Purple planets
Published on Jan 13, 2007 at 8:50 pm.
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Filed under astrobiology.
At the AAS Meeting, there were several sessions on astrobiology. Some of these sessions were on the nature of life, the origin of life, and detection of life. One of the biomarkers is the effect that it has on the spectrum of a planet. Oxygen and methane biomarkers. The only way that we know to get lots of atmospheric oxygen is through biological activity. Oxygen is the waste product of many organisms. Methane degrades quickly, particularly in the inner solar system, and definitely in the presense of oxygen. Methane can be generated by certain biological systems. However, Earth did not always have an atmosphere like today. In the distant past, there was far less oxygen and more carbon dioxide in the atmosphere. Some of the talks at the conference talked about these spectral signatures. However, you need good spectral resolution to see these spectral lines. We are right on the cusp of being able to make these measurements.
Most of the lifeforms that release oxygen on Earth do so via photosynthesis. Plants, for example, use sunlight to produce the chemicals that they need for life. Modern photosynthesis uses chlorophyll to harness the energy of light. Chlorophyll absorbs light and releases electrons to initiate a series of chemical reactions to produce the sugars that are the chemical energy source for life. Interestingly, chlorophyll tends to reflect more light near the middle of the visual spectrum — green light. That is why plants are green. Chlorophyll is a photoreactive chemical that does what it does by absorbing light.
However, chlorophyll is not the only photreactive chemical in life forms. If you are reading this, then you are using photoreactive chemicals in your eyes. These chemicals absorb light and yield other chemical reactions. Among the chemicals in your eye is rhodopsin, often called visual purple because of its color. You use the chemical to see, and it is most important for night vision. That is why bright lights cause you to lose your night vision — the rhodopsin is photoreactive, and bright light causes it to be used up. It takes a while to produce more so that you can see in low light conditions.  The retinal reactions of rhodopsin are quite different from those of plants, but they still use energy from light to do what they do.
Biologist believe that perhaps these retinal reactions were among the first ways that life adapted to use sunlight as an energy source. Certain single cell organisms from the archaea kingdom use chemicals similar to rhodopsin to generate an electrical potential that these organisms can use to generate the chemicals that they need for life. Some purple sulfer bacteria use this process in photosynthesis rather than chlorphyll reactions. Unlike the chlorophyll reaction that uses water and releases oxygen as a waste product, these organisms use sulfer and do not release oxygen. Evolutionary biologists seem to believe that these were among the first type of lifeforms on Earth. But, most annoyingly for astrobiologists, such organisms do not produce the atmospheric oxygen that would be such a clear biomarker. But, I suppose that one might see a slight spectral shift in the reflected light from a planet teaming with such organisms. At any rate, it makes detection of life on another planet quite a bit tougher if life is in its earliest stages.
Such a planet might be purple rather than green. Biology is way out of my field of specialty, so this was pretty new stuff to me. When I got back home, I was eagerly telling one of the biology faculty about this, and they were rather unimpressed. They already knew all this! Well, it was news to me, and I really do like learning new things, even things not directly related to astronomy.
-Astroprof
