Guest post by Bruno Hache, chemist:
Dune: the possible . . .
2. Plate tectonics and life:
We find the following statements in Sibylle Hechtel blog in the section about her interview of the Second International Convention of the Mars Society:
Chris McKay, of NASA’s Ames Research Center as to whether life existed on Mars:
‘‘Early Mars, when it was thought to have life, was warmer and had liquid water on its surface. The planet lost its atmosphere and became much colder.’’
Diana Valencia of Harvard University also said:
“Plate tectonics are essential to life as we know it, our calculations show that bigger is better when it comes to the habitability of rocky planets.”
Hechtel also says:
‘‘Plate tectonics are crucial to a planet’s habitability because they enable complex chemistry and recycle substances like carbon dioxide, which acts as a thermostat and keeps Earth balmy. Carbon dioxide that was locked into rocks is released when those rocks melt, returning to the atmosphere from volcanoes and oceanic ridges.’’
Chris McKay (see above) also said:
‘‘One hypothesis is that carbon dioxide (CO2) is unstable and forms calcium carbonate (CaCO3). On Earth, the subduction plates (plate tectonics) take CaCO3 into the interior where the core’s heat releases the CO2 - a prime “greenhouse” gas that helps retain heat - back to the atmosphere’’.
This mechanism for the recycling of CO2 by volcanic activity is explained in “some” details in the Surface Chemistry section:
“It is thought that Earth, being hotter, transported much of the iron downwards in the 1800 km deep, 3,200 °C (5,800 °F), lava seas of the early planet, while Mars, with a lower lava temperature of 2,200 °C (4,000 °F) was too cool for this to happen.. While the possibility of carbonates on Mars has been of great interest to exobiologists and geochemists alike, there is little evidence for significant quantities of carbonate deposits on the surface. ”
Also found on the Geology of Mars in the Magnetic Field and Internal Structure – Tectonics section:
“As a result of 1999 observations of the magnetic fields on Mars by the Mars Global Surveyor spacecraft, it was proposed that during the first half billion years after Mars was formed, the mechanisms of plate tectonics may have been active, with the Northern Lowlands equivalent to an ocean basin on Earth. ”
This is supported in more details by the following report:
Some measurements that have been recorded by the Magnetometer of the Mars Global Surveyor spacecraft in 1999 confirm the absence of a global magnetic field. The key consequence of the lack of magnetic field is:
“… unlike on Earth, the implied plate tectonic activity on Mars is most likely extinct.”
I was! But it caught my attention big time and I was not satisfied with the above explanations and needed to learn more to satisfy my scientist curiosity. Some important details were overlooked for the eye of the reader, especially for the scientifically-literate readers. My intention here is to expand on some of those details and propose an additional mechanism on how a rocky planet with oceans can further recycle CO2 from CaCO3.
Bruno expands about Diana Valencia’s comments:
Size matters because smaller planets have a smaller crust that can act as an insulator. The external crust on a planet like Mars, Venus or Earth is like an egg shell. But internal pressures do develop underneath the crust that acts like a pressure cooker. Once in a while, a breach in this crust will trigger a volcanic eruption. Without this crust and without the extra heat of the bigger planets core, smaller planet will lose the heat of their core faster than bigger ones. Once the core is no longer in the form of liquid metal, the loss of the magnetic field shut down the plate tectonics and concomitant volcanic activity (temperatures too close to 840 ˚C / 1544 ˚F), no more plate tectonics nor volcanic activity, no CO2 recycling, no Prime ‘‘Greenhouse’’ gas, not enough heat to keep water in a liquid form, no life is possible.
Is it also possible that the higher gravity will help retain the greenhouse gases?
I presume yes since the Giant planets such as Uranus and Neptune are quite gaseous in nature.
“Neptune’s temperature at its cloud tops is usually close to _218 °C… The temperature in Neptune’s center is about 7,000 °C, which is comparable to the Sun’s surface and similar to most other known planets.”
Therefore, the size of those Giant planets is big enough to retain gases by simple gravity as implied by Helium rain on Jupiter: Rain-like droplets of helium and neon precipitate downward through this layer…” Also to consider, those planets being far away from the Sun, are so cold, that, some if not most of those gases are in fact liquid in the upper atmosphere, which helps that those gases do not escape to Space!
For example of that gravity effect when considering a gas giant of our Solar System - Jupiter: http://en.wikipedia.org/wiki/Jupiter#internal_structure
“The presence of the core is also suggested by models of planetary formation involving initial formation of a rocky or icy core that is massive enough to collect its bulk of hydrogen and helium from the protosolar nebula. The core may in fact be absent, as gravitational measurements aren’t precise enough to rule that possibility out entirely. Assuming it does exist, it may also be shrinking, as convection currents of hot liquid metallic hydrogen mix with the molten core and carry its contents to higher levels in the planetary interior.”
McKay said above: “…CO2 recycling… and CO2 is unstable…”
Bruno would like to clarify that statement from McKay.
Bruno says: ‘‘In fact, CO2 is not unstable. But it is rather more stable at life form temperature when it is in the form of CaCO3. In turn, CaCO3 is unstable under high heat and when exposed to acids.’’
