Importing Volatiles
Importing Volatiles: Comets and Ammonia Asteroids
Zubrin and McKay listed this alongside mirrors and halocarbons in their original plan, and the appeal is that it solves several problems with one mechanism. Ammonia is a nitrogen carrier and a potent greenhouse gas in its own right. Comets carry water. The impact energy warms the planet directly. One redirected body contributes to atmosphere, hydrosphere and temperature simultaneously.
The arithmetic is unforgiving. It takes 3.89 × 10¹⁵ kg of gas to raise Martian global mean surface pressure by a single millibar. A large comet nucleus masses perhaps 10¹³–10¹⁴ kg, and only a fraction of that is usable volatiles that stay in the atmosphere rather than escaping or freezing out. Building tens of millibars means a sustained bombardment campaign lasting centuries.
Moving the bodies is its own industry. Outer solar system objects have to be found, characterised, fitted with propulsion or nuclear devices, and nudged onto multi-decade trajectories with enough precision that they hit a planet and not something else. Every one is a decades-long mission, and thousands are needed.
And every arrival is a planetary-scale impact event. Any surface infrastructure, any settlement, any deployed aerogel or biology has to survive a bombardment programme deliberately maintained for centuries. This is the entry on the list that is hardest to reconcile with also living there — which is why it appears in the literature as a thought experiment about resource limits rather than as an engineering proposal.
The case for
Three problems, one mechanism
Ammonia-rich bodies deliver nitrogen, water and kinetic heating in a single event, and ammonia is itself a strong greenhouse gas.
The material exists in quantity
The outer solar system holds vastly more volatile mass than Mars needs. The constraint is transport, not availability.
No planetary chemistry plant
Unlike gas factories, there is nothing to build on Mars. The work happens far away, and Mars simply receives the delivery.
The case against
Thousands of impacts, over centuries
At 3.89 × 10¹⁵ kg per millibar and 10¹³–10¹⁴ kg per body, tens of millibars means a bombardment programme with no realistic end date.
It is incompatible with living there
Deliberate planetary-scale impacts and a settled surface cannot coexist. Any occupied Mars would have to wait out the campaign somewhere else.
Each redirection is a decades-long mission
Finding, reaching and precisely steering an outer solar system body is at the outer edge of conceivable deep-space engineering — and thousands are required.
Much of the payload escapes anyway
High-velocity impacts blow a fraction of the delivered volatiles straight back out of the atmosphere, so the effective yield is well below the nominal mass.
| Per millibar | 3.89 × 10¹⁵ kg | Gas that stays in the atmosphere |
| Bodies required | 10²–10³ per 10 mbar | Depending on retained fraction |
| Mission length | Decades each | Detection to impact |
| Programme length | Centuries | Sustained, uninterrupted |
Could crashing comets into Mars create an atmosphere?
In principle. Ammonia-rich bodies deliver nitrogen, water and heat at once. In practice each millibar of Martian pressure requires 3.89 × 10¹⁵ kg of retained gas, so it takes thousands of large impacts sustained over centuries.
Why ammonia asteroids rather than ordinary comets?
Ammonia carries nitrogen — the one element Mars cannot supply for itself — and is a powerful greenhouse gas, so an ammonia-rich body addresses the buffer-gas shortfall and the temperature shortfall in the same delivery.
Is comet bombardment considered a serious proposal?
It appears in the founding terraforming literature and in resource-limit analyses, but not in current engineering roadmaps. The scale of the redirection programme and its incompatibility with a settled surface keep it in the thought-experiment category.