16 October 2013

Emeritus Professor Lionel Wilson explains how data from the spacecraft Dawn show that the same kinds of volcanic eruptions that happen on Earth now took place on some asteroids 4570 million years ago.

One strand of my research involves understanding volcanic events on the first asteroids that formed in the Solar System.  Analyses of new data from the Dawn spacecraft, which orbited the asteroid Vesta in 2011-2012, are supporting my predictions that asteroid volcanoes were fed by large bodies of molten rock that accumulated not far below the surface, just as on Earth.

Asteroids are small bodies, tens to a few hundred km in diameter, and were the first substantial objects to accumulate from the gas and dust that formed our Solar System 4570 million years ago. 

The earliest asteroids incorporated a radioactive isotope of aluminium that was a strong heat source.  This isotope was provided by a nearby exploding star that caused the collapse of a dusty gas cloud to form our System.  Because it has a short half-life, this isotope decayed away within a few million years.

Magma oceans or lava flows?

Even though these early asteroids were small (~50 to ~500 km in diameter) compared with planets like Earth, the aluminium heat source caused much of their insides to melt and separate into metal-rich cores and rocky mantles.  In recent years most scientists have assumed that most of the molten rock (magma) that formed in the mantles stayed there to form so-called "magma oceans". 

However my recent work has shown that most of this molten rock should have risen to depths of about 10 km and accumulated there in very large magma chambers, just like those that sit under volcanoes on Earth and the other large rocky planets Mars and Venus.  From time to time this molten rock should have risen through cracks to the surface in vigorous eruptions feeding lava flows.

Vesta: the only known early asteroid

Most of the earliest generation of asteroids have long since vanished.  The vast majority of them accumulated to form the rocky planets, and those that were left tended to collide and break up into smaller bits, some of which fall onto the Earth as meteorites, so it was difficult to find evidence to support my conclusions.

The only known surviving body of this era is the 525 km diameter asteroid Vesta, shown in the image.  Vesta was visited by the U.S. spacecraft Dawn between July 2011 and September 2012. 

Although battered by crater-forming collisions over its 4500 million year history, enough of Vesta's crust is preserved to allow spectroscopes on Dawn to investigate its chemistry and structure.  The data are consistent with a mixture of surface lava flows and the solid remains of sub-surface magma chambers, just as I expected.

Creating interest

Last December I published a major summary of my asteroid volcanism work in a joint review paper with my University of Hawai'i colleague Prof. Klaus Keil.  In it I explained why I thought the magma ocean idea was wrong.  This has created a lot of interest. 

I have been asked to present my arguments at a dedicated asteroids meeting in Washington, DC, later this month and to give an invited presentation reviewing these ideas at the December meeting of the American Geophysical Union in San Francisco.  I’ve also been asked to be the lead author on a chapter about asteroid volcanism in "Asteroids IV", the latest in a series of dedicated planetary science books by the University of Arizona press.


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