Remarkable waves observed in the atmosphere of Venus, 2015–2020
2020 August 2
Akatsuki’s atmospheric wave, 2015 December – 2016 January
A remarkable Venusian atmospheric wave was identified in the long-wave infrared (IR) images from the Akatsuki space probe of the Japanese Aerospace Exploration Agency (JAXA) during 2015 December, as announced briefly in this Journal and illustrated in popular magazines.1,2 The feature (shown in Figure 1) was a large bow-shaped wave centred upon the equator and extending north–south over some 10,000km across a wide range of latitude. (10,000km equates to a latitude range of 95°.) It was located near the dayside evening terminator (marked by a dashed line in Figure 1), and it remained nearly stationary with respect to the slowly rotating surface. It was slightly warmer than the surrounding atmosphere. Animations of the Akatsuki wave reveal at most an extremely slight movement in longitude, but the data were of short time span.3
Akatsuki observed the feature only during 2015 Dec 7–12, just before orbital insertion, and subsequently a change in its orbit terminated the observations. When it was able to observe the region again on 2016 Jan 16, the feature had disappeared.
The wave’s altitude of 65km corresponds to the boundary between the Venusian upper troposphere and lower stratosphere; at this layer the wind speed of normal clouds is of the order of 100ms–1 with respect to the ground. For atmospheric features recorded in the infrared, a near-stationary drift is unique. Synodic atmospheric rotation periods in the infrared region of 5–6 days, somewhat dependent upon latitude but more strongly upon wavelength (and hence upon altitude), are typical. The wave feature was extremely conspicuous at the very long infrared wavelength (λ) of 10 microns (1 micron (μm) = 1,000nm), but a trace of it could also be detected in the ultraviolet (UV): see Figure 1. (Akatsuki carried two IR cameras, for imaging in different spectral ranges.) A useful review of the type of information to be gleaned from observations in different wavebands is available.4
From the location of the wave near surface longitude 90°, the Akatsuki investigators considered it had been created by uprising airflow over the Aphrodite Terra mountain range, which reaches 5km in altitude. The feature was therefore regarded as a gravity wave (not to be confused with gravitational waves). Computer models run by the Akatsuki team suggest that air flowing over the elevated terrain could generate a gravity wave which propagates upward to the cloud tops, where it appears as a large bow-shaped wave.3,6 Gravity waves have been positively observed in the atmospheres of other planets, from the Earth (leeward of the Andes mountains, for instance) out to Pluto.
In the nocturnal hemisphere, a high IR albedo corresponds to a locally warmer area, because temperature sharply decreases with height above the strongly heated surface.5 However, upon the sunlit hemisphere, brightness in the near-IR implies greater reflectivity. On the side of the wave towards the terminator there was always an equally long, dark and irregular bow-shaped marking.
The Akatsuki investigators formally announced their findings in 2017 February.6 During the 2017 European Planetary Science Conference, J. Peralta (JAXA) suggested that near-IR imaging by amateurs might record such sharp cloud discontinuities having planetary-scale dimensions, and Kardasis subsequently made an attempt.
Infrared wavebands & the ground-based observer
The upper wavelength limit for red light is 780nm. Infrared wavebands may be conveniently grouped into the following categories,7 of which only the short wave region is accessible to amateurs:
Short wave 780–1,400nm (1.4μm)
Medium wave 1.4–3.0μm
Long wave > 3.0μm
Near-infrared images are nearly always bland in comparison with high-contrast ultraviolet ones. In the UV, the characteristic chevrons of the equatorial Y and psi markings always dominate the visible albedo patterns. Moreover, the atmospheric wave had been recorded by the spacecraft at a wavelength of 10μm in a waveband inaccessible to amateurs, so it was not originally considered to be a possible observational target for the BAA.
We have checked through the BAA image archive for the period covered by the Akatsuki observations, and found no records of linear N–S streaks, bright or dark, in either IR or UV images. Examination was made without further processing of the observations. Since IR features present very low contrast, they require special image treatment to reveal the subtle details.
Infrared observations by amateurs, 2016 October–December
The Akatsuki wave was not discussed in the recent long-term report for 2007–’17,8 but during the past year an equally remarkable phenomenon was recorded by several amateurs in the near-infrared as an extremely long and narrow discontinuity oriented N–S. Unlike the Akatsuki wave it did not display any noticeable bow-shape, but it did consist of a long bright streak followed by a dark line. Kardasis was the first to draw attention to it, and he recently gave advance details in the Section bulletin, Messenger,9 where he noted that P. Miles and A. Wesley (both based in Australia) had observed a similar feature in late 2016, during the 2017 E. elongation. A search was therefore made for more sightings, through the Section records as well as the ALPO Japan and PVOL websites.10,11 As a result, we now have several periods over five years during which apparently similar waves or discontinuities have been sighted.
Looking back through the records with the benefit of hindsight, a handful of sightings of a similar phenomenon could be identified from the late 2016 near-infrared images, although the feature appeared less prominent than in the Akatsuki images, did not exhibit a bow shape, and often looked fragmentary. Because amateur infrared images are usually heavily processed, they can sometimes exhibit one or more ‘step’ artefacts towards the terminator, so a linear vertical feature often represents this. For that reason, the very few sightings of a real wave in 2016 had not originally been noticed. Simultaneous visual and ultraviolet images were searched, but no sign of the wave was seen. The wave recorded by Akatsuki had been only weakly visible in the spacecraft’s extremely high-resolution UV images: if one had not first seen the IR results, it would probably have been overlooked.
Table 1 and Figure 2 summarise our findings from the BAA image archive. The latitude of the sub-Earth point (De) was small, and is stated at the foot of the table. A short-lived wave feature was recorded on 2016 Oct 24 & 29. It contained part of a bright arc oriented N–S, with a dark N–S feature adjacent to it on the terminator side. At this elongation there were many excellent daily images during October–December, but only those five days apart recorded the wave-like phenomena. Another recurrent bright feature had also been recorded on Oct 15 & 20: it does not appear to have been a wave or discontinuity, but is included in Table 1.
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