Friday, February 6, 2004, 16:00
WHGA Auditorium
Prof. A. Halliday, ETHZ
Abstract:
Planet formation
The Earth-like planets (including also Mercury, Venus and Mars) are
thought to have been built up gradually, initially by sticking together
of dust and rocky debris. When these objects reached the size of a
kilometre or so gravity would have started to exert a major influence
and a process called runaway growth would have consumed all of the
debris in the vicinity. The bigger the planet the stronger its gravity
and so the more it will attract other objects. However, this only
builds objects that are about 1% of the mass of the Earth. Nearly all of
the material in the inner solar system would then have been in the form
of numerous 1,000 km diameter planetary embryos. To get objects to be
as big as the Earth requires that these embryos repeatedly collide by
chance and gradually fuse into a much smaller number of discrete planets
as we have today. These collisions would have been incredibly energetic
and would have melted the colliding objects and even vaporized some of
the rock and metal. It is thought that the Earth's Moon formed from the
debris produced in such a collision. Isotopes tell us how the Earth
formed Because this collision process is somewhat random it is also hard
to predict. However, the various models that have been proposed differ
with respect to the amount of time over which it is expected that the
Earth would have taken to form. Some theories have predicted that the
Earth would have formed in much less than one million years. Other,
more widely accepted theories predict that it took ten to a hundred
million years. Still other schools of thought have proposed something
between these extremes. Extinct radioactive isotopes have proved
particularly powerful in defining just how quickly planet formation
occurred - allowing these theories to be tested. The isotopes of
tungsten and lead are especially useful because they have been affected
by the decay of radioactive hafnium and uranium respectively. The
biggest change in hafnium to tungsten and uranium to lead ratio takes
place during the core formation that accompanies planetary growth. This
allows the isotopic compositions of tungsten and lead to be used to
determine a rate of planetary growth. The data indicate that the Earth
formed over tens of millions of years and that the Moon formed late,
consistent with the theories of more protracted formation.
New clues
However, the story is not so simple. The two isotopic clocks used,
hafnium-tungsten and uranium-lead, actually give distinctly different
timescales for planet formation when calculated in the same manner.
There is only one likely explanation for this - that some portion of the
Earth's core formed as a result of the coagulation of earlier cores from
the colliding planets. This is different from the general view of core
formation - that the iron metal from each colliding planet first mixed
with the rocky outer parts of the Earth and then simply settled to the
centre of the Earth because of its higher density. Furthermore, it
means that the time-scales of formation of the Earth and Moon have been
under estimated. Recently it was estimated that the Moon-forming Giant
Impact took place at about 30 million years after the start of the solar
system. An age for the Moon of closer to 50 million years now appears
more likely. The story does not stop here however. The isotopic
compositions of tungsten and other elements in the Moon can be used to
deduce what the chemical composition of its impacting parent planet was
like. It turns out it was probably much more like Mars a relatively
volatile-rich and oxidized planet. The Earth and the proto-planets that
made the Earth most probably lost volatiles during growth. Given this
history a big question that remains is how did Earth acquire its water?
This is, in fact, one of the most puzzling remaining problems about
Earth-like planets.