Overview
Citation: Howe, A. R. (2022). Cloud Continents: Terraforming Venus Efficiently by Means of a Floating Artificial Surface. arXiv preprint arXiv:2203.06722. https://arxiv.org/abs/2203.06722
This is a speculative engineering proposal that outlines a method for terraforming Venus by constructing a floating artificial surface at approximately 50 km altitude, avoiding the need to remove the planet’s massive $\text{CO}_2$ atmosphere.
Motivation
While Mars is the typical candidate for terraforming, Venus possesses specific advantages for colonization:
- Gravity: Near-Earth surface gravity ($0.9g$), avoiding the health complications of long-term low-gravity exposure.
- Atmosphere: A thick atmosphere provides robust protection from cosmic rays and UV radiation.
- Proximity: Shorter travel time from Earth compared to Mars.
The challenge is that Venus’s surface is utterly hostile: 90 bars of $\text{CO}_2$ pressure and temperatures of 735 K. Previous terraforming proposals have focused on removing this atmosphere, which requires extreme amounts of energy or mass transport.
Contribution
Howe proposes working with the existing atmospheric conditions rather than fighting them. The core insight is that at ~50 km altitude, Venus’s temperature and pressure are already Earth-like. By building a sealed floating surface at this altitude, a habitable environment can be engineered above it without needing to remove the lower atmosphere.
The key advantages claimed are:
- In-situ resource utilization: The surface structure is built from atmospheric carbon (extracted from $\text{CO}_2$), and nitrogen (3.5 bars available) serves as the lifting gas.
- No mass export required: Unlike proposals that require exporting Venus’s atmosphere to space, this method leaves the $\text{CO}_2$ in place below the surface.
- Comparable timeline: The project could theoretically be completed in ~200 years, similar to other terraforming proposals but with significantly lower resource costs.
Critique of Previous Proposals
Howe reviews past terraforming methods to contextualize the efficiency of the Cloud Continent approach:
| Proposal | Method | Key Problem |
|---|---|---|
| Sagan (1961) | Seed clouds with algae to convert $\text{CO}_2$ | Venus is too dry; would produce tens of bars of $\text{O}_2$ requiring removal |
| Adelman (1982) | Asteroid impacts to strip atmosphere | Requires impactor mass exceeding the atmosphere ($> 5 \times 10^{20}$ kg) |
| Birch (1991) | Sunshade to freeze and bury atmosphere | Requires importing water equivalent to dismantling Enceladus |
| Landis (2011) | Floating cloud cities | Howe argues this should be expanded to a continuous planetary surface |
The Two-Phase Construction Model
The proposal involves two construction phases using locally sourced carbon and nitrogen.
Phase 1: The Shell (Sealing the Atmosphere)
The first phase creates a planetary-scale seal at ~50 km altitude to separate the hostile lower atmosphere from the habitable upper region.
- Structure: Interlocking hexagonal tiles, approximately 100 meters wide.
- Quantity: ~$7.2 \times 10^{10}$ tiles required to cover Venus’s surface area.
- Dynamics: Flexible joints are needed to accommodate zonal wind shears of 40-60 m/s at this altitude.
- Maintenance: Tears must be repaired quickly. A 1 $\text{km}^2$ tear would leak $\text{CO}_2$ at a rate of $8.04 \times 10^{11}$ kg/day.
Phase 2: The Honeycomb (Floating Landmasses)
Once the shell is sealed, a “honeycomb” structure several kilometers high is built on top to provide buoyancy and support for habitable land.
- Height: Approximately 6.86 km.
- Material: Carbon nanotubes or aggregated diamond nanorods, synthesized from atmospheric carbon via $\text{CO}_2$ electrolysis.
- Lifting Gas: $\text{N}_2$ (nitrogen), extracted from Venus’s atmosphere (3.5 bars available).
- Buoyancy Mechanism: The honeycomb cells are filled with $\text{N}_2$ and displace the heavier $\text{CO}_2$ outside the structure. Each layer is pressurized to ambient pressure to minimize structural stress.
- Load Capacity: A “Standard” design consumes 0.32 bar of $\text{CO}_2$ for construction and supports a mass load of 7,440 kg/m², sufficient for soil, infrastructure, and cities.
The Terraformed Environment
Once the surface is built, the environment above the shell is engineered to resemble Earth.
Atmosphere
A breathable nitrogen-oxygen atmosphere is created above the surface. Oxygen is a byproduct of the $\text{CO}_2$ electrolysis used to extract carbon for construction.
Temperature Control
No space-based sunshade is required. Instead, the surface Bond albedo is tuned to 0.62 (using mirrors on approximately 50% of the surface) to achieve Earth-like equilibrium temperatures.
Day/Night Cycle
Venus rotates extremely slowly (117 Earth days per solar day at the surface). However, the floating crust would move with the super-rotating atmosphere (~50 m/s at 50 km altitude), resulting in a day/night cycle of approximately 9 Earth days.
Topography
Hills and valleys can be sculpted by varying the height of the honeycomb structure, allowing for diverse landscapes.
Resource and Energy Requirements
The project relies heavily on in-situ resource utilization (ISRU), requiring external imports only for water.
Energy Budget
| Process | Energy Requirement | Time (using all solar flux at 20% efficiency) |
|---|---|---|
| $\text{CO}_2$ Electrolysis (carbon + oxygen) | $3.33 \times 10^{10}$ J/m² | ~30 years |
| Nitrogen Separation | Dominant cost | ~170 years |
Nitrogen separation from the $\text{CO}_2$-dominated atmosphere is the most energy-intensive step, driving the overall timeline.
The Water Problem
Venus is extremely dry (~20 ppm water vapor). Arable land requires significant water imports.
- Requirement: $2.30 \times 10^{17}$ kg of water (equivalent to a layer of 500 kg/m²).
- Ceres: Has water but lacks the angular momentum to support a space elevator for export (exporting water would halt its rotation).
- Mars: The optimal source. Water can be exported via space elevator with lower energy cost than from Earth (12.58 MJ/kg). Delivery can occur gradually after the surface is constructed.
Feasibility Assessment
Timeline
In a best-case scenario (energy-limited), the project could be completed in approximately 200 years, comparable to other terraforming proposals.
Technical Challenges
- Materials Science: Requires industrial-scale production of carbon nanostructures, which is currently only achievable at laboratory scales.
- Maintenance: The floating surface requires continuous monitoring and repair to maintain atmospheric separation.
- Coordination: A project of this scale requires unprecedented levels of interplanetary coordination and sustained effort over centuries.
Key Advantage
The method is argued to be more efficient than atmosphere-removal approaches because it works with the existing pressure gradient rather than fighting it. The $\text{CO}_2$ atmosphere below the surface actually provides the buoyancy that keeps the habitable layer aloft.