Let’s explore this hypothesis in more details with what we know about CaCO3 on Earth and having in mind Diana Valencia’s comments (above) about the size of planets and their life-form habitability.
An industrial process for the production of CaO routinely used by the chemical industry is called calcining. It exploits exposing CaCO3 to high heat, at temperatures above 840 ˚C (above 1544 ˚F). At this high temperature, the following chemical transformation happens and releases CO2:
CaCO3 + heat ‡ CaO + CO2 (g)
Since calcium carbonate is a more stable form of CO2 at life form temperatures, CaCO3 deposits constantly occur on Earth (Chalk, Limestone, and Marble).
On a planet with plate tectonics and volcanic activity, the heat of earthquakes and more specifically volcanic eruptions is sufficient to promote the above decomposition of CaCO3.
Mars having no plate tectonics and no volcanic activity as evidenced by the Mars Global Surveyor (above), CO2 cannot be recycled back in the atmosphere by the extreme heat of volcanic eruptions since plate tectonics have ceased to exist.
Acidic recycling of CO2 under volcanic eruptions:
This is another hypothesis I would like to POSTULATE on our comprehension on the plate tectonics and CO2 recycling. In planets with large amounts of oceans, plate tectonics and concomitant volcanic activity like we know on Earth, all the ingredients are there for CO2 recycling by an acidic mechanism as well. During a volcanic eruption, the ejection of SO2 is another one of the main gazes that is liberated besides CO2. When a volcanic eruption happens in deep sea like the sulfur vents, there is tons of water near the sulfur vents. This provides the ingredients necessary for the transformation of SO2 into H2SO4, sulfuric acid.
Carbonates in general also have another useful property (besides heat instability) in the context of volcanic activity and CO2 recycling. Carbonates such as CaCO3 are unstable to acid. Any volcano will eject tons of SO2, a chemical precursor to sulfuric acid, H2SO4 that we routinely get in acid rain for example. Acid rain in the form of dissolved H2SO4 OR with surrounding water from a sulfur-vent in deep sea will dissolve CaCO3 into more water-soluble CaSO4 and release CO2 into the atmosphere:
H2SO4 (aq)+ CaCO3 ‡ CaSO4 (aq) + H2CO3 (carbonic acid)
Carbonic acid is unstable and decomposes/dissociates spontaneously in the following manner:
H2CO3 ‡ H2O + CO2 (g)
To recycle the CO2 from CaCO3, the heat of a volcano is required which is probably the main stream mechanism for CO2 release on rocky planets.
However, this acidic mechanism to release CO2 is probably a minor component compared to the spontaneous high heat decomposition of CaCO3. This is especially true in early planets that do not yet have oceans, the ingredient necessary for the acidic mechanism to occur. The kinetics of the heat decomposition is most likely higher than the kinetic of the acid decomposition. The acid pathway simply needs water to react; there is not much water that will sustain temperatures above 840 °C (above 1544 ˚F)! But in the Sulfur vents Sibylle Hechtel refers to about the cyanobacterias, both the heat and the acid mechanism probably compete very well; there is plenty of water at depths of 10000 m!
This heat decomposition mechanism that we know on Earth supports that the absence of plate tectonics and volcanic activity on Mars makes CO2 recycling simply not possible on Mars, resulting in the apparently cold, dead planet that we observe today.
The greenhouse effect of CO2:
CO2 is an important greenhouse gas because it reflects IR radiation back to the ground, hence the Global Warming we slowly observe on Earth since the beginning of the industrial revolution. CO2 absorbs in the IR. Warm CO2 will release this heat back in all directions (3D) and most of that heat is lost to Space.
A portion of that released heat will be returned to the ground.
‘‘The molecular vibrations of CO2 (IR) return 5% of that heat back to the ground by [reflectance]. This heat retention eventually results in a temperature at which water is in a liquid form. Water needs to be liquid so it can act as the universal solvent essential to life. Water also has several IR wavelengths and will readily absorb heat.
In more details, H2O vapor reflects 36% of IR radiation; CO2 reflects 9% and O3 (ozone) only 3% of the IR radiation. Water has as a high heat capacity and this heat retention of water is synergistic with CO2. Both are believed to be essential to maintain temperature favorable for life on Earth.’’
Wikipedia also mentions:
‘‘The Earth’s average surface temperature of 15 ˚C (59 °C) is about 33 ˚C (59 °F) warmer than it would be without the greenhouse effect.’’
In summary, the existence of life on a planet is a succession of planetary scale events and requirements:
Bigger planets will be OR will stay warm enough to generate plate tectonics activity and concomitant volcanic eruptions.
If a planet is big enough, the planet will retain that core heat more efficiently.
Volcanic eruptions will liberate CO2 due to the plate tectonics on bigger planets.
CO2 helps to retain heat on the ground – the greenhouse gas warming effect.
With that heat, water is in a liquid form.
Water is essential to dissolve inorganic and more importantly organic molecules that are essential to life.
Based on those scientific observations and evidences presented in this document/short essay, if those planetary scale events and requirements are met, life as we know it on Earth can exist and flourish!
M. Sc. Chemistry
Large-Scale Chemist in the Pharmaceutical/Biotech industry
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