Did Comets Carry Prebiotic Materials to Early Earth?

“Don’t eat the snow!” As a little kid growing up in the central part of West Virginia, I heard that warning time and time again. Even though our surroundings were rural and rustic, it was ill-advised to scoop up a handful of freshly fallen snow to eat. At the time, chemical plants lined the banks of the Kanawha River, and the pollution from those plants contaminated the snowfall, making it unhealthy to consume.

Even though chemical impurities in snow and ice can be detrimental to existing life on Earth, astrobiologists think that chemical contaminants in icy comets played an important role in the evolution of the first life-forms. Support for this idea, they say, comes from recent studies carried out by investigators from France.1

Creating an Artificial Comet in the Lab

Frozen water makes up the bulk of comets, but these solar system objects also contain a substantial level of compounds such as methanol (around 30 percent) and ammonia (around 10 percent). Astrobiologists think that when ultraviolet (UV) radiation from the early Sun impinged on the comets, it catalyzed chemical reactions near the surfaces, creating biologically interesting compounds. Origin-of-life researchers postulate that these newly formed prebiotic materials would have been delivered to the early Earth, forging chemical evolutionary pathways that led to the first life-forms.

To test the plausibility of this idea, the French research team created an artificial comet in the laboratory by introducing gaseous water, methanol, and ammonia into a vacuum chamber at extremely cold temperatures (–200ºC). The researchers also introduced a magnesium fluoride crystal into the vacuum chamber. The gases condensed on the crystal’s surface, simulating the dust grains coated with cometary ice that would have been prevalent in the early solar system. The researchers irradiated the ice-coated magnesium fluoride with UV radiation from a hydrogen lamp and then warmed up the samples to room temperature. These steps were designed to simulate UV exposure from the early Sun and the warming that the cometary ice would experience as it approached our star.

After completing this protocol, they analyzed the ice coating and discovered that a number of interesting compounds formed, including sugar ribose, a key building block for RNA, a material that factors centrally into origin-of-life scenarios.

On the surface, this finding seems to provide compelling support for evolutionary explanations for life’s start. As I discuss in my books Origins of Life and Creating Life in the Lab, the generation of ribose is one of the most difficult problems confronting chemical evolutionary scenarios. Yet, the work of the French researchers points to a possible solution.

3 Issues with the Comet Simulation

But does it? Careful analysis of their work identifies a number of issues that raise questions about its relevance for the origin of life, leaving the scientific community with a huge hole in chemical evolutionary scenarios for life’s beginning.

1. Astrobiologists have yet to detect ribose or any other complex sugar molecule in comets or other astronomical sources. It is true that investigators have found low levels of sugar alcohols and sugar acids in the Murchison meteorite and have detected the two-carbon compound glycolaldehyde in cometary sources. Though sugar alcohols and sugar acids are structurally similar to sugars, these compounds have little or no biological relevance. And glycolaldehyde is a far cry from ribose, a five-carbon aldose. Astrobiologists’ failure to detect ribose in any astronomical source indicates that the reaction pathway the French scientists identified for ribose in the laboratory isn’t likely chemically productive in actual comets, and consequently, has no bearing on the origin-of-life problem.

2. The French researchers made ribose under unrealistic laboratory conditions that lack geochemical plausibility. Like most prebiotic simulation studies, the French researchers performed their experiments under highly controlled, chemically pristine conditions that would not have existed in the early solar system (or anywhere else for that matter). For example, they carefully excluded materials that would be present in cometary ices, inhibit the production of ribose, or react with this compound (and others) as soon as it formed.

The French scientists also employed UV radiation in an unrealistic way in their study.2 These investigators made use of a light source (a hydrogen lamp) that primarily emitted a single wavelength of UV light. In contrast, UV radiation from the Sun consists of wavelengths across the UV spectrum. This difference is significant. For a given chemical reaction, some wavelengths of UV radiation can catalyze the process, while other wavelengths in the spectrum inhibit the process, destroying the starting materials or the reaction products once they form. Failure to use a broadband source makes the results of this work of questionable relevance to the origin of life.

3. Even if this simulation was done under geochemically plausible conditions, there are a host of other unresolved issues. The researchers think that the production of ribose in cometary ice occurred via a chemical route known as the formose reaction. Origin-of-life investigators have extensively studied this reaction because a number of scientists have proposed this as the pathway for ribose production on early Earth. Unfortunately, these studies have led many origin-of-life researchers to conclude that this route isn’t a viable option on early Earth, so it shouldn’t be considered a viable option for ribose production in comets.

One of the big issues associated with the formose reaction has to do with its lack of selectivity. In addition to producing ribose, the formose reaction produces a large number of different types of sugars. This is very much the case for the UV-catalyzed reaction taking place in cometary ices. Ribose comprises only 4 percent of the sugars produced in this reaction and less than 1 percent of the overall reaction yield. In fact, sugars make up only 20 percent of the reaction’s products, with sugar acids and sugar alcohols (again, of no biological interest) making up 80 percent of the yield. In other words, ribose is a minor constituent within a complex mixture of sugars, sugar alcohols, and sugar acids. When confronted with a complex mixture such as this, origin-of-life researchers are unclear how subsequent prebiotic reactions requiring ribose could ever “find” it among the formose reaction products.

Another concern relates to the stability of the sugars produced by the formose reaction. Sugars, such as ribose, are chemically unstable and will either break down or react with other materials to form compounds of little use for the origin of life. The instability of this compound means that it would be short-lived in the comet once it formed, and would be largely unavailable for chemical evolution.

Other issues relate to the delivery of ribose (and other reaction products) to early Earth. Given their instability, it’s hard to envision how sugars—and other organic materials, for that matter—would survive during impact, when the comets crashed into Earth. Even if they did survive, the amounts delivered would be exceedingly small (ribose makes up about 0.00075 percent of the comet’s overall mass). Those low levels would be further diluted by the early Earth’s oceans.

Chemical Evolution or Intelligent Design?

At best, the French research team has identified a chemical route that can produce ribose in a cometary environment, but has failed to establish its relevance for the origin of life because the reaction conditions they employed in the lab aren’t geochemically plausible in the early solar system. In fact, these reactions only occur in the laboratory because the researchers employed carefully controlled conditions that were an intentional part of their experimental design. The control the researchers exerted in this study points to the necessary role intelligent agency must play if simple chemical materials are to be transformed and organized into the first life-forms.


  1. Cornelia Meinert et al., “Ribose and Related Sugars from Ultraviolet Irradiation of Interstellar Ice Analogs,” Science 352 (April 2016): 208–12.
  2. For a detailed discussion about the misuses of UV radiation in prebiotic simulation experiments, see my article, Fazale Rana, “Ultraviolet Light Illuminates More Problems for Chemical Evolution,” Today’s New Reason to Believe (blog), Reasons to Believe, March 17, 2016, http://www.reasons.org/articles/ultraviolet-light-illuminates-more-problems-for-chemical-evolution.

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