If you had to choose, which planetary system would you gauge most likely to house a life-bearing planet: Proxima Centauri or TRAPPIST-1? The question is a bit loaded given that there are seven TRAPPIST-1 planets, hence a much higher chance for success there than in a system that (so far) has produced evidence for only two worlds. But there are other factors having to do with the delivery of prebiotic materials by comet, which is the subject of a new paper from Richard Anslow (Cambridge Institute of Astronomy). “It’s possible that the molecules that led to life on Earth came from comets,’’ Anslow reminds us, “so the same could be true for planets elsewhere in the galaxy.”
So let’s untangle this a bit. We don’t know whether comets are vital to the origin of life on Earth or any other world, and Anslow (working with Cambridge colleagues Amy Bonsor and Paul B. Rimmer) does not argue that they are. What their paper does is to examine the environments most likely to be affected by cometary delivery of organics, which in turn could be useful as we begin to study exo-atmospheres for biosignatures. If we can narrow the kind of systems where cometary delivery is likely, that could explain future findings of life signs as opposed to systems without such mechanisms, ultimately supporting the comet delivery model.
Image: Comets contain elements such as water, ammonia, methanol and carbon dioxide that could have supplied the raw materials that upon impact on early Earth would have yielded an abundant supply of energy to produce amino acids and jump start life. Credit: Lawrence Livermore National Laboratories.
The idea of delivering life-promoting materials by impacts is hardly new. We’ve learned from asteroid return samples like those from Ryugu and now Bennu that the inventory of prebiotic molecules is rich. We’ve also found intact amino acids in meteorite samples, showing the survival of these materials from their entry into the atmosphere. The authors point out that comets contain what they call ‘prebiotic feedstock molecules’ like hydrogen cyanide (HCN) along with basic amino acids, and some studies suggest that by way of comparison with asteroids, comets have delivered two orders of magnitude more organic materials than meteorites because of their high carbon content. But survival is the key, and that involves impact velocity upon arrival.
HCN is particularly useful to consider because its strong carbon-nitrogen bonds may make it more likely to survive the high temperatures of atmospheric entry. What Anslow and team call the ‘warm comet pond’ scenario demands a relatively soft landing. This is interesting, so let me quote the paper on it. A ‘soft landing’:
…excavates the impact point and forms a dirty pond from the cometary components. Climatic variations are thought to cause the episodic drying of these ponds, promoting the rapid polymerisation of constituent prebiotic molecules. It is thought this wet-dry cycling will effectively drive the required biogeochemical reactions crucial for RNA production on the early-Earth, and therefore play an important role in the initial emergence of life… Relatively high concentrations of prebiotic molecules are required for there to be sufficient polymerisation, and so this scenario still requires low-velocity impacts. Specific prebiotic molecules are more (or less) susceptible to thermal decomposition by virtue of their molecular structure, and so the inventory of molecules that can be effectively delivered to a planet is very sensitive to impact velocity.
The question that emerges, once we’ve examined the delivery of molecules like HCN, is what kind of stellar system is most likely to benefit from a cometary delivery mechanism? To address this, the authors construct an idealized planetary system with planets of equal mass that are equally spaced to study the minimum impact velocity that can emerge on the innermost habitable planet. They then use N-body simulations to model the necessary interactions between comets and planets in terms of their position and velocity through time. The snowline marks the boundary between rocky and volatile-rich materials in the disk, as below:
Image: This is Figure 1 from the paper. Caption: Schematic diagram of the idealised planetary system considered in this work with equally spaced planets (brown circles, semi-major axis ai ) scattering comets (small dark blue circles) from the snow-line. The blue region represents the volatile-rich region of the disc where comets occur, and the green region represents the habitable zone. Low velocity cometary impacts onto habitable planets will follow the lower arrows, which sketch the dynamically cold scattering between adjacent planets. The dynamically hot scattering as shown by the upper arrows, will result in high velocity impacts.
That equal spacing of planets is interesting. The authors call it a ‘peas in a pod’ system and note what other astronomers have observed, that “…individual exoplanet systems have much smaller dispersion in mass, radius, and orbital period in comparison to the system-to-system variation of the exoplanet population as a whole.” And indeed, tightly packed systems with equal and low-mass planets have been shown to be highly efficient at scattering comets into the inner system and hence into the habitable zone. Giant outer planets, it’s worth noting, may form in these tight systems, their effects helping to scatter comets inward, but they are not assumed in the author’s model.
