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Monday, 26 March 2012

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

BAA electronic bulletin
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:

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

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(c) 2012 British Astronomical Association


Good Clear Skies
Colin James Watling
Various Voluntary work-Litter Picking for Parish Council (Daytime) and also a friend of Kessingland Beach (Watchman)
Real Astronomer and head of the Comet section for LYRA (Lowestoft and Great Yarmouth Regional Astronomers) also head of K.A.G (Kessingland Astronomy Group) and Navigator (Astrogator) of the Stars (Fieldwork)
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