New light on life

Astronomy

Scientists are looking at aspects of the reflected earth light that shines on the moon, to identify features that could indicate the presence of life, writes S Ananthanarayanan

There has been interest in getting a good external view of the earth, so that we are better equipped with what to look for when scanning for earth-like planets around distant stars. Satellites pictures are not good enough, we need a little more perspective – a picture from the moon is not bad. In 2009, a group in Tenerife, Spain, had managed pictures of the earth’s transmission spectrum, or the light that passes through the earth’s atmosphere, as seen on the moon. But Michael F Sterzik, Stefano Bagnulo & Enric Palle in Chile, the UK and again in Tenerife in Spain have looked at aspects of the reflected earth light that shines on the moon, to identify features that could indicate the presence of life!

Many planets in orbit around distant stars have been found in the last decade, which we hope resemble the earth. A planet, which has no light of its own, of a distant star, is not visible from the earth, as the glare of the mother star prevents any reflected light from the planet from being seen.

Ways to detect planets

While the first method to detect planets was through the slight wobble that they create in the position of the mother star, which affects the colour of the light it emits, a more effective way has been with planets that pass between the star and the earth, and cause a minute dip in the light received.

It should also be possible to detect the effect that the gases in the planet’s atmosphere have on the light that passes close to the planet’s surface. To draw conclusions from these effects, it was of interest to see what effect the earth had on light from the sun, as the light grazed the earth and passed on into space. The Tenerife experiment was conducted during a lunar eclipse, where the moon is dark, but still there is some light which has passed through the earth’s atmosphere. This light, which dimly lights up the eclipsed moon, also reflects back to the earth. The reflected light was examined for indicators of the earth’s atmospheric make-up.

Analysis in the visible and infra-red region showed signs of ozone, water, carbon dioxide and methane and also of calcium and of some gases arising from human activity. So, there is interest in refining the effect of the known gases of the earth, on transiting sunlight, so that we can make sense out of similar data from a distant planet. The other kind of external view of the earth is by its reflected light. Here again, a handy observation post is the moon, not in eclipse, but in a quarter moon position. The dark part of the moon, where it is ‘night’, is bathed in reflected light from the earth as earthlight, corresponding to moonlight, which we have on the earth. This earthlight reflects off the surface of the moon and can be detected, albeit faintly, by sensitive, large-diameter telescopes on the earth. The light detected is not transmitted light  and would carry not only the effect of the atmosphere, but also the effect of reflection by different surfaces, like the clouds, the sea, rocks or plants.

The main effect in the case of transmitted light was that the gases through which the light passed absorbed light at particular frequencies, characteristic of the gases. Analysis of the spectrum then showed the frequencies where there was absorption, and hence the presence of the gases. In the case of reflected light also, there is the effect of absorption, as the light does pass through atmosphere, but there is also the effect of reflection.

Light vis-a-vis sound waves

Apart from frequency, a property that light waves share with sound waves, light has another property, of the direction of periodic movement. A sound wave is a progression of compressions and rarefactions of air, set up by a vibrating object and detected by creating the same vibrations in another object, like an ear-drum or a microphone diaphragm. The variations in density are hence in the same direction as the motion of the wave.

But it is different in light waves, and the movement is like the waves on the surface of a pond. Here, the water bobs up and down, setting up similar movement of water, in expanding circles, but the movement of the water is not in the direction of the wave, it is transverse. In light, the wave is really one of electrical and magnetic effects, which rise and fall, not in the direction of the beam of light, but in the transverse plane. And here, they are different even from the waves on water – the water moves only up and down, the electrical and magnetic effects can in be in any direction in the plane that is at right angles to the beam of light. With certain substances, when a beam of light passes through them, the electro-magnetic effects are transmitted only in one plane, and the light that comes through is weakened, but richer in the selected plane of transmission. Light like this is called polarised light. If such light is again shone on the same material, but turned at right angles, then it will not pass! Some polarisation also takes place when light reflects off a surface, depending on the angle of incidence. This is the reason that Polaroid lenses can cut the glare on a sunny day – by controlling the amount of reflected light that it allows to pass.

Sterzik and colleagues report in the journal Nature that they used the 8.2-metre Very Large Telescope in Chile to analyse the polarisation spectrum of earthshine reflected from the moon. They find that the linear polarisation of light scattered by air molecules, aerosols and cloud particles and also by reflection is a more sensitive measure of distribution of atmospheric gases or land/water/vegetation features than usual spectroscopy. The results for the earth show unusual abundance gases like oxygen and methane, which are themselves bio-signatures, or indicators of life systems. There is also a sharp increase in long wavelength reflection, indicating the presence of vegetation. The analysis indicates the separate contribution of clouds and ocean and is sensitive to even 10 per cent changes in visible vegetation.

Circular polarisation

Christoph U Keller of Leiden University, The Netherlands, adds in a commentary, that measuring yet another property of the reflected light could more sensitively indicate the presence of life. An effect of reflection or passing through some substances, by polarised light, is that the electrical and magnetic components of light waves be put ‘out of step’. When this happens, the plane of polarisation keeps changing, in a spiral manner, as the wave moves on and this is called circular polarisation. While crystal structures or plates made from such materials can be created to turn the plane of polarisation, it is found that living tissue uniformly causes circular polarisation in only one sense, that is right-handed or left handed. Finding handedness in circular polarisation of light reflected by an exoplanet would then point a finger to living materials!