The opposition of Mars, 2020: Part I

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

For UK observers, 2020 was the most favourable year of the 15- or 17-year opposition cycle. An extremely comprehensive set of some 11,600 observations was obtained by 147 observers. In Part I we discuss the impact of dust storms upon albedo features and deduce velocities for the initial expansion of one such storm. Several striking albedo changes were found to have persisted since 2018–’19: in particular, the development of Indus between Oxia Palus and Mare Acidalium, and the canal-like Phasis west of Solis Lacus. The slow fading of several northern-hemisphere markings such as the Aetheria development, Trivium Charontis–Cerberus and Nodus Alcyonius continued. The opposition period saw clear Martian skies, but before and after closest approach, two large regional dust storms were observed, both of which commenced in S. Chryse–Xanthe/Valles Marineris, and stretched from Hellas in the east to Daedalia in the west. This initiation site has been the favoured one for some years now, whereas in the past many such events originated in NW Hellas and spread as far west as Argyre. Lists of both types of storms are presented in an appendix. A second appendix deals with the measurement of cloud velocities. In Part II, the behaviour of white clouds and the polar regions will be discussed.



Mars was at opposition on 2020 Oct 13 (disc diameter (D) = 22.3 arcseconds, ʺ), with closest approach on Oct 6 (D = 22.6ʺ). The sub-Earth latitude (De) at opposition was +5.5°, and D equalled 6ʺ or greater from 2020 Mar 19 till 2021 Mar 10. The 2020 perihelic opposition was the most favourable for UK observers in the 15- or 17-year cycle, as with seasonally comparable oppositions in the Section’s history: 1894,1 1909,2 1926,3 1941,4 1958,5 1973,6 1988,7 and 2005.8 Seasonal and other dates are listed in Table 1.

Observations covered the period from 2019 Oct 31 (drawing by D. Gray, at areocentric longitude Ls = 100°; the first image was by C. Foster, Nov 7) to 2021 Jul 29 (image by A. S. Kidd, Ls = 79°; last drawing was by R. J. McKim, Jun 5) from early northern summer in Martian Year (MY) 35 till late northern spring in MY 36: 94% of a 360° interval in Ls. An unbroken run of 435 consecutive days between 2020 Mar 2 and 2021 May 10 was achieved. In all, 537 dates were covered by 11,676 observations (9,908 images and 1,768 drawings) from 147 observers (Table 2). In the UK, high-resolution images were communicated by D. L. Arditti, P. Edwards, N. J. Haigh, A. S. Kidd, W. J. Leatherbarrow, M. R. Lewis, D. A. Peach (some in collaboration with E. Enzmann on La Palma, or via Chilescope) and D. B. V. Tyler, supported by many others. The Director obtained 220 drawings (see Figure 9 and Part II, Figure 11) and good totals were obtained by P. G. Abel and C. Nuttall. Of the overseas observers, H. Einaga, P. Gorczynski and M. Hood each secured over 250 observations, while M. Adachi, T. Arakawa, S. Ito, T. Kumamori, P. W. Maxson and K. Yunoki each made over 500. We regret the deaths of Peter Edwards and Ken Medway since the completion of this report.

The planet’s declination (+5.5° at opposition) was only slightly north of the celestial equator for much of 2020, enabling our Australian friends also to experience Mars at a reasonable altitude. On an historical note, members of the Société Astronomique de France renovated the 216mm reflector once used by E-M. Antoniadi, a former Director (1896–1917) of our Mars Section. They took comparative images through it and the restored 250mm refractor of the Flammarion Observatory, Juvisy.9

The COVID-19 worldwide pandemic limited overseas travel and the direct use of many professional instruments. W. P. Sheehan was almost alone in being able to use the 600mm refractor of Lowell Observatory on a few nights, while Pellier was able to visit Saint-Véran Observatory. Severe and enduring as it was, the pandemic did not in any way limit the determination of observers.

The writer maintained a frequently updated opposition webpage at the Section website.10 This was updated daily throughout the large regional dust storm of 2020 November and was the Director’s main method of communicating news. A few dust storm alerts were sent out by e-mail and posted upon the BAA Forum page. Several interim reports were prepared,11–14 the first containing an opposition preview. The writer prepared a preview in his ‘Sky Notes’ for the cancelled BAA Ordinary Meeting in 2020 March, as well as a tutorial on planetary sketching.15