The impactor’s size and velocity tell the tale. What we’d like to see is a minimum impact velocity below 15 kilometers per second to ensure the survival of the interesting prebiotic molecules like HCN. The simulations show that the impact velocity around stars like the Sun is reduced for lower mass planets, the effect being enhanced if there are planets in nearby orbits. Impact velocities drop even more for planets around low mass stars in tightly-packed systems (here again, think TRAPPIST-1), for here the comets tend to be delivered on low eccentricity orbits, a significant factor because impact speeds around low-mass stars are typically high. From the paper:
…the results of our N-body simulations demonstrate that the overall velocity distribution of impactors onto habitable planets is very sensitive to both the stellar-mass and planetary architecture, with the fraction of low-velocity impacts increasing significantly for planets around Solar-mass stars, and in tightly-packed systems. It will be these populations of exoplanets where cometary delivery of prebiotic molecules is most likely to be successful, with significant implications for the resulting prebiotic inventories due to the exponential decrease in survivability with impact velocity.
We learn from all this that we have to be attuned to the mass of the host star and the nature of planetary distribution there to be able to predict whether or not comets can effectively deliver prebiotic materials to worlds in the habitable zone. If this seems purely theoretical, consider that telescope time on future missions to study exoplanet atmospheres will be a precious commodity, and these factors may emerge as an important filter for observation. But we’ll also find out whether the correlations the authors have uncovered re lower mass, tightly packed planets are demonstrated in the presence of the biosignatures we are looking for. That may tell us whether comets are a significant factor for life’s emergence on distant worlds as well as our own.
The paper is Anslow, Bonsor & Rimmer. “Can comets deliver prebiotic molecules to rocky exoplanets?” Proceedings of the Royal Society A (2023). Full text. Thanks to my friend Antonio Tavani for the pointer to this work.
There are a lot of assumptions here.
1. That comets could be important to deliver carbon compounds when rocky worlds have abundant carbon.
2. That abiogenesis starts with RNA-World, and this is achieved in Darwin’s warm pond – but with wet and dry cycles,
Despite the disclaimer about the role of comets, there is this quote:
The delivery of organics, including the amino acids, is a variant on the Liller-Urey experiment, just moving the creation of the materials from a planet’s surface to astral bodies.
The RNA-World model is attractive in some ways, but chemists have so far failed to be able to devise a mechanism to synthesize the bases and sugars and then polymerize them. Conversely, experiments supporting the metabolism first model that originated in the rock pores in deep ocean hot vents are far more encouraging. [This doesn’t rule out the need to create RNA, and later DNA, to reach a full genesis, but the different locations for the 2 cases seem difficult to reconcile, IMO.
Given how easily many of these pre-biotic molecules can be created, it raises the question in my mind whether there is any need to build up an inventory of these compounds. As for a warm-pond scenario with periodic drying cycles, Earth has many such places, and therefore is there any need for a model where serendipitous comet impacts (almost soft landings) to deliver the material and conditions? Might the most important molecule comets deliver be water, rather than organics?
A possible counterfactual:-
Suppose we find life in the subsurface oceans of icy moons, possibly in the hot vents that emerge from the carbonaceous cores. Wouldn’t that suggest comets are potentially irrelevant? Suppose we eventually find a similar mechanism in a water world, even a hycean one. Wouldn’t that indicate the comet mechanism unnecessary?
Or suppose the role of comets is not to spread prebiotic molecules, but life itself, by intercepting microbial spores and then delivering them via planetary encounters via outgassing, à la Hoyle and Wickramasinghe’s hypothesis?
Lastly, testing the comet model might be difficult as any relationship might be very ambiguous when multiple mechanisms of abiogenesis may be in operation, even if it is true that comets do play a role in delivering water, organics, and even abiogenesis conditions, but competing with other mechanisms.
There are other endogenous sources and processes to build these molecules
True, if one ignores anaerobic environments like deep ocean vents, which are atmosphere-independent other than to restrict the pO2 in the surrounding ocean.
Climate-driven wet-dry cycling may be far too slow unless it is driving some weather type. Daily tidal cycling is probably a faster mechanism if this mechanism is viable and avoids the UV breakdown of complex organic molecules.
