The opposition of Mars, 2020: Part II

2023 August 10

Richard McKim

A report of the Mars Section. Director: R. J. McKim.

We report seasonal timings of the appearances and disappearances of the Equatorial Cloud Belt; the Elysium Mons, Tharsis Montes and Olympus Mons orographic clouds; and observations of the shadow cast by the Arsia Mons orographic cloud (or Arsia Mons Elongated Cloud). The sublimation of the south polar cap was followed in detail, and its recession was smooth and typical: after Ls ∼218° the recession curves of 2003, 2018 and 2020 were indistinguishable, but before then the 2003 cap was systematically larger. The seasonal separation of Novus Mons from the cap was judged to be complete at Ls = 238°, which was seasonally normal. The south polar cap had first appeared beneath the polar hood in 2020 February at Ls = 151°, and it was last seen in 2021 January at Ls = 353°. The north polar cap was visible up till 2020 February (Ls = 154°), and reappeared in late 2020 at around Ls = 338°.

 

Introduction

Continuing from Part I,²⁷ we discuss seasonal cloud and polar phenomena in 2019–’21, during Martian Years (MY) 35 and 36, for comparison with the 2018 opposition and earlier.²¹ We often quote Ls data for cloud phenomena, but seasonal comparisons with other years are most useful where they document cloud at the evening terminator: in other words, up till opposition. The most relevant comparisons are therefore with years having similar Ls at opposition: most recently, 2005 (opposition: Nov 7).⁸ A selection of the Director’s visual observations illustrating various phenomena is given in Figure 11.

White clouds

Equatorial Cloud Belt (ECB)

Phenomena in MY 35

We were last able to see the ECB commence in 2019 May at Ls = 020°. In 2020 we could observe its seasonal termination, though Mars was then distant. On Jan 15–23, to Foster the ECB was still causing the Syrtis Major to fade upon mid-disc. On Jan 31 – Feb 1 (Ls = 144°), Peach found the ECB still complete around CML = 250–260°, but it was no longer so effectively veiling the Syrtis. It looked very incomplete upon Foster’s images of Feb 5–13 (CML = 030–114°, Ls = 146–150°; a typical seasonal date for termination). By Feb 14–15 (Casely, Foster, Peach), there was no longer any trace of it.

Phenomena in MY 36

By 2021 March, there were fresh signs of ECB; again, Mars was distant. Images by Maxson showed the Syrtis Major obvious in blue light up to ca. Mar 3 (implying weak or absent ECB). But from Mar 5 (Ls = 013°), the Syrtis was no longer distinct, and Ito’s blue-light images showed veiling during Mar 14–23. The ECB was seen partially by the Director on Mar 17, under CM ∼081°, and was quite clearly complete on Jun 9 (Ls = 057°) in Ito’s blue image under CM = 223°. Hence the ECB had certainly commenced by Ls = 013°.

The Tharsis Montes, Olympus Mons, Alba Patera & Elysium Mons

We could not observe seasonal commencement of orographic clouds during the northern spring of MY 35, but later phenomena could be timed. Arsia Mons was the focus of attention, for we definitely detected the shadow of its banner cloud. The Tharsis Montes, Olympus Mons, Elysium Mons and Hecates Tholus showed a marked brightening for a week or so either side of the date of opposition.

In 2018, the orographics over the Tharsis Montes had all ceased on Jun 3 (Ls = 187°), as a global dust storm began. In 2020, the largest storms were regional.

Elysium Mons

Elysium Mons was bright in the evening from 2019 Nov 13 till 2020 Mar 25 (Ls = 172°), becoming faint by March. During 2018 it had persisted till May 7 (Ls = 171°). It was bright again on the morning side from 2021 Mar 2 (with morning cloud imaged as late as Jun 26 by Kidd (Part I, Figure 4), and bright on the evening side from Mar 22 – May 3 to several observers.

Olympus Mons

Olympus Mons, bright in the evening from 2019 Dec 31 till 2020 Apr 4 (till Ls = 178°), was fading in March. This was like 2018, when the cloud was followed till May 13 (Ls = 175°). It then appeared as a dark circular marking in the absence of seasonal cloud, particularly when near the evening terminator in August–September, when a reddish tint was obvious. The orographic cloud reappeared, weakly at first, from 2020 Sep 27 (Ls = 286°), continuing till Nov 12.

As already mentioned, there was a marked brightening of Olympus Mons a week or so either side of opposition, but it was still possible to distinguish the presence of weak evening cloud even at that time. It was easy to record the opposition brightening near the CM, but on other occasions one or other flank of the volcano could be seen as a bright elongated area near the limb. For example, J. Sussenbach on Oct 11 recorded its W. slopes shining under evening illumination.

From 2021 Mar 2, late afternoon orographic cloud was again returning to Olympus Mons, but the evening terminator was not accessible. The Director saw it well on Apr 12, and Kidd imaged it clearly as late as Jun 26 (Part I, Figure 4) and Jul 29 (Ls = 064–079°, D = 3.9–3.7ʺ). But by spring of 2021, the planet’s disc had already become too small for critical work: a comment applicable to all white cloud statistics.

