The Problem with Obliquity. From the writer of The Terraformers’ Toolkit.
Earth’s obliquity is the tilt in its rotation, so that it doesn’t quite rotate in the same plane as the solar system. This difference means that places away from the equator face toward or away from the sun at different times of year, giving us our seasons, winter, spring, summer and fall or autumn.
Early in the history of our solar system, many planets suffered collisions with planetesimals which left enormous impact craters, asteroid fragments, changed obliquities and altered spins. This left Earth with a 23.4 degree obliquity which means that life on Earth has evolved to survive with this.
Additionally Earth’s tilt or rotational obliquity has been stabilised over lengthy periods by the presence of our large Moon, without which Earth’s obliquity could vary enormously.
Plants and animals use the seasons to time their lives, including plant respiration, animal breeding, migration, feeding and hibernation. Any planet which we might consider terraforming would need to have a similar obliquity for life from Earth to survive.
Mars has two small moons which are not large enough to stabilise its obliquity, so that although it currently has a similar obliquity to Earth, it has varied in the past from 0° to 60°. Therefore, from our immediate planetary experience, it is possible to stabilise the tilt of a spinning planet by having a large moon. If large moons are key to stabilising obliquity, objects in the asteroid belt have been suggested as a possible moon to stabilise Mars’ obliquity.
Planets without seasons could have uniform conditions throughout the year, i.e., constant rainfall in one area and constant sun in another. This would make life hard in both locations. However, the climate of a zero obliquity planet would include far more glacial ice than a planet with even a moderate tilt. This is because any snow which fell could never be melted by a summer that never came. Equally, planets with high obliquity would be hostile to life because each Pole would face the star for months of the year, resulting in months of constant daytime heatwave, followed by months of freezing darkness.
Thus, before we can add life from Earth to a planet, it must have a stable Earth-like obliquity. Mars already has the right obliquity and little more needs to be done apart from warming the planet and adding an atmosphere and water. However, scientists are still uncertain whether Mars’ obliquity will be stable in the long term, or not. If it is deemed to be insufficiently stable on human time-scales, a larger moon will be needed. The material in the asteroid belt may not be massive or common enough to construct a large moon. One additional issue here is that Phobos and Deimos are too small and too close to Mars to be much use.
There are two possible ways to change a planet’s obliquity:
1. Asteroid impacts
The obliquities of both Mars and Earth are believed to have been set by impacts early in their histories. Finding asteroids which could be used as impactors to change a planet’s obliquity is limited by the range of objects in the asteroid belt and by their value as mining resources. However, if a number of smaller impactors are needed, they might be sourced from either the asteroid belt, Jupiter’s Trojans or the Kuiper Belt.
Once an obliquity of approximately 23 degrees has been attained, a large moon may be needed to stabilise this. Only the dwarf planet Pluto also has such a proportionately large moon. Therefore, a relatively large object would be needed to duplicate our Moon’s obliquity stabilising effect.
2. Momentum transfer
It might be possible to use objects from the Kuiper Belt or asteroid belt to transfer angular momentum from a gas or ice giant by tight turns around the planet needing tilt correction and the giant planet. However, an asteroid intended as an angular momentum transfer vehicle might come within the Roche limit of either planet and be destroyed in the process. One possible solution would be to follow a path around 3 rather than 2 planets to limit the tightness of its turns.
If the object transferred much of its momentum, it could be placed into a stable orbit around the planet as a moon, which may stabilise the obliquity once it has been corrected.
Any terraformed planet without seasons would be hostile to most of the life from Earth as animals and plants would not know when to perform critical functions. They would mistime their lives and fail to breed, not get enough to eat or lose contact with each other. This includes domesticated animals and plants important for human survival.