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| (Jan Erik Paulsen/Flickr/CC BY 2.0) |
You could catch a glimpse of something eerily beautiful if
you're in the right place at the right time and turn your attention to the sky:
Even though the Sun has already fallen below the horizon, wisps of cloud high
in the sky glow softly.
These are noctilucent clouds, or night-shining clouds, which
appear at dusk in the summer at high latitudes (but never observed from within
the polar circle). They are made up of ice crystal collections high in the
atmosphere, at altitudes of 76 to 85 kilometers (47 to 53 miles) above Earth's
surface.
These wisps are too faint to see throughout the day, but
once the Sun has fallen below the horizon, its last rays reach far into the atmosphere,
despite the fact that the star itself is no longer visible from the planet's
surface. These rays illuminate the clouds, making them shine against the
dimming sky. As a result, polar mesospheric clouds are another term for these
glowing clouds.
They're stunningly beautiful, as well as potentially
scientifically advantageous.
The sensitivity of these clouds has piqued interest, as they
occur only on the edge of viability in the upper atmosphere, where it's
extremely dry and incredibly cold, according to space physicist Richard
Collins.
They're a highly sensitive measure of temperature and/or
water vapor fluctuations in the upper atmosphere.
Collins and his colleagues proposed that these clouds were
caused by the presence of water vapor in the upper atmosphere. So they did the
only reasonable thing they could think of: they loaded water into a suborbital
rocket and fired it into the sky above Alaska to see if they could build a
noctilucent cloud.
They decided to launch the rocket into the winter pre-dawn
sky to avoid mixing their artificial clouds with naturally occurring ones, when
conditions tend to be much less favorable to natural noctilucent cloud
formation than in the summer.
The Super Soaker rocket mission was launched on January 26,
2018, with one sounding rocket carrying 220 kilograms (485 pounds) of water in
two canisters. Two more sounding rockets flew alongside, bearing TMA tracers to
map wind movement.
The water was released in a huge whoosh at an altitude of 85
kilometers. A faint noctilucent cloud was observed 18 seconds later by a laser
beam from a ground-based Rayleigh LIDAR. The cloud appeared to descend from a
peak altitude of 92 kilometers (57 miles) to 78 kilometers in three minutes (48
miles).
The team had to think beyond the box to figure out what had
happened and why the cloud had formed. They ran simulations of noctilucent
cloud formation because they didn't have direct measurements of cloud
temperature despite having atmospheric temperature measurements. The team was
able to infer the temperature change that occurred in the experiment thanks to
the simulated conditions under which the cloud formed.
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| (NASA's Wallops Flight Facility/Poker Flat Research Range/Zayn Roohi) |
The only way for the cloud to shape was for the temperature
to drop by around 25 degrees Celsius (45 degrees Fahrenheit). This suggested
that the temperature drop was caused by the water vapor itself, which was
emitted from the canisters. The temperature of the frost point, where water
vapor condenses into ice crystals, rose as humidity rose.
The combination of a decrease in temperature and an increase
in the frost point causes the two temperatures to cross, allowing water vapor
to freeze into ice crystals.
This is the first time, according to space physicist Irfan
Azeem, that anyone has experimentally demonstrated that the formation of polar
mesospheric clouds in the mesosphere is directly related to cooling by water
vapor.
This may explain why noctilucent clouds tend to accompany
space launches, according to the researchers. Water vapor is a natural effluent
in spacecraft exhaust, and scientists have found the connection many times
during the Arctic summer.
In the summer, the polar mesosphere experiences
"consistent and vigorous" upwelling, which undoubtedly transfers
water vapor from lower altitudes and causes the same effect as the Super Soaker
experiment. This could explain why space traffic production of mesospheric
clouds is more apparent in the summer than in the winter.
Despite the fact that space traffic water exhaust acts as a
reservoir for the formation of mesospheric clouds, this research shows that the
exhaust can actively cool the mesosphere and trigger the formation of
mesospheric clouds, even in the polar winter.
As a result, noctilucent cloud watchers can soon have a new
way to predict when they'll appear: by closely tracking space launch schedules.
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