Recent reports about the detection of carbon-based organic molecules on Mars by the instruments of the Perseverance rover included suggestions that some of the organics may well have fallen from space over the eons, and were then preserved on the Martian surface.
Given the cruciality of organics as building blocks of life –or even as biosignatures of past life — it seems surely important to understand more about how and where the organics might form in interstellar space, and how they might get to Mars, Earth and elsewhere.
After all, “follow the organics” has replaced the NASA rallying cry to “follow the water” in the search for extraterrestrial life in the solar system and cosmos.
And it turns out that seeking out and identifying organics in space is a growing field of its own that has produced many surprising discoveries. That was made clear during a recent NASA webinar featuring Samantha Scibelli of the University of Arizona, a doctoral student in astronomy and astrophysics who has spent long hours looking for these organics in space and finding them.
She and associate professor of astronomy Yancy Shirley have been studying the presence and nature of complex organics in particular in a rich star-forming region, the Taurus Molecular Cloud.
Using the nearby radio observatory at Kitt Peak outside of Tucson, she has found a range of complex organics in starless or pre-stellar cores with the Cloud. The campaign is unique in that some 700 hours of observing time were given to them, allowing for perhaps the most thorough observations of its kind.
The results have been surprising and intriguing.
A first take-away (surprising to those unfamiliar with the field) is that complex organics are often detected in these star-forming regions throughout the galaxy and cosmos — just as they were found in many regions of the Taurus cloud. The star-forming process, it appears, is key to understanding the mix of complex organics in interstellar space.
Also significant and unexpected is that these star-forming regions are where many simple organics (carbon dioxide, ammonia, water) are forged into more complex organics, carbon-based molecules of six atoms or more (think ethanol, glycine or acetaldehyde, an alcohol derivative that is contributing cause of hangovers.) When the star-forming process is completed, the organics have been incorporated into the resulting stars and planetary disks and spread further by comets.
Because organics, and especially complex organics, are so much a part of the story of the emergence and presence of life, these discoveries have become an increasingly high-profile part of the field of astrobiology — the search for life beyond Earth. In fact, these complex organics of the untolled star-forming “nurseries” of the galaxy and universe are now a necessary chapter in the scientific story of astrobiology writ large.
Complex organics are not life, but they are needed to build DNA, RNA and other essential and extremely complex molecules essential to life. Because of the cosmic work that goes in to making them complex, they are a step further along than simple organics and are further evolved and useful precursor molecules.
The inevitable question that arises is whether these complex organics dropped pre-formed on distant planets as they did on Mars and Earth? Or were they the more immediate chemical inheritance of faraway exoplanets or exomoons?
If so, they could potentially have helped move forward the emergence of life.
Perhaps the most surprising aspect of Scibelli’s work is when in the star-forming process the complex organics begin to appear. The answer is that they show up much earlier than expected.
Stars are born within clouds of dust and gas scattered throughout most galaxies. Turbulence deep within these clouds gives rise to knots or cores with sufficient mass that the gas and dust can begin to collapse under its own gravitational attraction.
As the cloud collapses, the material at the center begins to heat up. Known as a protostar, it is this hot core at the heart of the collapsing cloud that will one day become a star.
It was previously held that interstellar complex organics were formed in the presence of these hot cores. But Scibelli and Shipley found that they also appear far earlier in the process, within cold starless and prestellar cores that have not undergone that heat-generating and star-forming collapse.
Prior to Scibelli’s surveys, complex organics had been detected in only a handful of well-known, very evolved prestellar cores.
But in the Taurus Cloud, Scibelli found complex organics in 31 starless or prestellar cores. So their presence is not in question, but their origins are.
“A relatively well understood way to form hese larger molecules is when a forming baby star, or protostar, heats up the ice mantles in the surrounding cocoon of gas which can then go on to form rich complex chemistry in the gas-phase,” Scibelli wrote to me.
“But what drives the chemistry when no internal heat source is present? One of the big unanswered questions is – how exactly are these complex organic molecules forming so efficiently within cold core conditions?”
“It is still believed that smaller molecules will combine to form larger molecules in starless cores, but how these molecules actually get off the ices and are desorbed into the gas-phase so we can detect them with our radio telescopes is still debated,” she wrote.
Scibelli’s work pushes back the timescale of when the crucial complex organics appear in the star-forming process by a million years or more — not a lot in galactic time, but significant in terms of star formation.
“By studying complex organic molecules in starless and prestellar cores, I can uniquely probe the initial conditions of prebiotic material likely to be inherited by low-mass stars and planets,” she said. Planets form from disks of gas and dust around young stars, and their chemical composition is determined not only by ongoing chemical processes but also by the molecules made available from earlier evolutionary stages within the molecular clouds and cores.
As her colleague and advisor Shirley put in a UA release, all the complex organics in starless and prestellar Taurus cores “tell us that the basic organic chemistry needed for life is present in the raw gas prior to the formation of stars and planets.”
Scibelli will continue her work with a larger group of collaborators at the radio telescopes of the Green Bank Observatory in West Virginia, looking for three years for complex organics in one prestellar core. She is also completing another survey of two distant molecular clouds and she says she is also finding complex organics early in their star-forming processes.
Searching for intersellar complex organics has thus far been primarily the work of radio telescopes.
They are able to read the presence of interstellar chemicals in their gas phase by studying the frequencies and wavelengths at which they emit.
But the high precision and cutting-edge capabilities of the new James Webb Space Telescope are expected to allow researchers to identify complex organics within star-forming molecular clouds in the ice phase, too. This would be a first.
As Scibelli explained, radio telescopes can read the make-up of complex organics by using molecular spectroscopy and quantum mechanics to determine “which molecules are being excited at specific wavelengths in the radio regime of the electromagnetic spectrum.”
“Radio telescopes can detect individual rotational states of a molecule and therefore we know exactly what molecule is being detected due to its unique ‘spectral fingerprint’.”
“When molecules are in ices, you can detect their vibrational states in the infrared part of the spectrum. JWST will detect certain ‘vibrational modes’ that correspond to a certain molecule or group of molecules in the ice.”
An international group based in Leiden University in the Netherlands won time in the Early Release Program of JWST to study these ices and accompanying complex organics and dust under its Ice Age program. Their first report of findings is expected soon.
As described on the Ice Age website: “Ices and prebiotic molecules form well before planets themselves. They arise deep inside the clouds of molecular gas and dust that will eventually collapse to form planets. Here it is cold enough for ‘frost’ to form on dust grains. JWST will allow us to study in unprecedented detail the evolution of these icy grains through each physical stage of the star formation process, to see how much of these volatile materials is available to form planets.” (Volatiles are the group of chemical elements and chemical compounds that can be readily vaporized.)
The site goes on to report that life as we know it is made up primarily of hydrogen, oxygen, carbon and nitrogen, and all (except for oxygen) are “remarkably scarce on Earth,” which is dominated by rocky, refractory elements like iron, silicon, nickel, and oxygen.
This suggests that Earth was formed close to the Sun and was depleted in ices (and the volatiles they carry.) But then asteroids, comets and interstellar dust may well have brought in the needed molecules to give rise to life.
And then the site continues:
“If life on our planet was kick-started through the delivery of pre-fabricated components…. then it suggests that life could arise in a larger variety of environments than previously thought, both in our own solar system and in the diverse exoplanetary systems found around nearby stars.”