Wolf Read
Epona
Epona may not really exist, but it’s certainly a good example of how scientific principles can be used to imagine worlds and beings that might exist!
Figure One: A computer generated Eponan landscape offering a view from one of Epona’s warm temperate regions. The umbrella leafed “plants” are pagoda trees, and are grouped in the phylum Myophyta.
Illustration by Steven Hanly
Epona is a strikingly detailed fictional alien world. How detailed? Very. A Webster’s-sized dictionary might be enough to contain all the textual information amassed on Epona; and this grossly ignores the hundreds of Eponan art pieces detailing the world, from ink illustrations and paintings, to computer-generated renderings and animations, to sculptures. Quoting Larry Niven, who had a chance to view an Eponan interactive computer demo, “I have never seen a playground this size.”
The genesis of Epona came from a project created by members of Contact, a nonprofit educational organization with roots in science fiction (an article on Contact appears in the January 1992 issue of Analog). This experiment is called Cultures of the Imagination, or COTI. People participating in COTI are divided into two groups. One group creates an alien world with a sophont while isolated from the second group, which builds human history to the point of starflight. Development proceeds for three days during the annual Contact conference, and at the end, a first contact is simulated between the two isolated groups. Lots of fun.
After taking part in a COTI session, people sometimes mentioned that three days was not enough time to significantly develop an alien world and culture. In response, at Contact X, it was decided to build a world over three years. A regular newsletter for the Epona project was created, inviting Contact participants to join in the long-term development of an alien world. Many people, such as myself, responded, including artists, biologists, chemists, astronomers, anthropologists, and science fiction writers. The amount and variety of Eponan ideas, their interconnectedness, and the solid scientific detail supporting the framework was entirely unexpected.
Epona is the third world of nine which circle the star Taranis, originally 82 Eridani. Taranis is a yellow dwarf (G5 V main-sequence) star that is roughly 5 billion years old. As its original name implies, the star resides in the constellation Eridanis, and currently drifts in its galactic orbit some 21 light-years from Sol. Names of worlds follow a Gaelic tradition.
The four inner planets of Taranis, including Epona, are terrestrial in nature, being small, ranging from 0.1 to 2.0 Earth masses in size, and having average densities within the range of rock, from 3.8-6.4 g/cm3. The inner two worlds, Belenos and Grannos respectively, are similar to Mercury, with small bodies, high densities and little in the way of atmospheres. The large chunks of rock are tidally locked to Taranis, and Grannos’s long-ago atmosphere has been frozen into carbon dioxide ice on its night side. Epona follows, with a mass of 0.55 Earth’s, an oxygenated atmosphere that averages 0.577 bar at the surface, continents of silicate rock, temperate climate and seas of water. The fourth world, Sucellus, with a high density of 6.4 and mass of 2.0 is a sizable terrestrial world covered in a deep ocean and insulated by a carbon dioxide atmosphere of roughly four bars.
Figure Two: Epona’s family of worlds. Unlike the Earth, which holds the number one spot in size for terrestrial worlds in the Solar System, Epona is only the second largest rocky planet in the Taranis system, and, only massing about half that of the Earth, Epona has experienced significant internal cooling. As explained in the text, failure of the carbonate silicate cycle due to Epona’s solidifying mantle has significantly affected the climate, and subsequently biological evolution.
The next four planets are a family of gaseous giants, with huge masses, from 5.9 to 206 Earth’s, and light densities, all sitting around 0.7-2.4 g/cm3. The fifth world, Rosmerta, is the smallest of the family, a mini gas giant with a viciously hot world-encircling ocean being sustained under an equally challenging atmosphere that exceeds 1,000 bars in pressure. The small gas giant is followed by the largest, Borvo. Borvo is 65% the mass of Jupiter and is similar in nature, producing a powerful magnetic field, tightly holding a vast array of moons and presenting a surface of seething storms. Bormo, world number seven, is similar to Uranus, even possessing a strong axial inclination of 73 degrees. Bormanus, the eighth planet from Taranis, is similar to Saturn in density and mass. This icy-ringed world follows the most elliptical path of Epona’s sister planets, maintaining a mean eccentricity of 0.16.
The final world, Sirona, is Tritonian, being comprised of ices, though it is more massive than Mars. Occasional cryovulcanism maintains a thin atmosphere of nitrogen, methane and hydrogen.
Epona accreted a similar distance from Taranis as the Earth did the Sun, so the composition of the two worlds is quite alike. Epona has a lower abundance of heavy elements, accounting for a lower density. During the first 3 3 billion years of existence, Epona possessed a liquid iron-nickel core, a convective mantle and shifting lithospheric plates.
Epona’s tectonic activity did not last as long as it has on the Earth. Being a smaller world, Epona has a higher surface-to-volume ratio than the Earth, and thus has had its store of internal heat dissipated at a significantly faster rate. Nearly two billion years ago, Epona’s tectonism began to slow, and later froze up completely as the lithosphere continued thickening. Many types of mountain building stopped, and continental masses simply weathered and eroded away.
Highly weathered continents are OK as far as Epona’s biome is concerned, but breakdown of tectonism has one major side effect for life. Epona’s dead geology severs the important carbonate-silicate cycle.
In a “normal” state of affairs, as on Earth, carbon dioxide in the atmosphere combines with water to produce carbonic acid. This carbonic acid falls as rainwater and breaks down continental rock—a major feature of weathering. Bicarbonate ions, HCO3 (-), created by the chemical weathering, wash down the streams and into the oceans. In the sea, these ions reach saturation and precipitate from the ocean as carbonate ions, CO3 (2-), and accumulate in vast seafloor deposits, or are used by some animals to create hard shells.
Once converted to rock, the carbon atoms cannot escape, except by subduction. As the seafloor is pushed underneath the world’s numerous lithospheric plates, heating eventually releases the carbon as a gas again, which finds its way back into the world’s atmosphere via volcanoes and sea-floor spreading margins, where the cycle starts anew…
Except when subduction and volcanism fail, as with modern Epona, dropping carbon dioxide production to bare minimum as outgassing ceases. Lose one tie in a loop, and the entire system crashes, so to speak. No more CO2 for Epona.
All the world’s carbon dioxide will be lost when tectonism fails. How does this affect life? Very simply, and very profoundly. The carbon dioxide in the atmosphere provides Epona a buffer against a changing amount of sunlight from Taranis. See it this way: As a main sequence star ages, it becomes brighter. So, early out, Epona received less sunlight from Taranis than at present. With less Taranan flux, Epona should have been frozen, right? Nope.