With air travel on Earth severely restricted, 2021 February was ironically a busy month for interplanetary exploration. On 2021 Feb 9, the UAE Space Agency Hope probe went into orbit,16 followed the very next day by a spacecraft from China, Tianwen-1,17 while on Feb 18 the NASA Perseverance rover landed at the 49km-diameter Jezero crater (+18°, 282°),12,13,18 a feature possessing a fan-shaped delta and with clear evidence of flowing water in the geological past. Later, there were several flights by its little helicopter, and the first-ever successful extraction of oxygen from the Martian atmosphere.13 The former two missions consisted of an orbiter, lander and rover. The UAE mission involved insertion into a highly elliptical orbit which at closest approach usefully corresponds to a geostationary one, and at its furthest facilitates whole-planet imaging. A recent review summarises the type of meteorological data collected by present and past landers and rovers.19


Two strip maps, one labelled and the other not, showing features on Mars in 2020
Figure 1. Top: Apparition map for 2020 by T. Kumamori, from his RGB images with a 355mm SCT and ASI 290MM & ASI 462MC cameras, Aug 15 – Oct 25. Bottom: 2020 map by M. R. Lewis from colourised λ = 642nm images with a 444mm refl. and ASI 174MM camera, September–November. Both maps have planetocentric latitudes and cylindrical projections. Note: South is uppermost in all illustrations.


The observations


Albedo features

We continue to use telescopic nomenclature.20 Apparition maps by T. Kumamori and M. R. Lewis appear in Figure 1. Albedo changes due to the 2018 global storm have been discussed elsewhere.12 Of the latter, the developments at Phasis and Indus were the most strikingly enduring. Here we record how the albedo markings responded to the further storms of 2020. Near opposition, the different colours of the markings were beautifully seen, and with large apertures the maria could be resolved into innumerable tiny spots, this being the experience of the Director rather frequently during September–October, whenever the seeing was very good. The present report continues from that for 2018.21


Region I: long. 250–010°

See Figures 1 & 2. The features at the NW corner of Syrtis Major, around Antoniadi crater, were rather conspicuous but not abnormal. This area had experienced an albedo anomaly at the time of the 2018 global storm. To the north-east, Nodus Alcyonius remained faint. Hellespontus was darkened and greatly broadened by the 2020 June–July large regional dust storm, while the large November regional event deposited a complex pattern of fallout over S. and SE Iapigia.

The location of the Huygens crater was well seen, marked by a large dark patch upon its floor. The best images showed remarkably complex details upon the latter albedo marking.

Moeris Lacus formed a small, elongated spot on the E. side of the Syrtis, but the Nepenthes streak (which commences there) remained largely invisible, as it has done for decades. Interestingly, a broad but extremely faint and diffuse shading was seen in its place when the phase angle was high (on the morning side) in 2021 March. Such lighting conditions can sometimes exaggerate the contrast of certain halftone albedo markings, as observed with several other features in the past, but the Director has not previously noticed it with Nepenthes. Its appearance upon the images by Kurisu on Mar 23 and Enzmann & Peach on Mar 31 recall, albeit in a very much fainter form, its form around 1958 or 1960. A similar effect is apparent upon some 2020 February–April images, when there was an equally strong phase effect upon the evening side, while the Director had an incomplete visual impression in early 2020 November, due to diffuse and curved shadings E. of Isidis Regio. Will Nepenthes return one day?

Pandorae Fretum was essentially invisible in the early part of the apparition (though Noachis on its S. border was rather dark during the first half of 2020, and much darker than Deucalionis Regio), continuing to be so until after the large regional storm of June–July. As soon as the latter had subsided, by mid-July, Pandorae was seen to have become broad and dark, remaining thus for the rest of the apparition. The June–July storm darkened the E. half of Deucalionis Regio. The latter change persisted for the remainder of 2020–’21, being missed by several visual observers, but it was shown clearly by Giuntoli with a 100mm OG.


Region I of Mars, during 2020: a selection of images showing the longitudes described in the text during the apparition. Syrtis Major is central in most.
Figure 2. Region I. Images (RGB except where stated) and drawings in 2020, by P. G. Abel (305mm refl., ×375), L. Aerts (ASI 178MM camera), D. L. Arditti (Flea 3), P. Edwards (ASI 224MC), D. Gray (×385), C. Nuttall (×410), T. Olivetti (PG GS3 IMX252), D. A. Peach (355mm SCT and ASI 290MM), M. Radice (ASI 224MC) and M. Ratcliffe (ASI 290MM).


