Mars Section Report No. 6

1997 February 16–28

General

The disk diameter increased from 12.0 to 13.2 arcsec. New contributors were Mrs Elisabeth Siegel (Denmark), Mrs Komala Murugesh (India), Giovanni Quarra and his colleagues (Italy), and David Fisher and John Knott in the UK. Extremely bad weather conditions prevailed in the UK. Other observers receive them by post when the Director acknowledges observations. Observers should check the web site http://marswatch.tn.cornell.edu/irtf.html for remarkable current hi-res IRTF images by Bell and colleagues. Section members are now showing more detail in their drawings. Devadas has supplied the largest number of observations so far, and special credit must go to Quarra and colleagues for superb CCD work at five wavelengths from the ultrraviolet to the infrared; examples are attached [paper copies of this Circular only].

North Polar Region

The North Polar Cap was quite small, about 20 degrees across. Devadas made out the bright detached outlier Olympia. Warell reports clear skies from La Palma on Feb 22–23 and the observation of a rift in the cap around long.180 deg (the p. end of Rima Borealis?). Hyperboreus Lacus continued to be prominent, marking the only remaining really dark part of the NPC border. No sign of polar haze was detected in white light, though images by Quarra et al. , on Feb 8 and 9 in UV (330 nm) and B (420 nm) showed haze over the cap. In 1982 the BAA Mars Section found the onset of the polar hood about Ls = 152 degrees, and at 161 degrees in 1984. (Ls is 92 degrees at opposition on Mar 17.) Observers should pay special attention to this region. The Section received an email alert from M.Murakami of the OAA that under CML = 194–204 deg. on Feb 10 (Ls = 76 deg.), Minami and Nakajima found the following quarter of the cap more dull and yellow than the rest. Seeing was good. On the same date from the UK under CML = 270 deg., Shirreff (25-cm OG, poor seeing) found nothing odd about the cap, and Cave (Long Beach, USA, 32-cm refl., x660) on Feb 10–16 under CML = 323–26 deg., in very fine seeing found the cap uniformly sharp and bright, with a notch in the longitude of M.Acidalium (long. approx. 30 deg.). Quarra’s multi-filter images (Feb 8–9, CML approx. 280–3550 deg.) showed Olympia well, but no tonal differences. Later BAA observations under the same CM longitudes viewed from Japan were all negative. More details of the Japanese observations were given in the OAA’s CMO No. 185: red light CCD work on Feb 8 apparently also showeds the effect under CML = 191–200 degrees.

Surface features

The HST images are available on the MarsWatch Web site. The Director has studied these closely. The images of Mare Acidalium in December are almost indistinguishable from those in 1995. No largescale dust activity can have occurred since then, despite a report from Todd Clancy back in 1995 December which suggested that dust was then widespread as indicated by CO microwave observations. A copy of the BAA 1992–93 apparition map is attached to this Circular together with a print-out from the web of the long. 45-deg. image cited above [paper copies only]: the close resemblance between these apparitions will be at once apparent, so there is little need for a further description here except to add that the far northern regions can be better observed this year. Note that Pandorae Fretum remains dark.

Dust storms (yellow clouds) and white clouds

Jim Bell emailed the Director to draw attention to a fine NASA IRTF image from Hawaii now posted on the Internet. This was secured by Bell and others on Feb 17.Tempe appears normal in the near-IR at 2.331 microns (CML = 43 deg.), though Bell suggested that Dr L.J.Martin (Lowell Observatory) had recently seen a possible brief dust event there a few days earlier: more details have been requested by the Director.

Xanthe remained rather bright both at the evening terminator and on the morning limb; clasical Chryse less so. It seems that the martian sky is clear over the Pathfinder landing site. Tharsis, Thaumasia and Sinai (IAU) have all appeared bright on the morning limb, and the Tharsis orographics were most prominent near the evening terminator. HST pictures show the clouds’ appearances varied on a minute scale between images during September to January. Aeria and Tempe have exhibited diurnal cloud, too.

Elysium has shown up bright in whole or in part on the evening side as well as when rising. Hellas was dull white to Sturdy under CML = 268 deg. on Feb 15. Hellas was bright on mid-disk to Schmude on Feb 23; he could glimpse Hellas through the W47 (blue–violet) filter, but no albedo markings were seen. Schmude found Hellas especially bright (at most visible wavelengths) when it was on the morning limb. Siegel (20-cm SCT) on Feb 2 under CML = 6 deg. also estimated the Blue Clearing as 0. Quarra’s images (Feb 9) showed a UV/violet cloud over the location of Syrtis Major that rotated with the planet. Dark markings in violet light did not correspond with albedo features. Would observers please comment on the ‘BC’ effect? I have had few remarks about it so far.

Richard McKim, Director

1997 March 12