4.2 Detecting Life From Afar

summary

The search for life on exoplanets will focus on the byproducts of biology

The remote sensing of life from space is a technique that humans have honed since the dawn of space flight.
Molecular spectroscopy, the study of light absorption by different molecules, is a well-developed technology. It can already be used to detect trace quantities of organic gases from space. It works because compounds leave spectral “fingerprints” in light as it passes through them.
This technology can be turned to exoplanets that may harbor life to try to detect specific biological byproducts, like alcohols or other organic compounds.
There are two ways to make these measurements on an exoplanet: using the light passing through the atmosphere when the planet is in front of its star, and using light that reflects off of its atmosphere just before it goes behind its star.
In both cases the atmosophere will impart subtle changes to the light that can tell us about the atmosphere’s composition. These measurements have already been performed successfully on Jupiter-like exoplanets.

Recognizing the signs of life

The spectroscopic technology to detect life exists, but we still don’t know exactly what we are looking for. Life on other planets may be very different from life on Earth.
If we cannot expand our understanding of life then we will only be able to search for signs of life similar to our own.
Even the early Earth was very different atmospherically from modern Earth. Microbial life existed on the Earth’s surface long before they managed to transform the atmosphere by enriching it with oxygen.

Life in the lab

To expand our ability to search for life requires an understanding of how atmospheric chemistry changes as life develops.
Modern life forms on Earth exist in an atmosphere that has already been dramatically altered by ancient life.
To understand what a planet would look like as life develops requires us to develop chemical systems that replicate very early life. Such systems (called artificial minimal cells or protocells) will allow us to understand the environmental feedback of early life development and possibly provide a window into alternatives to Earth life.

The components of life on Earth

There are three basic components to the chemistry of Earth life: informational (RNA and DNA), compartmental (phospholipid membranes), and metabolic (enzymes and proteins).
The problem facing biologists attempting to replicate the development of life is that these three components seem to require mutually incompatible chemistries.
This was first realized in the famous Miller-Urey experiment, which managed to produce amino acids out of the basic small molecules expected in the early Earth but was also very complex and required an environment that was not hospitable to the other two major biological components.

The RNA world

A more recent theory of early life development is called the RNA world. In this theory, RNA was the precursor to life and the other two components developed later.
RNA can actually have enzymatic abilities, including autocatalytic power. In this way it can bypass the production of amino acids and proteins because RNA could both store information and carry out metabolic functions on itself and other compounds.
Six years ago a team of scientists working under John Sutherland managed to use a systems chemistry approach to create ribonucleotides (the building blocks of RNA) in a simulated early-Earth environment that was also conducive to amino acid development and the development of lipid precursors.
This has opened the door to studying early life development and its affect on the chemical composition of planetary surfaces and atmospheres.