|
For video click image A probe's close encounter with comet 67P - a relic from the birth of the solar system -
has revealed an abundance of organic compounds. they see life's raw materials...
The probe would follow its quarry, a comet called 67P/Churyumov-Gerasimenko, for two years as onboard instruments caught and analyzed the dust and gas streaming away from the comet.
Scientists sought hints about how our solar
system came to be - and about the origin of one class of molecules
in particular.
The Rosetta mission and others have shown just how ubiquitous organic molecules are in space, too.
When Hänni and her colleagues processed just one day's worth of the probe's data in 2022, they uncovered 44 different organic molecules.
Some were very complex, containing 20 atoms or more. Rosetta caught whiffs of glycine, one of the amino acid building blocks of proteins.
And more recently, Hänni used Rosetta data to identify dimethyl sulfide - a gas that, on Earth, is only known to be produced by living organisms.
What Rosetta did for comets, Japan's Hayabusa2 and NASA's Osiris-Rex are doing for asteroids.
Ryugu alone contains at least 20,000 kinds, including 15 different amino acids.
scooped material from the asteroid Bennu, the capsule containing the sample reentered Earth's atmosphere and landed in Utah in September 2023. The sample was then taken to Johnson Space Center in Houston
for
analysis.
How simple chemistry led to complex living organisms stands among the great unsolved mysteries of science.
The recent studies of asteroid and comet material add to the evidence that the first steps of the assembly process happen in space - and happen very readily.
Everywhere we look, space seems to teem with biology's raw materials.
Scientists have long wondered where these molecules come from.
For video click image Mark Belan/Quanta Magazine
By sending probes to sample primordial comets and asteroids, peering into planet-forming disks with telescopes, and re-creating spacelike conditions in labs and computer models, scientists are uncovering the origins of complex organic molecules.
Their findings indicate that planets like ours likely inherit much of their organic material from a time before the sun.
Fire and Ice
Last year, researchers glimpsed the earliest known occurrence of organic chemistry in the universe.
The James Webb Space Telescope observed a young galaxy, seeing it as it appeared just 1.5 billion years after the Big Bang, and detected polycyclic aromatic hydrocarbons - hefty molecules that look a bit like honeycombs.
These and other organic molecules likely formed in the twilight years of the earliest stars, perhaps as early as a few hundred million years after the Big Bang.
Dying stars exhale hot stellar winds:
In the hot, gas-rich environments around dying stars, she explained,
A total of 121.6 grams of sooty material from the asteroid Bennu made its way back to Earth in Osiris-Rex's science canister (left)
and the inside of the
canister's lid (right).
There's a second major formation pathway.
As generations of stars have lived, died and spilled their innards into space, some of their carbon has ended up in molecular clouds - patches of space where motes of gas and dust crowd close enough together to block out light.
Here, organic molecules form within the icy crusts of tiny dust grains.
Alice Booth, an astronomer at Harvard University, has found evidence that protoplanetary disks inherit organic molecules from the clouds they come from, and that these molecules grow more complex
as the disk develops.
Because icy grains are so cold, atoms drifting by can stick to them.
Scientists experimentally confirmed in 2020 that methane, among the simplest organic molecules, forms like this, as carbon and hydrogen land on the icy grains, one after another.
Methanol is thought to form in a similar way.
And a 2022 study showed that it's even possible to build up chains of the simplest amino acid, glycine, as carbon monoxide, carbon and ammonia condense on the granular surfaces.
Radiation powers these chemical reactions.
Cosmic ray particles and ultraviolet light,
With methanol as a starting ingredient, this kind of radiation-driven chemistry can produce an enormous diversity of molecules.
To date, scientists have detected more than 200 kinds of organic molecules in interstellar space.
The upshot, according to Booth, is that even billions of years before the sun was born,
Dynamic Disks
A key question is whether these molecules can survive the birth of a solar system.
New stars and planets form via the gravitational collapse of gas and dust clouds.
Only recently did scientists get their first glimpses of organic molecules within protoplanetary disks - the rotating frisbees of gas and dust that spin around newborn stars.
In one of these observations, Booth and her colleagues found abundant methanol within a nearby planet-forming disk.
