Introduction

Since the early summer, the Director has been receiving a steady stream of images and drawings of the present opposition. Preliminary results were conveyed by BAA Circulars, [1] an e-mailed dust storm alert notice sent to many active contributors, and the recent Council Report. [2] Observing tips by the Director appeared in a recent popular magazine. [3]

Encircling dust storm: 2007 June

This event was first imaged by David Moore over eastern Noachis on June 24. Next day (Figure 1A, below) the bright streak had extended in length, and impinged upon NW Hellas. Further activity began around Solis Lacus which soon extended along Valles Marineris, and Don Parker’s image (Figure 1B) shows several discrete, bright yellow clouds. Planet-encircling status was rapidly reached, and all albedo features faded considerably, though large features such as Syrtis Major were never completely lost. This encircling storm was very effectively followed upon a tiny disk, which at the start was less than 7 arcsec across! We shall not review the storm in any detail, suffice to write that atmospheric dust was settling by September, and that (as usual) the albedo features did not return fully to their normal intensity at once. NASA’s twin Mars rovers operating on the planet’s surface had to suspend operations due to the lack of power being generated by their solar cells, and a montage of sky images from the martian surface [4] showed the extent and duration of the storm from their point of view.

      The seasonal date of this event (at Ls = 264º) is typical, and close to that of the famous and long-enduring event of 1971 (Ls = 260º). The last planet-encircling storm to occur was that of 2001. [5] A complete catalogue of past dust storms was published by the Director. [6]

Local dust storm: 2007 November

Jésus Sánchez imaged a small bright cloud over Chryse Planitia (telescopic northern Xanthe) and others over Nilokeras on November 2. (Figure 1C) It expanded to obscure Nilokeras and southern Mare Acidalium, dust spreading into telescopic Chryse to the south. It quickly faded.

Changes in the dark markings

A number of notable changes have occurred as a result of the encircling storm, some becoming visible well before the dust had settled. The principal ones are as follows, but several other minor changes were noted.

1 A change in orientation of Solis Lacus has been apparent even in early August. The feature is now oriented Sp. oNf.,
   being drawn out on the W. side towards Phoenicus Lacus, somewhat as in 1926–29, [6] but it is considerably thinner
   and smaller than it was before the storm, or during those years. Compare Figure 1D, pre-storm, with Figure 1E, post-
   storm. Sirenum Sinus also became extended northeastwards into Gallinaria Silva.
2 A new streak (also since August) has appeared across Aethiopis, from the f. side of Elysium around the Hyblaeus
   secular darkening to the NW end of Mare Cimmerium. Rather similar features were observed in this location in 1933
   and in the late 1960s. [6] (Figure 1G)
3 Propontis now appears as two dark dots, separated by a light gap, greatly resembling its 1930s and 40s aspect. [7]
   (Figure 1F) For many previous oppositions it had appeared as a dark rod-like marking, orientated E–W.
4 The N. end of the Syrtis Major seems more rounded than previously, and the intensity difference between its
   p. and f. sides seems greater.

Polar regions

2007 has been an excellent year to witness various seasonal phenomena in the polar regions. The S. polar cap was visible in the early observations, together with its seasonal shrinkage, but lately the sub-Earth latitude has been northward, allowing a view of the transition from hood to ground cap. A first indication of change was the appearance of bright patches within the hood, especially on the morning side, and sometimes these appeared brilliant. The best images showed complex detail and swirls. The brilliant spots – often bluish in tone – are generally taken to be surface frost, the fresh ground cap apparently showing through the hood at such points, but some of these temporary bright features lay southward of the hood/cap boundary, and must have been cloud or temporarily frosted areas adjacent to that region. There were a number of observations of very obvious E–W rifts in the hood, including the well-known ‘slit’ over Baltia/northern Mare Acidalium, which was first observed by Dawes in 1864. (Figure 1H)

Meteorology: white crystal clouds

During the period of enhanced warming of the atmosphere by the dust particles, signs of active meteorology were low or absent, though white cloud activity had been observed earlier. Lately (2007 November) there have been signs of increasing morning and evening cloud activity. At around Ls = 50º (from 2008 late March onwards) we can expect to see the start of the formation of the Equatorial Cloud Band, as discussed in detail in our final Section Reports for 1995, [8] 1997 [9] and 1999. [10] This phenomenon is sensitive to atmospheric dust-loading, so the accurate timing of its seasonal appearance is of interest.

Contributors

The undersigned thanks all those who have contributed images and drawings so far:

References

1 BAA E-Circular No. 296 (2007 June 28); BAA Circular No. 808 (2007 July 1).
2 R.J. McKim, J. Brit. Astron. Assoc., 117, 249–250 (2007).
3 R.J. McKim, Sky at Night magazine, 2007 December, pp 65–69.
4 Sky opacity rose sharply during the onset of the storm so that at maximum only 8.4% of visible incident light was being
   transmitted. Opacity began to fall after July 19. Mission control decided not to move Spirit’s robotic arm for the 20 days prior to
   August 6 due to lack of power, and no driving was done until August 21. For further information see the JPL website:
   http://marsrover.nasa.gov/newsroom/pressreleases/index.html
5 For a preliminary account of the encircling storm of 2001, see R.J. McKim, J. Brit. Astron. Assoc., 112, 119–121
   (2002). A final  paper is presently in preparation.

6 R.J. McKim, Mem. Brit. Astron. Assoc., 44 (1999).

7 The following books reproduce a Mars map for the 1939 and 1941 oppositions: G. de Vaucouleurs, The Planet Mars,
   Faber and Faber, 1950; Physics of the Planet Mars, Faber and Faber, 1954.

8 R.J. McKim, J. Brit. Astron. Assoc., 115, 313–333 (2005).

9 R.J. McKim, ibid., 116, 169–186 (2006).

10 R.J. McKim, ibid., 117, 314–330 (2007).

Figure 1 : CCD and webcam Mars images, 2007.

A June 25d 10h 31m, CML = 298º, 300-mm refl., J. Melka. Small S. polar cap; nascent Noachis dust storm visible as light
    yellowish streak.

B July 19d 09h 32m, CML = 49º, 410-mm refl., D.C. Parker. Tiny SPC; bright dust clouds near Solis Lacus, over Candor and along
   Valles Marineris.

C November 2d 04h 54m, CML = 47º, 260-mm refl., J.R. Sánchez. Small bright dust clouds in Chryse Planitia and over Nilokeras.

D June 6d 08h 47m, CML = 100º, 356-mm Schmidt–Cass., D.A. Peach. Immediately before the planet-encircling dust storm,
   Solis Lacus remains unchanged from 2005.

E September 27d 08h 57m, CML = 85º, 410-mm refl., D.C. Parker. Solis Lacus orientation changed; Sirenum Sinus is extended
   to meet Gallinaria Silva; Olympus Mons appears as a warm-coloured dusky spot on the morning side; N. polar hood extensive.

F October 31d 11h 02m, CML = 155º, 356-mm Schmidt–Cass., E. Grafton. Propontis appears as two dark spots; the
   surroundings of Cerberus remain faint; irregular NPH; weak p.m. cloud over Arsia Mons.

G November 23d 06h 21m, CML = 236º, 356-mm Schmidt–Cass., P. Casquinha. The faint N–S dark streak (arrowed) crossing
   Aethiopis from Hyblaeus to NW Mare Cimmerium shows complex structure; NPH highly irregular with bright patches at S. edge.

H November 10d 08h 30m, CML = 26º, 410-mm refl., D.C. Parker. Chryse Planitia dust storm terminated; Baltia visible through
   NPH; weak p.m. clouds.

Richard McKim, Director