Wednesday, November 19, 2014

On The Origin of Comets

Comet 67P/Churyumov–Gerasimenko
This has been a big week for exploration of some elements of our solar system.

We landed on a "comet," whatever that is (Comet Landing In Progress).

In this series we will go through the episodes of comet origination and demise.

We will see that we really have no mature clue about when this particular comet came into existence, but let's ponder the hype that is out there:
Philae, the probe that landed on a comet as part of the Rosetta mission, has detected organic molecules in the comet's atmosphere. We don't know exactly what the molecules are yet, but they could hold a key to early life on Earth. Hell, this is a big reason we sent Rosetta all the way to a lonely comet in the first place.
(Rosetta & Company). From that typical detail we learn that a comet is lonely and we don't exactly know what the molecules are but we "know" they are organic:
The distinction between organic and inorganic carbon compounds, while "useful in organizing the vast subject of chemistry ... is somewhat arbitrary."
(Wikipedia, "Organic compound"). Why not look for RNA or DNA on the comet, neither of which are alive (Weekend Rebel Science Excursion - 37)?

But are those types of "molecular machines" (RNA / DNA) "organic" too?

The answer to that question is also meaningless, since "organic" now means only that a molecule contains an atom or atoms of carbon:
"organic: noting or pertaining to a class of chemical compounds that formerly comprised only those existing in or derived from plants or animals, but that now includes all other compounds of carbon." (Dictionary)
"An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon." (Wikipedia, Organic Compound)
In other words, carbon has many flavors (e.g. C, C2, C3, C4) and is originally made in the interior of stars, which are neither alive nor biological (they are abiotic).

Like planet evolution, comet evolution is a sub-function of stellar abiotic evolution.

In contrast, biotic evolution in our solar system is relatively recent (Putting A Face On Machine Mutation - 3).

Abiotic evolution is far, far more ancient than biotic evolution is, going back to the two predecessors of our current Sun:
Formation of the carbon atomic nucleus requires a nearly simultaneous triple collision of alpha particles (helium nuclei) within the core of a giant or supergiant star which is known as the triple-alpha process, as the products of further nuclear fusion reactions of helium with hydrogen or another helium nucleus produce lithium-5 and beryllium-8 respectively, both of which are highly unstable and decay almost instantly back into smaller nuclei. This happens in conditions of temperatures over 100 megakelvin and helium concentration that the rapid expansion and cooling of the early universe prohibited, and therefore no significant carbon was created during the Big Bang. Instead, the interiors of stars in the horizontal branch transform three helium nuclei into carbon by means of this triple-alpha process. In order to be available for formation of life as we know it, this carbon must then later be scattered into space as dust, in supernova explosions, as part of the material which later forms second, third-generation star systems which have planets accreted from such dust. The Solar System is one such third-generation star system.
This means that our Sun and our Earth have descended from an ancestor star that exploded and emitted a molecular cloud which then condensed to form a second ancestor star that did the same, i.e. formed a molecular cloud that condensed into the Sun and the planets of our solar system.
(On the Origin of the Genes of Viruses - 5). That evolutionary process is not complete yet, because the Sun still has major evolution to go through (On the Origin of the Genes of Viruses - 6).

That is to say that the original abiotic star in our solar system evolved before either carbon or our current Sun did, then that star produced carbon in its interior before going supernova to release the carbon.

The long term history of "comet organics" must take into account the carbon generating abiotic evolution of stars:
"Stellar evolution is the process by which a star undergoes a sequence of
Current phase of the Sun
radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years for the most massive to trillions of years for the least massive, which is considerably longer than the age of the universe ... All stars are born from collapsing clouds of gas and dust, often called nebulae or molecular clouds.
Next Phase of the Sun
Over the course of millions of years, these protostars settle down into a state of equilibrium, becoming what is known as a main-sequence star.

Stellar evolution is not studied by observing the life of a single star, as most
Final Phase of the Sun
stellar changes occur too slowly to be detected, even over many centuries. Instead, astrophysicists come to understand how stars evolve by observing numerous stars at various points in their lifetime, and by simulating stellar structure using computer models
(Wikipedia, "Stellar Evolution"). How that early evolution of our solar system's first star impacted upon the origin of comets is probably not exactly knowable.

Thus, neither is the full story of the recent history and abiotic evolution of comets.

Never-the-less, that history is said to be:
Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Longer-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper Belt to halfway to the next nearest star.
(Wikipedia, "Comet").  When the Kuiper Belt and Oort cloud came into existence depends entirely on what generated them.

Were they generated by The Big Bang, the galactic formation of the Milky Way, or one of the several stars in our solar system's early stellar evolutionary history?

That stellar evolution, which may have generated some or all of our current comets, could refer to the first star, the second star before the current one (the Sun), or to each of them.

The history of what happened depends on the particulars of each ancestor star's individual demise, which could have generated some or all of the Kuiper Belt and/or the Oort cloud when those stars exploded, i.e., when they went supernova.

The bottom line is that finding carbon on a comet near the Sun in the current solar system tells us nothing of that comet's origin, no, it can only tell us some of its history (see also Exploded Planet Hypothesis - 2).

Neither does it tell us about the origin of "early" life or water, it can only tell us some of the history of those concepts in this current solar system.

That is because "origin" and "history" are not the same concept; "origin" is primarily emergence into existence for the first time, while "history" is primarily what happens after that.

Abiotic evolution produces the galaxies, stars, planets, moons, and water, while biotic evolution only produces carbon-based life forms in the water on planets or moons:

1 comment:

  1. Another potential source is the explosion of a planet or planets in our solar system: link