Chapter 19: Life in the Universe

Relative Elements Abundance in Universe

Our study of the interstellar medium is in part driven by our desire to understand the origins of life. While planets are the most likely sites for life, many of the basic molecular building blocks of life form in deep space from material that has been ejected from stars. These molecules can be found in the interstellar medium, so analyzing their chemistry reveals important information about the origins of life. More than 130 molecular species have been identified in the interstellar medium in the past 30 years. Most of these have been detected by very low energy changes in their modes of rotation, which produce spectral features at radio frequencies; a few have been identified by their vibrations, which generate features in the infrared part of the spectrum. The molecules in the interstellar medium include ethyl alcohol (C₂H₅OH), the amino acid glycine (C₂H₅O₂N), and a number of other organic molecules.

Cyril Ponnamperuma (1923-1994) of the Exobiology Division at NASA's Ames Research Center points out the simple laboratory equipment with which he artificially produced ATP, the source of energy for all forms of life

While the majority of studies of the interstellar medium use spectroscopy to directly measure atomic and molecular spectral lines, we can also learn about molecules in cold and deep space by studying meteorites. Every now and then a meteorite "fossil" representing the primordial composition of the solar nebula drops right into our laps (hopefully not literally, although this has happened to a few unfortunate people). Radioactive dating of such meteorites reveals ages as old as 4.5 billion years — when the Earth and Sun were first forming. A small percentage of meteorites are rich in carbon materials; they are called carbonaceous chondrites. Only a few meteorites have been recovered quickly enough to have not been contaminated by molecules from Earth. These rare finds have provided most of our information on complex molecules from space. Dozens of amino acids have been found inside carbonaceous chondrites. In 1983, Cyril Ponnamperuma announced that all five critical bases for coding genetic information in RNA and DNA were found in a single carbonaceous chondrite.

Allende meteorite, carbonaceous chondrite

This Viking 1 orbiter image shows the thin atmosphere of Mars.

It was initially surprising that complex molecules could form in the cold and hostile environment of deep space. Since this discovery, experiments have replicated the production of organic molecules at the temperature of dry ice. Although there has been much speculation about life in comets, no evidence has been found of replicating molecules or primitive forms of life in comets or meteorites. However, these primitive bodies may have played a key role in the emergence of living things. They may have deposited organic material during the early bombardment of the Solar System, giving a head start to the pre-biological chemistry of all the inner planets, including the Earth.

Photograph of the Saturn moon Titan in False Color

Astronomers might learn about complex chemistry from our closest planetary neighbors. Outside the inner planets, Saturn's moon Titan may have produced a photochemical smog. With its reddish clouds of carbon compounds and its possible rains of liquid methane, Titan may provide an intriguing natural laboratory for further study of organic chemistry. This is not to say that all planets are oozing with organic slime. Sunlight encourages reactions that not only form organic molecules but also break down organic molecules. When energetic UV photons strike molecules, the molecules can break apart. On worlds like the Moon and Mars, where there is no ozone layer to block UV radiation, the rate of destruction is high and gases are insufficient to provide raw materials to form new organic molecules. Sunlight has apparently destroyed any molecules that may have formed on Mars in the past. Indeed, fossil and geochemical evidence suggest that life did not emerge on the Earth's land surface until ocean plants — which were shielded from sunlight by seawater — emitted enough oxygen to build up an ozone (O₃) layer in the Earth's atmosphere. This ozone layer then shielded land-based life from solar UV radiation.

Replica of the Viking 1 lander in the National Air and Space Museum, Smithsonian Institute, Washington DC

Although the Viking landers found no organic molecules on Mars, they did reveal interesting clues about the development of such molecules on planets. Unlike the Earth, Martian soil is exposed to strong solar UV rays, which apparently produce unfamiliar chemical reactive states in minerals. The soil contains material that can synthesize organic molecules from atmospheric carbon dioxide and release gas once nutrients are added. Because the Martian soil in its natural state contains almost no organic molecules most scientists believe that these processes are not caused by Martian life forms. In fact, laboratory experiments in 1977 duplicated most Viking results using simulated Martian soil (iron oxide minerals exposed to UV light in carbon dioxide) without any organisms. Nonetheless, the processes may indicate how chemical reactions create the material from which life could form on other planets.