Were the rocky worlds of the inner Solar System depleted in carbon as they formed, the so-called ‘carbon deficit problem’? There is evidence for a system-wide carbon gradient in that era, which makes for interesting interactions between our Sun’s habitable zone and the far reaches of the system, for as the planets gradually cooled, the carbon so necessary for life as we know it would have been available only far from the Sun.
How much of a factor were early comets in bringing carbon into the inner system? This question underlies new work by Charles Woodward and colleagues. Woodward (University of Minnesota Twin Cities / Minnesota Institute of Astrophysics) focuses on Comet Catalina, which was discovered in early 2016. He sees carbon in the context of life:
“Carbon is key to learning about the origins of life. We’re still not sure if Earth could have trapped enough carbon on its own during its formation, so carbon-rich comets could have been an important source delivering this essential element that led to life as we know it.”
Image: Illustration of a comet from the Oort Cloud as it passes through the inner Solar System with dust and gas evaporating into its tail. SOFIA’s observations of Comet Catalina reveal that it is carbon-rich, suggesting that comets delivered carbon to the terrestrial planets like Earth and Mars as they formed in the early system. Credit: NASA/SOFIA/ Lynette Cook.
Let’s zoom in on this a little more closely. Volatile ices of water, carbon monoxide and carbon dioxide are found mixing with dust grains in the outer system, an indication that the young Solar System beyond the snowline was, in the authors’ words, “not entirely ‘primordial’ but was ‘polluted’ with the processed materials from the inner disk, the ‘hot nebular product.'” Or to slip the metaphor slightly, we can say that comets were salted with materials that were originally produced at higher temperatures. Comets can offer a window into this process.
The work is anything but straightforward, for although we’ve learned a lot through missions like Giotto, Rosetta/Philae and Deep Impact (including, of course, abundant telescope observations from Earth and a sample return mission called Stardust), the interplanetary dust particles we’ve been able to analyze from comets 81P/Wild 2 and 26P/Grigg-Skjellerup differ considerably. The paper explains:
The former contains material processed at high temperature (Zolensky et al. 2006), while the latter is very “primitive” (Busemann et al. 2009). For these reasons, it is necessary to determine as best as we can the properties of dust grains from a large sample of comets using remote techniques (Cochran et al. 2015). These include observations of both the thermal (spectrophotometric) and scattered light (spectrophotometric and polarimetric). The former technique provides our most direct link to the composition (mineral content) of the grains.
The research team drew on data from the Stratospheric Observatory for Infrared Astronomy (SOFIA), a Boeing 747 aircraft carrying a 2.7-meter reflecting telescope with an effective diameter of 2.5 meters. At altitude (SOFIA generally operates between 38,000 and 45,000 feet), the observatory is above 99 percent of Earth’s atmosphere, which can block infrared wavelengths. SOFIA data show Catalina as a carbon-rich object.
The paper points out that carbon dominates as well in other comets we’ve seen, both those in closer orbits (103P/Hartley 2) and Oort Cloud comets like C/2007 N3 and C/2001 HT50. It also turns out that dusty material from comet 67P/Churyumov–Gerasimenko was rich in carbon, although the authors note that comets can show changes in their silicate-to-carbon ratio, sometimes even during the course of a single night’s observations. The paper adds:
A dark refractory carbonaceous material darkens and reddens the surface of the nucleus of 67P/Churyumov–Gerasimenko. Comet C/2013 US10 (Catalina) is carbon rich. Analysis of comet C/2013 US10 (Catalina)’s grain composition and observed infrared spectral features compared to interplanetary dust particles, chondritic materials, and Stardust samples suggest that the dark carbonaceous material is well represented by the optical properties of amorphous carbon. We argue that this dark material is endemic to comets.
All this suggests that carbon delivered by comets is a part of the evolution of the early Solar System. Each carbon-rich comet we study has implications for how life may have been spurred by impacts, making the investigation of carbon-rich Oort Cloud comets a continuing priority for SOFIA, which can be deployed quickly when comets are found entering the inner system.
