The Universe: A Biography by Paul Murdin

Author:Paul Murdin [Murdin, Paul]
Language: eng
Format: epub
Tags: General, Space Science, Essays, Science, History
ISBN: 9780500777251
Google: VphnEAAAQBAJ
Amazon: 0500024642
Publisher: Thames & Hudson
Published: 2022-04-19T23:23:37.155416+00:00

The faint young Sun problem

Calculations about the Sun have stood up well against the intense scrutiny of helioseismology provoked by the solar neutrino problem, so that its structure is well established at this point in its lifetime. But, of course, the Sun cannot have the same structure all its life – it is using up its fuel and radiating energy, and its interior changes accordingly. Among other things, the luminosity of the Sun changes.

The Sun is gradually becoming hotter, because the helium atoms that are made in the core occupy less volume than the hydrogen atoms that were fused. The core is therefore shrinking. Because the layers of the Sun become closer to the centre, they experience a stronger gravitational force. To maintain the balance between the internal pressure of the Sun and its downwards gravitational force, the internal pressure increases, which raises the internal temperature and pressure, which increases the rate at which the nuclear fusions occur, with greater power output. As a result, the Sun is 20–30 per cent brighter now than it was 4 billion years ago.

The internal structure of the Sun at the present time is so well established by results from solar neutrinos that it could almost be called a fact rather than a theory. In a way, this is not surprising because the theory is a combination of precise physics. It is a consequence of the way the Sun generates energy by nuclear fusion of hydrogen to helium in its core, and the balance of its gravitational force against the pressure of the gases inside. The calculations of all these processes are very robust. As a result, few people doubt the calculations of how the luminosity of the Sun has changed: it was fainter in the past. Putting this conclusion about the solar physics against what is known about the Earth’s climate in that early era creates what is known as the ‘faint young Sun problem’. For 2 billion years the young Sun was so faint it did not radiate the warmth to keep the Earth from freezing over. And yet, from about 4 billion years ago, the Earth was covered in oceans (see pages 222–23): there was no one alive at that time, but if there had been, people who looked at its surface would have seen storms of rain lashing onto the wavy surface of blue oceans, not blizzards of snow falling onto glistening white ice. The Earth should have been in an intense ice age, but it was not.

How can astronomers and geologists be so sure about this? The solar part of the faint young Sun problem seems to be in good shape. There is considerable doubt, however, about how the Earth’s climate changes, even over decades, as the current arguments show about the response of the weather to changes in carbon dioxide levels generated by anthropogenic emissions. What evidence is there about the climate from so long ago?

Some of the Earth’s oldest rocks are sedimentary rocks that are part of

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