The thick mesozoic atmosphere

It may be hard to imagine that the Earth’s air could be so thick that its density would be comparable to water. Nevertheless, there is no reason why a gas can not be compressed so much that it has properties similar to that of a liquid, and in fact compressing a gas into a liquid is a common industrial process.

In order to compress the air near the Earth’s surface, there has to be a substantial amount of overlapping air pressing down on the ground level air. Thus the high density ground level air is evidence of an extremely thick Mesozoic atmosphere.

Unlike water or other liquids that have nearly constant density between the top and the bottom, the density of a planet’s atmosphere increases as one travels from the darkness of space downward to the planet’s surface. In addition, there is also an increase in pressure as we move downward towards the surface. Close to the planet’s surface both the atmosphere’s density and the atmospheric pressure is the greatest due to the weight of all of the air above compressing the air at the surface.

A good approximation of the pressure at the Earth’s surface can be achieved by using the ideal gas law:

P V = n R T

P is the pressure in N/m², V is volume in m³, n is the number of moles, R is the ideal gas constant 8.31 J / mol*K, and T is the absolute temperature given in Kelvin. This is the ideal gas law in its standard form often used for closed containers. But our atmosphere is not a closed container so before we can use our ideal gas law equation we need to make the substitution:

n / V = ρ / M.

Where ρ is the density of the atmosphere and M is the average molecular weight of the gas. Making these substitutions gives the equation for calculating the pressure:

P = ρ R T / M.

We insert into our equation 667 kg / m³ for the density, 294 K (21 degrees Celsius) for the average Mesozoic global temperature, and 43.0 grams per mol for the molecular weight of the atmosphere. This shows that 150 million years ago the Earth’s atmospheric pressure near the surface was about 370 atmospheres.

An atmosphere is a unit for pressure. The present sea level atmosphere is said to have a pressure of 1.00 atmospheres. Other corresponding measurements of the present sea-level pressure are 1.01 E5 N/m² (1.01 x 10⁵ N/m²) and 14.7 PSI or pounds per square inch.

A pressure reading taken within a static fluid is an indication of the weight of fluid above that location. So a Mesozoic sea-level atmospheric pressure of 370 atmospheres would indicate that the Mesozoic atmosphere was 370 times thicker than what it is today.

Before concluding that the Earth’s thick Mesozoic atmosphere would crush all the species on its surface, stop to consider the pressure that currently exist at the deepest depths of the oceans. The average ocean depth is 3790 m and at this depth the pressure is 380 atmospheres. So for all practical purposes, the present day pressure at the average depth of the ocean is the same as the pressure at the bottom of the Mesozoic atmosphere. Yet there are numerous species that live at this depth and many more that live much deeper. Extremely high absolute pressure has no ill effect on our present creatures of the deep that have evolved in these environments; likewise, the extremely high pressure of the Mesozoic era had no ill effect on the terrestrial species of the Mesozoic era.

Understanding Pressure

.

To clear up possible confusion over pressure it may be helpful to recognize the distinction between absolute pressure and a difference in pressure.

If both the inside and outside of an enclosed container are at the same absolute pressure, no matter what the absolute pressure might be, there will be no net force on the sides of the container. For example, if both the inside and outside of a closed container are at 370 Atm. the walls of the container will not be under any stress.

In contrast to the example of where an extremely high but equal pressure is on each side of a wall, even a small or moderate difference in pressure between each side of a wall can produce a substantial force. For example, let’s imagine that the difference between the inside and the outside pressure on a typical window is ‘only’ 1/30 of an atmosphere. However, if this were attempted there would no longer be a window. 1/30 of an atmosphere would create a force on the window equivalent to laying the window on the ground and then asking several men to stand as close as possible on top of the glass surface. A typical window would break with a pressure difference of only about 4 10^-3 Atm.

The distinction between a difference in pressure and absolute pressure can be further illustrated by comparing the effect on submarines to the effect on marine biology. When a submarine on the water’s surface closes its hatch before its descent, the sea-level air pressure both outside the sub and inside the sub is 1.0 atmosphere. As the sub dives down into the ocean’s depth the water pressure outside the sub greatly increases while the air pressure inside the sub remains at 1.0 atmosphere. After the sub dives down 250 meters, the difference in pressure pushing inward on the sub is 25 atmospheres. Submarines must have thick steel hulls so that they can withstand the crushing pressure at these depths.

