How to listen for the building blocks of life in space
By Geoff Gilliard
Finding a needle in a haystack is child’s play compared to discovering new molecules in space.
But last October, Dr. Ilsa Cooke and her team at the University of British Columbia helped do just that when they, along with collaborators, discovered cyanopyrene in the Taurus molecular cloud 430 light years from Earth.
“Pyrene is a polycyclic aromatic hydrocarbon (PAH) — we think PAHs may make up more than 20 per cent of the carbon in space — and life is made of carbon,” says Dr. Cooke, who heads UBC’s Astrochemistry Laboratory. “By learning more about how PAH molecules form and are transported through space, we learn more about our own solar system and the life within it.”
So far Dr. Cooke and other researchers have uncovered 10 PAHs in space — but the field moves fast. Carbon — the fourth most common element in the Universe — exists in stars, asteroids and comets, as well as the atmosphere of most planets. It’s also the second-most abundant element in our bodies by weight. When Joni Mitchell sang “We are stardust” she wasn’t just being poetic.
“There’s nitrogen, carbon and hydrogen in space, all of which are common in organic molecules on Earth,” Dr. Cooke says. “Since we now know that many organic molecules can form in space, the question now is: can the most prominent building blocks of life — amino acids, nucleobases and sugars — form there as well? Because if they’re in space, they could be on other planets, too.”
Space: The most extreme chemical laboratory
The Taurus molecular cloud (TMC-1) is the closest large star-forming region to Earth, although in this case close is relative. It’s about four quadrillion kilometres distant — four followed by 15 zeroes.
Molecular clouds began forming from simple mixtures of atoms and molecules — mainly hydrogen — a couple of billion years after the Big Bang. As atoms collided, they joined together to form more complex compounds including PAHs. Over eons, gas and ice-covered dust grains were drawn together by gravity, densifying as they accumulated into protostars, then evolved into stars and their surrounding protoplanetary discs. Eventually, the systems’ stars ran out of fuel and died.
“We expect that many of the atoms contained in the molecules that are found in molecular clouds were once within stars and were cast out by supernovae or other means,” says Reace Willis, a graduate student with the Cooke group. “They then populated dense regions of space, and now they’re coming together to form the chemistry that we’re witnessing. We want to know whether we can find the same sorts of complex organic molecules that we see in TMC-1 in other regions of the interstellar medium — the gas and dust in the space between stars — and we already have for many of these molecules.”
“Pyrene might form within the molecular clouds itself, or perhaps it is more resilient than we previously thought and can survive its journey through space.”
Astrochemists don’t yet fully understand all the forms PAHs take in the interstellar medium, how they form or how they’re transported to solar systems like our own.
“The fact that we see pyrene in the cold molecular cloud TMC-1 is surprising,” says Dr. Cooke. “Traditionally, PAHs were thought to form in hotter environments and then travel through space to reach the molecular clouds. But pyrene could not survive this journey, it would get blown up by ultraviolet light. This suggests that pyrene might form within the molecular clouds itself, or perhaps it is more resilient than we previously thought and can survive its journey through space.”
Part of that mystery is that while some PAHs that have been identified in space are building blocks of life, on Earth many PAHs are carcinogenic, formed in hot environments, both from natural sources (volcanoes, wildfires) and human activities (burning fossil fuels, cooking). In TMC-1, where pyrene has been found, the temperature is about 10 Kelvin (-263 degrees Celsius).
The cold, harsh conditions of space mean that molecules can exist there that would not survive on Earth because they would be too unstable to survive exposure to our atmosphere. Small molecules such as carbon monoxide or methane form fairly easily because only one or two collisions are needed, whereas several reactions are needed to form the larger molecule pyrene from atoms and smaller molecules. In TMC-1 these collisions are infrequent because of the large space between atoms and molecules.
“On Earth those molecules would be removed quickly by collisions with other molecules in our atmosphere,” says Dr. Thomas Speak, a post-doctoral researcher in Dr. Cooke’s astrochemistry lab. “Although TMC-1 is defined as a dense molecular cloud, its ‘density’ is relative — approximately one thousandth of a trillionth of Earth’s atmosphere at sea level. The ratio of the density of gas in TMC-1 and Earth’s atmosphere is equivalent to comparing one drop of water and 25,000 Olympic sized swimming pools.”
