Planetary atmospheres and evolution
Planetary atmospheres are critical components that shape the environmental conditions of celestial bodies. The atmospheres of Earth, Venus, and Mars exhibit distinct characteristics influenced by their evolutionary histories. Earth has a balanced atmosphere primarily composed of nitrogen and oxygen, supporting liquid water and diverse life forms. In contrast, Venus has a dense atmosphere dominated by carbon dioxide, leading to extreme greenhouse conditions, while Mars features a thin atmosphere also rich in carbon dioxide but lacking sufficient pressure to sustain liquid water.
The evolution of these atmospheres began shortly after the formation of the solar system, approximately 4.6 billion years ago. Initial volcanic activity released various gases, which, combined with impacts from icy bodies, formed the early atmospheres. Earth's evolution allowed for weathering processes that sequestered carbon dioxide, while Venus experienced a runaway greenhouse effect that eliminated its water supply. Mars likely had a wetter and thicker atmosphere earlier in its history, but it lost most of its volatiles due to its smaller size and weaker gravitational pull.
Current exploration efforts, particularly on Mars and Titan (Saturn's largest moon), enhance our understanding of atmospheric dynamics and climate change by providing comparative insights. Titan, with its methane-based weather systems, offers an intriguing analogy to Earth’s water cycle. Overall, studying these planetary atmospheres helps clarify the complex interactions that govern climate across different worlds.
Planetary atmospheres and evolution
Comparisons of Earth’s atmosphere with similar atmospheres such as those of Venus, Mars, and Titan provide insight into atmospheric evolution and physics.
Background
The atmosphere of Earth defines the category of terrestrial atmospheres. Similar atmospheres are those of Venus, Mars, and Titan—the largest moon of Saturn. These atmospheres share the basic qualities of terrestrial atmospheres, yet they are quite different from one another. Using comparative planetology, the causes for these differences can be investigated, in turn providing further understanding of the atmosphere of Earth.
The Current Atmospheres of Venus, Earth, and Mars
Earth and its two nearest planetary neighbors, Venus and Mars, possess atmospheres with different qualities. Earth’s atmosphere has an average atmospheric pressure of 1.013 bars and an average surface temperature of around 15° Celsius. The chemical composition of the atmosphere is primarily nitrogen (N2; 78.1 percent by volume) and oxygen (O2; 21.0 percent). It also contains water vapor (H2O; less than 3 percent) and a small amount of carbon dioxide (CO2; about 380 parts per million). The atmosphere of Venus is thicker at 92 bars and has a surface temperature of 500° Celsius. Its composition is almost entirely CO2 (96.5 percent), with a little N2 (3.5 percent) and only trace amounts of H2O (about 50 parts per million).
Mars has a much thinner atmosphere than Earth or Venus. Its atmospheric pressure is 0.006 bar and its surface temperature is -58° Celsius. As with the Venusian atmosphere, the Martian atmosphere is mostly CO2 (95.3 percent) and N2 (2.7 percent) with a trace amount of H2O (less than 100 parts per million). Mars and Venus have no liquid water on the surface, although Mars possesses polar water ice caps. All three atmospheres experience greenhouse warming, about 500° Celsius for Venus, 35° Celsius for Earth, and 6° Celsius for Mars.
The differences between the three planets cannot be explained by current conditions alone. Rather, these differences arose as the planets evolved over the history of the solar system.
Evolution of the Atmospheres of Venus, Earth, and Mars
The solar system formed 4.6 billion years ago; the planets’ atmospheres formed in the first few hundred thousand years. Volcanic activity released CO2, N2, H2O, and other volatiles previously trapped in molten mantles, with additional gases supplied by collisions with icy, volatile-rich small bodies. Some atmospheric gases were lost at the top of the atmospheres to space and at the planetary surfaces to liquid, ice, and incorporation into solids. All three planets probably had roughly comparable early reservoirs of volatiles in roughly the same proportions, meaning all three planets had the same materials to draw on, and each could have developed an atmosphere resembling that of either of the other two. Each atmosphere, however, evolved along a different path.
On Earth, early conditions allowed water in the atmosphere to condense into rain and create liquid water oceans. The rain, the oceans, and exposed regions of crust created conditions in which CO2 could be trapped in solid carbonates through weathering. Weathering kept atmospheric temperatures within a range in which water remains liquid. As the solid Earth cooled further, volcanic activity decreased but weathering continued, removing CO2 and leaving behind an atmosphere largely of N2. The development of biological life also converted some CO2 to O2, with significant oxygen content appearing in the atmosphere around two billion years ago.