The paper is Woodward et al, “The Coma Dust of Comet C/2013 US10 (Catalina): A Window into Carbon in the Solar System,” The Planetary Science Journal (2021). Abstract / Full Text.
If comets were needed to add carbon, then that sounds like a bad sign for possible under-ice life on Enceladus and Europa unless there was cycling downward through the ice.
Oddly enough, there’s another recent paper on this. [googles] Yeah, just a couple of weeks ago.
TLDR: we haven’t been looking at chemical reactions in the primordial nebula enough — like, carbon combining with oxygen to make a volatile molecule that behaves very differently from atomic carbon.
Thank you for the heads up, I had missed that one. =)
More generally: comets as a delivery mechanism for Earth’s volatiles never made a lot of sense, and since the Rosetta mission they’ve been looking less and less likely.
Space science grabbed at comets because the models for Earth’s accretion suggested that young Earth would have boiled off most of its volatiles early on, leaving a dry and airless rock. But it was always a theory born of desperation rather than one that looked plausible. Now there’s a modest pile of evidence against it — AFAWCT, Earth’s volatiles seem to be primordial, and not delivered by comets or anything else.
So the trend is to look back at the models and look again at what might be missing. One possibility under discussion: early Earth may not have been well mixed. Planetesimals may have formed “lumps” in the accreting mantle that served as reservoirs of volatiles, only being released at the surface later after the planet had cooled somewhat.
We still don’t know. But “comets / asteroids may have delivered Earth’s carbon / volatile / whatever” is looking well past its sell-by date.
Isn’t this using some weasel words to suggest that comets are important for the origin of life on Earth? From ancient use of comets on portents of events, to whether interesting carbon compounds like amino acids, to Hoyle’s suggestion of viruses on comets, it seems that comets remain a focus of speculation about their relation to life on Earth.
Yes, I can quite understand that cometary impacts may have delivered extra volatiles to Earth, but does this have any relevance to abiogenesis? If they delivered volatiles to Earth, shouldn’t we also expect to find cometary material in lunar cold traps too? Would isotopic analysis clarify the relative contributions of accreted vs cometary fractions?
Abiogenesis seems to have occurred fairly quickly on earth after the surface cooled sufficiently. Was much of the early bombardment comets delivering needed carbon (and water), or was life established regardless of that delivery? Most of Earth’s current carbon is of carbonate rock mostly due to weathering of the volcanic emissions that created a dense atmosphere of CO2, H2O, and NH3. The biomass carbon fraction is quite small in comparison. Did life need additional carbon, that seems doubtful to me. In other posts, there have been suggestions that water worlds might be very frequent, and even worlds where the C:Si ratio is much higher. Does either of these scenarios suggest that these planets will be more frequently represented as living? Maybe biosignature surveys in the future could answer that question.
It is nice to know how available carbon is in comets. The carbon is potentially very useful for CNO bicycle fusion powered interstellar ramjets as well as fusion fuel pellet runway propulsion schemes.
Another good use of carbon is for manufacturing carbonaceous supermaterials and super-refractories. These will come in handy for relativistic spacecraft shielding, albeit, very thick shields.
Comets, TNO’s and Jupiters troyans might indeed become the prime estate both for off Earth colonies as it’s base for food production as well as fusion. Yet for shielding, ordinary water work perfectly fine, and is available everywhere so whatever have eroded off could be replenished for a possible return trip or second target of exploration.
In purely practical and immediate terms, the lack of carbon on the Moon is concerning when one’s keen to synthesise methane there.
There is plenty of organics on the moon most if not all at the moons poles.
Do you have a reference for that assertion? AFAICS, there is very limited evidence for organics trapped at the lunar poles, and certainly in very small quantities. Even water is not abundant at the poles. The rest of the moon is likely devoid of organics in the regolith.
Speaking of dust where it doesn’t belong … I mean didn’t belong: https://phys.org/news/2021-03-serendipitous-juno-shatter-ideas-zodiacal.html We always thought Martian dust storms were big, but we had no idea!
With data like this, the long-distance propagation of life starts to seem a less unlikely.
Are comets an even bigger threat to life on Earth than previously thought? Especially the long-period ones…