In contrast to the rigid submarine, the many species that live at the great depths of the oceans experience no ill effects despite living in an extremely high pressure environment. Unlike the submarine, these species do not attempt to maintain a pressure difference between the interior and the exterior of their bodies. Since the pressure inside the bodies of these deep-sea creatures is the same as the outside water pressure, there is no net force or strain on their bodies no matter what the absolute pressure might be.

This is true as long as the animal does not change depth too quickly and in so doing change the exterior pressure too quickly. Most people are familiar with the mildly painful experience of having their ears pop when changing altitude such as flying or driving up or down a mountain, or even just diving into the deep end of a pool. Scuba divers who make the mistake of rising to the surface too quickly may have the much more painful experience and possible death from the decompression sickness known as the bends. This effect is not limited to humans since even whales and possible other marine animals suffer from the bends if they rise to the surface too quickly. Because of decompression problems, marine biologists are still experimenting with different methods for bringing deep sea creatures to the surface without killing them. For clarification, a quick decompression does cause problems for many species, whereas there are no adverse effects for species to continuously exist in a constant-pressure environment.

As human beings living on the Earth's surface, we live at the bottom of a sea of air. At sea level this air produces a pressure on our bodies of 1.0 atm., 1.01 10⁵ Pa, or 14.7 PSI. The area on the face of an average adult's hand is about 0.0116 m² or 18.0 square inches so there is about 1200 N (270 pounds) of force bearing down on an average adult human hand. Since the pressure is the same for both inside and outside of us, the net forces balance out to zero. Rather than weighing us down, we are indifferent to this force.

Regarding fluid pressure, it does not matter if we are discussing air or water, they are both fluids. The creatures of the Mesozoic era were at the extremely high absolute pressure of 370 atmospheres. Yet just like the deep ocean species of today, this high absolute pressure was on both the inside and outside of their bodies so it produced no stress on their bodies. The extremely high absolute pressure of the thick Mesozoic atmosphere would have had no adverse effect on the dinosaurs.

Light Penetration

In order to imagine the thickness of the Mesozoic atmosphere, the density of the thick Mesozoic atmosphere has been compared to the density of water. Hopefully this comparison to water has also been helpful towards understand how the thick atmosphere provided buoyancy and to imagine what it would be like to move through such a thick atmosphere. However it still needs to be kept in mind that these two fluids are not the same in all respects. One difference between the two fluids is the way light is limited in how far it can penetrate into water.

Water is an electric dipole molecule. This means that even though the overall electric charge of the molecule is neutral, one side of the molecule is positive while the opposite side is negative. It is this unbalanced distribution of electric charge that gives water its many unique characteristics.

.

When we microwave our food for a quick meal, the microwave is interacting with the electric dipole molecules of water causing rapid oscillation. This rapid oscillation at the atomic level is what we perceive as thermal energy at our sensory level. It is because the microwaves often penetrate deep within the food before they transfer their energy to the water molecules that a microwave is able to cook so fast. The microwave example illustrates water’s ability to absorb electromagnetic energy, in other words, light.

Light from the sun does not lose energy as it travels to the Earth. But once it penetrates into the depths of the ocean much of its energy is absorbed by the water. Most of the sunlight’s energy is absorbed as it travels down through only the first 200 meters of the ocean water. Beyond this upper lit zone, the epipelagic zone, the light becomes too dim to even support plant life. If we sink even deeper the light continues to diminish until it becomes completely dark at about 1000 m; beyond this there is nothing but darkness into the depths of the sea.

It would certainly be a problem to the dinosaurs and the other species of the Mesozoic world if the thick Mesozoic atmosphere absorbed light in a similar fashion to the water’s of the oceans. But similar to today’s atmosphere, the Mesozoic atmosphere would contain only a small percentage of water vapor in comparison to the total volume of other gases. So similar to the present atmosphere, the vast majority of light would pass through the thick Mesozoic atmosphere to reach the surface and even penetrate the upper portion of the ancient oceans.

This is not to say that the Sun and stars of the Mesozoic sky would look exactly the same as they do today. Astronomers are well aware of how atmospheric turbulence deflects starlight causing stars to twinkle. During the Mesozoic era, starlight passing through such a thick atmosphere would be thrown about so much by atmospheric turbulence that individual stars may not have been distinguishable. Likewise because of the extreme thickness of the atmosphere the Mesozoic Sun would probably appear extremely hazy in comparison to how it appears today.

(to be continued)

Reference:

https://www.dinosaurtheory.com/thick_atmosphere.html