Weather on Venus?
On July 19, 2009, Frank Melillo took a picture of Venus through an ultraviolet filter from his backyard in Holtsville, New York state and saw a bright spot near the limb, almost at the same time as the first picture of the recent impact on Jupiter taken by an amateur in Australia (Anthony Wesley). Frank was imaging Venus based on an alert to the amateur astronomers put out through ALPO (Association of Lunar and Planetary Observers) that some Venus volcanoes were going to be approaching the terminator.
Surprisingly, Frank's image (Figure 1) showed a brighter than usual spot in the southern
hemisphere, far away from the volcanoes. The bright spot in Frank's image and two others
were also captured in an image taken about 10.5 hours earlier by George Tasourdis (Figure
1, right image)
Fortunately, the Venus Monitoring Camera (VMC) built at the Max Planck Institute for Solar System Research (Katlenburg-Lindau, Germany) for European Space Agency's Venus Express Orbiter was monitoring Venus during this period. VMC images on July 19th indeed show about 30% brighter than usual "spot" ~1000 km in size located at ~50°S (Figure 2). This bright region appears in one of the southern streaks seen almost every day on Venus which characteristically spirals towards the pole as a part of hemispheric vortex centered on the pole.
The atmosphere at the latitude of the bright spot rotates around the planet once every approximately four days, and interestingly, an image taken four days earlier (On Orbit # 1182 of Venus Express) shows the same spot which is noticeably brighter than the surrounding cloud, but not as bright as seen on July 19th. This suggests that the event that caused the confined portion of the cloud cover to appear brighter, likely occurred at least one atmospheric rotation earlier. Since the images are taken in reflected ultraviolet light, it is possible that changes in cloud properties (particle size, number, shape) can cause increased reflection, but that just raises the question, "what caused the cloud properties to change?"
The illumination and viewing geometry is important in understanding changes in apparent
brightness as the scattering properties of clouds can be quite different based on the
microphysical properties. This is evident every day on Earth - a cloud's apparent brightness
can vary a lot depending on where it is viewed from and how the sun is shining on it,
particularly when the sunlight falls on a cloud at a very shallow angle.
Bright spots were also seen in images taken on July 24th, 2009 by Dr. Don Parker from Coral Gables, Florida, also near the limb (Figure 3).
What Caused the Brightening?
A volcanic eruption is one possibility, a solar wind connection is another, an icy comet impact is plausible and intrinsic changes in the cloud without any external cause is also likely. The challenge is to identify the cause of the brightening from any intrinsic changes in the cloud cover due to atmospheric winds. Volcanoes are believed to have been active on Venus in its past, and perhaps even recently based on the secular decline in the amount of sulfur dioxide gas (a common product of volcanic eruption) estimated from ground or space based remote observations.
The conditions on Venus also make confirming a volcanic eruption difficult. With the atmospheric pressure about ninety times than that experienced at sea level on Earth, a volcanic plume needs to punch through a very stable (with respect to vertical motions) and thick layer of Venus atmosphere extending from ~48 km altitude to the level of the cloud tops at ~65 — 70 km altitude. Thus the eruption would need to be quite energetic if it were a Mt. St. Helens type eruption. However, most suspected volcanoes on Venus are "shield" volcanoes, like the currently eruptions on the island of Hawaii that produce effusive lava flows. If such an event were to take place on Venus the gaseous output can be carried vertically by atmospheric circulation. However, atmospheric transport by the circulation would not be consistent with the confined brightness increase seen in the clouds.
As Lori Glaze (NASA/GSFC) points out, the northern hemisphere volcanoes are located at a somewhat higher altitude where the surface atmospheric pressure is considerably lower and it would be somewhat easier for the plume to rise high. However, the East-to-West winds on Venus increase rapidly with altitude at all latitudes, so even if the plume were to rise quickly if the injection velocity is high enough, it would still get bent to become almost horizontal before reaching the ~70 km altitude of the cloud tops according to her calculations. A localized brightening due to a volcanic plume thus seems somewhat unlikely.
An Impact by a Small Water Ice Comet?
For the brightening to be due to an external impactor, the impact debris has to be highly reflective at ultraviolet wavelengths, e.g. water ice. Except for water ice, bodies impacting an atmosphere rich in carbon dioxide will likely create a dimming of the cloud brightness from the impact debris, not brightening.
A connection with the solar activity cannot be ruled out and was suggested as a plausible
explanation for an earlier sudden brightening event in January 2007 observed from Venus
Express. Dr. Don Parker, an experienced amateur astronomer imaged Venus serendipitously
during this brightening (13 January 2007) which showed a brighter than normal southern
hemisphere streak. This event quickly covered a much wider area (Figure 6) but the brightening
dissipated over the next few days and the cloud cover dimmed to its normal appearance.
Sudden changes in the Venus cloud cover have been observed in the past, but we do not yet understand Venus and its mysterious atmosphere very well at all. Something in the cloud properties is causing a portion of the cloud to brighten quickly and sometimes this spreads over a wide area quickly (January 2007) and sometimes it does not, instead getting stretched by the ambient flow and dissipating over a few days (July 2009). The fact that many ground based images tend to capture small bright spots near the limb, may be indicative of changes in reflective properties of the cloud cover as could occur due to increased particle density, changes in the particle shape or in the vertical cloud/haze structure.
An image taken by Paul Maxon (Figure 7) on July 20, 2009, just a day after the image by Frank Melillo also shows bright spots at high north and south latitudes near the limb, strengthening the notion that the scattering geometry is partly responsible for the enhanced brightness caused by some local changes in the cloud.
The most likely and natural explanation is that the local brightening was caused by perturbations in the atmospheric circulation. In other words the observers witnessed Venus weather. The lower atmosphere stores great amount of sulfur dioxide and water vapour, precursors of sulfuric acid that forms the cloud. These gases being brought to the cloud top by the gradual overturning of the atmosphere between the equator and high latitudes and give rise to fine and dense sulfuric acid aerosols which appears bright in UV images. This process can be enhanced by higher abundances of oxygen and water.
The gases can also be brought to the cloud level by local convection as occurs in the
spiral arms of a tropical cyclone and can also occur in the hemispheric vortex on Venus
as shown in Figure 7 which shows a comparative composite view of the vortex on Venus
and a tropical cyclone. Localized convection can be seen as bright spots in the spiral
arms of the cyclone near the bottom. Indeed, given the initial confined nature of the
brightening, it is likely that local convection was responsible for the changes in the
cloud cover, but we lack corroborating data at this point. The evolution of the bright
spot is controlled by advection that spreads the bright material over larger areas and
aerosol coagulation that converts small particles into larger ones. The changed reflective
or scattering properties make the cloud portion appear brighter than its surroundings
in reflected light.
Venus presents the simplest atmosphere to study in some respects – one hundred percent cloudy, no oceans and thus no major hydrologic cycle, no land-cover-ocean contrasts in terms of physical properties, almost a perfect sphere compared to most planets, lack of any seasonal change in sunlight falling on the planet due to an orbit with low eccentricity and almost perpendicular spin axis of the planet with a very slow rotation rate. Yet we do not understand its rapid deep atmospheric circulation and its organization into a giant hemispheric vortex over each pole. This vortex circulation shares many similarities with the circulation of a tropical cyclone despite the differences in size and source of energy. Thus, we can expect some convective activity in the spiral streaks on Venus similar to what is found in hurricanes, and it is possible that the bright cloud was a result of such activity.