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Rocks from Mars
Tapping, Ken
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Skygazing: Astronomy through the seasons, 2017-04-04
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ROCKS FROM MARS
Ken Tapping, 4th April, 2017
So far we have found getting on for 100,000
meteorites scattered on the surface of the Earth.
Some of these rocks from space are raw material
left over from the formation of the Solar System,
and are therefore just a continuation of the nowslowing process of planet building. However, some
of them are bits knocked off other planets. Around
150 of them are bits of the planet Mars. They were
ejected into space by volcanic eruptions in Mars’
early history, or more recently by meteoric
impacts. They spent millions of years or longer
orbiting around the Sun until they finally collided
with our planet. How then can we determine where
these rocks came from, and what can they tell us?
Meteorites are usually recognizable because their
surfaces look blackened, melted or glazed. This
happened during the last few seconds of their
journey here, as they smashed into our
atmosphere at tens of thousands of kilometres an
hour. Friction heated their surfaces to thousands of
degrees, melting them and blasting material away.
Now think about how they started their journey.
They were explosively ejected upwards with
enough speed to get into space. Mars has an
atmosphere, and the same sort of frictional heating
and melting happened there on the way up as
happened here when the rocks were on their way
down. In doing so samples of the Martian
atmosphere got trapped as tiny bubbles in the
molten surfaces of the rocks. These samples were
securely contained when the rock got into space,
and the melted rock solidified. On arrival at Earth,
frictional heating melted the surface again,
trapping some of our atmosphere. When the falling
rock had slowed to subsonic speeds, the airflow
would have cooled off the rock, so that by the time
it hit the ground it was no longer partially melted.
We can drill into the rock to find those tiny bubbles
of trapped atmosphere and analyze them. Trapped
samples of our atmosphere are easy to identify.
We know the composition of the Earth’s
atmosphere extremely well. Let’s assume we have
found some samples of some other atmosphere.
How do we determine what planet that particular
atmospheric mixture came from?
The Moon, where some meteorites have come
from, has no atmosphere. Neither does Mercury.
Venus’ atmosphere is a toxic mixture of carbon
dioxide, sulphuric acid and other chemicals. The
asteroids don’t have atmospheres. Jupiter, Saturn,
Uranus and Neptune have deep, dense
atmospheres covering whatever rocky or icy body
is concealed beneath, and it is unlikely anything
could be knocked off into space. If it did, the
atmosphere sample would contain gases like
methane, ammonia and hydrogen. Titan, Saturn’s
largest moon, has an atmosphere, of nitrogen, with
some methane and other hydrocarbons. The
bubbles of alien atmosphere in the meteorite are
none of the above. That leaves Mars.
Moreover, thanks to the landers and rovers now on
the Martian surface, we know the composition of
Mars’ atmosphere almost as precisely as we know
the composition of the Earth’s. It is 96% carbon
dioxide, 1.9% argon, 1.9% nitrogen, and traces of
oxygen, carbon monoxide, water and methane.
We can be sure we have a Martian rock sample.
Most of the Martian meteorites are made up of
igneous rocks blasted from deep inside the planet.
However a few fragments of rock came from closer
to the planet’s surface. Some contain carbonates
and sulphate minerals. These require mild surface
temperatures and liquid water. In the Solar System
we have only two worlds that fit the bill: Earth, and
Mars as it was long ago. However, as yet no
Martian meteorite contains solid evidence of life.
Mars lies low in the Southwest after sunset. Jupiter
rises soon after dark and Saturn in the early hours.
The Moon will be Full on the 10th.
Ken Tapping is an astronomer with the NRC's
Dominion Radio Astrophysical Observatory,
Penticton, BC, V2A 6J9.
Tel (250) 497-2300, Fax (250) 497-2355
E-mail: [email protected]