[BAA-ebulletin 00661] Mars: a remarkable terminator projection
(c) 2012 British Astronomical Association http://www.britastro.org/
The 2011-12 Mars apparition has been well observed, and some 63 observers have contributed their work directly to the BAA Mars Section so far. There are many interesting points, including the detection of a number of tiny dust events at the edge of the retreating N. polar cap. As one would have predicted, no large dust storm events have been witnessed, and there have been few visible changes in the albedo markings, so that the pattern of events has been mainly normal and seasonal to date. The same seasonal trends in polar caps and white clouds were witnessed, though at much lower resolution, during the series of aphelic oppositions in the decade of the 1990s. However, this E-bulletin is intended to draw attention to one very notable and unusual phenomenon: not unique, but rather rare.
An unusually high cloud at the martian morning terminator has been spotted on amateur webcam images taken during 2012 March 19-21. The Director had noticed a small projection on images by Efrain Morales Rivera (Puerto Rico) on Mar 19 under CM = 151; by CM = 156 it had vanished. The same observer saw it again next day, as did others. On Mar 21 Jim Phillips (USA) emailed details of a much larger projection captured by his own images of that date (CM = 153), and confirmatory images by Don Parker (USA) taken on Mar 21 (CM = 146) were soon forthcoming. On the latter images it appeared as a nearly detached cloud lying along the terminator, anchored at the S. end. The cloud was visible in red, green and blue images, so it would seem that suspended dust is involved. There are doubtless many other images of this feature in the Director's unopened emails and posted on various websites, but it seemed more important to send out this alert now, before trying to assess them all. Certainly there are other records of a small feature in the same location captured by many European observers some ten days earlier, but its sudden increase in size on March 20 and 21 (Ls = 85-86) was really remarkable. Marc Delcroix recorded it on March 12 (CM = 152-155), for example. He has measured the images of Mar 20 to obtain a current position of lat. -45, long. 193, placing it at the border between Electris and Eridania.
Terminator projections due to both white cloud and suspended dust have been reported by many past observers, particularly when using large telescopes visually, and when the planet was close to quadrature so that the phase defect was at a maximum. It is therefore unusual for such a prominent projection to be visible just weeks after opposition, though the direction of maximum phase defect does lay within the S. following quadrant of the planet's disk as we can see from the ephemeris for P (the position angle of the aerographic N. pole) and Q (the position angle of the greatest defect of illumination) from the BAA Handbook:
Date P Q
2012 Mar 14 16 128
2012 Mar 24 14 119
Recall that position angle is measured eastwards from the north point. Terminator projections due to dust activity have been described at some length in the BAA report on the 2003 opposition, for example, which is readily available as a pdf document at our website:
http://www.britastro.org/mars
There was an obvious projection to be seen in the longitudes following Hellas during the large regional dust storm that appeared just prior to the arrival of Mars Express at the planet in 2003 December. This projection was easily seen visually by the writer and was captured on several CCD images. At the time the phase defect was large.
My BAA Memoir, Telescopic Martian Dust Storms: A Narrative and Catlogue (Mem. Brit. Astron. Assoc., 44 (1999)) has an extensive discussion of the history of terminator projections up to and including the opposition of 1993. Of those that appeared at high southern latitude the most remarkable that springs to mind both in terms of length and apparent height is one observed by E-M.Antoniadi over Eridania with the 83 cm OG of Meudon observatory on 1933 April 14 (Ls = 96). Note the quite similar seasonal date to 2012. He estimated its height at 35 km. Antoniadi's drawing much resembles Parker's image of March 21. The BAA Memoir also gives references to the literature concerning the calculation of heights of projections. The prevailing view is that the old telescopic observers unwittingly exaggerated the height of the projections they recorded; owing to the falling off of illumination at the terminator this is an easy thing to do.
Given that there exist previous observations of terminator projections at this location it is very unlikely that they have arisen as the result of planetoidal impact, although there has already been some discussion of this possibility on the internet.
Observers are requested to scrutinise the region carefully and to send their results to the undersigned. Good observing!
Richard McKim
Director, BAA Mars Section
2012 March 22