UNS Unity, February 24, 2100AD

Introduction - the astrophysical background

Alpha Centauri A ("Big Alpha") is a G2V main sequence star, similar in spectral class to the Sun, but about a billion years older. Chiron is a 1.8 Earth mass planet orbiting at 1.08 AU from Big Alpha, receiving 23% more sunlight than Earth does at the present.

Like the Sun, Big Alpha was once about 30%-40% dimmer than its present luminosity. Stars grow continually more luminous over their lifetime on the main sequence - this was a matter of some concern to astronomers in the past (Hart 1978) who reasoned that if the Sun had been much less luminous than it is today, then the Earth should have been locked in a permanent ice age. But the geological record shows that Earth was not significantly cooler than it is today. This was known as the Faint Young Sun Paradox.

It was Walker and Kasting who pointed out (1981) that if the Earth had had a much greater amount of CO₂ in the atmosphere, then the CO₂ greenhouse effect could account for the difference. CO₂ is emitted into the atmosphere from volcanoes and weathered out of the atmosphere by the reaction

CaSiO₃   +   CO₂   --->    CaCO₃   +   SiO₂
calcium           carbon           lime                 silica
silicate           dioxide

The carbonate rocks are eventually subducted into the mantle by continental drift, broken down into silicate rocks and CO₂, and the cycle begins again. The crucial feature of this cycle is that the weathering reaction is temperature dependent: the higher the temperature, the faster CO₂ is removed from the atmosphere. So the carbonate-silicate cycle acts as a planetary thermostat, keeping temperatures constant even while stars vary over geological time.

On Earth, the consequence was that life evolved in an atmosphere of up to 1000 millibars CO₂ or even more. The first photosynthesising organisms using atmospheric CO₂ may have evolved up to four billion years ago and have dominated the biosphere ever since. But on Chiron, the temperature was already too high to support a dense CO₂ atmosphere, and carbon was a much less available element in the warm early seas. This has led to a very different evolutionary path.

Chiron today - atmospheric composition

The atmosphere consists of >160 kPa N₂, <15 kPa O₂ and <20 Pa CO₂. The low oxygen content results in fewer forest fires, a higher proportion of anoxic environments - encouraging a large anaerobic ecosystem reducing nitrates to break down organic matter, about which more later (see report on "red fungus") - and a plant ecosystem dominated by the need to conserve carbon.

meteorology and climatology

The warm tropical seas of Chiron are breeding grounds for hurricanes, which are also encouraged by the high gravity and rapid rotation. The dense nitrogen atmosphere only partly offsets this. The equatorial cloud belts, however, help to regulate the climate by reflecting sunlight.

oceanography

At over 20% higher insolation than Earth, Chiron has very small polar ice caps. The effect of this on the oceanic circulation is profound. Instead of cold oxygen-rich polar water sinking at the poles and being carried in a current along the ocean floor to the equator (as on Earth today), the circulation is driven in reverse, with warm saline oxygen-poor water sinking at the equator and flowing to the poles (as on Earth in the Cretaceous period). As a result, the bottom waters are highly anoxic (note: such seas are called euxinic after the Black Sea).

soil composition

Compared to Earth, silicates are much less common in the soils of Chiron. As in the tropics on Earth, warm water leaches the silica from clays, leaving a poor alumina-rich soil (this does not prevent rain forests from growing, but will inhibit agriculture). The arctic regions have a higher proportion of acidic soils with a high proportion of organic matter (podzols) which is equally hard to farm. The temperate soil zone, which on Earth is favoured with rich aluminosilicate clays, is much narrower here on Chiron, and the soils are more likely to be sandy or lime-rich. Bogs are also common.

ecology

Although basically similar to Earth life, in that it is based on carbon compunds in water, the organisms of Chiron have evolved a biochemistry very different Earth. The scarcity of carbon in the environment, and of dioxygen in the soil, has forced plants to try to make do without O₂, and economise on the use of carbon in structural parts and as an energy storage material. They do this by using a biochemical reaction unknown on Earth

N₂   +   H₂O   +   5/2O₂   -->   2H⁺    +   2NO₃^- ( -7 kcal mol^-1 )

as can be seen, this reaction is exergonic, but (thankfully) is unknown on Earth. Chironian plants seem to have a special enzyme to encourage this reaction, possibly with the aid of sunlight. They use the nitrate obtained this way to store energy as organic nitro-compunds (see report on "gun-cotton trees"); to reduce carbonates to carbon; and to carry out respiration in anoxic environments.

The prevalence of anoxic environments rich in organic material, combined with the presence everywhere on Chiron of nitrated compounds has led to an astonishing variety of underground organisms (see report on "red fungus") which live in the absence of oxygen (though they can use oxygen if it is present) and "breathe" nitrate:

(CH₂O)   +   NO₃   --->    H₂O   +   CO₂   +    1/2N₂O   +   1/4O₂

This ecosystem apparently has symbiotic relations with the plants and with Chironian animal life (see report on "worms"). The prevalence of nitrate in the environment has serious repercussions (see below).

The nitrous oxide is present in only small amounts as it combines with ozone in the stratosphere to break down into N₂ and O₂

                               light
N₂O   +  O₃   --->   2NO    +   O₂ ; 2NO   --->   N₂    +   O₂

This process prevents the build-up of an ozone layer.