In September and October 2018, ESA’s Mars Express orbiter observed a spectacular water ice cloud — named the Arsia Mons Elongated Cloud (AMEC) — extending as far as 1,800 km (1,118 miles) westward from the 20-km- (12.4-mile) tall Arsia Mons volcano on Mars. In new research, planetary scientists studied the AMEC cloud using images from different orbiters, including Mars Express, NASA’s Mars Atmosphere and Volatile Evolution (MAVEN), Viking 2, Mars Reconnaissance Orbiter (MRO), and ISRO’s Mars Orbiter Mission (MOM).
The white, elongated cloud in the vicinity of the Arsia Mons volcano, close to the planet’s equator. Image credit: ESA / GCP / UPV / EHU Bilbao / CC BY-SA 3.0 IGO.
Arsia Mons is the southernmost in a trio of giant Martian shield volcanoes known collectively as Tharsis Montes; the others are Pavonis Mons and Ascraeus Mons. It is rivaled only by Olympus Mons in terms of its volume.
This shield volcano was built up over billions of years, though the details of its lifecycle are still being worked out.
Arsia Mons is the only low-latitude location on Mars where clouds are seen — and the only one of numerous similar volcanoes in the region to possess such a veil of cloud — throughout the spring and summer seasons.
The AMEC cloud is difficult to observe in its entirety due to the fast, changeable dynamics of the Martian atmosphere and the constraints of many spacecraft orbits, limiting our knowledge of how and why it forms and changes over time.
This digital-image mosaic of Mars’ Tharsis plateau shows the extinct volcano Arsia Mons. It was assembled from images that NASA’s Viking 1 orbiter took during its 1976-1980 working life at Mars. Image credit: NASA / JPL / USGS.
“To clear these hurdles, we used one of Mars Express’ secret tools — the Visual Monitoring Camera (VMC),” said Dr. Jorge Hernández-Bernal, a researcher at the University of the Basque Country.
Dr. Hernández-Bernal and colleagues combined the VMC observations with those from two other Mars Express instruments — OMEGA and HRSC — and from several other spacecraft, namely MAVEN, MRO, Viking 2, and MOM.
“We were especially excited when we dug into Viking 2’s observations from the 1970s,” Dr. Hernández-Bernal said.
“We found that this huge, fascinating cloud had already been partially imaged that long ago — and now we’re exploring it in detail.”
Hernández-Bernal et al. reveal the daily and seasonal behavior of the AMEC cloud, using new observations from the Visual Monitoring Camera on ESA’s Mars Express, along with data from other Mars orbiters. Image credit: ESA.
The findings revealed that, at its largest, the AMEC cloud measures some 1,800 km in length and 150 km (93 miles) across.
It is the biggest orographic cloud ever seen on Mars, meaning that it forms as a result of wind being forced upwards by topographic features — such as mountains or volcanoes — on a planetary surface.
In this case, Arsia Mons perturbs the Martian atmosphere to trigger the formation of the cloud.
Moist air is then driven up the flanks of the volcano in updrafts, later condensing at higher, and far cooler, altitudes.
The cloud undergoes a rapid daily cycle that repeats every morning for several months.
It begins growing before sunrise on the western slope of Arsia Mons before expanding westwards for two and a half hours, growing remarkably fast — at over 600 kmh (373 mph) — at an altitude of 45 km (28 miles).
It then stops expanding, detaches from its initial location, and is pulled further westwards still by high-altitude winds, before evaporating in the late morning as air temperatures increase with the rising Sun.
“Many Mars orbiters cannot begin observing this part of the surface until the afternoon due to the properties of their orbits, so this really is the first detailed exploration of this interesting feature — and it’s made possible by not only Mars Express’ diverse suite of instruments, but also its orbit,” said Dr. Agustin Sánchez-Lavega, a researcher at the University of the Basque Country.
The findings are published in the Journal of Geophysical Research: Planets.
J. Hernández-Bernal et al. An Extremely Elongated Cloud Over Arsia Mons Volcano on Mars: I. Life Cycle. Journal of Geophysical Research: Planets, published online December 20, 2020; doi: 10.1029/2020JE006517