An Extremely Elongated Cloud Over Arsia Mons Volcano on Mars: I. Life Cycle

Abstract

We report a previously unnoticed annually repeating phenomenon consisting of the daily formation of an extremely elongated cloud extending as far as 1,800 km westward from Arsia Mons. It takes place in the solar longitude (Ls) range of ∼220°-320°, around the Southern solstice. We study this Arsia Mons Elongated Cloud (AMEC) using images from different orbiters, including ESA Mars Express, NASA MAVEN, Viking 2, MRO, and ISRO Mars Orbiter Mission (MOM). We study the AMEC in detail in Martian year (MY) 34 in terms of local time and Ls and find that it exhibits a very rapid daily cycle: the cloud growth starts before sunrise on the western slope of the volcano, followed by a westward expansion that lasts 2.5 h with a velocity of around 170 m/s in the mesosphere (∼45 km over the areoid). The cloud formation then ceases, detaches from its formation point, and continues moving westward until it evaporates before the afternoon, when most sun-synchronous orbiters make observations. Moreover, we comparatively study observations from different years (i.e., MYs 29-34) in search of interannual variations and find that in MY33 the cloud exhibits lower activity, while in MY34 the beginning of its formation was delayed compared with other years, most likely due to the Global Dust Storm. This phenomenon takes place in a season known for the general lack of clouds on Mars. In this paper we focus on observations, and a theoretical interpretation will be the subject of a separate paper. HERNÁNDEZ-BERNAL ET AL.

A Long‐Term Study of Mars Mesospheric Clouds Seen at Twilight Based on Mars Express VMC Images

“…We have analyzed 4621 images in 1891 independent observations spanning from MY28, Ls 344º, to MY35, Ls 229º (November 2007 to June 2020). The coverage trough time is not homogeneous (Ravanis et al, 2020), and it is especially low in MYs 31 and 32. Due to the MEX orbit, most of the observed areas correspond to morning twilight (see supporting Fig.…”

“…We base our study on the MEX/VMC image dataset (Ravanis et al, 2020), which includes more than 2000 independent observations (each observation lasts several minutes and contains a number of consecutive images) in MYs 29-35, mostly from Mars Express apocenters, where VMC captures full disk images of the planet.…”

“…To include these effects we have developed a numerical method, detailed in the supporting material text S2. For image navigation, geometry, topography, and Mars time calculations, we used the methods described in Hernández-Bernal et al (2020).…”

“…A long term study of Mars mesospheric clouds seen at twilight based on Mars Express VMC images. Hernández-Bernal et al 2021 manuscript accepted for publication in Geophysical Research Letters This document is distributed under CC BY-SA 3.0 IGO license In this paper, we study twilight clouds (Figure 1) with the aid of the Visual Monitoring Camera (VMC; Sánchez-Lavega et al, 2018; see also Ravanis et al, 2020) onboard Mars Express (MEX). VMC is probably the best suited among available instruments for this kind of study, as it has a wide field of view, takes regular full disk images showing the terminator of Mars, and its archive covers 6 Martian Years (MY; from MY 29 to MY 35).…”

“…More recently, the Curiosity rover has also imaged clouds at twilight (Newman, 2019). In this paper, we study twilight clouds (Figure 1) with the aid of the Visual Monitoring Camera (VMC; Sánchez-Lavega et al, 2018; see also Ravanis et al, 2020) onboard Mars Express (MEX). VMC is probably the best suited among available instruments for this kind of study, as it has a wide field of view, takes regular full disk images showing the terminator of Mars, and its archive covers 6 Martian Years (MY; from MY 29 to MY 35).…”