Whether HCN is important or not, it is created on Earth. While it may be more resistant to heat, that is not a reason to invoke comet sources.
The rest of the paper is about dynamics and could be applied to any material delivered by comets, such as water. We are already using isotopic analysis to try to understand the source of Earth’s water, and no doubt will be doing the same to the subsurface Martian glaciers. I get the impression that the hypothesis that comets could be a major source of organic materials for abiogenesis is just a way to raise the impact score of this paper, given the renewed emphasis on the search for life in the universe.
Life may not just have started on planets but minor planetary bodies as well. Ceres for instance could happily host life and allow it to be thrown around the solar system from impacts and probably more successfully due to its low gravity.
Just shows how habitable for microbes Ceres or minor planets can be.
https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018GL081473
I think you are pushing the term “habitable for microbes” for Ceres. The cited reference is simply an attempt to model what they say is evidence for subsurface brines. It says nothing about the brines themselves, other than they are liquid – i.e. could be slush.
You may recall that we use salt curing and preservation specifically to prevent microbial growth in preserved foods. There are halophile bacteria, but there are other conditions that are needed to allow their growth and reproduction. One factor is temperature. (We also use ice boxes to stop bacterial growth (as well as keep icecream frozen. )
The paper states that temperatures need to be above 250K to maintain a liquid brine. For every 1K below 273K, metabolic activity declines and the interior of any cell might well freeze unless it is equally salty. Unless bacteria have evolved a way to grow and reproduce in sub-zero brines on Ceres, I would suggest that “habitability for microbes” is not a characterization I would use. [Note on Europa, and Enceladus, the subsurface oceans are not very salty, and there is heat coming from the rocky core as well as that from the effect of Jupiter’s gravity on Europa. In contrast, Ceres is a small worldlet, with very limited forms of internal heating the article authors require very salty brines to maintain any non-solid mantle beneath the ice.
For a over a billion years the temperature and pressure in the mud layer was quite amicable for life. Plenty of time to cook those carbon and nitrogen compounds into basic life building blocks.
Aren’t you saying Ceres is still habitable, not that it may once have been habitable? Mars was once habitable, as was likely Venus. But that doesn’t mean that either is today, although Mars may be in its crust, and the MIT Venus team are hoping to test whether there is extant life in the clouds today.
But let’s speculate that Ceres was once habitable, and moreover life did emerge in the distant past, but that it is now extinct. What might we hope to find in the frozen ocean? Fossil carbon compounds that might indicate life was once extant? Fossil microbes or remains encased in ice?
Your thoughts on this would be appreciated.
Alex, the temperatures in the depths are much higher and quite within the range of life as we know it. The pressures throughout ceres are no higher than ten kilometers on our planet with plenty of salts to reduce the melting point. If life got started on ceres it still there and we should look for it especially around salt vents and it would be much easier to probe than say europa.
https://www.aanda.org/articles/aa/full_html/2020/01/aa36607-19/aa36607-19.html
Michael, I concede that the temperatures may still be high enough for life at depth. Modelling the internal structure of Ceres: Coupling of accretion
with compaction by creep and implications for the water-rock
differentiation.
As for life being extant, or even was ever present, this article is far more circumspect Ceres: Astrobiological Target and Possible Ocean World, suggesting that biosignatures of long-lasting biomolecules like lipids might be detectable in the ice. This may be assuming access only to surface ice rather than the still liquid brine ocean many kilometers below the surface.
What I think is more certain is that the energy flow in this worldlet is very low, metabolism is anaerobic and energy inefficient, and therefore any life is going to be sparse.
Interesting sample return mission for Ceres.
https://iopscience.iop.org/article/10.3847/PSJ/ac34ee
You brought up a good point, sixty-six percent of the earth is deep ocean. The majority of habitable planets will likely have at least that or much more. Could water worlds be a gentle place for comets to be absorbed?
What would it take for a comet to produce organic oils from a soft impact? What temperatures would produce the most oils from the mix of comet material? Oil separates from water and that should be the best place for long term mixtures to form. Super earth water worlds could have an ocean wide soup of organic oils floating on its ocean surface.
Even rouge super earths with volcanic activity could cook up a very healthy stew…