Heavy metal frost and a dark mystery: Venus is a strange place

Wednesday, October 22, 2014, 6:24 PM - Venus may be known best for its sulphuric acid rain, crushing atmospheric pressures and hellish temperatures, but the planet is turning out to be even more bizarre, as researchers discover that its mountains are covered in heavy metal frost, and perhaps something even stranger, which is sparking a mystery about our sister world.

Venus has always held its mysteries. With its surface hidden by thick layers of cloud, it was once thought to be a tropical jungle paradise, even more alive than Earth. However, once we began investigating the planet with telescopes, spacecraft and landers, those ideas were quickly dropped. We now have detailed maps of the planet's terrain and even pictures taken from the surface. We have an excellent idea of what's happening in the planet's atmosphere, and we've even identified regions where we might be able to live at some point in the future. There are still some mysteries that endure though.

For decades, ever since the Magellan spacecraft mapped the surface of Venus with radar altimetry, scientists have puzzled over unusual patterns in the data that was returned as Magellan flew over the planet's mountains.

“There is general brightening upward trend in the highlands and then dark spots at the highest locations," said Elise Harrington, a student at Simon Fraser University who has been going over the Magellan data while interning at the Lunar and Planetary Institute, in Houston, TX, according to a Geophysical Society of America news release.

"Like on Earth, the temperature changes with elevation," Harrington explained in the release. "And the cooler temperatures at altitude lead to ice and snow, which create a similar pattern of brightening for Earth - but in visible light. Among the possibilities on Venus are a temperature dependent chemical weathering process or heavy metal compound precipitating from the air - a heavy metal frost."

Venus' highlands appear bright in backscattered (SAR) radar, here from Magellan, and their highest elevations appear dark. Credit: Harringtong/Trieman/NASA/JPL-Caltech

Hearing about frost on a planet as hot as Venus - where it's over 460 degrees C on average - is pretty strange, but Venus isn't exactly your standard planet. The runaway greenhouse effect caused by the combination of Venus' thick atmosphere and its proximity to the Sun have produced conditions that we'd be hard-pressed to find here on Earth (outside of a laboratory). This results in some very unusual chemical reactions. Previous studies (like this one from Washington University in St. Louis) have identified metals like lead sulphide (also called galena) and bismuth sulfphide (or bismuthinite) as being possibilities for what make up this heavy metal frost.

However, the dark spots that are seen in higher elevations still defy explanation.

3D perspective of Ovda Regio. Credit: NASA/JPL-Caltech

Confronted with low-res altimetry data from the Magellan mission (with its 8 by 12-kilometre resolution) Harrington, and her Institute supervisor, Allan Treiman, sought to improve on that for their study. They contacted University of Alaska geophysicist Robert Herrick, who had generated higher-resolution elevation data - with 'block's just 600 metres on a side - from Magellan, and they combined his dataset with high resolution (75x75-metre) radio reflectance data straight from Magellan's Stereo Aperture Radar (SAR) instrument.

Focusing in on two regions of the highlands of Ovda Regio, which forms the western part of the Venus' largest continent, Aphrodite Terra, Harrington and Treiman applied this data and found the same pattern of brightening with higher elevation. However, the high resolution data turned up many more of the mysterious dark spots.

"The previous author saw a few dark spots," Harrington said in the GSA news release. "But we see hundreds of them."

Even more strange, they see reflectance data right up to about 4,700 metres elevation, and then the radar signal goes completely dark.

"No one knows what explains the sudden darkness," Harrington said, according to the GSA release. "We think this might spur some more interest in Venus."