Solar System Chemistry: Snow Line, Thermodynamics, and the Proto-planetary Disk

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Home / Solar System Chemistry: Snow Line, Thermodynamics, and the Proto-planetary Disk
Our solar system is visually-striking, but what is the chemistry behind it? Image courtesy of NASA

Our solar system is visually-striking, but what is the chemistry behind it? Image courtesy of NASA

When you use a telescope to gaze into the night sky, you may notice the craters of the moon or the red vistas of Mars, but what we see doesn’t tell the full story.

These astronomical bodies aren’t just lovely – they can teach us about our own origins; why we inhabit the Earth, and how life skirted Mars.

Origins of Life and Chemistry

Life’s beginnings played out from a combination of chemistry, energy, and time. Chemically speaking, life required water, carbon molecules and a combination of nitrogen, sulfur, phosphorus, iron and trace metals. However, a major reason for life-as-we-know-it is our location.

Location, Location, Location… and Thermodynamics in the Proto-planetary Disk

When you boil water, the steam rises and condenses — water is volatile and has an appreciable vapor pressure. Its thermodynamic phase diagram (A phase diagram illustrates the preferred physical states of matter at various temperature and pressure levels.) is good reason for evidence of its stability under a variety of conditions. There is a wide range of temperatures at which water can turn progressively from ice to liquid to steam.

We can loosely extend the analogy of water to the solar system’s birth environment, the aptly named proto-planetary disk comprised of simple molecules that would eventually take the shape of our biology, our planet and the solar system.

The ‘disk’ consisted of a hot, chaotic mass that was undergoing pre-fusion processes. During the pre-fusion process, the mass was rotating and expelling gas and dust-the gas and dust that would eventually become the planets, comets, meteors and us.

Currently, models of solar system formation show the proto-planetary disk as possessing zones that may resemble habitable and non-habitable zones-in much the same way that our solar system does. In some zones, liquid water is available on the planets, and in other zones, water freezes, and so on.

Earth’s Bio-molecular Diversity

While the Earth looks unusual by the virtue of its diverse biosphere, its unique qualities are reflected in the bio-molecules.  Although other solar system bodies possess many of the same molecules as Earth, molecules like water (H2O) exist in three phases (solid, liquid, and gas) on Earth.

The property to solvate (attract and associate) the molecules of life is due in part to the phase transitions. Nitrogenous bio-molecules are amply represented as amino acids and in RNA/DNA as well. Moreover, we may be able to trace the origins of the bio-molecules to the proto-planetary disk (the Earth’s ‘original nursery’).

Evidence central to pre-biotic molecules is the concept of ‘frost-or-snow lines.’ The proto-planetary disk ‘snow-line’ can determine whether the molecules important for life will be present. Carbon monoxide is an important starting point for the synthesis of simple amino acids.

Net reaction scheme for synthesis of glycine from carbon monoxide, nitrogen gas, and hydrogen. by John A. Jaksich

Net reaction scheme for synthesis of glycine from carbon monoxide, nitrogen gas, and hydrogen. Image by John A. Jaksich

In a paper published in March, 2014, in The Astrophysical Journal Letters, the authors (Rebecca Martin and Mario Livio) argue why it would be simple for  the evolution of snow lines (specifically for  carbon monoxide) to take part in pre-biotic molecules.

The authors  cite  nitrogenous molecules as  co-occurring with carbon monoxide in an observed ‘snow line’ of an exo-solar system, TW Hya. (TW Hya is the most studied of exo-solar system to date and is in the proto-planetary phase. This system is approximately 17o light years away, in the constellation Hydra.)

Using the work of Qi, D’ Alessio,  Öberg and others, the authors argue that cometary material could have delivered nitrogen-bearing carbon molecules to the early Earth from the colder reaches of the solar system. These regions of the solar system would have been delineated by a ‘snow line’ or more specifically the Kuiper belt.

Frost-line illustration in solar system.

The frost-line in our solar system is critical – chemical elements have different properties at different temperatures. Image courtesy of NASA

The ‘snow-lines’ for our solar system stand at the asteroid belt for water,  Neptune for carbon monoxide and the depths of the Kuiper belt for nitrogen.

These lines correspond to temperatures and pressures (in space) at which the substances would ‘naturally’ exist in their ice form.

Central to the temperature and pressure are the reasons for it: the amount of matter at time of formation.  It contained no more and no less.

Solar System Chemistry

In any solar system, each planet is formed under certain conditions – but in order for the planet’s chemistry to develop the conditions under which life can develop, the planet must meet certain environmental criteria. Whether there’s life or not, however, all planetary bodies contain unique chemical elements, and provide us with information about our origins and the development of our solar system.

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