Return to HelliconiaWe're now equipped to tackle atmosphere and weather in a more complex simulation of a fictional planet - again Brian Aldiss' Helliconia. As we've seen previously, Helliconia's seasons are primarily driven by the distance to Freyr, the hot star of the binary pair, and so we can expect many properties of the world to change as we go from periapsis to apoapsis with respect to Freyr.Aldiss doesn't talk much about atmosphere except that it exhibits some Greenhouse effect (so it must have CO2 and water at least) and that it seems readily breathable by human beings (which argues for a roughly similar oxygen fraction). The biochemistry of organisms seems to be reasonably similar, so nitrogen must be available in the atmosphere. Let's therefore assume that it is basically equal to an Earth atmosphere, except that trace gas content may differ. This enables us to run the atmosphere structure code including the Chapman mechanism.
Atmospheric compositionAldiss gives us one important piece of information - Helliconia's temperature at periapsis is +10 deg C (283 K), i.e. somewhat colder than Earth. Thus we may reduce the amount of water in the atmosphere correspondingly, using the Clausius-Clapeyron relation. Using the following atmosphere parameters for the planet
results in the following atmosphere properties
i.e. the atmosphere has less Greenhouse effect than Earth but a slightly higher transport coefficient.
Atmospheric albedoWhile the atmosphere of Helliconia is (by assumption) not much different from Earth, the incoming radiation spectrum very much is - and moreover it is different between Summer and Winter, as the high UV component of Freyr-light is much reduced in winter.
The plot shows that Freyr emits very hard UV light that however is completely captured by the atmophere. Especially in summer, a sizable fraction of the total never reaches the planet's surface, and even eye-balling the graph, it seems that the fraction of energy captured by the atmosphere is much higher than on Earth. Doing the numbers, this turns out to be quite true: Whereas Earth's atmosphere captures about 18% of the incoming energy in the atmosphere, the fraction is a whopping 33% during Helliconia summer and still 22% during Helliconia winter (when Freyr's contribution is smaller). It is particularly relevent that the summer and winter fractions are different - as this in essence means the overall albedo of the atmosphere is higher in summer (again, the atmosphere as such does not need to change, it is the relative UV content of the incoming radiation that is different, which again happens only in a binary star system). Stratosphere structureRunning the code to do the temperature profile of the atmosphere reveals the following picture:
What happens is that the high UV photon flux of Freyr makes the Chapman mechanism work much higher in the atmosphere than on Earth, so the whole stratosphere is shifted upward and the troposphere follows suit - active convection, weather and cloud formation is possible nearly up to 20 km in altitude in Helliconia's atmosphere - largely thanks to Freyr's hard radiation flux. (The stratosphere shifts somewhat downwards during winter, but still it does not nearly reach something similar to Earth.) Continue with Helliconia Summer. Back to main index Back to science Back to worldbuilder Created by Thorsten Renk 2023 - see the disclaimer, privacy statement and contact information. |
