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Cusps and Collars

by Perry Pezzolanella

Venus is the brightest planet in the morning and evening sky, shining almost as bright as magnitude -5 when it comes within only 25 million miles of Earth, the closest of any planet. It can then appear as a large, one arcminute fingernail thin crescent that can be discerned by an observer with exceptionally sharp vision (20/15 or better) without optical aid. With all these outstanding characteristics, it would be expected that Venus is a rewarding planet to observe with a telescope. Unfortunately, it is the most disappointing of all the planets since it lacks the rich details of Mars and Jupiter.

Venus is completely covered by clouds that hamper even the most seasoned observer with the best equipment. It is rare to see faint, diffuse markings of low contrast. Venus is so bright that it dazzles the eyes so it is best to use a filter to dim it, some of which can increase the odds of observing cloud detail visually. There are different types of clouds we are familiar with on Earth: wispy cirrus, sheets of stratus, puffy cumulus, and towering thunderheads, along with haze, dust, fog, and smoke. These clouds have texture, which is obvious to anyone who has flown above them or observed them when the Sun is low along the horizon. The winds and jet streams bend and shape the clouds into long streaks, bows, and swirls while sunlight shining on them at a low angle gives a feeling of depth. Most of the clouds that affect our weather occur from the surface (fog) to around 10 miles up (cirrus). On Venus it is far different with clouds ranging from 30 to 45 miles above the surface in three distinct layers. Venus has a hefty atmosphere compared to Earth and the thicker cloud deck adds to the texture that a low Sun angle creates. This can make it easier to see detail within the clouds of Venus, but can we?

The winds of Venus at the cloud tops are ferocious, reaching 220 miles per hour. It takes only four days for a cloud to go completely around the planet if it could hold together. The direct heating of the Sun near the equator creates cumulus clouds that are stretched into streaks as the upper-level atmospheric circulation draws them towards the poles. A zonal flow parallel to the equator eventually predominates near the polar vortex. Convection is the boss on Venus and governs the circulation. The rising air near the equator eventually starts to descend at latitudes between 60º-70º and flows back towards the equator beneath the visible cloud deck. This region of sinking air appears as dusky collars that surround the brighter polar hoods. Venus stands nearly upright on its axis which makes the collars appear foreshortened because they are closer to the poles. The poles are dominated by a whirlpool pattern of powerful polar vortices, which are vivid with a bright high haze and are known as cusp caps.

Visual observing of dazzling Venus is tricky, so a neutral density or polarizing filter is needed in hopes of seeing any cloud detail. It is also best if Venus is a plump crescent, half phase, or slightly more than half phase, around 30%-60% lit to increase the chance of seeing anything. The best areas to study are around the poles, or the horns if Venus is a crescent. One or both poles may appear brighter than the rest of the planet and it might be possible to detect a dusky collar. It will be challenging but using a violet (W47) or deep blue (W38A) filter may help improve the odds. An ultraviolet filter will not help as these are used for photography only. It is fun to experiment with color filters to see what each color might reveal, especially if combined, but violet and deep blue are the best bet. If you want to see any detail in the clouds with certainty, then photography with an ultraviolet filter is the way to go. With skill and luck, it might even be possible to image the changing cloud patterns over time.

Venus may be a hostile and frustrating world to explore visually but with patience the reward of seeing the fleeting details of the cloud cusps and collars is worth the effort. Imaging Venus regularly may make it possible to observe and record the changing and evolving weather patterns on a world so similar and yet at the same time so extreme to Earth.