Did comets deliver the essential elements for life on Earth? It’s looking more & more like they might have. At least one comet may have, anyway: 67P/Churyumov-Gerasimenko.
A new study using data from ESA’s Rosetta mission shows that the comet contains the life critical element, phosphorous.
Researchers from University of Turku in Finland led this research. The lead author of the study, Esko Gardner, an astrophysicist & software engineer.
The title of the work is “The detection of solid phosphorus & fluorine in the dust from the coma of comet, 67P/Churyumov-Gerasimenko,” and it’s published in Monthly Notices of Royal Astronomical Society.
The raw elements for life are referred to as CHNOPS, which stands for Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorous & Sulphur. Combinations of these six chemical elements make up a great majority of biological molecules on Earth. Together, they account for nearly 98% of Earth’s living matter.
Scientists had previously found the other 5 in comets, so finding phosphorous could be the ultimate or final piece of this puzzle.
The team of researchers found the phosphorous & fluorine in the inner coma of comet, 67P/Churyumov-Gerasimenko. They were present in solid particles collected only a couple of kilometers from the comet by Rosetta’s COmetary Secondary Ion Mass Analyser (COSIMA) instrument.
The particles were collected on the instrument’s target plates which were then photographed remotely. The individual particles were selected in the images, then measured with a spectrometer.
This all was done from Earth before Rosetta ended its mission with a controlled crash into Comet 67P at the end of September 2016.
Phosphorous found in comets before. In a 1987 paper, researchers announced the detection of phosphorous in dust from the Halley’s comet. But it had been likely atomic phosphorous contained in an uncertain mineral & chemically unavailable.
Then in 2006, NASA’s Stardust spacecraft returned the samples from the comet 81P/Wild. Researchers found a single particle containing phosphorous which was associated to calcium.
Scientists concluded that phosphorous was “most likely contained within an apatite particle.” It had been also likely unavailable.
This new finding is different: this time, researchers found phosphorous ions in solid particles in either minerals or metallic phosphorous.
“We have shown that apatite minerals aren’t the source of phosphorus which means that the discovered phosphorus occurs in some more reduced & possibly more soluble form,” said the project leader Harry Lehto from the Department of Physics & Astronomy at University of Turku.
The press release accompanying the research states that “this is the first time that life necessary CHNOPS elements are found in solid cometary matter.”
The new finding is vital for the understanding of how life came on Earth. Phosphorous is critical to life on Earth but if it’s locked into an apatite mineral, it’s largely out of reach. If it’s in its gaseous form, it is also unsuitable.
But newly discovered phosphorous is more available. As the authors write in paper, “In the process of forming life, water-soluble reactive phosphorus compounds were required to convert nucleotide precursors by phosphorylation to the active nucleotides.”
Phosphorous is one among the missing pieces in the puzzle of life on Earth. There was a lack of molecules containing soluble-phosphorous on early Earth. Experiments have shown that soluble-phosphorous could serve a critical role in the origin of biological molecules.
“It has been experimentally shown that soluble P, HCN & H2S can function as a suitable feed stock for the pre-biotic synthesis of nucleotides, amino acids & phosphoglycerine backbones,” the authors write.
But the most efficient way of manufacturing these biological molecules involves highly reduced-forms of phosphorous. These forms of phosphorous occur mostly in meteoritic materials or possibly in elemental phosphorous.
Until now, meteorites or possibly geochemical processes were known to be liable for the pre-biotic phosphoric materials necessary for life.
But this study shows that along with CHNO & S, P is now known to be present in comets, and those comets could have delivered P to early Earth.
But there’s still a drag with the comet delivery idea. If the impact is just too energetic, materials are often destroyed or altered. The team behind this new research thinks they’ll have a solution to that.
“It is conceivable that early cometary impacts onto the planet-surface are less energetic as compared to the impacts of the heavy stony meteorites, thus preserving the pre-biotic molecules in a more intact condition.”
The researchers are appropriately cautious about their results yet. The results show that the elements for life can come from comets but those compounds must be soluble & available. They cannot be locked into minerals.
“The solubility of the detected cometary phosphorus from 67P/C-G isn’t clear, but we will conclude that it can’t be Apatite, which is a common mineral source of phosphorus in meteorites. Additionally, other phosphate minerals are unlikely because we couldn’t find a clear cometary contribution of PO2 & PO3?”
The authors think a comet sample return mission is important to advance the comet CHNOPS delivery idea.
“The presence of all the CHNOPS elements gives a strong premise for a future cometary sample return mission to a comet. This might confirm the presence of all compounds & their possible mineral sources & the possible solubility of the matter. This is able to allow a comprehensive analysis of the relative amounts of these CHNOPS elements.”
NASA’s Stardust mission captured & returned comet dust materials from the coma of Comet 81P/Wild 2. And scientists learned a lot from these samples. But those samples, though important, suffered from some limitations.
In the report “The Comet Coma Rendezvous Sample Return (CCRSR) Mission Concept – The Next Step Beyond Stardust” the authors acknowledged that “these samples have important limitations, however, because they were collected in modest numbers at harsh hyper-velocities & represent a one-time random sampling of the coma (a ‘grab’ sample).”
How could that be improved? In the same report, the authors described what an improved comet sample return mission would look like.
“This mission utilizes a spacecraft designed to rendezvous with a comet, make extended observations in the cometary coma (but not land on the comet), gently collect multiple coma samples representing different source sites & return them to Earth for study.”
The benefits? “First, the samples will be captured at far lower velocities, eliminating sample destruction & alteration during collection, and leading to the return of much more pristine material, especially organics & fragile minerals.”
This improved mission would also collect samples from the coma & from jets of off-gassing volatiles. It might also collect tons more material making the sample more relevant statistically.
For now, that updated mission is simply a concept. And there are not any shortages of worthy mission concepts out there. It’s just a matter of selecting the most-worthy ones.
