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


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 storms1,2,3,4,5,6. Highly concentrated and localized patches of auroral emission controlled by magnetic field anomalies in the crust have been observed at an altitude of 130 kilometres7. 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 exosphere8,9. 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 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 detected7. Importantly, both explanations defy our current understanding of Mars’ upper atmosphere.

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Figure 1: A high-altitude plume at the Martian terminator.
Figure 2: Plume top altitude and its rapid changes.
Figure 3: Plume reflectivity and radiative transfer model comparison.
Figure 4: Atmospheric temperature profile, and water and carbon dioxide condensation temperatures.


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This work was supported by the Spanish MINECO project AYA2012-36666, FEDER, Grupos Gobierno Vasco IT765-13 and UPV/EHU UFI11/55. The IAA (CSIC) team was supported by the Spanish MINECO through the CONSOLIDER programme ASTROMOL CSD2009-00038 and AYA2011-30613-CO2-1. F.G.-G. is funded by a CSIC JAE-Doc contract co-financed by the European Social Fund.

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Authors and Affiliations



A.S.-L. coordinated the study and performed plume measurements, photometric calibration and participated in the models study; A.G.M. studied aurora and together with S.P.-H. performed radiative transfer modelling; E.G.-M. performed with A.S.L. the geometric modelling; M.A.L.-V. and F.G.-G. performed the GCM model calculations and aurora studies, and F.G.-G. performed the evaporation calculations; J.M.G.-F., C.P. and M.D. performed plume measurements; W.J., D. Parker, J.P. and D. Peach performed ground-based images. All authors discussed the results and commented on the manuscript.

Corresponding author

Correspondence to A. Sánchez-Lavega.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Images of the 2012 plume event (ringed) on 12–20 March.

Dates in March (and authors) are as follows: a, 12 (M.D.); b, 15 (D. Peach); cf, 20, plume in rotation (W.J.). Time indicated at top left of each panel is in UTC.

Extended Data Figure 2 Images of the 2012 plume events (ringed) on 22 March and 13 April.

a, First event on 22 March, 04:12 UTC (image by W.J.); b, second event on 13 April, 20:03 UTC (image by D. Peach).

Extended Data Figure 3 Images of the 2012 plume event (ringed) at different wavelengths on 21 March.

a-c, Images by J.P., df, images by D. Parker, with filters indicated: a, d, B (blue); b, e, G (green); c, f, R (red). Time indicated is in UTC.

Extended Data Figure 4 Martian viewing geometry.

a, Angle definitions with the simulated protrusion of altitude H located at point c and out of the illuminated part of the disk near the limb. b, Top view, taking as a reference the planet’s terminator and definition of γ and α angles when the cloud is on the equator (but the latitude of the subsolar point is not zero). Green arrows represent the projected cloud altitude as seen from Earth in the extreme situations when the cloud is on the terminator and follows the grazing sunlight, and when it follows the planet’s radius. To simplify the figure, and without loss of generality, the sub-Earth point is placed on the arc linking the subsolar point and the cloud base. c, General side view of the geometry of the planet’s projected shadow.

Extended Data Figure 5 Hubble Space Telescope images of the event on May 17 1997.

Wavelengths and times in UTC were: a, 255 nm (17:27); b, 410 nm (17:35); c, 502 nm (17:38); d, 588 nm (17:50); e, 673 nm (17:41); f, 1,042 nm (17:47); g, Colour composite. Plume ringed in af, arrowed in g. Table at bottom identifies each image and its HST number, and also shows filters used, giving their central wavelength and bandwidth (FWHM).

Extended Data Figure 6 Radiative transfer model fit for the 2012 event.

This is an example of the degeneracy of the model solution due to the narrow wavelength range covered in the 2012 event. Model fit as follows: solid lines, CO2; dot-dashed lines, H2O particles; blue, data for an effective radius reff = 0.1 μm; red, data for reff = 1.0 μm; stars, wavelengths used in the calculations. As in Fig. 3, open black triangles show the observed reflectivity of the 2012 cloud. The error bars represent the average quadratic deviation of the measured reflectivity in the integration box.

Extended Data Figure 7 Assessment of the radiative transfer model fit for the 1997 event.

ai, Colours show values of χ2 (for measured I/F versus model calculation, colour scale at right) for the effective radius (reff in μm) versus optical depth (τN at 502 nm), and for different particle types and values of the indicated particle variance (νeff, shown in parentheses) as follows. a, CO2 (0.5); b, CO2 (1.0); c, CO2 (2.0); d, DST (dust, 0.5); e, DST (dust, 1.0); f, DST (dust, 2.0); g, H2O (0.5); h, H2O (1.0); and i, H2O (2.0). The calculations are for a vertical extension of the cloud with D = 100 km, and they provide the best-fitting values of the whole free parameter space.

Extended Data Figure 8 Assessment of the radiative transfer model fit for the 2012 event.

As Extended Data Fig. 7 in terms of variables plotted, particle types and particle variance, but for the 2012 event.

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Sánchez-Lavega, A., Muñoz, A., García-Melendo, E. et al. An extremely high-altitude plume seen at Mars’ morning terminator. Nature 518, 525–528 (2015).

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