Eye On The Night Sky: Terrestrial Planet Atmospheres

Looking through the Earth’s atmosphere. Image courtesy of NASA.

by Steven Ransom-Jones

Around four-and-a-half billion years ago, the planets in our part of the solar system, now known as the terrestrial planets, were formed and they looked remarkably similar. Earth, Mars, Venus and Mercury all have rocky compositions, contain a variety of metals and have solid surfaces.

Three of these planets, Earth, Venus and Mars, followed an extremely similar path until around four billion years ago, including massive volcanic activity and the development of almost identical atmospheres deriving from the gases of these eruptions. Each still has an atmosphere circulates, and contains clouds and is energized by energy from the sun. Four billion years ago, these atmospheres were comprised almost exclusively of carbon dioxide with a little oxygen and mere traces of water vapor, ammonia and methane.

How did three similar planets with almost identical atmospheres end up so different?

Let us look at some of the driving forces on our atmospheres:

1. Each body in the universe radiates heat at a wavelength that is a characteristic of its temperature. The fusion reaction in the sun causes it to radiate at a wavelength (based on its temperature) in the visible part of the spectrum. Planets will absorb some of the energy from the sun and reflect the remainder. This absorption warms a planet, causing it to radiate in the infrared part of the spectrum.
2. Certain gasses are transparent to visible light but will absorb infrared radiation. These gases, including water vapor and carbon dioxide, will absorb infrared radiation and become warmer.
3. Where the sun is directly overhead (at the equator), you can imagine a cylinder of rays warming a circle on a planet’s surface. Where the cylinder of rays hit the sloped surface of the poles, the energy is spread out over a much larger area, so that there is less heating.
4. The warmer surface of the equator, combined with the heat-absorbing gasses will heat the atmosphere above the surface and cause the air to rise.
5. The cooler surface at the poles will cause the warm air to descend and cool. As a result, we have air circulation, driven by solar energy, properties of atmospheric gases and different surface temperatures.
6. Where gases condense to form ice at the cooler poles, this will cause further temperature differences as the ice will reflect more of the visible light, resulting in less heating of the surface and the formation of polar caps, reinforcing both the temperature differences and the air circulation.
7. Heating an atmosphere will cause the gas particles to move faster. Not all gas molecules will move at the same (average speed) at a given temperature but there will be a distribution around that average with some significantly faster or slower.

Now, can we use this knowledge to unravel the mystery of how three almost identical atmospheres changed so radically.

Earth and Venus have similar sizes (within 10 percent) and orbit the sun at comparable distances (93 million miles for the Earth and 67 million for Venus). Mars is a little more distant at 142 million miles and has about a half of Earth’s diameter.

Earth

The Earth’s atmosphere is comprised of 78 percent nitrogen, 21 percent oxygen with some carbon dioxide, water vapor and traces of other gases. Around four billion years ago the Earth cooled to a point where the water vapor started to condense and it started to rain (and I mean really rain!). This deluge formed the oceans and allowed basic organic carbon chemistry and simple life to flourish. Photosynthesis increased the amount of oxygen in the atmosphere as plants converted carbon dioxide into carbon for growth and released oxygen.

As these plants died and became buried, the carbon was removed from the atmosphere (for a few billion years, at least) and converted into fossil fuels. Carbon dioxide was also locked into sedimentary rocks such as limestone absorbed by the oceans. The emergence of life actually stabilized the atmosphere into something that can sustain life.

Venus. Courtesy of NASA.

Venus

Venus, being slightly closer to the sun than Earth, enjoyed a little more solar radiation. This differences was just enough to prevent water vapor from condensing. Water vapor, like carbon dioxide, absorbs the infrared radiation from the planet’s surface, causing the temperature in the atmosphere to rise even further until a point is reached where the carbon in the rocks on the planet’s surface is “baked” out, releasing yet more carbon dioxide and accelerating the temperature rise. This “positive feedback” has resulted in an extremely inhospitable atmosphere:

• It remains unbreathable with a composition of 96 percent carbon dioxide and 4 percent nitrogen.
• The pressure is incredibly high; on the surface a person would endure a crushing pressure 92 times what we feel when standing on the Earth (or the equivalent of diving in the ocean to a depth of 3,000 feet).
• The temperature is a fairly constant 864°F, hotter than the surface of Mercury and high enough to melt lead.
• Venus does enjoy clouds, but these are made of sulphur dioxide or sulphuric acid at the very top of the atmosphere as water does not exist on this planet.
• The winds are also pretty strong, at 185mph, encircling the planet every four  days.

Mars 

Looking through the thin Martian atmosphere. Courtesy of NASA.

Mars has a much lower surface gravity (38 percent of the Earth) and enjoys only a quarter of the solar radiation. The lower gravity (lower escape velocity) has allowed, over a period of billions of years, much more of the atmosphere to escape into space (see observation 7 above).

While we see evidence of oceans covering almost a third of the surface, most of the water has condensed into the planet. We have now discovered that subterranean caches of brine probably exist under the poles. Some of the carbon dioxide has bound into rocks. With the removal of water vapour and carbon dioxide, the atmosphere absorbs much less infrared radiation, accelerating the cooling process (almost opposite to what we see on Venus).

The Martian poles comprise of solid carbon dioxide and a little water ice. Each Martian year, as spring arrives, the polar caps evaporate, exposing the dust of the Martian surface under them. With an atmospheric pressure one percent of the Earth, the heating effect on the atmosphere is amplified (less radiation heating much, much, much less atmosphere, but with more carbon dioxide released from the poles) and strong winds result. These winds lift the exposed dust into the atmosphere, allowing the dust to absorb more solar radiation, accelerating the winds. The result is huge dust storms; the storm that started in June this year covered a quarter of the planet’s surface.

In the autumn, cooling causes the carbon dioxide to return to the polar caps in solid form and for a much calmer winter.

Our experience on Mars would be of a thin, cold (-195°F to +70°F range), dusty, carbon dioxide-based environment.

Conclusion

What we experience in our solar system is a form of the “Goldilocks” effect with conditions on Earth allowing life to appear and that life sustaining those conditions. On Mars and Venus, positive feedback has caused the same original atmospheres to evolve in very different ways.

The lessons that we have learned from our own solar system allow us to understand, for a given stellar radiation, what types (size and orbits) of planets may be able to sustain life as we know it. This will help us to understand which of the newly discovered exoplanets may be of most interest to study.

As a footnote, unfortunately, Mercury, being significantly closer to the sun and having a much lower mass did not evolve a significant atmosphere.

“Eye On the Night Sky” is a monthly column by the Door Peninsula Astronomical Society. For more information on the organization, visit DoorAstronomy.org.