Arsia Mons

General

In professional literature, the banner cloud of Arsia Mons (which trails its summit on the west) is nowadays referred to as the Arsia Mons Elongated Cloud (AMEC). The long cloud of Arsia cast an observable ground shadow shortly before, at and shortly after opposition. See Figures 12–13.

Arsia Mons was clearly brightened in the evening by its orographic cloud, 2020 Feb 25 – May 31 (Ls = 157–211°), which became weaker after mid-May. (For comparison, in 2005 it was visible until Jun 1, or Ls = 222°.) The cloud appeared again from Aug 1 when Haigh (confirmed by della Vecchia on Aug 2) caught it at the evening terminator in blue light. It was well seen in September–October and persisted – fading after early November – till Nov 20, this second period spanning Ls = 250–318°. (The corresponding second period in 2005 was Jul 22 – Oct 25;⁸ Ls = 254–312°.)

By 2020 late August, the banner cloud was sufficiently developed to cast a visible shadow (see below). As always, the orographic cloud was more prominent in the evening; by mid-September it had become conspicuous by early afternoon.

Gasparri on Sep 6–7 recorded how white cloud at Arsia Mons had appeared on both sides of the summit to give a ‘butterfly’ effect: an occasional phenomenon we have recognised in amateur images since 2005. See the images by Peach for Sep 12 (Figure 13C). The effect was visible for some weeks. The Oct 18 images by Pellier (Figure 13D) show the effectiveness of the UV waveband in recording orographic clouds.

Terminator projections & shadow of the AMEC

On 1988 Sep 30 – Oct 2 (Ls = 281–283°), I. Miyazaki had photographed a dark streak between the morning Arsia Mons and the terminator (Figure 12A). In the absence of higher-resolution work, it was suggested at the time to have been the shadow of the volcano.²⁸ What must have been the same phenomenon was imaged again during 2003 Aug 29 – Sep 11 (Ls = 250–259°): see Figures 12B–C.²⁹ In 2018, BAA observers recorded the morning AMEC and its shadow from Sep 30 – Oct 29 (Ls = 260–279°).²¹

Turning to 2020, the AMEC sometimes projected beyond the terminator. A visual sighting was the first that came to our attention, by Pozharov (50cm refl., ×216, Russian Federation) during Aug 30 (Ls = 268°) to Sep 3, from around CM = 171–176° (Figure 13A). The Aug 23 images by Kumamori and Yunoki (CM ~180°) are possible earlier records, but the effect is marginal. Next came an image by Arditti (UK) on Sep 7 around CM = 181° (Figure 13B). To him the projection was apparent upon both monitor-screen and processed images. From the USA, Ratcliffe reported the same effect on Sep 14 (CM = 189°). (For there to be a constant Martian time for the effect, as the terminator approaches the evening limb before opposition, there has to be a slow increase in the critical CM longitudes.) In Arditti’s image, the presence of what appears to be a cloud shadow adjacent to the bright strip of cloud adds weight to the visual impression that the cloud projected beyond the terminator. Such a situation can even make a true shadow appear to be a surface depression.³⁰

On Aug 31, Pozharov again saw the projecting cloud, and then also its shadow. He made sketches over 37 minutes, showing how the cloud first projected beyond the limb, and how the shadow then appeared: Figure 13A. These phenomena disappeared as the planet rotated. He repeated the observations on Sep 1 & 2/3, and although there were slight differences from night to night, the sequence of events was the same. Flanagan and Grafton (USA) on Sep 16 also caught the shadow of the AMEC cloud right at the terminator. J. Sussenbach apparently caught the shadow in the afternoon on the date of opposition (Oct 13).

From 2020 Oct 9–13, numerous Australasian observers caught the AMEC shadow in the morning: see Figure 13E for an Oct 12 example by Heffner. An image by Edwards on Oct 14 (Figure 13F) shows the shadow clearly again in mid-afternoon. Visibility in the afternoon or evening that day was confirmed by images from Radice, Rodriguez and E. Sussenbach. Foster caught the shadow on Oct 19 precisely upon the morning terminator, and Gasparri imaged it clearly next day. On Oct 22, della Vecchia (Figure 13G) recorded the shadow well off the terminator, and his animation showed how it first appeared and subsequently disappeared with rotation. On the same date, it was captured by Leatherbarrow and Pratt from the UK under less than perfect conditions, and by J. Sussenbach (Netherlands). In a very high-resolution Oct 27 Chilescope (1m Cass.) image, Peach (confirmed by E. Sussenbach) recorded it as a rather fine dark band (Figure 13H); this was repeated on Oct 30 when the shadow was partly lost in extensive morning cloud. Chappel, Flanagan, and Grafton (USA), on Nov 1–4 (till Ls = 309°), caught it in the morning. After that, the writer could find no more sightings.

(Log in to view the full illustrated article in PDF format)