title: 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.
authors: 1] Departamento de Física Aplicada I, ETS Ingeniería, Universidad del País Vasco, Alameda Urquijo s/n, 48013 Bilbao, Spain [2] Unidad Asociada Grupo Ciencias Planetarias UPV/EHU-IAA (CSIC), Alameda Urquijo s/n, 48013 Bilbao, Spain.
authors: A
authors: A
authors: Sánchez-Lavega
authors: European Space Agency, ESTEC, 2201 AZ Noordwijk, The Netherlands.
authors: A García
authors: AG
authors: Muñoz
authors: 1] Departamento de Física Aplicada I, ETS Ingeniería, Universidad del País Vasco, Alameda Urquijo s/n, 48013 Bilbao, Spain [2] Fundació Observatori Esteve Duran, Montseny 46, 08553 Seva (Barcelona), Spain.
authors: E
authors: E
authors: García-Melendo
authors: 1] Departamento de Física Aplicada I, ETS Ingeniería, Universidad del País Vasco, Alameda Urquijo s/n, 48013 Bilbao, Spain [2] Unidad Asociada Grupo Ciencias Planetarias UPV/EHU-IAA (CSIC), Alameda Urquijo s/n, 48013 Bilbao, Spain.
authors: S
authors: S
authors: Pérez-Hoyos
authors: Fundació Observatori Esteve Duran, Montseny 46, 08553 Seva (Barcelona), Spain.
authors: J M
authors: JM
authors: Gómez-Forrellad
authors: Commission des Surfaces planétaires, Société Astronomique de France, 3, rue Beethoven, 75016 Paris, France.
authors: C
authors: C
authors: Pellier
authors: Commission des Surfaces planétaires, Société Astronomique de France, 3, rue Beethoven, 75016 Paris, France.
authors: M
authors: M
authors: Delcroix
authors: 1] Unidad Asociada Grupo Ciencias Planetarias UPV/EHU-IAA (CSIC), Alameda Urquijo s/n, 48013 Bilbao, Spain [2] Instituto de Astrofísica de Andalucía, CSIC, Glorieta de la Astronomia, 3, 18008 Granada, Spain.
authors: M A
authors: MA
authors: López-Valverde
authors: 1] Unidad Asociada Grupo Ciencias Planetarias UPV/EHU-IAA (CSIC), Alameda Urquijo s/n, 48013 Bilbao, Spain [2] Instituto de Astrofísica de Andalucía, CSIC, Glorieta de la Astronomia, 3, 18008 Granada, Spain.
authors: F
authors: F
authors: González-Galindo
authors: Association of Lunar and Planetary Observers, 200 Lawrence Drive, West Chester, Pennsylvania 19380, USA.
authors: W
authors: W
authors: Jaeschke
authors: Association of Lunar and Planetary Observers, 12911 Lerida Street, Coral Gables, Florida 33156, USA.
authors: D
authors: D
authors: Parker
authors: Association of Lunar and Planetary Observers, Charleston, 570 Long Point Road STE 230, Mount Pleasant, South Carolina 29464, USA.
authors: J
authors: J
authors: Phillips
authors: British Astronomical Association, Burlington House, Piccadilly, London W1J 0DU, UK.
authors: D
authors: D
authors: Peach