2020-04-02_Science_Illustrated

12 | SCIENCE ILLUSTRATED

GEOLOGY More than once, our planet has experienced a

‘Snowball Earth’ global ice age, during which its entire surface has

been covered in ice. It happened 2.4 billion years ago, after new

oxygen-producing bacteria made the atmosphere colder. The

oxygen broke down a layer of methane that had kept Earth warm,

but some 2.2 billion years ago an event suddenly defrosted the

world again. Geologists used to believe that the cause was a series

of intense volcanic eruptions, but now scientists from the Johnson

Space Center in the US are offering a different explanation.

The scientists studied minerals from an asteroid crater by the

name of Yarrabubba in south-western Australia. The minerals

contain uranium, which decays into lead over time. By analysing

the relationship between the two elements, the scientists

determined the age of these minerals, and the dating showed that

the crater was formed 2.229 billion years ago, which is consistent

with the time when the planet began to defrost. The

scientists now think that this strike was the triggering factor.

To examine the idea more closely, calculations were

made of what would happen if a 7km-wide asteroid

struck a 2km-to-5km ice sheet. The calculations showed

that such an impact would spread dust across

thousands of kilometres, sending 500 billion tonnes

of water vapour high up into the atmosphere.

Both effects might have caused a warmer climate.

Dark dust on the surface would hold on to more

solar heat, while the water vapour would

increase the weakened greenhouse effect. So

the asteroid strike could have helped Earth pass

into a new era that allowed life to develop

further on ice-free land and in the open sea.

Asteroid strike in WA

defrosted the frozen Earth

The dating of a crater from an asteroid strike in Western Australia offers a new

explanation of why a global ice age subsided more than two billion years ago.

Asteroid strike could heat Earth in two different ways

More than two million years ago, an asteroid strike marked the end of a global ice age.

The strike released dust and water vapour, each contributing to the heating of the frozen planet.

Strike

Dust

Outflux

Water vapour

Dust layer

Intensified

greenhouse effect

The Sun The Sun

Ice sheet

SOLAR ENERGY REFLECTED FROM EARTH

The ice-covered Earth reflected almost all solar radiation

back into space. The atmosphere couldn’t hold on to the

heat either, because the greenhouse effect was so low.

DUST AND VAPOUR HOLDS ON TO HEAT

The asteroid strike covered the ice in dust, which absorbs more

solar energy. At the same time the quantities of water vapour

escaping into the atmosphere intensified the greenhouse effect.

Influx Outflux Influx

BEFORE STRIKE AFTER STRIKE

THE LATEST FINDINGS AND DISCOVERIES