Mars Section Circular No. 4

 

1999 February 1–29

 

 

 

General

 

This Circular summarises the period 1999 February 1–28 (but with an incursion into early March). On Feb 1, Ls = 91 deg., D = 7.9 arcsec., lat. of centre of disk = 18 deg. N, with the planet’s declination at 10 deg. south, some 5 degrees lower in the N. hemisphere skies than last month!

      At my suggestion, Jim Bell included Circular No. 3 in the last Marswatch Electronic Newsletter, with a circulation of 1,455 (only 1,355 more than mine). More good news is that next month marks the start of the main mapping mission of Mars Global Surveyor. The craft has achieved its final, more circular orbit, and will hopefully take many high-resolution images. On the MGS web site there is now the ‘Aerobraking Image Set’ available for viewing and downloading. Recommended.

      National Geographic has just published a new Mars book with many recent images. I have not seen it, but surely it will soon be out of date! (I am hoping somebody will send me a review copy!) A general interest article by A.D.Andrews in the Irish Astronomical Journal was copied to me by Tony Kinder, BAA Librarian. Titled ‘The Leviathans of Tharsis’ its scope is self-evident, but I was surprised to find the BAA 1986 map, one drawing by myself, and several other drawings by past BAA observers used as illustrations (Reference: 26 (1), 45–64 (1999)). John Rogers draws attention to several pieces in Nature for 1999 February 18 (397, 560, 584, 586, 589 and 592) dealing with volcanism on the early Mars, recent volcanism deduced from crater counts, dune-fields, etc., as revealed by the MGS spacecraft during the early phase of its mission.

      On Mars throughout February the N. polar cap appeared rather small, and at some longitudes the dark rim seemed hard to see, or was absent. Parker’s CCD images sometimes showed some white haze just S. of the NPC, and this was the case in the most recent images of March 3 (CM = 5–12 deg.). White cloud activity presently remains high, and the Equatorial Cloud Band phenomenon (ECB) has been very conspicuous in the CCD images, especially in blue light. Elysium was brighter in the afternoon, and Hellas brightened considerably in early February, being markedly lighter than in the previous month, sometimes with a brighter spot in its NW corner. The ‘Syrtis Blue Cloud’ has been imaged again by Parker. More observers are becoming active, and are sending in work: thanks to S.Beaumont, A.W.Heath, Dr T.J.Richards and Dr R.Topping for recent contributions.

      Rik Hill e-mailed to say that his group at LPL were observing the planet every fortnight (primarily to monitor atmospheric water vapour) with the 61-in telescope at Catalina. He reports: ‘Our observations indicate the possibility of enhanced dust south during the period a week to either side of 1 Feb.’ The Director e-mailed early data about the Valles Marineris storm reported below, although this event occurred a little later in February, as will be seen. I have asked Rik for more details of his work.

 

Dust storm over Valles Marineris!

 

Rather than report the mundane, this month’s Circular is devoted to the story of a regional dust storm which was observed in the second half of February. On February 21 David Gray (42-cm Dall–Kirkham Cass., with x262–x415, Spennymoor, County Durham, Great Britain) observed considerable obscuration of the markings and kindly notifed me at once by telephone. I immediately issued an e-mail alert to about 20 observers in Europe, the USA, Australia and Japan, and telephoned several UK observers who are not on e-mail. The response was gratifying, although I have had no news from Japan as this issue goes to press. Although we may not have caught the precise start of this regional event, its decline and fall (as Gibbon might have written) was well followed. I recently e-mailed Todd Clancy to ask whether the dust has had any measurable effect upon martian atmospheric temperatures, as revealed through his radio waveband work. Selected details of the observations follow:

      Feb 19, 0700–0730 UT, CM = 101–108 deg, D.C.Parker, visual, 15-cm refl., from Chiefland, Florida: Don e-mailed this observation after hearing about the storm. He may have witnessed its start by noting a bright cloud on the terminator: ‘It was very bright in integrated and blue light, but not especially bright in red. On one occasion of excellent seeing, however, I thought it had a peculiar hook shape, reminiscent of dust’. This must have been over Xanthe at the CML quoted, perhaps close to Aurorae Sinus. (I await sight of Don’s sketch.)

      Feb 19, 1756 UT, CM = 261 deg, T.J.Richards (Victoria, W. Australia): A nice CCD image with his 18-cm OG, showing the Syrtis Major side of the planet completely normally and in fine detail.

      Feb 21, 0210–0300 UT, CM = 12–24 deg, D.Gray: Most of the SW (S. following) part of the disk was distinctly light, even in poor moments of seeing. A brighter, elongated core was seen, especially in red (W25). This core was like a ‘V’ on its side, with the apex occupying the N. part of Margaritifer Sinus, one fork running along or close to the Valles Marineris, and the other running off towards the SW limb, covering part of Mare Erythraeum. Margaritifer Sinus was quite invisible. Sinus Sabaeus was well-marked, but the ‘Forked Bay’ area (Sinus Meridiani) and the S. part of Mare Acidalium both seemed a little obscured too. A more diffuse brightness covered the equatorial deserts from the CM westward (Chryse, Xanthe, etc.), and this region was bordered on the E. by a dusky curving streak. Nothing could be seen of Aurorae Sinus to the west. Seeing was almost continually good throughout.

      Feb 21, 0600 UT, CM = 68 deg, Hernandez (USA): N. Margaritifer Sinus and Mare Erythraeum are faint. Agathodaemon (W. half of Valles Marineris, between Aurorae Sinus and Tithonius Lacus) is dark. Chryse–Xanthe bright in red light. To the west, Solis Lacus is dark, and that region appears normal. (I have not yet seen the drawing made by Carlos; it was most useful that he observed a few hours after Gray, so that his observation places limits on the W. side of the dust storm.)

