cools the mesosphere to induce cloud formation. », Concludes the study in its summary. ” This is the first time that someone has experimentally demonstrated that the formation of noctilucent clouds is directly related to cooling by the water vapor itself », Adds Irfan Azeem. These data have direct implications in terms of space traffic since rocket launches inject a large amount of water vapor into the upper atmosphere. Thus, this could therefore explain at least in part the upsurge in noctilucent clouds for several decades.