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

We report a previously unnoticed annually repeating phenomenon consisting of the daily formation of an extremely elongated cloud extending as far as 1800 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 hours with a velocity of around 170 m/s in the mesosphere (~45 km over the areoid). The cloud formation then ceases, it detaches from its formation point, and continues moving westward until it evaporates before the afternoon, when most sun-synchronous orbiters observe. 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, whilst in MY34 the beginning of its formation was delayed compared to 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

Regolith Depth, Mobility, and Variability on Vesta from Dawn's Low Altitude Mapping Orbit

Regolith, the fragmental debris layer formed from impact events of all sizes, covers the surface of all asteroids imaged by spacecraft to date. Here we use Framing Camera (FC) images [1] acquired by the Dawn spacecraft [2] from its low-altitude mapping orbit (LAMO) of 210 km (pixel scales of ~20 m) to characterize regolith depth, variability, and mobility on Vesta, and to locate areas of especially thin regolith and exposures of competent material. These results will help to evaluate how the surface of this differentiated asteroid has evolved over time, and provide key contextual information for understanding the origin and degree of mixing of the surficial materials for which compositions are estimated [3,4] and the causes of the relative spectral immaturity of the surface [5]. Vestan regolith samples, in the form of howardite meteorites, can be studied in the laboratory to provide complementary constraints on the regolith process [6]

Dynamics of the extremely elongated cloud on Mars Arsia Mons volcano

Starting in September 2018, a daily repeating extremely elongated cloud was observed extending up to 1800km from the Mars Arsia Mons volcano. We study this Arsia Mons Elongated Cloud (AMEC) using images from VMC, HRSC, and OMEGA on board Mars Express, IUVS on MAVEN, MCC on Mars Orbiter Mission (MOM), MARCI on MRO, and Visible Camera on Viking 2 orbiter. We study the daily cycle of this cloud, showing how the morphology and other parameters of the cloud evolved rapidly with local time. The cloud expands every morning from the western slope of the volcano, at a westward velocity of around 160m/s, and an altitude of around 45km over martian areoid. The expansion starts with sunrise, and resumes around 2.5 hours later, when cloud formationresumes and the elongated tail detaches from the volcano and keeps moving westward until it evaporates before afternoon, when most sun-synchronous missions observe. This daily cycle repeated regularly for at least 80 sols in 2018 (Martian Year 34). We find in images from past years that this AMEC is an annually repeating phenomenon that takes place around the Solar Longitude range 220º-320º. We study the AMEC in Martian Year 34 in terms of Local Time and Solar Longitude, and then compare with observations from previous years, in search for interannual variations, taking into account the possible influence of the recent Global Dust Storm

Dynamics of the extremely elongated cloud on Mars Arsia Mons volcano

Starting in September 2018, a daily repeating extremely elongated cloud was observed extending from the Mars Arsia Mons volcano. We study this Arsia Mons Elongated Cloud (AMEC) using images from VMC, HRSC, and OMEGA on board Mars Express, IUVS on MAVEN, and MARCI on MRO. We study the daily cycle of this cloud, showing how the morphology and other parameters of the cloud evolved with local time. The cloud expands every morning from the western slope of the volcano, at a westward velocity of around 150m/s, and an altitude of around 30-40km over the local surface. Starting around 2.5 hours after sunrise (8.2 Local True Solar Time, LTST), the formation of the cloud resumes, and the existing cloud keeps moving westward, so it detaches from the volcano, until it evaporates in the following hours. At this time, the cloud has expanded to a length of around 1500km. Short time later, a new local cloud appears on the western slope of the volcano, starting around 9.5 LTST, and grows during the morning. This daily cycle repeated regularly for at least 90 sols in 2018, around Southern Solstice (Ls 240-300) in Martian Year (MY) 34. According with these and previous MEx/VMC observations, this elongated cloud is a seasonal phenomenon occurring around Southern Solstice every Martian Year. We study the interannual variability of this cloud, the influence of the Global Dust Storms in 2018 on the cloud’s properties (Sánchez-Lavega et al., Geophys. Res. Lett. 46, 2019), and its validity as a proxy for the global state of the Martian atmosphere (Sánchez-Lavega et al., J. Geophys. Res., 123, 3020, 2018). We discuss the physical mechanisms behind the formation of this peculiar cloud in Mars

The Agilkia boulders/pebbles size-frequency distributions: OSIRIS and ROLIS joint observations of 67P surface

