[BAA-ebulletin 00661] Mars: a remarkable terminator projection

[BAA electronic bulletins service]

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
[Richardmckim at btinternet.com]