An extremely high-altitude plume seen at Mars’ morning terminator

Abstract

The Martian limb (that is, the observed 'edge' of the planet) represents a unique window into the complex atmospheric phenomena occurring there. Clouds of ice crystals (CO2 ice or H2O ice) have been observed numerous times by spacecraft and ground-based telescopes, showing that clouds are typically layered and always confined below an altitude of 100 kilometres; suspended dust has also been detected at altitudes up to 60 kilometres during major dust storms. Highly concentrated and localized patches of auroral emission controlled by magnetic field anomalies in the crust have been observed at an altitude of 130 kilometres. Here we report the occurrence in March and April 2012 of two bright, extremely high-altitude plumes at the Martian terminator (the day-night boundary) at 200 to 250 kilometres or more above the surface, and thus well into the ionosphere and the exosphere. They were spotted at a longitude of about 195° west, a latitude of about -45° (at Terra Cimmeria), extended about 500 to 1,000 kilometres in both the north-south and east-west directions, and lasted for about 10 days. The features exhibited day-to-day variability, and were seen at the morning terminator but not at the evening limb, which indicates rapid evolution in less than 10 hours and a cyclic behaviour. We used photometric measurements to explore two possible scenarios and investigate their nature. For particles reflecting solar radiation, clouds of CO2-ice or H2O-ice particles with an effective radius of 0.1 micrometres are favoured over dust. Alternatively, the plume could arise from auroral emission, of a brightness more than 1,000 times that of the Earth's aurora, over a region with a strong magnetic anomaly where aurorae have previously been detected. Importantly, both explanations defy our current understanding of Mars' upper atmosphere.

A cometary origin for martian atmospheric methane

“…This mechanism may also explain the recently reported appearance of high-altitude dust plumes on Mars (Sánchez-Lavega et al, 2015).…”

“…Cometary meteor showers could also be responsible for the two highaltitude dust plumes observed over Mars (Sánchez-Lavega et al, 2015). These two optically visible dust plumes occurred at elevations >200 km above the martian surface, well above martian weather or dust storm phenomena (<60 km altitude).…”

“…2). The second dust plume (Sánchez-Lavega et al, 2015) was noticed on 12 March 2012, three days after the closest approach between Mars and the orbit of comet 275P/Hermann. The areal extent of meteor showers is in agreement with the areal extent observed for martian methane plumes.…”

Variability in the super-Earth 55 Cnc e

“…The highest volcanic plumes on Jupiter's moon Io (radius, RIo = 1821 km) have heights of ∼ 300-500 km or 0.16-0.27 RIo (Spencer et al 1997;McEwen et al 1998). Recently, the highest plume on Mars was reported at the day-night terminator with a height of ∼250 km (0.07 RMars) and a width of 500-1000 km, i.e with aspect ratio over 2:1 (Sánchez-Lavega et al 2015). The source of the Martian plume is currently unknown though apparently non-volcanic.…”

“…However, whereas extremely small planets (nearly Mercury-size) subject to intense irradiation can undergo substantial mass loss through thermal winds, super-Earths are unlikely to undergo such mass-loss due to their significantly deeper potential wells (Perez-Becker & Chiang 2013;Rappaport et al 2014). Thus, ejecta from volcanic eruptions on even the most irradiated super-Earths such as 55 Cnc e are unlikely to escape the planet and would instead display plume behaviour characteristic to the solar system (Spencer et al 1997(Spencer et al , 2007McEwen et al 1998;Sánchez-Lavega et al 2015). The extent and dynamics of the plumes if large enough can cause temporal variations in the planetary sizes and brightness temperatures and hence in the transit and occultation depths.…”