Nature Letter reports a remarkable terminator projection on Mars

2015 March 22

Richard McKim

In 2012 March and April a large, recurrent terminator projection extending over 500km in dimension was seen at around 45°S latitude upon Mars by a number of observers, including several amateur astronomers monitoring the planet. In Figure 1 we show a fine rotational sequence made by Jim Phillips (Charleston, South Carolina, USA) with a 203mm apochromatic refractor.

First announced in BAA E-bulletin no.661 (2012 Mar 22), the event was described, illustrated and compared with previous phenomena by the writer in the Journal [123(1), 6-7 (2013)]. Located near longitude 195°, it resembled an earlier event seen by E.-M. Antoniadi in 1933. Terminator projections are hard to measure precisely, but dust clouds are not observed above 60km, and Antoniadi’s result fell within this range. On a drawing or image they look misleadingly high when seen against the terminator, but the height is reduced by perspective to a more modest figure.

A long list of such data is contained in the writer’s BAA Memoir (‘Telescopic Martian Dust Storms: A Narrative and Catalogue’, Mem. Brit. Astron. Assoc., 44 (1999)): phenomena of this type are often identified as dust clouds. Indeed, a very early example witnessed by Lowell & Slipher in 1903 was identified as a dust storm by Lowell from its colour and low height. It was located over Chryse Planitia, an area prone to such events. Further projection phenomena were looked for without success in 2012 but we did not see them again.

Measurements have subsequently shown that the projection seen in 2012 was (at least at its maximum development) not just projecting beyond the terminator but beyond the morning limb of the planet. At the time the phase was quite high, being just a few weeks after opposition. Projection above the limb would imply an enormous height, far too high to be dust, and probably too high even for a water-ice cloud.

Six of the amateur astronomers who secured images (including the UK’s Damian Peach) teamed up with three professionals headed by A. Sanchez-Lavega to measure their images in detail, and thereby to describe this unique event  in a Letter to Nature published this February (http://www.nature.com/nature/journal/v518/n7540/full/nature14162.html). They consistently obtained heights of over 200km, which at first sight looks improbable, placing the event well into the martian ionosphere, but there can be no doubt about the validity of these results.

No martian clouds whether dust, carbon dioxide or water ice have been seen above 100km altitude in the past. No conclusion as to the composition of the phenomenon was reached, but the authors have suggested that either 0.1 micron particles of carbon dioxide or water ice may have been involved, or even possibly an auroral effect of a type previously unrecognised. If so it would have to have been remarkably bright in order for it to be seen from Earth. Some of the early observers did draw terminator or limb projections which appeared to be far too high to be plausible: perhaps now is the time to objectively re-examine these older records, and moreover to be on the lookout for modern-day recurrences.
The Nature authors used the images by the amateur co-authors as well as others available online at the ALPO and ALPO-Japan websites. It is likely that the BAA database (which is not online) contains other pertinent data, for we receive a good deal of quality data that the observers do not post on the Web. However, the final analysis of the 2012 observations by our team of over 100 observers is not yet complete, and the writer will examine the data in great detail in compiling that report.

It is clear that we still have a lot to learn about the Red Planet, and that there is everything to be gained by continuing our telescopic monitoring programme.

Richard McKim, Director, Mars Section