Over two weeks ago a bright white spot appeared in the atmosphere of Venus, but where did it come from? A new report from the Venus Express team sheds some light on the matter.

In our initial report of the discovery, amateur observer Frank Melillo who first spied the marking on 19 July commented: "I have seen bright spots before but this one is an exceptionally bright and quite intense area."

The spot, thirty percent brighter than 'normal' and 1,000 kilometres wide was soon confirmed by other amateurs, and fortunately the Venus Express Monitoring Camera (VMC) was also looking in the right place at the right time to examine the spot close-up and to monitor its evolution over the coming days.

Venus' atmosphere is permanently smeared with white clouds, and the new bright region stuck its head up in one of the southern streaks, which typically spirals in towards the pole as part of the vortex-like circulation pattern that is centred there – rather like water spinning down a plug hole. In the spot's location at a latitude of 50 degrees south, the atmosphere rotates around the planet once every four days – a fair lick compared with Venus' somewhat lethargic 243 day long turn about its own axis. Interestingly, the VMC images reveal the white spot in images captured by the spacecraft four days earlier, albeit dimmer, brightening by the 19 July and suggesting that the event causing the confined marking occurred one atmospheric rotation earlier than Melillo and others first noticed it.

But what caused the cloud properties to change so dramatically? In a new report released last night, and lead by Sanjay Limaye of the University of Wisconsin and Dimitri Titov of the Max Planck Institute in Germany, four possible scenarios are presented: a volcanic eruption, solar wind interaction, an icy comet impact or a previously unseen weather phenomenon.

Volcanic eruption?
While an attractive idea that below Venus' impenetrable cloak of poisonous gas clouds lie hundreds, thousands, or even millions of active volcanoes spewing sulphur dioxide into the atmosphere, the very nature of the planet's atmosphere makes confirming such a possibility a logistical nightmare. Not only are direct observations of the surface in visible light impossible from space, with an atmospheric pressure nearly one hundred times that of Earth a volcanic eruption would have to pack a serious punch to knock through the thick cloud layers to the 70 kilometre altitude at which the spot was seen. This also doesn't marry with the idea that Venus volcanoes are shield volcanoes, a type of volcano that emits lava very gently down shallow-sloping sides, rather than emitting gas high into the atmosphere. "The possibility of actually detecting or confirming an eruption through a bright cloud appears somewhat difficult until we have other data," Limaye tells Astronomy Now.

Solar activity?
Previous observations of a white marking on Venus have been attributed to solar activity, whereby charged particles from the solar wind have interacted with the planet's atmosphere to cause brightening, only to disappear a few days later.

Impact?
Since the white spot appeared at roughly the same time as the dark spot on Jupiter, could Venus' scar also be attributed to an impact of an asteroid or comet? On Jupiter, darkening occurred, while on Venus the region in question became brighter. That means the impact debris would have to be highly reflective at ultraviolet wavelengths. Although water ice could cause brightening, there would need to be an awful lot of it, and bodies impacting into an atmosphere rich in carbon dioxide would create a dimming in the cloud and not the scale of brightening seen at Venus.

Weather?
We know how quickly and unpredictable weather on the Earth can be, and Limaye and colleagues suspect that we may have simply witnessed some unusual Venusian weather. "Sudden changes in Venus' cloud cover have been observed in the past, but we do not yet understand Venus and its mysterious atmosphere very well at all," he says in the online report. The atmosphere is continually churning over, cycling sulphur dioxide and water vapour from the lower atmosphere into sulphuric acid clouds in the upper atmosphere. These sulphuric acid aerosols, which appear bright in ultraviolet images, can also be brought to cloud level in cyclones like the giant vortex at the planet's south pole.

As it stands with the current information available, it seems that the most likely and natural explanation for the local brightening was caused by perturbations in the atmospheric circulation. "In other words, the observers witnessed Venus weather," concludes the report.