Looking in space for what's not there

One of the ways we are searching for life on other planets in the Solar System and beyond is to look for things that are only there because of living things, like lots of oxygen.

Now we are also looking for substances that, because of living things, have become almost or completely absent.

The original atmosphere of our world comprised lots of carbon dioxide, nitrogen and methane. There was little if any oxygen. There are life forms on Earth today that can survive in such an atmosphere. However, most living things, including us, cannot. Back then, in the absence of oxygen, our oceans contained soluble compounds of iron. Then, 2.4 to 2.7 billion years ago, (around two billion years after our planet formed) things changed in an event known as the Great Oxidation Event.

Around that time, a new form of life, cyanobacteria—the ancestors of plants—appeared. Unlike the earlier life forms, these used sunlight to process carbon dioxide and other compounds to power their life processes. The mechanism is referred to today as photosynthesis.

The waste product resulting from these processes was oxygen. As these new living things multiplied, the oxygen they released into the oceans caused great changes. There were life forms for which oxygen is a poison. Most of them vanished, although in various airless niches on Earth, their descendants are still around.

The increasing concentration of oxygen caused the iron and other chemicals dissolved in the water to change. The iron became insoluble iron oxide, which sank to the ocean floor as thick accumulations of red sediments that eventually became red rocks. Once most of the oxidizable materials were exhausted, oxygen built up in our oceans and atmosphere and carbon dioxide almost vanished, leading to the world we live in today.

If carbon-based life forms have appeared on worlds orbiting other stars, maybe they had great oxidation events too. This possibility offers another way to seek living things on those worlds.

Most of the extra-solar planets we have found have been through our detecting the tiny dimmings of stars as their planets pass in front of them. If a planet has an atmosphere, some starlight reaches us after passing through it, in the process picking up the signatures of whatever gases it is made of.

Searching for oxygen in the atmospheres of other planets is one way we search for life. This element is highly reactive and will rapidly disappear unless it is continuously topped up by plants and similar life forms. However, we can look for chemicals that are consumed by living things, like carbon dioxide.

Venus has lots of carbon dioxide in its atmosphere. With a surface hot enough to melt lead and tin, it is unlikely life forms anything like the ones on Earth could develop or survive there. Mars, also called the “Red Planet” because through a telescope it looks red or orange, is a more intriguing case.

That red colour is due to an abundance of iron in its surface soils and rocks. The planet's surface shows dry riverbeds, deltas and lakebeds. There are huge water-worn canyons. Today the planet is an arid, cold, almost airless desert. Long ago it was completely different, warmer, wetter and with a denser atmosphere. The red deserts and rocks suggest lots of iron and suggests that long ago Mars might have made the transition from a carbon dioxide to an oxygen atmosphere, driven by the appearance of life. However, something changed.

Today, Mars' atmosphere, although very thin, is 95% carbon dioxide, with just traces of oxygen. As of yet, we have found no signs of abundant life living on Mars.

Having two things we can search for in our quest for life on other planets should make our quest more reliable. The search has started.

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• Venus and Mercury lie low in the dawn sky.

• Jupiter and Saturn shine in the southern sky after sunset.

• The Moon will be new on Jan. 11.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.