Tuesday, 7 February 2012

Earth – Goldilocks Planet

Earth Data
Mass: 5.973 x 1024 kg
Diameter: 12 756 km
Surface gravity: 1.00 gee
Axial tilt: 23.5°
Mean surface temperature: 22 Celsius
Rotation period: 23.93 days
Orbital period: 365.3 days or 1 year
Inclination of orbit to ecliptic: 0.0°
Orbital eccentricity: 0.017
Distance from the Sun: 0.98–1.02 AU
Satellites: 1

Moon Data
Mass: 7.163 x 1022 kg or 0.012 of Earth’s
Diameter: 3477 km or 0.27 of Earth’s
Surface gravity: 0.17 gee
Mean surface temperature: -42 Celsius
Rotation period: 27.32 days
Orbital period: 27.32 days
Inclination of orbit to Earth equator: 18.3–28.6°
Orbital eccentricity: 0.05
Distance from the Earth: 356 410–406 697 km
Water is abundant throughout the Solar System. But nowhere except on planet Earth are the conditions right for that water to exist as a liquid on the surface. Mars is too cold, its atmosphere too thin. Venus is too hot. But Earth is just right – and for this reason it has been called the Goldilocks planet. Third planet from the Sun and largest of the terrestrial worlds, blue Earth is the gem of the Solar System, almost certainly the only world conducive to complex lifeforms. It has many of the features shared by its terrestrial cousins – volcanoes, impact craters and tectonic activity – and is unique in that its volcanoes are still active. Its single satellite is one of the largest in the Solar System. In fact it is a giant in comparison to its parent planet. It seems to be the outcome of a great cosmic accident – the coalesced debris of a collision that the Earth endured when it had recently formed.

Active Planet

While every planet is unique is some way, perhaps none is as different from the others as the Earth is. Three-quarters of its surface is covered in water. Most of this water resides in great oceans, on average about 4 kilometres deep, while the rest of it is locked up at the poles, frozen solid. Like Venus, the Earth’s pull is strong enough to retain an atmosphere. But ours is very different. With 78 per cent nitrogen and 21 per cent oxygen, it has almost none of the carbon dioxide that made Venus so inhospitable.

In terms of general composition and overall structure, at least, Earth is not too dissimilar from its terrestrial cousins. Like these other planets, the Earth has a metal-rich core, a rocky mantle and a rocky crust. But there are some marked differences with our planet’s interior. Its core is composed of two parts: an inner and an outer. The inner core – where temperatures may exceed 6000 Celsius – is made of nickel and iron. This is kept solid, despite the heat, by the pressure, all 3.7 million atmospheres of it. The outer, cooler core, under less pressure, has the same composition but is mainly liquid. Both cores account for one-third of the planet’s mass, though they occupy only one-eighth of its volume. It is in the core where convective motions generate the Earth’s magnetism as they do on Mercury. Above the core is the mantle, composed of minerals such as pyroxene, garnet and olivine. It is mostly solid. However, high pressures and temperatures give the mantle a rubber-like consistency and enable it to flow like a fluid. The uppermost 400 kilometres of the mantle is liquid – a region known as the asthenosphere. Lastly, resting on this pliable bed is a rigid transition zone called the lithosphere, which gradually blends into the Earth’s crust. The crust is not a solid block, however, as it is on Venus, Mercury and Mars. It was shattered long ago, by impacts, into several continental plates.

Because the Earth’s interior is still hot, its plates cannot remain still. They drift sideways, driven by convection, to create the well-known continental drift. It is this phenomenon that makes the planet so active. Where plates come together, the crust crumples up and forms mountains. And where two plates are being pushed apart, volcanoes fill in the gap with magma that eventually solidifies and forms new crust. Because of Earth’s very nature – volcanoes, faults, mountain-building, oceans and rain – the planet has an exceedingly young surface, geologically speaking. Long gone are the scars from impacts endured during the heavy bombardment – and there would have been very many – for most of the crust is only 100 million years old. It is constantly recycled by geological activity and water erosion. Today, only a few impact craters dot the planet – all formed in the recent past.

Dead Moon

The Moon, by contrast, is as different from Earth as it could possibly be. It is too small to retain an atmosphere, and has never supported liquid water. About 84 per cent of its surface consists of a highly cratered, ancient highland. Some of the rocks there – primarily a calcium–oxygen–aluminium compound called anorthosite – are 4300 million years old. The craters indicate that this part of the Moon has changed little since then. The remaining 16 per cent of the surface is taken up by relatively smooth and dark maria (singular mare), Latin for ‘seas’. It is the contrast of these dark patches with the brighter highlands that creates the illusion of the Man in the Moon. The maria are not real seas, of course – at least, not of water. They are in fact seas of solidified lava, regions where very fluid magma oozed onto the surface through cracks in the crust and spread out. Thus the maria are similar to the lowland plains on Mercury and, as on that world, are mostly associated with the large craters called impact basins. Because the maria have relatively few impact craters they are younger than the highlands – but still geologically ancient.

Our Moon is unique among the terrestrial worlds. Of the other rocky planets, Mercury and Venus have no satellites, and those of Mars are puny pebbles, barely kilometres across. Why is Earth any different? The answer, it seems, can be found by looking elsewhere in the Solar System – at Mercury.

Formation of the Moon

We have seen that Mercury’s high density might be explained if the planet suffered a collision with a gigantic protoplanet as it was approaching full size. In the early 1970s, astronomers put forward a similar suggestion to explain the origin of Earth’s large Moon. They call it, quite plainly, the giant impact hypothesis. This scenario is by no means certain. But it does explain a lot of characteristics of the Earth–Moon system and about the bodies in general.

