HABITABLE ZONE

The Terrestrial planets (Mercury, Venus, Earth, Moon, Mars) are similar to each other in mass, diameter, and distance from the Sun. Because of this, it is expected that their atmospheres should share many similar qualities. Despite this, their atmospheres show significant differences.

Remarks:

There are other differences between the planets, but we consider the above as the key points for developing an understanding of the atmospheric evolution of the Terrrestrial planets.


ORIGIN OF THE ATMOSPHERES

Immediately after formation, Terrestrial planets essentially had no atmospheres (if they had captured some hydrogen and helium from the Solar Nebula, it was rapidly lost to space). Whatever atmosphere a Terrestrial planet has today was either captured or generated after the planet formed; the Terrestrial planets have secondary atmospheres.

There are two suggestions for the generation of secondary atmospheres:

It is not clear which of the above theories are correct.


Outgassing

Current studies of Terrestrial volcanoes show that they do emit large amounts of volatile materials such as water, carbon dioxide, nitrogen, and sulfur dioxide (at left is the Santa Maria volcano in Guatemala), however, it is not clear if enough volatile material can be trapped during the formation of the planets.

For example, consider water. On the Earth, there is enough water to cover the planet to a depth of around 3.6 kilometers. The oceans thus contain a mass of water of

Mass ~ 1.5x1021 kilograms.

The current rate of outgassing of water from volcanoes is

Outgassing = 1011 kilograms per year


Comets

Comets are roughly half water ice and half rocky material. A 2 km comet with density 2 grams per cubic centimeter, thus has mass

M ~ 8x1012 kilograms.

So, roughly 4x108 comets are needed to explain the Earth's oceans.

There are many hundreds of billions of comets in the Solar System (in the Oort cloud) and so, there is an ample supply of comets but, is the rate of cometary impacts sufficiently large to warrant considering comets as a viable source for the Earth's oceans? Based on recent cratering history, the rate of crater formation by 1 km objects is roughly one every few tens of thousands of years.


ATMOSPHERIC RETENTION

Why does Venus have an atmosphere while Mercury does not?

There are two competing effects which determine whether a planet retains an atmosphere:

  • the strength of the gravitational field at the surface of the planet (as measured by the escape speed of the planet)

  • the speed with which the gas particles move around (as determined by the temperature and masses of the particles which make up the atmosphere).

There are therefore two important points:


EQUILIBRIUM SURFACE TEMPERATURES AND THE GREENHOUSE EFFECT

We now define the Equilibrium Temperature. Assume:

For a planet with an atmosphere, because of the presence of the atmosphere, not all of the solar radiation strikes the planet. Some of it is reflected by the cloud layer and returns to space. We measure this effect by defining the Albedo for the planet. The Albedo, A, is the fraction of the solar radiation which is reflected to space. This means that a fraction (1-A) of the radiation reaches the Earth.

Actual Atmospheic and Equilibrium Temperatures

Venus

Earth

Mars

Actual Temp

>850 F

~60 F

-60 F --> -70 F

Eq. Temp

-20 F

-4 F

-70 F

The albedo for each planet is 0.65, 0.35, and 0.15 for Venus, Earth, and Mars, respectively. For Mars, the equilibrium and actual atmospheic temperatures are roughly the same while for Venus and Earth, the temperatures differ significantly. Why? Because both Venus and Earth have significant atmospheres and both exhibit the Greenhouse Effect.

The Greenhouse Effect

The Greenhouse Effect arises because the atmosphere of the Earth allow the bulk of the visible light from the Sun to penetrate to the Earth's surface. The absorbed sunlight heats the surface of the Earth causing the Earth to re-radiate the absorbed energy into space. The problem is that the surface temperature of the Sun is 5,500 Celsius while the Earth heats to a temperature on the order of -20 Celsius (with clouds but without an atmosphere), a temperature well below the freezing point of water! The lower temperature of the Earth means that less energetic radiation is re-emitted; the re-emitted radiation falls in the infrared (IR) portion of the spectrum. The problem is that the atmosphere of the Earth absorbs some of the re-emitted IR (the Greenhouse gases, for example, carbon dioxide, water vapor, methane absorb the IR). This traps some of the outgoing radiation and re-directs it back to the Earth which causes the surface temperature of the Earth to rise (to ~ 15 Cenlsius).

Both Venus and the Earth show Greenhouse Effects; the Earth, however, has a much milder Greenhouse effect than found on Venus. The mild Greenhouse effect is important because it is what makes the Earth comfortable as we now discuss.


Faint Young Sun Paradox

The luminosity of the Sun increases as it has ages; 3.8 billion years ago the Sun was ~25 % fainter than today. This is a conundrum because there was liquid water on the Earth at least 3.7 billion years ago and a simple argument leads to a prediction for the Equilibrium Temperature, Te, which, for the Earth at that time would be below the freezing point of water, Te = -40 C!. Note that Te is determined by simply finding the temperature for the Earth where it radiates exactly the same amount of energy per second as it receives from the Sun in the absence of clouds and an atmosphere. Further, if we were to include an atmosphere with the composition of our current atmosphere, the temperature would rise but would still be less than the freezing point of water.