Region II: long. 010–130°

See Figures 1, 3 & 5. The great development at Indus (connected to SE Mare Acidalium), which had been a result of the 2018 global storm, survived the 2019 January regional event.21 About 2020 Feb 21, there was a short-lived local dust storm over W. Deucalionis Regio (see later), and a few days afterward an image by Peach on Feb 26 showed the configuration again changed, due to fallout from that event. Now the Np. (p. = preceding) part of Margaritifer Sinus had faded, and Hydaspis to the west was again extended and darkened, giving the area an even more peculiar appearance. However, Margaritifer and its connection with Oxia Palus slowly darkened while Hydaspis faded slightly, doubtless owing to a small regional storm in April (see below), so that by late that month the area looked more normal, though the darkened and broadened Indus persisted throughout. The latter development was still clearly visible as late as 2021 July. In Figure 5, we show the sequence of changes since 1999.

Solis Lacus and the Phasis development remained prominent. Tithonius Lacus was not as dark as it sometimes is, suggesting that post-global storm fallout in the canyons of Valles Marineris had raised its albedo.

The orographic clouds over the Tharsis Montes (as well as Olympus Mons) are detailed in Part II. With the Martian atmosphere clear and transparent, it was possible to see – especially a fortnight or so either side of opposition – the ‘opposition effect’ upon the slopes of the Martian volcanoes, when they brighten considerably. All these volcanoes, as well as Elysium Mons and Hecates Tholus, showed it.


Region III: long. 130–250°

See Figures 1 & 4. Dust fallout that had affected Nodus Alcyonius in 2018 also caused a further fading of the long-enduring Aetheria development, while Cerberus and Trivium Charontis remained very faint. A long way from opposition, in 2020 May, the faint Cerberus had appeared a little darker (relatively), but this was merely a further example of the phase effect. In the south, the dark spot Caralis Fons (the floor of Newton crater) was very prominent, as it has been for years, attached to the Sf. (f. = following) end of Mare Sirenum by a prominent halftone streak. (Caralis Fons was not recorded prior to 1926, and has now been visible for many years.)


The Martian atmosphere

Dust storms


In a recent paper, Shirley, McKim et al. (2020) used dynamical considerations to predict whether global storms might occur during 2020 and at the following three oppositions.22 Our prediction that there would be no global storm in 2020 proved to be correct. At the time of submission of the present paper, the other predictions remain to be tested.

In the following, we review the smaller events followed by two large regional storms.


Local & small regional storms, 2020 January–October

Some of the smaller events of 2020 are collected in Figure 6.

On 2020 Jan 26–29 (Ls = 141–143°), Foster imaged dust in CebreniaElysium, which appeared strongly yellowish compared to the evening orographic cloud at Olympus Mons, and brighter in red and infrared than in blue light. On Jan 23, Elysium had seemed to show only orographic cloud, but there are no other useful observations. On Feb 21 (Ls = 154°), he found bright dust over W. Deucalionis Regio Thymiamata / AramMargaritifer Sinus (Figure 6A). Invisible on Feb 17, the time course of this local dust could not be followed, but by Feb 26 fallout had again altered the albedo configuration at N. Margaritifer Sinus (see also under ‘Region II’, above, and Figure 5).

In March, there was a brief local event in southern Zephyria.11 Foster on Mar 8 (Ls = 163°) imaged three separate small yellowish dust clouds, the activity lasting till Mar 10, as illustrated in Figure 6B. Significantly, activity at this rarely active site had repeated at the same value of Ls as in 2018 April.21

From Mar 17–29 there was local activity in the south of Hellas. A tongue of S. polar hood (SPH) also protruded into the basin, as it had done at this epoch in 2018. MacNeill’s Mar 20 image appears in Figure 6C. On Mar 23, a tongue of SPH stretched north to the Argyre basin, and on Mar 25 Foster found Argyre much brighter and strongly yellower than before: this marked another short-lived local dust event, which had already faded slightly the following day.

The largest event was a regional storm during Apr 10~16 (Ls ~181–184°; Figure 6D). Nothing was visible in Maxson’s or Peach’s images on Apr 8, but on Apr 10, Maxson recorded a large, bright cloud over SE Margaritifer Sinus / E. Mare Erythraeum. The time course of this event was not completely followed by us but was observed by the cameras of the Mars Reconnaissance Orbiter (MRO). On Apr 11–12, dust was observed by Casely, Lonsdale and MacNeill south and east of Aurorae Sinus, running along central and E. Valles Marineris, affecting Mare Erythraeum, and substantially penetrating into Chryse, crossing the equator. The E. limit was at Margaritifer Sinus, whose N. end was briefly effaced. Go showed the event to be fading on Apr 16. MacNeill and Wesley on Apr 17 showed no trace of active dust, but the shape of N. Margaritifer had reverted to its post-global-storm aspect (see under ‘Region II’, above, and Figure 6D).

A small dust cloud existed in N. Chryse during May 21~23, which did not expand. Arakawa’s and Kumamori’s images showed it most prominently on May 23.


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