Prebiotic Synthesis Experiment:

Numerous key organic compounds necessary for life existed in the protoplanetary Solar disk prior to the formation of the planets.

Part I

Recent research into the origin of life has been favoring ever-colder and ever-dryer conditions for the stable formation of RNA molecules.

Part I.a.)

While the "plausibility" criterion for a given chemical pathway suggested as being involved in prebiotic synthesis generally requires that the chemical environment has to be one which could have plausibly been present on the early Earth, such a criterion ignores that the early Earth isn't the only place that life could have formed. Such is an a priori assumption completely unsupported, and one might go so far as to say contradicted, by all experimental and observational data thus far gathered. The Earth is but one planet among hundreds known and among billions thought to exist in our galaxy alone. Furthermore, there is no particular reason to hypothesize that the generation of life is something which must occur on a planet at all! Especially when one considers how early in the geological record life first began to appear on Earth, other environments such as other Solar planets, exoplanets, planetesimals, asteroids, comets, moons, protoplanetary disks, protostellar nebulae and the stelliferous nebulae (star-forming regions of giant molecular clouds) to which they belong, as well as any interactions between such systems, must all be considered as potential environments for the various stages of prebiotic synthesis, and "plausibility" determined only when a particular chemical pathway has been demonstrated to be likely (or not) in one of such environments. An early-Earth-based "plausibility" criterion is, frankly, an unnatural constraint which ignores the fact that in our universe, no planet is an island.

Many stages of prebiotic synthesis wouldn't have worked out in hydrothermal vent scanrios or on a hot early Earth.

The hydrothermal vent model, though seemingly ruled out, would give us a mathematically elegant and thermodynamically sensible sequence of OGT≥80°C, OGT=65±15°C, OGT≤50°C and a cellular LUCA model gives us a progressive OGT≈0°C, OGT≤50°C, OGT=65±15°C thermotolerance sequence before a general decline to OGT≤50°C with a few modern lineages OGT=65±15°C or even OGT≥80°C, while a precellular LUCA model merely gives us an OGT≤50°C; OGT≥50°C threshold separating the cryophilic-to-mesophilic nucleobases, RNA, LUCA, and protocells, and the thermophilic-to-hyperthermophilic first true lifeforms, followed eventually by diversification into various levels of thermotolerance.

(Szostak, 2012; part 2)

(Sleep, 2010)

Cool Early Earth model.

Here, however, we run into another problem. Water is highly corrosive, and would've been deadly to the first RNA-based replicators (Barras, 2014)

(Ricardo et al., 2004)

Hydrolysis (Benner et al., 2012)

Part I.b.

The earliest stages of life on Earth very likely antedate the Earth Herself. This idea has been variously referred to as “molecular panspermia", “quasi-panspermia", or “pseudo-panspermia".

"Essential to the spontaneous origin of life was the availability of organic molecules as building blocks. The famous ‘prebiotic soup’ experiment by Stanley Miller (Miller 1953, Miller-Urey experiment) had shown that amino acids, the building blocks of proteins, arose among other small organic molecules spontaneously by reacting a mixture of methane, hydrogen, ammonia and water in a spark discharge apparatus. These conditions were assumed to simulate those on the primitive Earth. Already in 1922 Oparin had proposed that the early Earth had such a reducing atmosphere (in his classic ‘The Origin of Life’ from 1936 he expanded on these ideas). Observations of Jupiter and Saturn had shown that they contained ammonia and methane, and large amounts of hydrogen were inferred to be present there as well (it is now known that hydrogen is the main atmospheric component of these planets). These reducing atmospheres of the giant planets were regarded as captured remnants of the solar nebula and the atmosphere of the early Earth was assumed by analogy to have been similar." -- The Origin of Life by Albrecht Moritz

(Moritz, 2010)

The conditions of the Miller-Urey experiments more closely resemble the conditions of the Solar nebular than the conditions of the primordial Earth (Hill & Nuth, 2003).

"Indigenous purines and pyrimidines have been detected in several carbonaceous chondrites. The pyrimidine uracil and the purines adenine, guanine, xanthine, and hypoxanthine (Stoks & Schwartz 1979, 1981) were detected in the CM carbonaceous chondrites Murchison and Murray, as well as in the CI meteorite Orgueil, in total concentrations of about 1.3 parts per million (ppm). Upper limits exist (detection limit of 0.01 ppm) for the concentrations of thymine and cytosine, as well as other heterocyclic compounds, in the Murchison meteorite (van der Velden & Schwartz 1977)."