So the theory goes, shortly after the Earth had finished accreting and its crust had started to cool – around 4500 million years ago – the planet was struck a devastating blow. A rogue protoplanet, at least as massive as Mars and possibly three times that, slammed into the primordial Earth in a gigantic off-centre collision. The impact melted much of Earth’s crust and mantle and liquefied the impactor almost completely. In the case of Mercury’s similar event, most of the debris from the collision eventually fell towards the Sun. This left behind the iron-rich planetary casualty that Mercury is today. But the Earth is more massive than Mercury. Most of the pulverised remnants were captured by Earth’s gravity. Some of this material – possibly most of it – eventually fell back to the planet. But a large part of it stayed in orbit long enough for it to accrete into larger and larger chunks – just as the planets themselves had been built earlier. This accreting body would eventually become the Moon. At first, the Earth and Moon would have been very close together, only around ten Earth radii apart. Since the event, though, the two bodies have been slowly moving away from each other. Today, 384 400 kilometres separate them – a distance that increases by about 3 centimetres every year. Your fingernails grow by thesame amount in the same time.

In many ways, the giant impact – if it really happened – marks the true naissance of the Earth and Moon. If it had not been for the collision, today’s Earth would be a bit smaller than it actually is, and lacking a satellite. Since that birth, the Earth and its satellite have followed radically different evolutionary tracks.

Evolution of the Moon

The Moon’s history is doubtless much like Mercury’s. Its oldest rocks date to about 4300 million years, and so the Moon was molten for its first 200 million years. Denser materials sank, while the lighter materials such as anorthosite were buoyed up.

Once the surface had hardened, it gradually began to accumulate craters – courtesy of heavy bombardment. Some of these impacts formed the gigantic basins such as Imbrium and Orientale, which fractured the crust locally to considerable depths. Magma from the still molten interior found its way through the weakened lunar crust to the surface where it formed the maria. This flooding occurred around 3900–3000 million years ago, in a series of stages rather than all at once. But, by 3 billion years ago, the Moon’s interior had cooled to the degree that no more lava would ever reach the surface. With no atmosphere, or water, and with an end to flooding, the surface changed very little in the last 3 billion years. The only events that have occurred since then are the impacts, though at a much lower rate than during the heavy bombardment. This steady rain of rock pulverised the surface rocks into the fine powder, the regolith, that now covers the lunar landscape. And in the meantime the Moon’s interior has frozen solid. Like Mercury, it is a geologically dead world.

Evolution of the Earth

Earth started out much the same way as the Moon – a molten ball of rock. While the planet was cooling, gases previously locked up in rocks were released from the surface. This outgassing took place on the early Moon too. But, because the larger Earth has six times more gravity, the freed gases clung to the surface as they did on Venus. They formed a primitive atmosphere.

The commonest gases vented by the early Earth included carbon dioxide, methane, ammonia – and water. At first, the Earth’s surface would have been too hot for the freed water to exist as a liquid there. It remained suspended in the atmosphere in vapour form. But, as the planet cooled down, there came a point where its surface temperature dropped below the boiling point of water. At last, the water that fell as rain stayed on the surface – and the first oceans began to appear. This process started quickly, within 100–200 million years of Earth’s formation. Comets and asteroids, flung into the inner Solar System by the giant planets, especially Jupiter, may also have brought significant quantities of water to the new-born planet, nurturing its oceans still further. Life appeared in those oceans after about 1 billion years. And it was also at about this time that the Earth’s surface – cracked into plates by overwhelming cosmic impacts – began to spread sideways, carried by convective motions in the warm mantle beneath. This created the first mountain chains.

With the appearance of the oceans, the atmosphere began to change. Atmospheric carbon dioxide dissolved in the water, where it chemically combined with other materials to form such minerals as limestone. Slowly, the Earth’s oceans cleansed the skies of carbon dioxide – a process that had been unable to occur on Venus’ hot, waterless surface. After another 500 million years – 3 billion years ago – the atmosphere consisted primarily of those other materials vented by volcanoes: a noxious cocktail of methane, ammonia and other hydrogen-rich compounds. Back then, there was very little trace of free oxygen or ozone in Earth’s skies. Ultraviolet radiation from the Sun steadily attacked the hydrogen-rich gases in the atmosphere and broke them apart into their constituent atoms. Nitrogen was freed from ammonia, carbon was released from the methane, shattered water molecules gave up their oxygen – and the released hydrogen escaped into space. Some of the oxygen combined with the carbon to form carbon dioxide, which again was absorbed by the oceans. Meanwhile, other oxygen atoms combined in threes to form ozone. Slowly, the atmospheric stock of nitrogen grew as oxygen and carbon were consumed, but in time the atmosphere stabilised as the ozone began to shield the planet from the Sun’s ultraviolet attack. Since then, the evolution of our atmosphere has been dominated by one major force: life.

About 2 billion years ago, plant activity and photosynthesis all over the planet suddenly bloomed. Photosynthesis absorbs carbon dioxide from air or water, uses sunlight to manufacture planet-nourishing carbohydrates and releases oxygen as a by-product. With the onset of mass photosynthesis, free oxygen began to accumulate in the atmosphere. One billion years ago the oxygen levels were about 10 per cent what they are now. But the amount of free oxygen increased dramatically and reached modern levels about 600 million years ago. At that time there was also a huge proliferation of complex life, the Cambrian explosion. The Earth was at last in full swing.

As far as we know, life is unique in the Solar System – certainly complex life is. But there is one other planet on whose surface liquid water almost certainly existed in the past. Could life have arisen there too? Let’s turn now to the fourth planet from the Sun: Mars.

Source :
Mark A. Garlick. The Story Of The Solar System. University Press: Cambridge. 2002.



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