This report was published in the BAA Journal for 2002 June

Introduction

As I write these lines in late 2002 March, Mars is a diminutive speck low in the evening sky with a disk diameter of 4 arcsec. It is now difficult to see markings on the disk unless the seeing is very good, but it is hoped the observations will continue a little longer. Temporarily occupying the same low-power binocular field as Comet Ikeya–Zhang, it is an attractive target for photographers.

      A summary of the Section’s observations during last year was published by the Director in the BAA Journal for 2001 October. As noted therein, BAA Circulars 777 and 779 contained early news of the great dust storm of 2001. An earlier Interim Report appeared in the Journal for 2001 June. A Section Circular was sent to all members in December, and posted on the Section’s web page. This final interim report is intended to give a summary of the dust storm, and much of it is adapted from the aforementioned Section Circular. A full report will be submitted later.
 

The Great Dust Storm of 2001
 

Origins

Before opposition MGS imaged local dust storms, and one example in Hellas is illustrated in Astronomy and Geophysics, 2001 August, p.26. The image is dated April 8. There is further MGS evidence of local dust activity in Hellas in mid-June. By late June ground-based observers also detected local activity in Hellas. On the 24th, atmospheric dust was confined within the N. part of the basin, but on the 26th a long, bright, twisting ribbon of dust had extended into Ausonia, marking the emergence of an important event at Ls = 185 deg, right at the start of southern spring. The HST imaged Mars the same day but this activity was beyond the evening limb. Only the slightly dusty nature of Hellas could be seen on the latter image. But MGS temperature data (using the thermal emission spectrometer) show a warming beginning in Hellas on the 24th. The event developed quickly and dust expanded from Hesperia (long. 270 deg.) and Hellas (long. 300 deg.). Wei-Leong Tan’s image of the 27th (280-mm Schmidt–Cassegrain, Singapore) shows additional dust in Libya with diffuse dust starting to mask the Syrtis Major and points east. Rapid expansion of the event occurred chiefly to the E and NE, and additional activity occurred over Elysium. Within a few days the Oriental Astronomical Association (OAA; Japan) were calling it a ‘global’ storm, but this is to misuse the agreed classification system. The dust covered much of one hemisphere, but in longitudinal extent it was still only a large regional event. Over the next few days Syrtis Major was effaced. So far, this was not much different from the course of many past regional storms, such as 1988 June.
 
    

Images of Mars near opposition in 2001, by Ed Grafton, of Houston, Texas, obtained with a 356-mm Schmidt–Cassegrain at f/60, and ST6 CCD camera (south at top). Top row, left–right: May 10d 08h 21m; May 19d 08h 32m; May 29d 06h 57m; June 3d 07h 28m; Bottom row, left–right: June 20d 06h 10m; July 5d 04h 58m; July 11d 03h 31m; July 31d 02h 17m. The surface features are well-marked, and are shown in great detail. On July 5 small dust clouds are visible around Solis Lacus. On July 11 the whole disk, apart from the far south, seems shrouded in dust. On July 31 bright dust clouds are seen over Thuamsia and environs, and the isolated dusky spot in the north marks the place of Olympus Mons, partly visible through the dust.

Development
 

On July 3 a significant development began: a new bright dust core appeared in Daedalia. This was observed from Melbourne, Australia, by Maurice Valimberti (152-mm OG) and others. This new storm expanded rapidly, and its incidence showed that a global forcing condition was already operating despite the very early seasonal date. Over the next few days Don Parker (410-mm reflector, Miami, Florida, USA) and others imaged the new event’s expansion primarily to the east over Solis Lacus, Valles Marineris and Mare Erythraeum (etc). Many small new bright clouds appeared around the region. The storm front crossed Noachis to link with the Hellas regional event, and the latter event had by then expanded east to meet the new event around the longitude of Thaumasia/Mare Sirenum. By July 11 (storm day 16) the planet was encircled by dust and contrast was low everywhere. Albedo features rapidly faded from view. A pair of HST images for June 26 and September 4 published in Sky & Telescope, 2002 January, give a graphic representation of the extent of the dust.

      By mid-July the colour of the planet was more yellow than orange and even to the naked eye the colour was noticeably different. The dust veil extended down to a latitude of about 40 deg. north, so that the north polar hood was not veiled. The NPH became less active in August, though this could be due to the southward movement of the subsolar point. The dust had a significant warming effect upon the martian atmosphere, to the extent of 40 K or more between latitude 20 N up to the S. pole, as measured by MGS from orbit. All evidence of white cloud activity was suspended for months. Indeed, even the limb brightening was less sharp and less marked during the storm.

      Viewed from the Earth it was hard to see what was happening in the far south. The SPC was tilted away from the Earth and the cap, though large, was foreshortened. Dust did not cover the SPC (see the aforementioned HST image for September 4) but nevertheless did extend to rather high southern latitudes.

      During July the only specific bright clouds were over Hellas and also over Daedalia. Several observers reported renewed activity over Daedalia, and this source remained active for some time. In August and September many observers sketched or imaged Olympus Mons as a dusky spot, showing that the storm could not have been much higher than its summit caldera. As Masatsugu Minami (OAA) points out, the spot represents the caldera making a hole in the surrounding swirling yellow clouds rather than an albedo feature as such. Sure enough, as the dust settled, the caldera became indistinguishable. Richard Schmude communicated measurements of the Olympus Mons dark spot (made from various July and early September CCD work) which suggested, by comparison with altitude contour maps, that the opaque dust layer extended to some 9 km in that region.