The answer to the question of Why do we have liquid oceans? then requires that our atmosphere in the past must have had a much different chemical composition than today so that the Greenhouse Effect could maintain liquid oceans or, perhaps, the Sun was much brighter in the past than we now believe.


EVOLUTION OF THE ATMOSPHERES OF THE TERRESTRIAL PLANETS

We consider:

After this, we consider the atmospheres of Venus and Mars (and address the question of why the atmospheres of the three planets are so different).


Atmosphere of the Earth

We first look at the current atmosphere of the Earth. Recall that the current atmosphere of the Earth has a pressure of 1 bar which is ~ 100 times larger than Mars and ~ 1 % that of Venus. The composition of the Earth's atmosphere is 78 % Nitrogen molecules ad 21 % Oxygen molecules with trace amounts of other things, in particular, the greenhouse gases water, carbon dioxide, methane, and CFCs.


The atmosphere is conveniently divided into regions in terms of how the temperature behaves (whether it is increasing or decreasing):

  • Thermosphere: In the thermosphere, Solar radiation is able to ionize (strip electrons off of atoms forming the ionosphere ) and temperature increases with altitude (because atoms absorb Solar radiation). The ionosphere is the layer which traps radio signals and allows them to be heard around the world (it is also the layer which gets disturbed and disrupts radio communication during Solar storms).

  • Mesosphere: There are no strong absorbers of Solar radiation in the mesosphere so temperature decreases with altitude there.

  • Stratosphere: The next layer of the atmosphere is known as the Stratosphere and is broken up into layers composed of different materials (i.e., it is stratified from which follows its name). The stratosphere is the layer where Ozone lives. In the stratosphere, because Ozone absorbs Solar ultraviolet radiation, temperature increases as you move upward in altitude through the stratosphere.

  • Troposphere: The lowest layer of the atmosphere, the troposphere is where atmospheric convection occurs and is the layer which contains most of the water. The troposphere is the layer where weather is generated. In the troposphere, temperature declines with altitude. At the top of the troposphere, clouds form (because it gets too cold for water to be vapor). This traps the water in the troposphere, the so-called Cold Trap. Because the ozone layer lies in the Stratosphere, the water in the troposphere is shielded from the Solar UV radiation and is not destroyed by photodissociaion.


What Happened to Venus and Mars?

Terrestrial planets (the atmosphere ones) are roughly the same sizes and same distances from the Sun and yet, they have grossly different kinds of atmospheres and conditions on their surfaces. Do we have any ideas as to what leads to the huge differences? Surprisingly, there may be simple explanations.

Venus, Earth, and Mars

In the beginning, we believe that the material which was outgassed from the interiors or carried in by comets onto the Terrestrial planets was similar. That is, the Terrestrial planets started out roughly the same. Originally, they were dominated by water, carbon dioxide, sulfur dioxide, carbon monoxide, suflur, cholorine, nitrogen, molecular hydrogen, sufur, nitrogen, and chlorine, ammonia, and methane. As the Earth evolved, oceans formed in the Early Archean period (the time before 2.5 billion years ago) as the Earth cooled. What are the consequences of the formation of extensive liquid oceans?

On the Earth and Mars:

On Venus:


What About the Free Oxygen in the Earth's Atmosphere?

Today, we see that the atmosphere of the Earth contains ~21 % free oxygen. As noted above, at birth there was no free oxygen. This is good because chemical reactions thought to produce amino acids are inhibited by oxygen Where did the oxygen come from?

Oxygen Production:

    (i) Photochemical dissociation - breakup of water molecules by ultraviolet produced free oxygen at ~ 1-2% levels. At these levels, ozone can form to shield Earth surface from ultraviolet (UV) radiation.

    (ii) Photosynthesis - carbon dioxide + water + sunlight ===> organic compounds + oxygen molecules. Produced by cyanobacteria (photosynthetic prokareyotes--blue-algae), and eventually higher plants supplied the rest of oxygen to the atmosphere.

In the Archean period (pre-2.5 billion years ago), there was little or no free oxygen in the atmosphere (< 0.001 % of the current level of oxygen, PAL). What little oxygen produced by cyanobacteria was probably consumed by the weathering process. Only after rocks at the surface were sufficiently oxidized could free oxygen remain free in the atmosphere.

During the Proterozoic era (~2.5 to 0.5 billion years ago), the free oxygen rose to 1 % to 40 % of PAL. Most of the oxygen was released by cyanobacteria, which showed a strong increase in abundance (in the fossil record) about 2.45 billion years ago. The present level of free oxygen probably was achieved around ~400 million years ago.

