One of the many mysteries of Earth's nearest planetary neighbour Venus has been cracked, Nature journal reports.
Scientists have long puzzled over conspicuous patches in the Venusian clouds that appear dark at ultraviolet (UV) light wavelengths.
They now think these are solid particles or liquid droplets that get transported from deep in the atmosphere up to the planet's cloud tops.
But a riddle remains: scientists still don't know what they are made up of.
The features are distributed within the thick clouds of sulphuric acid and sulphur dioxide that shroud the hothouse planet.
"These (UV features) have been observed since 1929. We see them in images from the Pioneer Venus probe and in ground-based observations," said Dr Dmitri Titov, from the Max Planck Institute for Solar System Research in Germany.
It had previously been unclear whether these were caused by differences in the height of the cloud tops, temperature differences or variation in composition of the clouds.
Darkness and light
Data from the European Space Agency (Esa) spacecraft show that areas near Venus' equator which appear dark in ultraviolet light are regions of relatively high temperature.
The scientists think this is where intense convection brings up the mysterious dark material from below.
Bright regions at Venus' mid-latitudes are areas where the temperature in the atmosphere decreases with depth, which prevents air from rising. The effect is most extreme in a wide belt around the poles, which has been dubbed the "cold collar".
At low and mid-latitudes, the cloud top is located at a constant altitude of about 72km in both the dark and light regions, which suggests the light and dark patches do not result from changes in elevation.
Instead, the most likely cause is the uneven distribution of a mysterious chemical in the atmosphere that absorbs ultraviolet light, creating bright and dark zones.
Although the exact chemical species that creates the high-contrast zones remains elusive, a complex compound of sulphur is now a favourite.
But a full answer may have to wait for a subsequent Venus mission.
"It seems that Venus Express will not completely solve this," Dr Titov told BBC News.
"This species is very strange because it doesn't have particular features - just very broad ones. So we can't say exactly what it is made of. It's probably some kind of chemical hidden inside cloud droplets."
He added: "We need to send balloons (to Venus). The balloons will be ideal because they will be flying in this region. And if we have a chemical laboratory... on board the balloon we will really understand what this is."
Balloons were deployed in the Venusian atmosphere during the Soviet-French Vega 1 mission in 1985. And both the US and Europe have carried out technical studies on a next-generation Venus mission which could feature a balloon or lander.
Some researchers have even speculated whether Venusian microbes could survive high in the planet's atmosphere, where the temperature and pressure are quite Earth-like.
Here, they say, the ultraviolet-absorbing chemical could act as an "umbrella" to shield life forms from the destructive UV rays coming from the Sun.
Further down, conditions are quite different. The planet's surface is heated to an average temperature of 467C (872F) - hot enough to melt lead.
And the dense atmosphere generates a surface pressure 90 times greater than that on Earth.
Another key question for the mission is whether Venus is still volcanically active. Venus Express has found a highly variable quantity of the volcanic gas sulphur dioxide in the atmosphere.
Some observers think this could be to do with recent volcanic activity on the surface.
But others say the lack of rain on Venus to scrub the atmosphere clean of sulphur dioxide means Venus Express could be detecting events that happened millions of years ago.
"We're not ready to say definitively one way or another on the basis of this evidence before we analyse all the data," said Fred Taylor, Venus Express interdisciplinary scientist at the University of Oxford.
He told the Oxford Science Blog: "However, there's plenty of indirect evidence for volcanic activity on Venus so, in my opinion, it's about how much activity is going on and the role it plays in the planet's climate. I think it's probably just a matter of time before we 'see' a volcano erupting."