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 its 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.
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.
Many of life's building blocks arise naturally in extraterrestrial bodies. Most of the amino acids in meteorites do not occur in life on Earth and must have an extraterrestrial origin. Scientists are confident of this conclusion because atoms can link into molecules with either "left-handed" or "right-handed" symmetry. These terms refer to shapes that are mirror images of each other like a left hand and a right hand. Earth-life uses only left-handed molecules, whereas the complex molecules in meteorites occur equally in left and right-handed varieties.
Comets are icy messengers from the outer Solar System and have also been shown to contain complex molecules. Scientists can analyze meteorites directly using the powerful techniques of molecular biochemistry, but analysis of cometary material is more indirectrelying on spectroscopy of the reflected light. Since massive molecular species have very complex and subtle spectroscopic signatures, identifying amino acids indirectly in comets is beyond the capabilities of current technology. A current ESA-lead mission Rosetta hopes to overcome this obstacle by making a rendezvous with comet 67P/Churyumov-Gerasimenko and making a detailed chemical analysis of its nucleus and coma. In 2015, the spacecraft successfully reached the comet and dropped the Philae lander onto its surface. although the lander has limited solar power, much will be learned about this comet as it approaches the Sun and becomes active. This is not the first time a spacecraft has approached a comet, just the first time detailed samples have been taken. In 1984, spacecraft made close encounters with the nucleus of Halley's Comet, finding a dark surface with a large amount of carbon-rich material, and in 2005 the Deep Impact mission collided with the comet 9P/Temple.
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.
Astronomers might learn about complex chemistry from our close 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 geo-chemical evidence suggests 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.
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 material from which life could form on other planets.