Public Lectures for the Session 2006-2007 held in Room E7of the Renold Building, University of Manchester.
Delivered to the Manchester Astronomical Society

16 November 2006

''Planetary Volcanism: the importance of Venus"

Prof. Lionel Wilson
(Emeritus Professor of Earth and Planetary Sciences, Lancaster University, from where he took early retirement to become Visiting Professor at the Hawaii Institute of Geophysics & Planetology, and Editor-in-Chief, Journal of Volcano & Geothermal Research)

Venus is only slightly smaller than Earth and its core diameter,12% in radius, compared with 16% for our own planet, would lead us to expect the two planets to have had similar thermal histories and similar, possibly ongoing, volcanic activity. Although the current probe, Venus Express, was more interested in surface temperature and the composition of the atmosphere, its sensors could detect volcanic activity if it occurred during the mission.

Planets lose heat from their cores via conduction and convection through the mantle. Some years ago, the Magellan radar mapping of the surface gave resolution in the order of 75m and showed that lava flows similar to those on Earth were present on Venus. Different types of surface texture, comparable with those on Earth were identifiable that allowed a good understanding of volcanic eruptions on Venus. The surface is basaltic, like Earth's ocean basins, the surface of Mars and the lunar maria and is covered with relatively young solidified basalts.and lava flows. Unlike Earth, none of the other terrestrial planets show evidence of plate tectonics and their crusts are not recycled.

Venus has a few tens of big shield volcanoes, like Sapas Mons, 400km in diameter and 1.5km high but with comparatively shallow slopes. There are also thousands of smaller shield volcanoes less than 2km in diameter. Compared with those on Earth, Venus's lava flows are larger and much more extensive but it is not known why.

Terrestrial basalt eruptions commonly start as fire fountains, up to 500-700m high, along fissures through which very fluid molten rock and gasses, including water, are shot into the air before flowing for considerable distances. Eruptions can last for weeks or even years. At the end of the eruption, material can flow back into the vent and mask it from view. On Mars, fire fountains are thought to have shot 3km high because of its lower surface gravity, whilst on Venus the high atmospheric pressure seems to have restricted the height of fire fountains and seem to have caused the basalts to form proportionally larger droplets than on Earth, Mars and the Moon as deduced from radar imaging.

On the Moon, sinuous channels like the Herodotus Valley, were caused by long eruptions on surfaces that remained hot for a long time - at least two weeks- that softened the surface and allowed gravity dictated sinuous rilles to form. Some are visible on Venus. But there are also many pancake domes, indicative of viscous lava eruptions.

There are relatively few large impact craters on Venus > 50km that are filled with lava and it is thought that the very dense atmosphere has protected the surface from asteroid impacts. However those that are present indicate a steady cratering history onto a surface that is perhaps only 600My old.

Synopsis by Kevin J. Kilburn (Secretary)

Maintained by Michael Oates
Page modified 10 December, 2006