      Feb 22, Gray: Very windy and with poor seeing. Mare Acidalium on the CM. Bright cloud is again seen from about the CM to the following limb.

      Feb 22, CM = 126–128 deg, F.J.Melillo (USA): Near-blank red light CCD images, but on a small scale, and at too high CML to catch the storm.

      Feb 23, 0520 UT, CM = 40 deg, Gray: In fine seeing he found the Aurorae Sinus area very faint (and it is drawn very ill-defined), but the Margaritifer Sinus and Mare Erythraeum regions have darkened again, and are as dark as M. Acidalium. Agathodaemon is dark, as is Solis Lacus. Chryse–Xanthe is light, as is the whole of the following part of the disk: Candor, Tharsis, Tempe. All this light cloud has rendered Nilokeras faint, and Ganges nearly invisible. By 0600 UT the Chryse–Xanthe area was less bright. As ECB had been observed in these longitudes before the event, there was probably a mixture of white cloud and dust.

      Feb 23, CM = 119 deg, S.Whitby (USA): With a 15-cm refl. Chryse–Xanthe bright on the evening side, more so in red than in blue, but observer unable to see anything else due to small aperture.

      Feb 24, CM = 81–93 deg, Parker (USA), 41-cm refl., CCD work (and below): seeing evidently not very good, but Solis Lacus dark. Strong ECB right across the disk, including the evening Chryse–Xanthe. The Aurorae Sinus region is not well placed.

      Feb 26, CM = 15–25 deg, Gray: The S. features look much as on Feb 23 (Gray), with Aurorae Sinus still weak. Equatorial cloud present. I have not yet seen David’s drawing of this date.

      Feb 26, CM = 75 deg, Parker: Aurorae S. is present but not dark: seeing is not very good. ECB evident.

      Feb 26, T.Stryk (ALPO, observation passed on to me by Jim Bell): observer remarked upon the region of Candor–Tharsis, that was bright in red light. Certainly Candor is also light in Parker’s February CCD images generally, but whether it was enhanced during the storm I have not yet decided.

      Feb 27, CM = 52–67 deg, Parker: As Parker, Feb 26.

      Feb 28, CM = 33–61 deg, Parker: seeing looks better than on 24, 26, 27. Aurorae Sinus looks normal in shape, but still perhaps less dark than usual, as it is definitely not as dark as the (now clearly normal) E–W dark band of the Mare Erythraeum. The ECB is incomplete at this CML, with some cloud in S. Chryse–Xanthe, then unconnected cloud over Ophir–Candor–Tharsis on the a.m. side. What is perhaps significant is that the images show the ‘canal’ known as Hydaspes visible as a halftone streak curving its way from the W. side of Margaritifer Sinus to somewhere about the SW side of Niliacus Lacus. This feature is rarely seen, though it was recorded as being dark during 1858–1871. Did it darken by surface excavation at the NE edge of the storm? I wrote the above description before I read Don’s own notes: he independently noticed the Hydaspes, and drew attention to it.

      March 3, CM = 5–12 deg, Parker: good seeing. My impression of this CCD image is that some residual dust shows up very weakly as a thin E–W streak in red light just N. of Mare Erythraeum, and that there is a distinctly brighter cloud (dust) in red light just NE of Aurorae Sinus. Extensive but diffuse brightness is seen in Chryse–Xanthe (perhaps brightest in red light), and in Tharsis and Tempe (the latter regions were brightest in green and blue light). There is an ECB from the p. terminator to the f. limb. The albedo markings look very nearly normal to me on these images.

      In character and evolution the February storm was not very different from those of 1984 April (Ls = 132), 1990 October (Ls = 308) and 1997 June (Ls = 139), described in past BAA Section Reports by the undersigned. If we accept the Feb 19 observation as marking the start, the storm was highly active and near maximum on the 21st, but was already dispersing by Feb 23. On Feb 28 and Mar 3 only traces of dust remained.

      Thus the storm began near Ls = 99 deg., which is seasonally a bit early compared with former telescopic events which definitely began over Valles Marineris: my historical research shows that the earlier storms (1924–1990 and ?1879) occurred during the interval Ls = 132–357 deg., with most events near the extreme limits. On the other hand, events which began in neighbouring Chryse/Xanthe have occurred in the intervals Ls = 96–225 and 308–344 (from records between 1903 and 1992), and the 1999 February event thus falls nicely within the first interval. Anyone else out there with more observations of the longitudes in question? Steve Lee e-mailed me to say that his group were to have obtained time on the HST on March 3, under CM 300 deg., and their results will be interesting to compare with Parker’s on the same date (see above). HST imaging is to be much less intensive than in 1997 this apparition, as noted in an earlier Circular: the role of the ground-based observer is again underlined by the our coverage of the dust event. (And who says it is always cloudy in Great Britain?)

 

The next Circular

 

As I may be out of the country in early April I will attempt to issue a report covering March 1–April 15 during the third or fourth week of April. Therefore please report up to April 15 by, say, April 22.

 

Reporting data to the Section

 

I am always happy to receive CCD images by e-mail. Any urgent and important drawing can be scanned and sent as an image file, but I do not want to receive routine drawings by e-mail, because the vast majority are sent as hard copies, and that is the form I like to compare (and archive) them in. Send mail to Cherry Tree Cottage, 16 Upper Main Street, Upper Benefield, Peterborough PE8 5AN, Great Britain; home telephone 01832 205387; home e-mail Rmckim5374@aol.com. (Do not use the former e-mail address for my place of work (mckim@oundle.northants.sch.uk).) Urgent faxes can be sent to my place of work on 01832 274052.

 

 

Richard McKim, Director

 

1999 March 7