Geoscience Reference
In-Depth Information
environment that laid them down: from fine shale and mudstone deposited in still water,
through sandstones to coarse conglomerates washed down by raging torrents. Others, such
as chalk and limestone, are chemical deposits accumulated as living systems took carbon
dioxide from the atmosphere and precipitated it in sea water, turning, as it were, the sky
into stone.
Even individual mineral grains have their story to tell. Mineralogists can strip them apart
atom by atom in mass spectrometers so sensitive that they can reveal different ratios of iso-
topes (different atomic forms of the same elements) even among trace constituents. Some-
times these can help date the grains so that we know if they came out of still more ancient
rock. They can also reveal the stages of growth of a crystal, for example of diamond, as it
passes through the Earth's mantle. In the case of isotopes of carbon and oxygen in minerals
derived from marine organisms, it's even possible to estimate the temperature of the sea
and the global climate when they formed.
Other worlds
The trouble with the Earth is that it is the only one we've got. We can only see it as it is
today, and we can't tell if it is here simply due to some happy accident. That's why Earth
scientists are taking a renewed interest in astronomy. Powerful new telescopes sensitive to
infrared and sub-millimetre wavelength radiation can stare deep into star-forming regions
to see what may have happened when our own solar system was born. Around some of the
young stars they have revealed dusty haloes known as proto-planetary discs, perhaps new
solar systems in formation. But the search for fully formed Earth-like planets is more diffi-
cult. To see directly such a planet in orbit around a distant star would be like trying to spot
a small moth close to a powerful searchlight. But indirect methods have led to the discov-
ery of planets in recent years, mainly by detecting tiny wobbles in the motions of the par-
ent stars due to gravitational effects. The clearest effects and therefore the first discovered
seem to be due to planets far bigger than Jupiter orbiting far closer to their stars than the
Earth is to the Sun. So they could hardly be termed Earth-like. But evidence is beginning to
accumulate for solar systems more like our own, with multiple planets. Though small and
hospitable planets like Earth will be hard to detect.
To see such planets directly would take telescopes in space that we can scarcely dream of.
There are ambitious plans underway in both the USA and Europe for a network of linked
infrared telescopes. Each would have to be far bigger than the Hubble Space Telescope,
and four or five of them would have to fly in close formation to combine their signals to
resolve the planet. They would have to be as far out as Jupiter to get beyond the dusty in-
frared glow of our own planetary system. But then, they might be able to detect vital signs
in distant planetary atmospheres and, in particular, they might detect ozone. That would
