Apparently the vegetable kingdom in Mars, instead of having green for a dominant colour, is of a vivid blood-red tint—H.G. Wells

According to Nancy Y. Kiang (biometeorologist at the NASA Goddard Institute for Space Studies) green aliens are so passé. Well, she may have a point. In a fascinating article in Scientific American (April, 2008), Kiang tells us that “light of any color from deep violet through the near-infrared could power photosynthesis.” For instance, the cooler type M stars (red dwarfs) are feeble and planets receive less visible light. Plants might need to be close to black in color to absorb as much light as possible. Young M stars fry planetary surfaces with ultra-violet flares, so many organisms would likely be aquatic to survive. Our sun is type G, and on Earth green generally dominates the color of living plants. Around F-stars, hotter and bluer than our sun, plants might get too much light and would need to reflect much of it, so they would tend to absorb blue light and might look green to yellow to red or violet.

Photosynthesis, says Kiang, adapts to the spectrum of light that reaches an organism; and the spectrum results from the parent star’s radiation spectrum, combined with the filtering effects of the planet’s atmosphere. Kiang further adds that photosynthesis can produce very conspicuous biosignatures (see more below): 1) biologically generated atmospheric gases such as oxygen and its product, ozone; and 2) surface colors that indicate the presence of specialized pigments such as green chlorophyll.

When I first learned about photosynthesis in Grade 3, I thought it was a magical process. Scientists who make it their specialty still do. It is truly one of God’s wonderful gifts to life in this universe. Well, think about it: photosynthesis converts light energy (sunlight) into chemical energy through living organisms. The raw materials include carbon dioxide and water and the end-products include oxygen and (energy rich) carbohydrates, like sucrose, glucose and starch. The process is arguably the most important biochemical pathway on Earth since nearly all life either directly or indirectly depends on it. And like all marvelous things in nature, the pigments that harvest sunlight don’t operate in isolation. They operate “like an array of antennas, each tuned to pick out photons of particular wavelengths,” says Kiang. Chlorophyll preferentially absorbs red and blue light. Carotenoid pigments, responsible for the vibrant reds and yellows of autumn, pick up a slightly different shade of blue. All this energy is funneled to a special “hub” chlorophyll molecule, which splits water and releases oxygen.

How plausible is it for photosynthesis to arise on another planet? The process is so successful on Earth that it remains the foundation for most life (exceptions being organisms that live off methane of oceanic hydrothermal vents, etc.). The majority of life on earth depends on sunlight. Photosynthesis evolved early on in Earth’s history, with the first fossil evidence dating to about 3.4 billion years ago. “The rapidity of its emergence suggests it was no fluke and could arise on other worlds too,” Kiang contends and adds, “As organisms released gases, they changed the very lighting conditions on which they depended,” which meant that hey had to evolve new colors. We can see this in the evolutionary range in pigmentation of simple unicellular life, from the near-infrared absorbing first photosynthetic bacteria to the early blue-green algae, red and brown algae and finally the more evolved green algae. “Studying Earth life to guide our search for life on other worlds is the essence of astrobiology,” said Carl Pilcher, director of the NAI at NASA Ames. “This work broadens our understanding of how life may be detected on Earth-like planets around other stars, while simultaneously improving our understanding of life on Earth.”

Predicting alien plant colors takes experts ranging from astronomers to plant physiologists to biochemists. While the longest wavelength observed in photosynthesis on Earth is about 1,015 nm (in purple anoxygenic bacteria), the laws of physics set no strict upper limit. The limiting factor, according to Kiang, isn’t the feasibility of novel pigments but the light spectrum available at a planet’s surface, which depends mostly on the star type. Astronomers describe what’s called a “habitable zone” around each star. This is a range of orbits where planets can maintain a temperature that supports liquid water. In the solar system of our G star, this includes the orbits of Earth and Mars. The habitable zone of an F star, a hotter star, would be farther out and that of a K and M star, would be closer.

Aside from colors reflected by plants, the following features may provide signs of life (e.g., biosignatures) according to NASA:

1. Oxygen plus water: even on a lifeless world, light from the parent star produces a small amount of oxygen in a planet’s atmosphere by splitting water vapor. The gas dissipates quickly (e.g., rained out or through oxidation of rocks and volcanic gases). Abundant oxygen therefore signals an additional source;
2. Ozone: easier to detect, ozone provides an indicator of oxygen, being its product;
3. Methane plus oxygen: these two are considered an awkward combination, hard to achieve without photosynthesis;
4. Seasonal cycles: fluctuations of methane suggest life, given that levels tend to remain constant otherwise;
5. Methyl chloride: produced on Earth from burning of vegetation and the action of sunlight on plankton and seawater chlorine. An M star’s relatively weak radiation might allow the gas to build up to detectable amounts;
6. Nitrous oxide: released when plant matter decays.

• Anaerobic ocean life: where the parent star is a young star of any type and the organisms may not produce oxygen and the atmosphere may be mostly other gases like methane;
• Aerobic ocean life: where the parent star is older and photosynthesis has evolved, building up atmospheric oxygen;
• Aerobic life on land: the parent star is mature and plants cover the land (like Earth);
• Anaerobic land life: the star is a quiescent M star, so the UV radiation is negligible and plants wouldn’t produce oxygen.