Evidence from the Rock Record: (i) iron is extremely reactive with oxygen. If we look at the oxidation state of Fe in the rock record, we can infer much about atmospheric evolution. In the Archean period, we find minerals in sediments that can only form in non-oxidizing environments: Pyrite (Fool's gold), and Uraninite. These minerals are easily dissolved out of rocks under present atmospheric conditions. Banded Iron Formation (BIF) - Deep water deposits in which layers of iron-rich minerals alternate with iron-poor layers, primarily chert. Iron minerals include iron oxide, iron carbonate, iron silicate, iron sulfide. BIF's are a major source of iron ore, because they contain magnetite which has a higher iron-to-oxygen ratio than hematite. These are common in rocks 2.0 - 2.8 billion years old, but do not form today. Red beds (continental siliciclastic deposits) are never found in rocks older than 2.3 Billion years, but are common during the Phanerozoic time. Red beds are red because of the highly oxidized mineral hematite that probably forms by oxidation of other Fe minerals that have accumulated in the sediment.

Conclusion - amount of free oxygen in the atmosphere has increased with time.

Biological Evidence: Chemical building blocks of life could not have formed in the presence of free oxygen. Chemical reactions that yield amino acids are inhibited by the presence of even very small amounts of oxygen. Oxygen also prevents the growth of many primitive bacteria such as photosynthetic bacteria, methane-producing bacteria and bacteria that derive energy from fermentation.

Conclusion - Today, most primitive life forms are anaerobic suggesting that the first forms of cellular life probably also had similar metabolism. Today, such anaerobic life forms are restricted to anoxic (low oxygen) habitats such as swamps, ponds, and lagoons. Atmospheric oxygen built up in the early history of the Earth as the waste product of photosynthetic organisms and by burial of organic matter away from surficial decay. This history is documented by the geologic preservation of oxygen-sensitive minerals, deposition banded iron formations, and development of continental "red beds" or BIFs.


Where is Venus's Water?

Because there is no Ozone layer, the temperature simply decreases as you move up in altitude around Venus. There is not a water trap and the water vapor is free to rise up into the high levels of Venus's atmosphere where it is broken up by Solar radiation. The hydrogen atoms from the water then escape to space and the oxygens combine with other atmospheric gases to form different molecules. Venus thus loses its water. After the water is lost, the Greenhouse effect eases and the temperature drops to the mild ~ 800-900 F of today and the pressure drops to 90 bars.


THE GAIA HYPOTHESIS


MARS AND WATER

An upshot of the above scenario is that in the past Mars could have had a much thicker atmosphere and been much more earth-like (there are models which suggest that the early Mars had an atmospheric pressure of 2 bars!). This is interesting because, today, the atmospheric conditions on Mars are such that liquid water cannot exist on the surface of Mars. We do see evidence, however, for water on Mars. For example, there is in water in the northern residual polar ice cap:

The polar caps on Mars have two parts; regions that show seasonal variations and residual caps. The seasonal caps are thought to be composed of frozen carbon dioxide. The residual caps are smaller and brighter than the seasonal caps and show a very marked north-south asymmetry. The southern residual cap is frozen carbon dioxide while it is believed that the northern residual cap is water ice (supported by the observation that water vapor is observed over the residual cap in the northern summer and the temperatures of the caps).

Polar Caps and Clouds


In addition to the water in the northern residual polar caps, there is also evidence for water in the low-lying clouds above canyons, and in large glaciers lying scattered rocky debris:

Clouds Above Canyons

Glaciers on Mars

Huge glaciers up to half a mile thick which lie close to the equator of Mars are thought to be the remnants of an ice age on Mars. It is thought that the glaciers formed up to 100 million years ago and represent evidence of climate change on Mars. Hundreds of glaciers have been identified by researchers using ground-penetrating radar which allows them to see through the rocky layers of debris covering the ice. The largest glacier is 13 miles long and more than 60 miles wide. It could be a source of water for astronauts on Mars. When the glaciers formed, Mars' climate was much colder because the angle Mars' spin axis makes with its orbitl axis was much greater than it is now. This allowed ice sheets to extend far beyond the polar regions and towards, possibly even reaching, the Equator.


There, presumably, is also a permafrost layer on Mars even today as implied by Outflow Channels (large channels which can be up to 100 kilometers and thousands of kilometers long--likely formed by catastrophic flooding), "Islands", and Splosh Craters (oozing mud formed by impacts which melted the permafrost layer). The outflow channels and islands were produced by massive floods on Mars. Presumably what happened was that some event (possibly the impact of a large object) caused a rapid, large-scale melting of the permafrost layer which caused floods.

These is ample evidence that water exists on Mars much of it below the surface which can be melted and lead to transient flows.


There is also evidence that in the past water existed in liquid form on the surface of Mars under quiescent conditions ===> grossly different atmospheric conditions in the past than presently. There is thus evidence that the climate of Mars may have been more Earth-like in the past than it is today. This leads to the hope that perhaps life existed on Mars in the past.


Life on Mars?


More recent missions to Mars have greatly improved our picture of the Martian surface and the conditions on Mars today. Curiosity was launched on Nov 26, 2011 reaching Mars on Aug 5, 2012 (landing in Gale crater).



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