By using the images acquired by the OSIRIS (Optical, Spectroscopic and Infrared Remote Imaging System) and ROLIS (ROsetta Lander Imaging System) cameras, we derive the size- frequency distribution (SFD) of cometary pebbles and boulders covering the size range 0.05- 30.0 m on the Agilkia landing site. The global SFD measured on OSIRIS images, reflects the different properties of the multiple morphological units present on Agilkia, combined with selection effects related to lifting, transport and redeposition. Contrarily, the different ROLIS SFD derived on the smooth and rough units may be related to their different regolith thickness present on Agilkia. In the thicker, smoother layer, ROLIS mainly measures the SFD of the airfall population which almost completely obliterates the signature of underlying boulders up to a size of the order of 1 m. This is well matched by the power-law index derived analysing coma particles identified by the grain analyser Grain Impact Analyser and Dust Accumulator. This result confirms the important blanketing dynamism of Agilkia. The steeper SFD observed in rough terrains from 0.4 to 2 m could point out intrinsic differences between northern and southern dust size distributions, or it may suggest that the underlying boulders 'peek through' the thinner airfall layer in the rough terrain, thereby producing the observed excess in the decimetre size range. Eventually, the OSIRIS SFD performed on the Philae landing unit may be due to water sublimation from a static population of boulders, affecting smaller boulders before the bigger ones, thus shallowing the original SFD

Orbital Observations of Dust Lofted by Daytime Convective Turbulence

Over the past several decades, orbital observations of lofted dust have revealed the importance of mineral aerosols as a climate forcing mechanism on both Earth and Mars. Increasingly detailed and diverse data sets have provided an ever-improving understanding of dust sources, transport pathways, and sinks on both planets, but the role of dust in modulating atmospheric processes is complex and not always well understood. We present a review of orbital observations of entrained dust on Earth and Mars, particularly that produced by the dust-laden structures produced by daytime convective turbulence called “dust devils”. On Earth, dust devils are thought to contribute only a small fraction of the atmospheric dust budget; accordingly, there are not yet any published accounts of their occurrence from orbit. In contrast, dust devils on Mars are thought to account for several tens of percent of the planet’s atmospheric dust budget; the literature regarding martian dust devils is quite rich. Because terrestrial dust devils may temporarily contribute significantly to local dust loading and lowered air quality, we suggest that martian dust devil studies may inform future studies of convectively-lofted dust on Earth

Science goals and new mission concepts for future exploration of Titan's atmosphere geology and habitability: Titan POlar Scout/orbitEr and In situ lake lander and DrONe explorer (POSEIDON)

In response to ESA’s “Voyage 2050” announcement of opportunity, we propose an ambitious L-class mission to explore one of the most exciting bodies in the Solar System, Saturn’s largest moon Titan. Titan, a “world with two oceans”, is an organic-rich body with interior-surface-atmosphere interactions that are comparable in complexity to the Earth. Titan is also one of the few places in the Solar System with habitability potential. Titan’s remarkable nature was only partly revealed by the Cassini-Huygens mission and still holds mysteries requiring a complete exploration using a variety of vehicles and instruments. The proposed mission concept POSEIDON (Titan POlar Scout/orbitEr and In situ lake lander DrONe explorer) would perform joint orbital and in situ investigations of Titan. It is designed to build on and exceed the scope and scientific/technological accomplishments of Cassini-Huygens, exploring Titan in ways that were not previously possible, in particular through full close-up and in situ coverage over long periods of time. In the proposed mission architecture, POSEIDON consists of two major elements: a spacecraft with a large set of instruments that would orbit Titan, preferably in a low-eccentricity polar orbit, and a suite of in situ investigation components, i.e. a lake lander, a “heavy” drone (possibly amphibious) and/or a fleet of mini-drones, dedicated to the exploration of the polar regions. The ideal arrival time at Titan would be slightly before the next northern Spring equinox (2039), as equinoxes are the most active periods to monitor still largely unknown atmospheric and surface seasonal changes. The exploration of Titan’s northern latitudes with an orbiter and in situ element(s) would be highly complementary in terms of timing (with possible mission timing overlap), locations, and science goals with the upcoming NASA New Frontiers Dragonfly mission that will provide in situ exploration of Titan’s equatorial regions, in the mid-2030s

The Color of 4 Vesta and Lithology Diversity: First Results from Dawn Survey Orbit

The FC cameras onboard the Dawn spacecraft are expected to map the asteroid 4 Vesta in seven different colors from Survey Orbit in August 2011. We will present the first immediate results of the spectral mapping of the visible surface from FC images along with their association with surface compositional units. The first medium resolution observations of Vesta have been performed in July 201