The Green Planet

On the day Sydney looked like Mars (this is the view from my office this morning), the obvious question is: If plants had evolved on the red planet, would they be green?

Scientists analysing the first data sent back from the Phoenix Mars lander last year announced that an asparagus plant would grow in a pot of Martian soil. The dirt from the northern arctic plains of Mars, said Samuel Kounaves from Tufts University, had all the nutrients a plant would need and was a little alkaline, just the way asparagus likes it.

A month or so later, however, there were reports of a chemical called ‘perchlorate’ in the soil, which certainly wouldn’t encourage asparagus growth – it’s a bit like bleach. But that chemical may be scattered in distribution and not a big problem.

Elsewhere on Mars, the lander was picking up signs of water, albeit without confirmation that water still exists on the planet. It was already known that the atmosphere is primarily carbon dioxide. So everything exists, or existed, to grow an asparagus.

But if there was ever life on Mars and it evolved into plant-like things, what colour would they be? Because there is plenty of sunlight reaching Mars and our sun is one the bright ones, you could reasonably expect lovely green asparagus.

However, as I suggested on radio recently, picture yourself on some far-flung planet not so close to a star. You might look out the kitchen window at your freshly mown black lawn and neatly cropped purple box hedge. A little further in the gloom you may be able to make out a few maroon shrubs and a very shady looking tree or two. If your planet doesn’t have an energetic young sun like Earth, your world is likely to be always a little dull. But couldn’t the plants at least provide a little colour to brighten your day?

According to Nancy Kiang, a NASA scientist working in New York, plant-like things growing on another planet are unlikely to be green. Green is a compromise colour and one that wouldn’t work in a planet orbiting a sun in the twilight of its life.

Dr Kiang is trying to help space explorers target planets that might be suitable for plant-like life and suggest to them what these living things might look like. The key defining characteristic of ‘plant-like’ is photosynthesis – the process of using energy (here on earth, the sun’s energy) to turn carbon dioxide and water into sugars and oxygen.

Many planets won’t have the bright star we have to provide all that light energy. Red dwarfs, for example, produce only a small fraction of the visible light we get from our Sun. So plants on a planet orbiting a Red Dwarf would need to hoard all the light they can get, rather than reflecting some of it back the way plants on earth do.

In any case, a light hoarding plant on a planet near to a Red Dwarf would reflect hardly any light and might look black or purple. However Kiang says it is unlikely you will find a blue plant on any planet because blue light is particularly yummy for a plant.

Kiang thinks there are plenty of planets capable of hosting plants in range of odd colours. So when you are next gazing at planets with your telescope, look to see if they are covered in dark coloured vegetation. Just don’t expect to see a blue moon, or planet.

Back on earth, most photosynthetic plants don’t absorb green light, reflecting it back to us and give them their distinctive colour. There are purple bacteria, and brown and red seaweeds (see next section), but mostly our photosynthetic life is a shade of green.

This might seem a bit odd given that sunlight reaching the Earth’s surface is rich in green light. So why are most plants on earth green, and what is the lesson for us earth-dwellers?

Firstly, life is compromise. Red light is rich in photons, and blue light more energetic, so plants may have evolved to use the more ‘nutritious’ reds and blues. A purplish variant may have been able to use more of the sun’s energy but it may also have been easy to spot, and eat, by some vigilant insect or wildebeest. A purple plant may also have become too warm to function at its best – compare your own performance in a (non-air-conditioned) black versus white car on a sunny day.

Secondly, life is the result of successful adaptations, not some a priori grand plan or design. As the environment changes, some plants are better adapted to that change and survive to produce more offspring. Not all possibilities, and combinations of possibilities, are explored. The living world we inhabit represents one track of evolution, one of a many possible worlds.

Thirdly, contrary to Voltaire’s Dr Panglos, there is no reason to believe this is the best of all possible worlds? It’s just one of them. There may be a planet out there with purple plants and that planet may be more what Dr Panglos was imagining.

For now though, we are stuck with Earth. Interestingly, although we call it the green planet, it is mostly blue due the predominance of the oceans, and in any case only the big obvious plants are green.


alice said…
Pretty good post. I just stumbled upon your blog and wanted to say that I have really enjoyed reading your blog posts.

green world
Tim Entwisle said…
Thanks for the feedback - it's fun writing it!
jenna said…
I liked your post. Keep posting interesting matters here.

green planet
Tim Entwisle said…
Thanks Jenna. Posting like this one on 'The Green Planet' are fun and fascinating to write so glad they are interesting.
joannakrupa said…
A purplish variant may have been able to use more of the sun’s energy but it may also have been easy to spot, and eat, by some vigilant insect or wildebeest.

Beauty & Fashion
Tim Entwisle said…
That's true - we need to think about more than just maximising photosynthesis. Being eaten, not pollinated, or maybe crushed, are all things that will reduce your evolutionary success. If ALL plants were purplish, of course, then the more 'efficient' plants wouldn't be easy to spot...