The tilt of the axis of a planet can change over time. Earth’s axial tilt, or obliquity as it is formally called, oscillates between 22.1 and 24.5 degrees on a 41,000-year timescale. In contrast, Mars’s obliquity ranges from 15 to 35 degrees. Even the relatively small scale of the Earth’s obliquity can cause climate cycles. Extreme obliquity could have made conditions challenging for life, particularly complex multicellular life, to arise.
In the last couple of million years, Earth has experienced climate troughs and peaks on a 100,000-year cycle. The obliquity contributes to these cycles, but so too does change to something called the eccentricity of Earth’s orbit. When we think about the Earth orbiting the sun we typically think of it tracing a perfect circle, but that is not right. All orbits are elliptical, but some ellipses are closer to circles than others, and it is fortunate for us that Earth’s elliptical orbit is reasonably circular. Its departure from a circular orbit is described as its eccentricity. As the distance from the sun increases during this orbit, less sunshine reaches the Earth and temperatures can drop, while during times the orbit takes the planet close to the sun, things warm up. This long-term fluctuation in temperature should not be confused with the annual winter/summer fluctuation we feel at high latitudes. The eccentricity of Earth’s orbit changes with time, and this is a major driver of the 100,000-year climate cycles that have driven ice ages on Earth over the last 2.5 million years. If variation in our planet’s eccentricity were greater, it could be possible that Earth would periodically leave the habitable zone, negatively impacting life on Earth.
Despite all the things that could have prevented it, our planet lies in the habitable zone around our sun, not too far away and not too close. It has a stable orbit that is not too eccentric, and it spins on an axis and at a rate that keeps water liquid across much of the globe. Even small changes to these parameters might have meant oceans, rivers and lakes did not form, and instead the Earth might have been a ball of ice, or a hothouse where the only water was in the atmosphere as steam. Yet it is not just the atmosphere above our heads and our position in the solar system that has helped life thrive but also the solid land beneath our feet.
The Earth is approximately spherical, and consists of layers, a little like a scotch egg. If you have never eaten a scotch egg, you should. It consists of a boiled egg surrounded by a thick layer of sausage meat which in turn is covered in breadcrumbs, before the whole delicious sphere is deep-fried. Like a scotch egg, our planet has four layers. The centre, analogous to the yolk of the egg, consists of a solid inner core made primarily of iron. Surrounding this is a molten outer core, which consists of iron and other heavy metals. The egg white represents the outer core. The outer core generates the Earth’s magnetic field, another key feature of our planet. Surrounding the outer core is the mantle, analogous to the sausage meat layer of the scotch egg. The mantle is a rocky layer consisting of silicates, molecules containing some atoms of silicon and oxygen. On top of the mantle sits the crust, the thinnest layer – the breadcrumbs on the outside of the scotch egg. The breadcrumbs give a scotch egg a rough texture, and you might think this parallels rugged mountains on Earth’s surface. Intriguingly, however, if you could shrink our planet to the size of a scotch egg, it would feel as smooth as a billiard ball. Even tall mountain ranges like the Himalayas and the Andes would not interrupt the smoothness on a shrunken-down model of the Earth the size of scotch egg.
The Earth’s magnetic field acts as a shield that helps protect our atmosphere and has been essential for life. Without the magnetic outer core, life may have long since died out on Earth. To understand why, we need to revisit the sun. It is not only planets that have atmospheres, so too do stars, and our sun is no exception. The outermost layer of the sun’s atmosphere is called the corona, and for reasons that we only partially understand it is extraordinarily hot, much hotter than the sun’s surface, reaching a temperature of a million degrees Celsius. One reason it may be so hot is because of the sun’s magnetic fields. At such a high temperature, electrons are ripped away from their atomic nuclei and a plasma of charged ions is formed. Particles in this plasma are then accelerated to very high speeds by the sun’s magnetic field, before they form solar winds that hurtle out through the solar system at speeds of between 200 and 500 miles per second.
Anatomy of the Earth
When these charged ions collide with gas molecules in a planet’s atmosphere, the collisions can result in the gas molecules being jettisoned into outer space. Mars loses a greater proportion of its atmosphere to solar wind than Earth. Over the last 4 billion years, the solar wind has stripped Mars of the thicker atmosphere it had once had. The atmosphere on Mars is now thin as there is little of it left.
For the first half a billion years of Mars’s life the solar wind did not ravage the planet’s atmosphere so destructively. Back then, Mars had a magnetic field, a bit like Earth has today. If a planet has a magnetic field, it can deflect the solar wind, protecting the atmosphere from such significant losses. On Earth, the solar wind enters only the upper two layers of our atmosphere, the thermosphere and exosphere, but when it does it can cause spectacular sights: the Northern and Southern Lights. These remarkable light displays, caused by collisions between the sparse gases in the higher atmosphere with the protons, electrons and helium nuclei that form the solar wind, are a result of our planet’s magnetic field reducing the rate at which gas is lost from our atmosphere to outer space. The obvious question is why does the Earth have a magnetic field, and why did Mars have one which petered out so early in its existence?
The outer core is the egg-white layer of Earth in my scotch-egg analogy. It consists of molten iron, silicates, sulphides (molecules containing atoms of sulphur) and an unknown quantity of radioactive metals. Humans have not visited the centre of Earth, and there is much we do not know about it, but the circulation of molten iron in the outer core creates what scientists call a dynamo, and it is this that is responsible for our planet’s magnetic field. If the outer core were to solidify or shrink, the magnetic field that protects our atmosphere would shut down and our atmosphere would slowly be lost to outer space. This raises the question, why is the outer core molten?
Some of the core’s heat is left over from the formation of the Earth. The accretion of material from the early solar system and the collision with Theia generated lots of heat. As the outer layers of our planet cooled and solidified as rock, the core became insulated, and heat only dissipates slowly. The action of gravity and the decay of radioactive metals will also generate heat (although the relative contribution of each process is not known). There are consequently multiple things happening that have kept our outer core molten, and as long as it remains thus the Earth will have a magnetic field. Mars’s outer core, being smaller than the Earth’s, cooled and solidified earlier in its history. Its dynamo lasted only about 500 million years. From that point on, its atmosphere was destined to be stripped by the solar winds, and life, if it had evolved, was not to thrive as it has on Earth.
The molten core means the Earth is a huge magnet and, just like a smaller magnet you might attach to your fridge, it has a north and south pole. The magnetic north pole, where northern lines of attraction of the Earth’s magnetic field enter the planet, is currently on Ellesmere Island in Canada. The magnetic south pole is just off Antarctica, in the direction of Australia. These poles slowly meander, and 600 million years ago may have been close to the equator, rather than being nearer the poles. Many times in the history of Earth the magnetic field has even switched orientation, with magnetic north becoming magnetic south, and vice versa. The history of these switches can be detected in various rock deposits around the globe. Scientists have been unable to identify any pattern in these switches. They can occur as frequently as once every 10,000 years, or as infrequently as every 50 million years, but on average occur every 300,000 years. The last switch in polarity occurred 773,000 years ago, and may have taken 7,000 years to complete. The field has become close to flipping a further fifteen times since, but it hasn’t quite happened. There were several species of bipedal apes alive when the last reversal happened, including the first members of the Homo genus (our relatives) living in Europe. I suspect they were oblivious and largely unaffected by our planet’s changing magnetic field. We would notice such a change now as it would play havoc with our electronic devices. As well as switching poles, the Earth’s magnetic field also fluctuates in strength with time, sometimes being stronger and other times weaker.