(Peeters et al., 2003)

(Martins et al., 2008)

(Ehrenfreund & Cami, 2010)

(Ziurys, 2008)

(Coggins & Powner, 2017)

The purine bases adenine and guanine have been detected in meteorites, although the only pyrimidine-base compound formally reported in meteorites is uracil (Stoks & Schwartz, 1979), however cytosine cannot be ruled out (Shapiro, 1998; Peeters et al., 2003; Martins et al., 2006) and ultraviolet irradiation of low-temperature ices (the dominant phase of H₂O in cold astrophysical environments is ice, and most ices in such environments are H₂O-rich) has been shown to produce not only amino acids, quinones, and amphiphiles, but have also, with the introduction of pyrimidine, been shown to produce uracil (Nuevo et al., 2009), cytosine, and even thymine, though the abiotic synthesis of thymine is less straightforward compared to other pyrimidine-base compounds (Sandford et al., 2014). (That this would logically make the prebiotic synthesis of RNA easier than that of DNA could explain why RNA has a larger repertoire of functions than does DNA in modern cells.) Additionally, ribose and related sugars have been produced experimentally in astrophysical ice analogues (Meinert et al., 2016).

Cytosine can be synthesized from cyano-acetylene and cayanate, this is unlikely to have occured in acqueous media as cayanate is rapidly hydrolized into CO₂ and NH₃.

Cyano-acetylene is an abundant interstellar molecule and can by produced by a spark discharge in a CH₄/N₂ environment.

hydrolysis of cyanoacetylene leads to cyanoacetaldehyde

reaction of cyanoacetaldehyde with urea produces cytosine in 30-50% yields

Hydrolysis of cytosine leads to uracil

(Robertson & Miller, 1995)

(Pudritz, 2016)

(Sandford, Bera et al., 2014)

(Kuga et al., 2015)

Organic methyl cyanide in comet-forming region of MWC 480.

Clear liquid turns brown as amino acids form peptides. Jennifer Blank. Experiment.

Amino acids + impact = peptides (Blank & al. 2001)

Planetesimals & chondrules

Being that ribonucleotides can form sugar-phosphate backbones in clays when frozen (cite) we should consider whether it be possible that the first RNA molecules were produced by the sublimation/condensation cycles of clayey, H₂O-rich spinning grains along the snowline of the protoplanetary disc.

Figure out minimum grain size for clay sheet formation.

Krijt, Ciesla, & Bergin 2016

Ciesla & Sandford, 2018

Bergin 2017

Lunine, 2006

Zhang & Jiang, 2009

Blake & Bergin, 2015

Testi &al. 2014

Schwartz, 2006

Sephton, 2004

Cooper &al. 1992

Botta, 2005

Pasek, 2008

Georgelin &al. 2013

Ribonucleic peptide (PNA), uses peptides instead of phosphorylated ribose.

Part I.c.

What all of this means is that hypothetical geochemical pathways for the production of key organic compounds here on Earth, though interesting, are completely superfluous, since the biochemical building blocks of life were already being produced astrochemically before the formation of the Earth. Occam's razor therefore dictates that such geochemical hypotheses for the prebiotic synthesis of organic compounds should be regarded as irrelevant to the origin of life problem.

Part II

This experiment should employ granular protoplanetary particle analogues (GPPPAs or G3PAs) mimicking the ice-mantled grains that would've existed on the snowline of the protoplanetary Solar disk, with each G3PA consisting of a clayey, silicate-heavy grain core surrounded by an H₂O-rich ice mantle.

Part II.a.

Part II.a-1.

Grain composition — clays/silicates, adenine, guanine, pyrimidine, uracil.

Part II.a-2.

Part II.b.

Part III

Ideally, the G3PAs would be of variable composition and each G3PA should not only be spinning on its own axis, but also moving around the source of the UV radiation in an orrery-like fashion so that each G3PA has to move through the sublimated material of the preceeding G3PA. This would simulate ice-mantled grains of variable composition in the protoplanetary Solar disk passing their sublimated material to grains following in their wake while acquiring such material from the grains proceeding them.

Part IV

Part V

References