Mars Section Report No. 10

1997 April 16–30

General

D: 12.9 to 11.5 arcsec, Ls: 105 to 111 deg. UK weather and seeing conditions were a little less satisfactory than in the first half of the month, but many observations came to hand. Vanessa Cave (USA), David Graham (UK), Terry Platt (UK) and Antoine Van de Jeugt (Belgium) sent in their first contributions of the apparition. Congratulations to Section member Dr Patrick Moore on the 40th Anniversary of his ‘Sky at Night’ TV programme. Earlier still, fifty years ago, at the 1947–48 apparition, Patrick made his first contributions to the Mars Section, then directed by P.M.Ryves. And he has sent in observations at every apparition since! Meanwhile, NASA’s Mars Pathfinder is still on course for a July 4 landing.

North Polar Region

The NPC summer remnant looks static now. Olympia was seen by Cave on April 16, when it was drawn rather small by him. Both Cave and Teichert on April 30 and 13–17 respectively saw a tiny white remnant near long. 10 or 20 deg. (March drawings just to hand from Niechoy also show some NPC outliers, and both he and Graham found Ortygia rather bright on March 7 and 10, respectively.) Some observers report seeing the NPC surrounded by a lightish region.

Surface features

Gaskell confirmed the Director’s view of the apparently more conspicuous Lunae Lacus (Circular No. 9). A pity the region has not been imaged by the HST recently. Gaskell also commented upon the rich red colours visible in the Arcadia region when observing with a friend’s 51-cm reflector. Cave continued to see Juventae Fons as very tiny and dark in excellent seeing. Tanga on April 13 (42-cm OG, Turin observatory) was fortunate to see ‘three tiny (but incredibly dark) dots corresponding to Ascraeus, Pavonis and Arsia [Mons] (the last one on the [following] limb).’ This calls to mind one of the 1995 HST images where one of the summits of the volcanoes poked through morning haze. The Director was lucky to see a similar instance of this phenomenon in the 1995 apparition, but he did not catch it in 1997.

Dust storms (yellow clouds)

Nothing positive to report.

White clouds

(An incomplete report!) The Tharsis and Nix Olympica orographic clouds remained visible. The ALPO (Martian Chronicle, 1997 April edition) found the Olympus Mons orographic cloud less prominent, only visible through the W47 filter (blue–violet light). Southern limb haze was noticeable. Hellas has been bright throughout the martian day: frosted floor now? Patrick Moore, April 24, found Hellas as bright as he had ever seen it, and brighter than the NPC. Teichert also found Hellas brighter than the NPC in green, blue and red light. On April 29 the Director followed Hellas as it appeared on the morning side. The S. limb was whitish under CML =3D 252 deg., as observations began; under CML 268 deg. Hellas was differentiated from the general S. limb whiteness as a very bright patch. Eridania, Ausonia and Argyre were light. Knott on April 20 drew Edom lightish in blue (but not in white) light when somewhat W. of the CM. Elysium and Tempe appeared lightish near the disk centre and brighter at the limb or terminator. Libya–Isidis and Chryse–Xanthe are similar. No reports of the ‘Blue Clearing’ have been received, all W47 filter observations being negative.

Special comments upon the new HST images

As recommended in a recent Circular the Director has studied the three ‘colour’ WFPC images released onto the Internet. All were taken on March 10, and released on March 20. There are other images too, filter versions, but I will just discuss the colour set here, all of which have incredible, diffraction-limited resolution (one pixel =3D 22 km on Mars!). Each colour image combines blue (433 nm), green (544 nm) and red (763 nm) filter images. The accompanying Press Release from the Space Telescope Institute states that a preliminary analysis reveals some dust as well as water ice crystal clouds over Terra Tyrrhena [Mare Tyrrhenum], Noachis Terra [Noachis] and Hellas, as well as Vastitas Borealis. However, inspection of the colour images shows little evidence of airborne dust.

(A) CML =3D 160 deg. Olympus Mons is just E. of the CM, its caldera marked by thin white clouds and the usual orographic cloud streaming over its western slopes in a NW direction. Remarkable detail is seen in the evening Tharsis orographic clouds which cover a good part of the E. limb. Euxinus Lacus is very small on the CM. NPC outliers are seen in several places. To the far south, Caralis Fons is clearly seen poleward of Mare Sirenum; the NW end of the latter Mare remains obscure (since 1986, now).

(B) CML =3D 210 deg. Elysium straddles the CM. The extraordinary pallor of Trivium Charontis and Cerberus (I) is noticeable at once. Has the region faded even further than in 1995? Trivium Charontis comprises a few tiny, complex spots on my monitor screen, whilst Cerberus is invisible apart from a tiny dark spot. Many observers have missed these features completely this apparition (and last). Interesting that Elysium does not appear on 19th Century sketches before the 1830s and 40s, when observers such as Galle, Beer and Maedler drew it. Neither Herschel nor Schroeter, nor any of their predecessors ever unambiguously figured it. It seems that the area is now a site for dust deposition, and is enjoying a period of obscurity! Had Cerberus–Trivium Charontis ever previously been as dark and as intense as it was in the first decade of the present century, it would have surely been recorded even in the 1700s. Thus perhaps the desposition of dust there marks a return to the circulatory regimes that existed then, when no evidence was found for large dust storms. On this HST image, the other points of interest are the dark, comma-shaped Propontis I, looking as it did in 1993–95, and the widespread shading between the lightish Elysium and Propontis I, which is all that remains of Phlegra–Styx, the classical E. border of Elysium. Note also the bright orographic cloud over Elysium Montes near the CM, and the irregularities in Mare Cimmerium that closely resemble the spoch 1988–95. Olympia is foreshortened near the CM, and the polar ices are highly irregular.

(C) CML =3D 305 deg., with the following edge of Syrtis Major central. Hellas is bright, bluish-white, presumably frosted over, but also shows a dusky E–W streak dividing the equally bright N. and S. halves. The detail in the Syrtis includes the Huygens crater complex, and intricate irregularities on all sides. Moeris Lacus seems unchanged since 1990. Further north, the Nodus Alcyonius is a dark, elongated streak, but contrary to the impression I reported from contemporary CCD images in a recent Mars Section Circular, it is not connected to Casius/Utopia. Nilosyrtis is a faint chain of small irregular spots, Boreosyrtis is complex and remarkably like its aspect in 1990–95 (and in the USGS albedo–topographic charts), whilst Nepenthes remains absolutely invisible. The E. end of Sinus Sabaeus is pale, but reaches Iapigia. Edom crater is visible, but not light on the morning side, while bright white evening cloud covers Elysium. Aethiopis–Isidis shows very thin diffuse white cloud seeming to merge with the limb haze on the E. side. The Deuteronilus seems less well-marked than it appears in the ground-based visual and CCD work. The NPC dark collar is some way south of the summer residual frost; this may well have some bearing upon past ground-based measures of the summer cap. The NPC summer remnant is complex, with the broad Chasma Boreale cutting largely across the cap from the longitude of Mare Acidalium. Olympia appears as a pale, irregular cap outlier, rather reduced in size.

Richard McKim, Director

1997 May 10