GOTHAM
Dr. Cooke is co-principal investigator of GOTHAM, a collaboration of astronomers and astrochemists who study the Universe using radioastronomy to listen for the radio signals from stars, planets and molecular clouds. GOTHAM is an acronym for Green Bank Telescope Observations of TMC-1: Hunting Aromatic Molecules. The telescope is so sensitive that its site in West Virginia sits in a federally mandated 34,000-square kilometre quiet zone where cell phones, wifi routers and iPads are banned so their signals don’t interfere with the whispers of the Universe.
Radioastronomy can detect individual molecules millions of light years away because a molecule can give off energy as it rotates around its centre of mass. That rotational energy is released as a radio signal when the molecule changes the way it rotates.
“It’s like a fingerprint. The exact frequency is specific to the molecule, so we can tell different molecules apart.”
“These rotational emissions are really low energy,” says Dr. Speak. “If you take all of the radio observations that have ever been carried out on Earth, and you summed up the energy of all of the photons in the radio waves that are being collected, it would be less energy than one snowflake hitting the ground.”
So far, each of the more than 300 molecules detected in space by radioastronomy has a unique electromagnetic signal depending on its mass and the bonds within the molecule.
“It’s like a fingerprint,” Dr. Cooke says. “We’re looking at rotations at a specific wavelength range. The exact frequency is specific to the molecule, so we can tell different molecules apart.”
GOTHAM began searching for PAHs in TMC-1 after the Japan Aerospace Exploration Agency collected samples from the near-Earth asteroid Ryugu that contained large amounts of pyrene.
“The pyrene in Ryugu was shown to be a sample of what’s made in the cold regions of space, which supports the notion that solar systems inherit carbon molecules that form long before their birth,” Dr. Cooke says.
Pyrene molecules, however, can’t be detected by a radio telescope.
“The four fused aromatic rings of pyrene molecules are symmetric so there is no overall change as it spins around its axis, meaning it doesn’t emit radio frequencies that we can detect,” Dr. Speak explains.
So instead, GOTHAM researchers listened for cyanopyrene — pyrene with a carbon and nitrogen atom attached, substituting one of its hydrogen atoms.
Carbon and nitrogen have different abilities to grab hold of the electrons being shared in that bond, with nitrogen atoms able to pull slightly more towards them. Adding the nitrile group (CN) to the parent pyrene molecule allows the cyanopyrene molecule to emit radio waves when it rotates.”
Creating space in a lab
To determine whether small PAHs can survive the journey across the Universe from a circumstellar envelope to molecular clouds, the UBC astrochemists are investigating whether PAHs form from the bottom up or the top down.
“Do the molecules start as small hydrocarbons that react with each other to form larger structures?” Dr. Cooke says. “The other hypothesis is top down — are large species of PAHs, or even fullerenes, blown apart, leaving just fragments?”
To try and answer that question, the lab is building instruments to investigate reactions that could occur in TMC-1, and explain or predict the presence of molecules such as pyrene. In one of these instruments, a flow cell apparatus, gas is flowed into a reactor to study the reaction of interest. Then a laser shines ultraviolet radiation into the chamber to start a chemical reaction. The researchers will use a mass spectrometer to probe the reactions’ reliance on physical conditions like temperature and pressure to understand if they possibly take place in the interstellar medium. The reactions will occur in a matter of microseconds or milliseconds, depending on the complexity of the molecules and the temperature, although the experiments will monitor these processes on the seconds or minutes timescale.
“The main things we’re interested in learning from the experiment are, does a reaction happen, if it happens how fast does it happen and at what temperature, what does the reaction make and in what proportions,” says Dr. Speak.
The findings will be entered into an astrochemical computer model containing thousands of reactions. Starting with very simple atoms, the reactions will be run through simulations to learn how the molecules evolve over time.
By understanding how the reactions occur over a range of temperatures and pressures, Dr. Cooke’s team will come one step closer to solving the mystery of PAH formation in the interstellar medium.
With files from Alex Walls, UBC Media Relations
