Understanding the geological evolution of Epona is the key to appreciating its distinctiveness - those differences that render the world alien and not just another Earth.
Epona is 55% the mass of the Earth and 85% the size roughly speaking a compromise between Earth and Mars. Its surface to volume ratio is thus greater than the Earth but smaller than that of Mars and so, given a similar inventory of radioactive elements, it will lose its internal heat at an intermediate rate between the two. At an age of 5 billion years, the Taranis system is 400 million years older than our own. Epona has had a lot of time to grow old and that is in fact what has happened.
Epona was formed at a distance from its primary of only slightly greater than the equivalent (in terms of insolation) of Earth from the Sun. Its material constitution was similar, having a liquid iron core, rocky outer layers and a watery surface. Up to ~1.7 billion years ago the geological activity on Epona was strong and continuous. Crustal plates, like on the Earth, moved slowly over the planet, being formed at volcanically active spreading centers and recycled back into the depths of the planet at subduction zones. Continents, ocean basins and a differentiated crust rich in mineral deposits were formed. Conditions were perfect for life which soon originated and gradually converted the atmosphere to an oxygen-rich composition. Slightly faster than on the Earth, 'multicellular' life forms evolved and advanced on to the land. Epona resembled a slightly smaller Earth.
The key to this ~ 3.3 billion year period of warmth and stability was the constant volcanic and tectonic activity which permitted a closed carbonate-silicate cycle. Carbon dioxide, an important greenhouse gas, dissolves in rainwater to form carbonic acid which weathers silicate rock to form carbonates ๓a particularly common form being limestone. However, the continual burial of rock to depths where the heat is such that the CO2 is driven out recycles this essential gas back into the atmosphere. What's more is that the weathering part of the cycle is sensitive to surface temperature allowing a stabilizing negative feedback loop to operate that maintains a pleasant climate. If the planet heated up, weathering would increase, drawing down CO2, reducing the greenhouse effect and thereby cooling the planet. If the planet were to cool, weathering slows down, but the rate of volcanic release stays the same ๓the amount of CO2 increases and the planet heats up. Stars slowly increase in brightness as they age and so the equilibrium proportion of CO2 in the atmosphere gradually falls over billions of years, as the quantity of radiation received by the planet increases. The important thing to note though is that an operative carbonate-silicate cycle stabilizes the climate on timescales much shorter than this. Although conditions may fluctuate, the oceans neither boil nor completely freeze over (so long as the planet did not form close to the inner edge of the ecosphere). This is an important feature of the Earth's climate and prior to ~ 1.7 billion years ago the habitability of Epona was also assured by this geochemical mechanism.
However, since that time, because of the planet's smaller size and more rapid heat loss, its crust has cooled and thickened to the extent that it became static. The continuous conveyor belt of crustal creation, destruction and recycling ceased. Over the millions of years since, mountain building slowed or stopped altogether, and the land surface started eroding down at a faster rate than it was being uplifted. Mountains crumbled into the sea; the land largely flooded and the oceans became more saline and filled with silt. The carbonate-silicate cycle was cut, its return volcanic branch from mineral to gaseous phase now operating at a much lower rate. Temperature regulation faltered, the much reduced CO2 greenhouse effect could not prevent ice sheets advancing from the poles down to low latitudes. Where life was not physically obliterated from the land surfaces by glaciers, the biota mostly died off and reverted almost to bacterial level when the concentration of CO2 fell too low for productive photosynthesis. Although life was still reasonably abundant and diverse in the seas, the cutting of Epona's ability to self-regulate its atmosphere and temperature resulted in its biosphere crashing into an impoverished and moribund state.
Not a very hopeful prospect . . . but not all is lost! Although its crustal plates have thickened and are permanently locked together, the interior of Epona is far from stone cold. Hot spots build up at the base of the crust and, every 100 million years or so, cause a quite abrupt and violent spasm of terminal volcanism. Huge quantities of lava and gas are released; new volcanic highlands form and ash darkens the ice fields below; the CO2 concentration in the atmosphere is boosted over ten-fold; quite rapidly, once a certain trigger point is reached, ice retreats back to the poles exposing fresh land surfaces. The biosphere is temporarily rejuvenated- these warm times of plenty giving the land biota a new lease of life for about 10 million years before the volcanism ceases and the next ice die-out occurs.
There have been about 20 of these cycles of biospheric 'near-death-experience' and 'revival' and our exploration of Epona takes place towards the end of one of the last of these warm spells. In the soon-to-come geological future, the weakening volcanism will no longer be able to break through the ever-thickening crust. Epona will become quiescent for good.
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