Radar data reveals cavernous underground lava tube on Venus

By Will Dunham

WASHINGTON, Feb 9 (Reuters) - A fresh examination of radar data for Venus obtained by NASA's Magellan spacecraft in the 1990s indicates the presence of a large underground cavity created by a lava flow, the first subsurface feature ever detected on Earth's planetary neighbor.

Researchers said the radar data was consistent ​with a geological feature called a lava tube that is found in certain volcanic locations on Earth. Lava tubes also exist on the moon and are thought ‌to be present on Mars.

Venus has carefully guarded its secrets, with its surface shrouded by thick noxious clouds. But radar can peer through these clouds.

Scientists had theorized the presence of lava tubes on Venus, considering its history ‌of volcanism.

"Moving from theory to direct observation marks a major step forward, opening the door to new lines of research and providing crucial information for future missions aimed at exploring the planet," said Lorenzo Bruzzone, a radar and planetary scientist at the University of Trento in Italy, senior author of the study published on Monday in the journal Nature Communications.

The researchers analyzed data acquired by Magellan's Synthetic Aperture Radar remote-sensing instrument from 1990 and 1992 at locations bearing signs of localized surface collapses suggestive of lava tubes beneath. They used a recently developed data-analysis method aimed at identifying ⁠underground cavities like lava tubes.

The structure they detected, interpreted as ‌part of an empty lava tube, is on the western flank of the Nyx Mons, which is a shield volcano - a broad and gently sloped volcano whose overall shape resembles a warrior's shield - measuring about 225 miles (362 km) wide and located in the planet's northern hemisphere. The area ‍has a large number of pits caused by surface collapses.

"Our knowledge of Venus is still limited, and until now we have never had the opportunity to directly observe processes occurring beneath its surface," University of Trento radar scientist and study lead author Leonardo Carrer said.

Venus has a diameter of about 7,500 miles (12,000 km), slightly smaller than Earth. Magellan mapped 98% of the Venusian surface. Its data helped give scientists ​a basic understanding of the surface of Venus, which possesses more volcanoes than any other planet in our solar system and exhibits extensive evidence of past lava flows on ‌its surface.

"Lava tubes are natural underground tunnels created by volcanic activity. They usually form within basaltic lava flows, where low-viscosity lava continues to move beneath a solidifying surface," Bruzzone said.

The Magellan instrument's side-looking observation geometry was able to spot radar reflections coming from underground cavities.

The apparent lava tube identified in the data has an estimated average diameter of about six-tenths of a mile (1 km), a roof thickness of at least 490 feet (150 meters) and an empty cavity reaching a height of at least a quarter of a mile (375 meters). Due to the Magellan data's limitations, only the initial portion of the structure could be directly observed. The researchers suspect it is much longer, stretching perhaps several miles.

Its dimensions make it both wider and taller ⁠than lava tubes found on Earth or predicted for Mars. It lies at the upper end of ​the size range expected - and in one case already observed - on the moon. The researchers said the structure's ​size is not entirely surprising considering that lava channels observed on the Venusian surface are larger and longer than those present on other planets and moons in our solar system.

"Intense volcanic activity is known to have played a major role in shaping the surface and geology of Venus, as well ‍as in driving exchanges between the planet's interior ⁠and its atmosphere. Recent studies suggest that some volcanoes on Venus may still be active today, a possibility that future missions to the planet are expected to clarify and better constrain," Bruzzone said.

By its very nature, a lava tube would not be related to ongoing volcanism, Bruzzone said.

Venus is the second planet from the sun, ⁠Earth third and Mars fourth. Venus has received much less scientific attention than Mars, but two important missions are looming - the European Space Agency's Envision and NASA's Veritas.

Both spacecraft will carry advanced radar systems capable of capturing ‌higher-resolution images. Envision will carry an orbital ground-penetrating radar capable of probing the Venusian subsurface to depths of several hundred yards (meters).

"The coming decade promises to be ‌a pivotal one for Venus research," Carrer said.

(Reporting by Will Dunham, Editing by Rosalba O'Brien)