2.3 The Goldilocks planet

So why are we describing all this cosmology in a Chapter about the evolutionary origins of the fungi? Our reason is that all this cosmology was essential to make the Earth-Moon binary system such a very special habitat for life. A habitat that is unique in its:

  • Orbital position: not too hot and not too cold.
  • Molten iron core.
  • Nature as a binary planetary system.

As mentioned, our orbital position is crucial to maintaining liquid water on the surface of the planet. You only have to think of our seasonal changes to realise how exact that positioning must be. The Earth's axis is tipped over about 23.5° from the vertical and it’s our annual orbital motion around that which causes our seasons. We're European northern hemisphere people, and used to wintry ice and snow from December to February and warm summers from June to September. But these seasons only result from the old planet being angled slightly towards the sun in summer, or away from the sun in winter. Just that small rotational shift towards or away from the sun is sufficient to generate an enormous temperature differential. The highest recorded temperature in Europe (in Seville, Spain) is 50˚C; compared to the lowest recorded European temperature of -55˚ C (at Ust-Shchugor, Russia). You will appreciate that if Earth’s orbit was very slightly closer to the Sun, let’s say by about one Earth-radius, surface temperatures would be intolerably high all the time. Alternatively, if the planet was a similar distance further from the Sun, it would be in permanent deep freeze.

That highly improbable impact between proto-Earth and Theia not only topped-up the Earth’s liquid iron core but it imparted increased rotation to both planet and core. The molten core spins now, and the spinning iron generates our magnetic field, the magnetosphere, producing a magnetic field strong enough to protect Earth from the solar wind.

The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun and consists mostly of high-energy electrons and protons. Mars is unprotected and the solar wind has stripped away up to a third of its original atmosphere. Even the dense atmosphere of Venus is being eroded by the solar wind, so much so that space probes have discovered a comet-like tail that stretches back to the orbit of the Earth.

But Earth is safe, protected from the solar wind by its magnetic field, which deflects charged particles but also serves as an electromagnetic energy transmission line to the Earth's upper atmosphere and ionosphere through the Aurora Borealis and Aurora Australis. By preventing the solar wind stripping away the ozone layer our magnetosphere also protects the surface of Earth from  biologically harmful ultraviolet (UV) radiation emitted by the Sun as well as the high-energy particles of the solar wind.

This is crucially important to biology; UV wavelengths shorter than 280 nm (also known as ‘germicidal UV’ or UV-C) is entirely screened out by ozone at around 35 km altitude. For radiation with a wavelength of 290 nm (UV-B, which is most damaging to DNA), the intensity at Earth’s surface is only 10-8 that at the top of the atmosphere, thanks mainly to high-altitude ozone.

And that leaves the contribution of our enormous Moon, by far the largest satellite in the Solar System (in proportion to its planet) and constructed from the rocky remnants of the Theia-Earth impact (Fig. 3). The presence of the Moon gradually reduced the rotation rate to a level that reduces temperature variations on the Earth’s surface to life-supporting limits. But it also stabilises our axial tilt, which ensures annual seasons and challenging environments to drive evolution. Similarly, the Moon generates tidal effects in both rocks and water, and the latter also produce variable shoreline environments that spur chemical and biological evolution.

Resources Box

The story of Planet Earth

For more information about the origins of the Earth we recommend a collection of publications and websites.

CLICK HERE to visit a page providing details of these.

CLICK HERE to listen to Eric Idle's Galaxy Song (from Monty Python’s The Meaning of Life, 1983). Note that this track is in Windows Media Audio (WMA) format. If you are using a Mac computer, you may need to download and install the Windows Media Components for Quicktime from this URL: http://www.microsoft.com/windows/windowsmedia/player/wmcomponents.mspx

Updated December 16, 2016