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

Examination of amateur observations of Mars shows atmospheric plumes 200 to 250 kilometres high that are observed in the morning but not in the evening over a period of more than a week; our current understanding of Martian atmospheric dynamics and plume formation cannot account for the creation of such enormous plumes. Mars's mystery plume In March and April 2012, amateur astronomers observing Mars recorded what seemed like an eruption of some kind, a protrusion from the planet's disk that took about 10 hours to develop and lasted for more than a week. Agustin Sánchez-Lavega and colleagues have now analysed these observations in detail and describe the highest-reaching plume-like feature ever detected on Mars. It reached an altitude of 250 kilometres — previous high-altitude features reached no higher than 100 kilometres — and extended more than 500 kilometres in both the north–south and east–west directions. Suggested explanations for this mysterious plume include clouds of CO 2 or H 2 O ice or a phenomenon related to auroral emissions, but our current understanding of Martian atmospheric dynamics and plume formation cannot account for the creation of such enormous plumes. 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 (CO 2 ice or H 2 O 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 1 , 2 , 3 , 4 , 5 , 6 . 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 7 . 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 8 , 9 . 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 hou