Tech UPTechnologyVenus: Greenhouse Effect Run Away

Venus: Greenhouse Effect Run Away

 

Venus is sometimes described as the sister planet of Earth . It is slightly smaller than our planet and has a very similar density and gravity . Both are part of the inner solar system, they are rocky planets and have a similar composition. However, there is something that makes them completely different: their atmosphere .

On Earth we have an atmosphere dense enough to retain enough heat and apply enough pressure for the surface to hold large amounts of liquid water , while the atmosphere of Venus is the densest and most massive known of any rocky body in the solar system, and absorbs the sun’s heat to bring the average temperature of the planet above 400 ºC , making the presence of liquid water impossible.

Seeing the current state of both atmospheres and how inhospitable that of Venus is, it would be natural to ask ourselves the following question: has it always been like this? Or was it more similar to Earth at some point in the past? Everything points to the fact that it has not always been like it is now , since its appearance was closer to that of our atmosphere at the beginning of the solar system.

We believe, although the evidence is not as conclusive as on Mars, that Venus had an ocean of liquid water on its surface. However, a few hundred million years after the formation of the planet, with the increase in brightness of the Sun and the evolution of the atmosphere of Venus, temperatures grew too high and this ocean ended up evaporating completely , becoming part of the atmosphere of the planet (and contributing more to its warming, since water vapor is a very good greenhouse gas). Over time , this water vapor would have been lost , because it would have escaped into space or because its molecules would have broken down into their atomic components and recombined to form other different molecules.

One of the indirect pieces of evidence we have for this whole story is the higher concentration of deuterium in the atmosphere of Venus , when compared to Earth’s atmosphere. This deuterium, which is nothing more than a heavy isotope of hydrogen, would come from primordial water. Being heavier than hydrogen, it would have accumulated more efficiently in the atmosphere , making it less likely to escape into space. On Earth, most of this deuterium would be in the water of the oceans , not being detectable in the atmosphere.

On Venus, what is known as a runaway greenhouse effect occurred, in which the greenhouse effect is triggered and temperatures continue to rise , until the oceans themselves evaporate. In any atmosphere there will be a certain greenhouse effect, that is, there will be a certain absorption of the light emitted by the planet’s surface by the atmosphere . This emitted light will depend on several factors: on the energy received from the star (which will depend in turn on the distance and the type of star), on the energy reflected directly to the outside (which will depend in turn on many factors, such as the greater or lesser presence of ice or clouds that, being white, reflect most of the light they receive) and the energy that the surface gives off , after absorbing the incident light and re-emitting it. The greater or lesser absorption of the light emitted by the atmosphere will depend on its composition, the presence of clouds, etc.

Depending on the combination of these factors that occur on a particular planet, we will have an equilibrium temperature for its atmosphere. If we modify the composition of the atmosphere, increasing the proportion of greenhouse gases , we will modify this balance . The same thing is happening on Earth: by increasing the amount of carbon dioxide present in the atmosphere we have increased the equilibrium temperature of our atmosphere. As long as we continue to increase the presence of this gas, this equilibrium temperature will continue to increase. Temperature that, in fact, we have not reached , because the atmosphere warms up more slowly than we increase the CO2 levels in it. This is why, even if we stopped emitting carbon dioxide today, the temperature of the atmosphere would continue to rise .

Of course, increasing CO2 and thus the temperature of the atmosphere can have consequences that we cannot control. This temperature increase causes more water to evaporate, adding more water vapor to the atmosphere and accelerating that same heating. It also causes the area covered by ice to decrease and the Earth’s surface to absorb more and more energy , heating up more quickly. These feedback effects, which we don’t fully understand yet, can make the problem of global warming worse, but they are not the worst thing that could happen. They are not the runaway greenhouse effect that affected Venus.

There comes a time when, although the temperature increases, the amount of energy that the planet returns to space does not increase , so that it is impossible to achieve any kind of balance. When this happens, the energy received from the star and absorbed by the planet will always be greater than that emitted into space, causing an increase in temperature until complete evaporation of the oceans is achieved. This happened on Venus billions of years ago. Fortunately, even though the Earth is currently experiencing global warming, and even though it has experienced other warming events in its past, everything points to the fact that a process like the one on Venus could not take place , especially not due to human causes. It would take thousands of times more CO 2 in the atmosphere for this to happen.

References:

T.M. Donahue et al, 1982, Venus was wet: a measurement of the ratio of deuterium to hydrogen, Science, 216, DOI: 10.1126/science.216.4546.630

J. F. Kasting et al, 1988, Runaway and moist greenhouse atmospheres and the evolution of Earth and Venus, Icarus. 74 (3), doi:10.1016/0019-1035(88)90116-9

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