‘Earth has a unique chemical signature — we humans are now impacting this rarity’

Dimitar Sasselov teaches astronomy at Harvard University. Speaking at Times Evoke, he discusses why life on Earth is unique:


What is the core of your research?

My work stems from my background in physics and astrophysics — I specialise in the interactions between life and matter. I’ve been focusing recently on exoplanets — planets which orbit around other stars, astronomers having found over 5,000 of these in our galaxy and stellar neighbourhood — and the photochemistry in their atmospheres and surfaces. This is to understand the transformation from chemistry to life on a rocky planet like ours.

HOW THINGS COULD HAVE BEEN: The image from Mars (L) shows us four billion year-old sediments from an ancient lake on the rocky planet. (Picture courtesy: D. Sasselov). In contrast, water bodies and lakes emerged and grew on Earth (R), both drawing from and supporting a plethora of vibrant life. (Puctyre courtesy: iStock)




Can you explain the ‘Origins of Life’ project?

This initiative at Harvard University began over a decade ago. This comprises researchers who are focused on a particular approach to understanding the transition to life. It involves studying photochemistry, with energy from starlight, the sun (particularly UV light), planetary conditions — in this case, lakes of water which are shallow, where sunlight can penetrate — and chemistry to the point of protocells forming and surviving in such a lake. Our team also involves astronomers who try to detect signatures of life beyond Earth. This involves various distances and time scales — the photochemistry we study happens faster than a second. But the transition takes thousands of millions of years and the distances in exploring distant planets are in millions of light-years.




What are the basic building blocks of life?

We understand these from the perspective of life as we know it on Earth where all living creatures are created with the same molecules — we’re almost certain life elsewhere will also be chemical in nature, made up, like us, of molecules which form polymers. These will evolve amino acids which arrange themselves in chains, retaining particular information, like keeping track of chemical reactions.

We call these DNA and RNA but similar molecules may do this on other planets. We’ve also discovered how the photochemistry which leads to life’s basic building blocks involves a particular mineral that accumulated commonly on Earth before life emerged. And another important discovery came through our participation in NASA’s Keppler mission — this shows that rocky planets like ours are extremely common in our galaxy.



What is a star’s ‘chemical signature’?

Astronomers break down the light that comes from distant stars and planets into individual colours — in these, we see the different fingerprints of molecules and atoms. This is called ‘spectroscopy’. We see how our sun, for instance, has particular sets of atoms in diverse abundances, hydrogen and helium being the most common, while Mars and Jupiter have heavier elements. We can now do these studies for very distant solar systems and for those which experience ‘transit’ — this is when a distant planet passes in front of the face of its star. This helps us discover the planet itself — a dip in the brightness of a star tells us about a planet orbiting it and we can then measure its size. We can also measure the chemistry inside its atmosphere.

What kind of chemical signature does Earth have — and is this changing now?

Earth’s chemical signature is unique — it has typical signatures of a rocky planet, like nitrogen, carbon dioxide and water vapour. But it also has a very strong signature coming from a large amount of oxygen. Normally, this wouldn’t be present in a rocky planet’s atmosphere but it is here as our Earth is inhabited — it has a living biosphere. Its life is the source of that oxygen. Now, there is an important change — carbon dioxide has increased very steadily and quickly, particularly over the last five decades. This is leading to a greenhouse effect where the temperature on Earth’s surface is rising as a whole.

The trouble is, humans have an illusion that we are in control of this whereas in reality, we are not going to be able to manage these effects. The truth of the matter also is that while this is terrible for the human species, in the long term, Earth will recover. Humans may not but our planet’s biosphere will carry on for millions of years. We are not displaying the wisdom needed to come together as a species to ensure our own survival. It is crucial we do this now — valuing what makes our planet so special is part of this endeavour. Earth’s living biosphere is unique — it has not been seen anywhere else. Our team is searching for such a signature in space but the fact is, we already live on an extraordinary planet.

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