Читать книгу Exploring the Solar System - Peter Bond - Страница 33

The Solar System is continually evolving and changing. The collision of comet Shoemaker‐Levy 9 with Jupiter in July 1994 illustrated that impacts and planetary evolution are continuing today. More significantly, the Sun is also evolving, as nuclear fusion continues to create helium from hydrogen in its core.

Since its birth, 4.54 billion years ago, the Sun has grown 30% brighter and this change will continue (see Chapter 2). Over the next 1.2 billion years, its surface temperature will increase by about 150°C and its luminosity will increase by another 10%. By this time, the oceans will have boiled away. Over the next 2 billion years, even the water vapor will be lost, turning Earth into an arid planet comparable to Venus today.

Models suggest that, about 7 billion years into the future, the Sun will swell into a red giant with a diameter perhaps 200 times larger than today's value – large enough to reach almost to Earth's present orbit. However, an increase in the solar wind will cause up to 25% of the Sun's mass to be blown away.

This drop in mass will cause the orbits of the planets to expand outwards, so that Venus may recede to Earth's current orbit, while Earth may lie near the present orbit of Mars. However, this outward retreat will probably be partially balanced by solar tidal drag, which will cause our planet to spiral slowly inward. Earth's fate will hang in the balance.

Further out, Mars will briefly become warm enough to melt its icy volatiles, leading to a temporary spell of warmth with a dense atmosphere. However, the planet's gravity is not strong enough to maintain the situation for very long.

Jupiter's ice‐rich Galilean moons will also develop thick atmospheres of water vapor, but again, these wet greenhouse conditions will be fairly short‐lived. On Saturn's giant moon Titan, an ocean of liquid ammonia may survive for several hundred million years, perhaps providing a brief interlude when primitive life may evolve.

With its hydrogen now exhausted, the Sun will shrink and become 100 times less luminous as it switches to helium for its energy source. However, the fusion process that converts helium to carbon will only prolong its active life for a few hundred million years. As the helium becomes exhausted, the Sun will expand once more into a red giant. Riven by sudden pulsations in size, it may well consume Earth – if it still exists.

100 million years after the second red giant phase, the Sun will eject its outer layers, forming a beautiful (from the outside!) planetary nebula. All that will be left is a tiny, extremely hot, superdense core known as a white dwarf.

The final layout of the Solar System is hard to predict, but it may be that the scorched remnants of Earth and Mars, along with the outer giants, will continue to orbit the fading dwarf star, largely undisturbed, for hundreds of billions of years.

Meanwhile, our galactic environment will also have changed dramatically. About 4.5 billion years from now, the Andromeda galaxy and our Milky Way will collide, combining to form a single, football‐shaped elliptical galaxy. By then, the Sun will be an aging star nearing the red giant phase and the end of its life.

Models suggest that the Solar System likely will reside 100,000 light years from the center of the new galaxy – four times further than the current distance. Any human descendants observing the future sky will experience a very different view. The band of the Milky Way will be gone, replaced by a huge bulge of billions of stars.