Clearance

These notes are intended as preliminary only, and do not constitute a full report. From late August onwards a gradual clearing was underway, but it was very slow, and throughout September the ground markings remained hard to see well. By October the general albedo features could be easily recognised, but contrast was not back to normal until mid or late November. Signs of atmospheric dust were even then still detectable: some dust hung over Edom crater and a patch of dust hung over Argyre, whilst Hellas was still bright and yellow. In early December the Director and others still saw N. Hellas to be bright in yellow and red light, but there were no other bright dust clouds. By then all the features were sharp and well-defined, even if some were apparently not as dark as in the pre-storm period, and furthermore the SPC was again well contrasted as the southward tilt of the axis increased, though by then greatly shrunken.
 

Historical context

The most important point about the present storm (denoted 2001A) is that it was seasonally the earliest ever recorded amongst all the past encircling events. It was also one of the most enduring storms, and optically one of the most dense. It also may mark the return to the dusty climatic period that was witnessed throughout nearly the whole of the 1970s and into the early 1980s. My historical work clearly established the fact that encircling storms were witnessed every martian year from 1971 to 1977. It showed that 1975 contained a planet encircling storm, a fact not widely appreciated, and this year together with 1971 and 1973, when coupled with the Viking data up to late 1977 show the emergence of a great storm every year on Mars (with two in ‘77). This epoch was unprecedented and Viking’s cameras recorded an atypical Mars: a fact well worth reiterating. During this epoch both Hellas and Thaumasia/Daedalia dominated the scene as emergence sites, indeed they remain the only such sites for the emergence of encircling storms.

      The development of the 2001 event was similar to many past great storms. It began with what appeared to be a large regional event originating in the Hellas longitude. In this respect the storm was not seasonally especially early. Historically, regional storms from Hellas had begun at even lower. One more discovery from my Telescopic Martian Dust Storms Memoir (Mem. Brit. Astron. Assoc., 44 (1999)). I divided up the last century and a half into epochs: 1866–97; 1898–1929; 1930–61; 1962–93. I wrote on page 144 in connection with the Hellas emergence site: ‘In the last three epochs, the R(egional) type storms have begun progressively earlier in the season.’ It seems that the 2001A event’s timing has also been unusually early. Is there a simple physical mechanism for this phenomenon, associated with the gradual net accumulation of dust grains at a site?

      Event 2001A had the opportunity to become planet-encircling only after a secondary dust emergence event commenced in Daedalia on July 3, and together with the initial dust outbreak carried dust around the globe. All the great storms have become encircling in this way when a secondary source supported the initial outbreak elsewhere: for example, the 1973 event began in Daedalia/Claritas, but it had secondary outbreaks over Hellas and Meridiani Sinus.

      The 2001A storm showed a more vigorous expansion to the north than some of the most recent encircling events. Thus the area of Syrtis Major was blotted out from view quickly. The duration of any storm is always hard to judge by imaging data alone. The remaining suspended dust in Hellas had settled, at least according to ground-based visual and imaging work, in December. Richard Schumde (Georgia, USA) used a polarizing filter attached to a SSP-3 solid-state photometer (with a 90-mm telescope) to measure the degree of polarization. This work suggests that the effects of the storm lasted 165 + 10 days (June 24 to December 15). The durations of the planet-encircling storms of the 1970s–80s dusty epoch were (my data): 1971 161 days; 1973 91 days; 1975 100 days; 1977A 60 days; 1977B 158 days; 1982 110 days. Thus 2001A was of comparable length to the global event of 1971.

      For visual observers 2001A was probably the most opaque storm since 1971, although the 1971 storm was more global in its dust coverage. Of course, the 1977 and 1982 Viking data were a little incomplete.

      The MGS data for the 2001A event have been written up in the paper ‘Thermal emission spectrometer observations of martian planet-encircling dust storm 2001A’, by M.D. Smith, B.J. Conrath, J.C. Pearl, and P.R. Christensen, and it was recently accepted for publication in the journal Icarus.

Changes on the martian surface
 

As is usual with big dust storms, some changes upon the martian surface were apparent. Prior to the storm, the markings were much as they had been in 1999. Nepenthes was invisible, Cerberus and Trivium Charontis nearly so; Nodus Alcyonius, the Amenthes darkening and Solis Lacus were all prominent. After the storm, Parker remarked that Syrtis Major appeared thinner, and the writer found it more tapering to the north in his visual post-storm work; at the same time, Deltoton Sinus had become more visible. And there was a new dark marking just W. of Solis Lacus, greatly resembling the old ‘Phasis’ canal shown on the maps of G.V.Schiaparelli and N.E.Green from 1877. This was imaged from early September onwards, when Solis Lacus itself was still mostly hidden by bright dust clouds. (This feature was also observed during the mid-1980s through the early 1990s.) Solis Lacus itself was smaller after the storm, differently orientated, and the feature known as Nectar, connecting Solis Lacus to Mare Erythraeum was greatly faded. Other minor changes were also observed.

Concluding remarks
 

The writer is now starting work upon the final Section Report and urges all members to send him any unsubmitted observations, however few, as soon as possible. Thanks to all those who have contributed so far to make this a memorable apparition.

Richard McKim, Director