This methanol could only have formed on grains of carbon monoxide-rich ice, which would have filled the cold molecular cloud from which the protoplanetary disk came but would then have vaporized in the warm disk.
So, Booth said,
Since its 66 highly sensitive radio telescopes became operational in 2013, the Atacama Large Millimeter Array (ALMA) in Chile has picked up the glow of organic molecules
infusing
protoplanetary disks.
But the chemical assembly process probably doesn't end in the cloud.
According to Booth,
As material moves around in the disk, it experiences dramatically varying conditions.
The disk's surface is exposed to heat and radiation, while its midplane is shielded and cooler. Just as wet-dry cycles on Earth might have helped drive organic complexity at the origin of life, the flow of dust and gas through different parts of disks might drive new kinds of organic complexity.
Researchers have wanted to computationally model the churn and tumble of disk material, but it's so computationally costly to do so that,
That's changing now.
In 2024 a team of scientists including Booth published initial results of computer models showing that complex organics can form rapidly in protoplanetary disks.
In particular, the molecules assemble in the same "dust traps" where planetesimals, the asteroid-size building blocks of planets, coalesce.
In our own solar system, comets are among the most primordial material - leftovers from the protoplanetary disk.
The inventory of small organic molecules that Rosetta detected so far in comet 67P mostly lines up with what scientists would expect of material inherited from molecular clouds in interstellar space.
But some organic molecules from 67P are more complex than scientists expected, and it's still an open question where that complexity came from.
Nora Hänni, a chemist at the University of Bern, has been identifying and analyzing the complex organic molecules detected in comet 67P by the European
Space Agency's
Rosetta probe.
Asteroids are less pristine than comets, having often endured heating and the effects of liquid water.
But these effects can produce dramatic new organic complexity.
For decades, scientists have known that meteorites called chondrites, which originate from asteroids, contain a staggering diversity of organic molecules.
Life uses just 20 or so.
Osiris-Rex and Hayabusa2 have confirmed that the asteroids Bennu and Ryugu are as complex as those meteorites.
And at least some of this complexity seems to have arisen before the asteroids themselves:
The Chemistry of Life?
Organic molecules on the early Earth took a new, remarkable step up in complexity.
They 'somehow' organized themselves into something alive. Some hypotheses for the origins of life on Earth involve a starter kit of organic material from space.
The "PAH world hypothesis", for instance, posits a stage of the primordial soup that was dominated by polycyclic aromatic hydrocarbons.
In general, understanding how complex organics form in space and end up on planets might give us a better idea of whether life has arisen on other worlds, too.
If the raw materials of life on Earth formed in the interstellar medium,
For now, such ideas remain largely untestable.
But because life itself represents a new level of organic complexity, astrobiologists are hunting for complex organics as a possible biosignature, or sign of life, on other worlds in our solar system.
The European Space Agency's Juice mission is already on its way to study Jupiter and three of its icy moons, and NASA's Europa Clipper mission launched toward one of those moons, Europa, in October.
Both will use onboard instruments to search the atmospheres for organic molecules, as will the future Dragonfly mission to Saturn's moon, Titan.
Yet it's tricky to determine whether a given organic molecule is a biosignature or not.
If scientists were to find sufficiently complex organic molecular assemblages, that would be enough to convince at least some researchers that we've found life on another world.
But as comets and asteroids reveal, the nonliving world is complex in its own right.
Compounds thought to be biosignatures have been found on lifeless rocks, like the dimethyl sulfide Hänni's team recently identified on 67P.
Hänni wants to use the abiotic complexity of comets to help rule out false-positive biosignatures and inform future searches for life.
Life aside, these studies about the organic chemistry of space also open a window onto processes we might never observe directly.
Glein and his colleagues have used James Webb Space Telescope observations of methane in the outer solar system as evidence that some icy Kuiper Belt objects could have warm, wet interiors.
The organic molecules on Europa could reveal the chemistry of its subsurface ocean.
And Schmitt-Kopplin has used the organic assemblages of meteorites to study impact shocks and geochemical processes at the birth of the solar system.
Life is just one act of the enormous cosmic drama recorded in organic molecules.
|
