By Meghan Bartels
It's baaaaaaack …
A weird long cloud has formed so
many times over the same Martian volcano that scientists have given up and
named it.
Meet the Arsia Mons Elongated
Cloud, or AMEC. Its long bright trail has become a familiar feature over the
peak known as Arsia Mons, to the southeast of the more famous Olympus Mons. Although
the cloud comes and goes over the volcano, scientists say it isn't formed by
the volcano itself. And it is timely: Scientists affiliated with Europe's Mars
Express orbiter were waiting for it to show up again on its yearly cycle.
"We have been investigating
this intriguing phenomenon and were expecting to see such a cloud form around
now," Jorge Hernandez-Bernal, a Ph.D. candidate at the University of the
Basque Country in Spain and the lead author of the ongoing study, said in a
statement released by the European Space Agency (ESA), which runs the
spacecraft.
Photos: Red Planet views from
Europe's Mars Express
"This elongated cloud forms
every Martian year during this season around the southern solstice, and repeats
for 80 days or even more," Hernandez-Bernal said. "However, we don't
know yet if the clouds are always quite this impressive."
So far, scientists have caught
the cloud clocking in as long as 1,100 miles (1,800 kilometers), according to
ESA. The tail-like structure is made of water ice, and despite its location
over Arsia Mons, it isn't formed by the volcano itself, scientists said, but
instead by the way local winds interact with the topography.
And the Arsia Mons Elongated
Cloud doesn't just come, stick around for a while, then dissipate. It forms and
fades over the course of a few hours each local morning, then returns the next
day. That makes the weird cloud difficult to study from orbit around the Red
Planet.
But Mars Express is uniquely
qualified to do so. It carries an instrument called the Visual Monitoring
Camera, which can photograph an unusually wide swath of the planet in a single
frame. And the spacecraft's orbit lines up to put Arsia Mons in its view during
the morning hours when the cloud is visible.
"The extent of this huge cloud can't be
seen if your camera only has a narrow field of view, or if you're only
observing in the afternoon," Eleni Ravanis, a graduate trainee on the
Visual Monitoring Camera team, said in the statement. "Luckily for Mars
Express, the highly elliptical orbit of the spacecraft, coupled with the wide field
of view of the VMC instrument, lets us take pictures covering a wide area of
the planet in the early morning. That means we can catch it!"
The scientists last spotted
Arsia Mons' tail-like cloud in September and October 2018. At the time and
again now, the days are the shortest of the year in the Red Planet's northern
hemisphere and the longest of the year in the southern hemisphere. Arsia Mons
itself is located just a bit south of the Martian equator and stretches to an
altitude of about 12 miles (20 km).
Scientists hope that by
continuing to study the strange cloud, they can begin to understand how long it
has been making its appearances and why it appears only in the mornings.