When terrestrial glaciers retreat from permafrosted terrain, the barren
earth undergoes a process of reclamation involving runoff (sometimes
massive), precipitation (often minimal) and plant succession (well-ordered).
In the course of this reclamation, each successive vegetable community
literally prepares the ground for the next.

Asteroids may be considered the ultimate cases of permafrost. Their "soils"
are regolith of the most barren sort imaginable, having been exposed to hard
vacuum and cosmic radiation. Despite these unpromising initial conditions,
I wonder if postglacial vegetation -- with human assistance and
monitoring -- might be one approach to preparing asteroids for human
habitation.

First, human beings will have to create an atmosphere and at least a modicum
of radiation shielding. One strategy might be to put an approximately
spherical asteroid into a "bag" composed of a reinforced shell of ice five
to ten meters thick. Provisions should be made to protect the exterior from
vacuum sublimation, but the interior should be allowed to liquefy and
perhaps take on a plant and animal community derived from freshwater arctic
species. The shell should outsize the asteroid by at least a kilometer to
allow for an atmosphere (which would further abate radiation), and it should
be securely anchored to the asteroid itself.

The best asteroid candidates for this treatment might be old cometary cores,
which might contain ices of water and perhaps methane; this would greatly
simplify the provision of the ice shell and an atmosphere. Cometary cores
might also have Apollo-type orbits, which might simulate seasonality that
would aid in the adjustment of plants to the novel asteroidal environment.

A particular challenge in this approach would be binding of water to the
surface, especially in the case of very small asteroids that have only a
trace of gravity. Fortunately, lichens and mosses cling tightly to the
rocks they inhabit, and they would tend to trap moisture directly. Also,
many postglacial plant taxa have evolved strategies to extract moisture from
fog, which would likely be a regular feature of the atmosphere of a shelled
asteroid.

The rate of progress in soil creation would probably be somewhat greater
than in the terrestrial postglacial case, due to the ready availability of
regolith. Other things like atmosphere balancing would take more time,
especially because there would be a substantial "learning curve" for the
humans who were conducting the experiment.

At length, we might observe the eventual creation of a classic boreal forest
on such an asteroid. Such a development might require an experimental
duration of 600 years (typical in terrestrial cases, but I suspect that it
would be far less, since the main inhibition to tree growth would be the
biological impoverishment of the regolith-soil. Soil remediation is
primarily a bacterial activity, and that might require less than a decade.

As a first candidate for this experiment, I suggest Deimos, the outer moon
of Mars. It is of a convenient size (less than ten miles average diameter),
and it is near a large planet. A living outpost so close to Mars might be a
great asset in the exploration of the Red Planet.

Jim McCauley