Being closer to the Sun, the early Venusian atmosphere was warmer than was Earth’s. This warmth was enhanced by the of an increasingly thick atmosphere of CO2 and H2O. As it grew warmer, virtually all the water on the planet vaporized, preventing weathering. This caused a runaway greenhouse effect, in which atmospheric CO2 and greenhouse warming steadily increased while H2 was preferentially lost at the top of the atmosphere, gradually reducing the water supply. This process created the thick, nearly waterless CO2 atmosphere seen today. The runaway greenhouse effect may date from the origin of Venus’s atmosphere, but another possibility is that a moist greenhouse phase preceded the later phase. The moist phase would have involved condensation, liquid water, and weathering similar to those experienced on Earth at warmer temperatures. This period of moist greenhouse may have lasted for up to two billion years.
Evidence also exists for an earlier, moist or wet phase on Mars. This evidence is apparent in large fluvial erosion features, the detection of substantial subsurface ice, and similar observations. The early atmosphere was probably thicker and wetter than the contemporary Martian atmosphere, with significant rainfall. Debate is ongoing as to whether this rainfall resulted in large oceans or transient floods, relatively short-lived lakes, and more ice and glacial features.
Mars’s wet period probably lasted for less than a billion years, with a substantial amount of liquid water lost to subsurface and the polar ice caps. Only one-half the radial size of Earth, solid Mars cooled more rapidly, so its volcanic activity ended earlier, ending the supply of volatiles. Volatiles continued to escape to space from a planet with a gravitational field 2.6 times weaker than that of Earth. Increasing this loss was a period of heightened asteroid bombardment around 3.9 billion years ago and solar-wind stripping after the disappearance of the Martian around 4.0 billion years ago. These losses reduced the Martian atmosphere to its current thin veneer. Still, the variable obliquity and of Mars over geological time scales may have permitted occasional bursts of subsurface water to surface, creating more recent erosion features.
Observations of Mars and Titan
The program of Mars exploration starting with Mars Pathfinder in 1996-1997 has been critical to reconstructing the past and current states of the Martian climate. These observations are also studied to determine if changes in solar output influence the climate on Mars (where human influences are nonexistent), contributing to the scientific understanding of Earth’s climate and solar output as well. The Martian atmosphere also provides an additional testing ground for general circulation models, potentially improving models of Earth’s atmosphere. Mars exploration programs, such as the famed Preservance Rover program, continued throughout the 2020s.
The Cassini-Huygens mission to Saturn and Titan explored another earthlike atmosphere in the solar system. Titan’s atmospheric pressure is about 1.5 times that of Earth, and the moon has a surface temperature of around -179° Celsius with a largely N2 atmosphere (more than 96 percent). Titan has a phase-change cycle, including rain and surface liquid lakes—but of rather than water. These similarities to Earth make Titan an inviting target for comparative planetology.
Context
Comparative planetology has clarified the evolution of the terrestrial atmospheres. Venus is a planet where greenhouse warming occurred unabated; Mars was too small and too cold to maintain a thick, warm atmosphere. Both resulted in dry climates, likely inhospitable to life. The evolution of Earth struck a more hospitable balance, resulting in abundant liquid water and abundant life. Continuing to compare Earth’s atmosphere and atmospheric history with those of Venus, Mars, and Titan will promote further understanding of the physics and chemistry that govern climate change.
Key Concepts
- comparative planetology: scientific approach that tests theories by comparing data and observations of multiple planets
- terrestrial atmosphere: a globally significant atmosphere covering a rocky planetary body
- volatile: an element or compound with a low melting or boiling point
- weathering: a chemical process in which atmospheric carbon dioxide is converted to carbonates through reactions with water and silicate minerals
Bibliography
Atreya, S. K., J. B. Pollack, and M. S. Matthews, eds. Origin and Evolution of Planetary and Satellite Atmospheres. Tucson: University of Arizona Press, 1989.
De Pater, Imke, and Jack J. Lissauer. Planetary Sciences. New York: Cambridge University Press, 2001.
Grinspoon, David H. Venus Revealed: A New Look Below the Clouds of Our Mysterious Twin Planet. New York: Basic Books, 1998.
Lorenz, Ralph, and Jacqueline Mitton. Titan Unveiled: Saturn’s Mysterious Moon Explored. Princeton, N.J.: Princeton University Press, 2008.
Read, Peter L., and Stephen R. Lewis. The Martian Climate Revisted: Atmosphere and Environment of a Desert Planet. Berlin: Springer-Verlag, 2004.
Visscher, Channon. "Planetary Atmospheres: Chemistry and Composition." Planetary Science, 24 Feb. 2022, doi.org/10.1093/acrefore/9780190647926.013.17. Accessed 21 Dec. 2024.