"Some say the world will end in fire,

Some say in ice.

From what I've tasted of desire

I hold with those who favor fire.

But if it had to perish twice,

I think I know enough of hate

To say that for destruction ice

Is also great

And would suffice."

Robert Frost (1874-1963)

The end of the world may come slowly, but it´s inevitable. Our sun, exhausting the hydrogen-fuel of the core, will successively burn the outer layers and doing so becoming hotter and expanding in size. In estimated 7,59 billion years a red giant will engulf the earth - or whatever still be left over of the once blue planet.

Fig.1. Frontispiece of Thomas Burnet’s "“The Sacred theory of the earth...[]", like a life form, earth is born and even if it will exist for eons, in the end it is doomed to death...

Already in 1,6 billion years the hotter sun will evaporate the oceans, and plate tectonics, whiteout enough water acting as lubricant in subduction zones, will stop. Without plate tectonics erosion will become a dominant factor. The increased radiation of sun will modify the chemical composition of earth´s atmosphere. The light hydrogen will also "evaporate" into space and the heavy oxygen will accumulate on the surface of earth. In this denser atmosphere rare, but strong, rainstorms will cause large mudflows in the last mountain ranges. Mountains will be eroded and basins filled with sediments and earth´s surface will become a plain desert. The iron in the sediments will react with the oxygen and earth´s colors will change into a permanent red, like planet Mars today. In the dense atmosphere temperatures will still rise, dissolving gypsum and other sulphur-bearing rocks. The free sulphur will react with the traces of vapor left in the atmosphere and it will rain sulphuric acid from earth´s sky.

In 7,5 billion years the expanding sun will gravitationally lock earth and one side will now face always towards sun. In the sunny side the temperature of earth´s surface will rise to 2.200°C, on the dark side of earth the temperature, without an isolating atmosphere, could plunge to -240°. Basalt, one of the most common rocks on earth, melts at 1.100-1.200°C, on the sunshine side it will be so hot that a molten magma-ocean forms… and it will start to evaporate. Between the hot side and cool side of earth the evaporated elements, like iron and silica, will form rain and like today snowflakes form a landscape composed of snow, iron- sodium-, magnesium- and potassium-flakes will form an eerie landscape composed of these elements. Rock-glaciers will descend from the mountains to the shores and icebergs of rock will float into the magma-ocean.

On the dark side of earth a smooth ice cap, composed of frozen water, carbon dioxide-, nitrogen- and argon-ice, will form over time. May on the borders still some liquid water will persist, but probably not even the toughest microbes could survive in these last, toxic seas.

Fig.2. In the end earth will display a strange and exotic geology, but probaly no geologist will be there to study it...

The outer frozen layers of earth, composed of lighter elements, will evaporate over time completely, only the iron core will persist. However as the expanding sun will slow done the orbital speed of the planets in just 400.000 years the last rusty remains of earth, now to slow to contrast the sun´s gravity force, will plunge into the fiery corona of the red giant and dissolve.

But the end of one world may is the birth of a new one. The outer planets and moons of our solar system will receive in 12-17 billion years enough energy to melt and oceans of water will form on many icy moons. However the sun will continue to shine only for some few million years, so it´s improbable that ever higher life forms will evolve. Finally the sun will devolve into a white dwarf and eternal darkness will descend onto the frozen remains of the solar system.

Bibliography:

SCHRÖDER, K.-P. & SMITH, R.C.(2008): Distant future of the Sun and Earth revisited. Monthly Notices of the Royal Astronomical Society. Vol. 386 (1): 155-163

VAAS, R. (2007): Flammendes Finale. Bild der Wissenschaft, Nr.11: 44-51