Visibility times of Martian afternoon clouds
2022 August 7
This paper reports local times when afternoon clouds become telescopically visible for several locations on Mars. With the exception of dust storms, the times are consistent with historical records. It is concluded that late afternoon orographic clouds form near at least one of the four large Tharsis volcanoes throughout the Martian year. During 2018 and 2020, Argyre appeared as a northward projection on the South Polar Cap (SPC), near Ls = 170°. This is in line with the analyses of BAA observations reported by R. J. McKim for 1999 and 2001.
This paper summarises formation times of telescopically visible clouds throughout the Martian year. Diurnal clouds (those that form and dissipate each day) are examined here. The word ‘cloud’ refers to a white patch that is bright in blue and ultraviolet light; it is believed that a feature with this characteristic is a condensate cloud. Clouds in many areas are examined and visibility times are reported. They display regular patterns. Nearby dust storms, however, may prevent clouds from forming. In this study, the Martian year is broken into 20-degree increments of areocentric longitude (Ls), starting with 0–20°. The beginning of northern spring, summer, autumn and winter are at Ls = 0, 90, 180 & 270°, respectively.
Under clear Martian skies, a few of the large volcanoes become bright near opposition.1–3 For example, a Hubble Space Telescope image made on 2005 Nov 7 shows the Elysium volcanoes as bright spots.1 Much of this brightening may be due to the phase angle dropping.3 The brightening is most obvious when orographic clouds are not present, during southern spring and summer. During the other two seasons, clouds usually mask this brightening.
The ‘W’ Cloud & Equatorial Cloud Belt (ECB)
During northern spring and summer, many of the late-afternoon clouds discussed in this report become part of either the W cloud or the Equatorial Cloud Belt (hereafter ECB) systems. Slipher (1962) shows several photographs of the W cloud.4 It forms in the Tharsis area. Figure 1A shows an image of it and Figure 1B shows the outline of a ‘W’. The top-right point of the W is near Arsia Mons, and the bottom-right point is near Ascraeus Mons. This cloud system has been observed since at least 1907.4
The ECB is a second type of seasonal cloud system that develops. Two images of it are shown in Figure 1. It is usually more transparent than the W cloud and is best seen in blue and ultraviolet light. It lies near the equator and is visible at most longitudes.
A brief history of the repeating nature of white clouds
The repeating nature of cloud development on Mars has been known for many decades. For example, Slipher (1921) points out that the clouds he observed over Syrtis Major in 1920 were similar to those Lowell observed in 1903 and 1905.5 Thomson (1924) reports a diurnal cloud over Syrtis Major in 1918 as being similar to one observed in 1903.6 British observers saw a ‘zig-zag’ streak near Ceraunius in 1935,7 which was probably associated with the W cloud. Capen (1966; p.23) reports that ‘recurrent clouds have been observed to form over the same areas of the Martian surface for the past 62 years’.8
Smith & Smith (1972) carried out an extensive study of blue-light photographs made at the Lowell and Pic du Midi observatories.9 They focused on Hellas, Elysium and Nix Olympica (the bright spot associated with Olympus Mons) and studied photographs made between 1924 and 1971. They conclude that the white spots (probably clouds) follow a seasonal cycle for all three features, but only Elysium and Nix Olympica brighten as the day progresses. They also report that Elysium and Nix Olympica reach peak activity during Ls = 90–130°. Spacecraft results are also consistent with clouds following regular seasonal patterns. For example, Smith (2009) reports that the water ice aerosols forming the Equatorial Cloud Belt developed at nearly the same seasonal date between 2002 and 2008.10
McKim summarises BAA results for all Mars apparitions between 1980 and 2012.11–26 Later in this paper, the general trends in evening cloud visibility are compared to the results here.
Goal of the study
The goal of this study is to estimate cloud visibility times in selected areas. McKim (2018) describes the difficulty of comparative meteorology and stresses the need for a database of ground-based observations over many years.25 He describes over three decades of cloud observations.11–26 Furthermore, the current ALPO Japan website has thousands of Mars images spanning over 20 years.27 Therefore, the writer believes a sufficient number of ground-based images is available to construct a table of afternoon-cloud visibility times throughout a Martian year.
In spite of the large set of images now available, those from 2007 June–November could not be used because of the planet-encircling dust storm then taking place.28,29 This created uncertainty in cloud formation times for Ls = 320–360°. The writer believes cloud activity returned to normal by early December of 2007.
The areas described here include Arsia, Pavonis, Ascraeus and Olympus Mons; Alba Patera, the Elysium region, Syrtis Major, Libya, Edom Promontorium (Edom Pr.), Eden, Arabia, Aeria, Argyre I (Argyre), and Zephyria. The results are believed to correspond to times when there are no large dust storms. Therefore, they may be used in measuring the impact of dust storms on cloud formation times. One may also determine year-to-year differences in formation times.
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