GHG Interventions on Mars¶
Warming Mars to temperatures compatible with liquid water requires raising the surface temperature by at least \(\sim 60\,\text{K}\) (from \(\sim 210\,\text{K}\) to \(\sim 273\,\text{K}\)). The most practical near-term pathway is injecting synthetic super-greenhouse gases (GHGs) into the Martian atmosphere — a strategy first rigorously analysed by Marinova, McKay, & Hashimoto (2005) and McKay et al. (1991).
Why super-GHGs on Mars?¶
The natural Martian greenhouse effect is weak: the thin CO₂ atmosphere produces only \(\sim 5\,\text{K}\) of warming above the bare-rock equilibrium, compared to \(\sim 33\,\text{K}\) on Earth. This is because:
- Mars's CO₂ atmosphere is 170× thinner than Earth's by mass, so it absorbs very little outgoing IR.
- There is no water-vapour feedback — Mars's water inventory is locked in ice.
- The dominant IR window (\(8\)–\(12\,\mu\text{m}\)) is essentially transparent on Mars.
Super-GHGs are effective precisely because they absorb strongly in this window region (Pierrehumbert & Gaidos, 2011).
Radiative forcing model¶
The forcing from a gas at atmospheric concentration \(C_i\) (ppb by volume) is:
where \(\eta_i\) (W m⁻² ppb⁻¹) is the Mars-specific radiative forcing efficiency from Marinova et al. (2005). These differ from IPCC Earth values because the absence of water-vapour overlap bands and the thinner Martian CO₂ column change the spectral windows available for absorption.
The total forcing from all injected species:
Concentration from injected mass¶
Given an annual injection rate \(\dot{m}_i\) (kg yr⁻¹) of species \(i\) with molar mass \(M_i\) (kg mol⁻¹), the steady-state atmospheric concentration (in ppb) after accounting for atmospheric loss with lifetime \(\tau_i\) (yr) is:
where \(M_\text{atm} = P \cdot 4\pi R^2 / g\) is the total atmospheric mass (\(\approx 2.5 \times 10^{16}\,\text{kg}\) at current Martian pressure, Mahaffy et al., 2013).
The transient concentration at time \(t\) under continuous injection evolves as:
Compound registry¶
| Compound | Formula | Lifetime \(\tau\) (yr) | Notes |
|---|---|---|---|
| CF₄ | Tetrafluoromethane | >50,000 | Strongest long-lived candidate; GWP 6,630 (IPCC AR6) |
| SF₆ | Sulfur hexafluoride | 3,200 | GWP 23,500; spectral absorption in 8–10 µm window |
| C₂F₆ | Hexafluoroethane | 10,000 | GWP 11,100 |
| NF₃ | Nitrogen trifluoride | 500 | GWP 17,400 |
| C₃F₈ | Octafluoropropane | 2,600 | — |
| CHF₃ | Trifluoromethane | 228 | — |
| CH₂F₂ | Difluoromethane | 5.2 | Short-lived |
| CH₄ | Methane | 12 | Low GWP; synergistic with CF₄ in some spectral bands |
| N₂O | Nitrous oxide | 114 | GWP 273 (IPCC AR6) |
Lifetimes from IPCC AR5 Annex II and Ravishankara et al. (1993). Mars atmospheric lifetimes may differ from Earth values due to the different UV environment and the absence of OH radical sinks.
Greenhouse factor update¶
After each year of simulation, the greenhouse factor \(\gamma\) is updated based on the total accumulated forcing:
This \(\gamma\) is then fed into the temperature ODE for the next year's integration (see Climate Model).
Terraforming phase 1 target¶
A commonly cited first milestone is reaching \(\sim 273\,\text{K}\) mean surface temperature and \(\sim 1000\,\text{Pa}\) surface pressure — conditions under which liquid water is stable at low elevations and the CO₂ caps begin to fully sublime, releasing additional CO₂ that further amplifies the greenhouse effect through a positive feedback (McKay et al., 1991).
Marinova et al. (2005) estimated that injecting \(\sim 300\,\text{ppb}\) of CF₄ into the Martian atmosphere would provide \(\sim 25\,\text{W\,m}^{-2}\) of additional forcing — enough to initiate significant polar cap sublimation, releasing a much larger CO₂ reservoir that then provides additional forcing through the cap feedback.
Implementation¶
GHG compounds are registered in src.interventions.compounds. Concentration calculations are in src.interventions.forcing, and the annual injection schedule is managed by src.interventions.controller. See the Interventions API for the full interface.