Tilted Earth-like planets

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What would be the climate of an Earth-like planet, tilted sideways? Would it be hospitable to life at all?

A paper entitled “Climate at high obliquity” (Ferreira et al. 2014) addresses these questions with a climate model including the influence of oceans. It has been done before, but with simpler approaches, for instance with an ocean modelled as a slab of water without currents (the so-called swamp model, with the ocean allowed to absorb heat but not to redistribute it).

Why study a tilted Earth? Axial tilts are actually common in planets, due to the violent nature of the last stage of planet formation, full of collisions and close encounters that can jolt their rotation axes. Even in our own Solar System, Uranus is tilted sideways and Venus is actually upside-down. Earth’s spin has a rather moderate tilt of 23.4 degrees, but even that is believed to be partly due to the stabilising influence of the Moon.

Planet_axis_comparison

Axial tilt of Earth, Uranus and Venus. [Wikipedia]

The axial tilt plays a huge factor in the climate of a planet. On Earth, it causes both the seasons and the difference in mean temperature between the equator and the poles.

An Earth-like planet more tilted than ours would be rather weird: above a limiting value of 53 degrees of inclination, the polar regions receive more solar heat over the whole year than the equator. This is a massive change in the climate, since the principal driver of both atmospheric and oceanic circulation on Earth is the redistribution of heat from equatorial to polar regions. It would not simply cause a reversal of the climate of equator and poles, because although the poles will get more heat overall, they still spend half the year in total darkness.

What climate might we expect on an Earth-like planet tilted entirely sideways (with an axial tilt of 90 degrees)? On a waterless planet, the seasonal variations would be enormous, with huge swings between scorched desert and dark freezes so deep that the atmosphere itself would be in danger of collapsing (like it does on Mars every year). But Earth is primarily an ocean planet, and oceans dominate the climate.

The results of Ferreira et al. are very surprising, at least to those of us less familiar with ocean climatology: the variations in the climate of a tilted Earth are milder than that of the present Earth. The planet remains actually quite amenable to life, with lower seasonal and geographical temperature gradient than on Earth. On an Earth-like planet tilted completely sideways, the polar oceans might not even freeze over, in spite of the six-month long periods of obscurity.

The reason is two-fold: First, on our own Earth, the polar regions constantly receive low levels of Sunlight. They might have a midnight sun in summer and complete darkness in winter, but over the whole year, sunlight reaches them at such a low angle that they get far less total energy than the equatorial regions.

By contrast, on a tilted Earth, all regions get “their time in the Sun”. The Sun reaches zenith in the sky in all regions of the planet, at the north pole in the northern summer, near the equator in spring and fall and  at the south pole in the southern summer. The relative illumination of the different latitudes is more homogeneous than on Earth.

The seasons on a tilted Earth.

The seasons on a tilted Earth.

This first reason combines with a second: the very high thermal inertia of the oceans. It takes an awful long time to warm or cool an ocean of even moderate depth, months for the first dozens of meters of depths and years for deeper layers. This is more time than the interval between two summers on an Earth-like planet. Therefore the ocean can keep summer heat through the winter, and absorb most of the temperature fluctuations. It keeps the poles cool in summer and warm in winter, even with the Sun blocked at zenith or completely absent. The simulations suggest that, as long as the depth exceeds 50-200 m (a trifle compared to the mean depth of 3000 m of Earth’s ocean), the global ocean would prevent wide seasonal variations.

As a footnote, landmasses on such a tilted Earth would react very differently. In small islands, the climate would remain oceanic, but the centre of larger landmasses in the polar regions would undergo brutal seasonal variation, with frozen winter and extremely hot summers. In the sky of these regions, the Sun would “spiral up” like it does on Earth near the pole, but instead of culminating at a height of 23 degrees above the horizon, it would climb up all the way to the zenith, then linger at a fixed point for a few days, before starting its spiralling descent.

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About Author

I am a professor of planetary science at the University of Exeter. My specialty is the study of exoplanets, in particular the observation and modelling of exoplanet atmospheres. I have done my PhD a the University of Geneva and worked in Chile, France and Switzerland.