Tikalon Blog by Dev Gualtieri

Martian Atmosphere

January 27, 2020

Early observations of Mars revealed a seasonal variation in its color and luminance. It was easy for planetary astronomers to imagine surface vegetation blooming in the Martian spring, thriving in the Martian summer, retreating in the Martian autumn, and becoming dormant in the Martian winter. However, some observations were contrary to this idea of a vegetated Mars. In 1784, William Herschel (1738-1822), who is famous for his discovery of the planet, Uranus, deduced that Mars had very little atmosphere when he observed its transit across stars.

Despite Herschel's observation, people still had hope for life on Mars, since Mars and Earth are so similar. A Martian day is just 37 minutes, 22 seconds longer than an Earth day, and its axial tilt to its orbital plane, which is responsible for the Martian seasons, is 25.19°, quite close to Earth's 23.44°. A Martian year, however, is 2.1354 Earth years.

Many in the 19th century believed that most planets harbored life. In 1862, French astronomer and author, Camille Flammarion (1842-1925), published his La pluralité des mondes habités (The Plurality of Inhabited Worlds), a book that promoted this idea.[3] Unfortunately, Flammarion was dismissed from his position at the Paris Observatory within a year of its publication, an event perhaps caused by the book's content.

Many in the early 19th century believed that the darker features on Mars were oceans, and the lighter features were land, and the idea of life on Mars was reinforced by the mapping of Martian "canals" and publications about Mars by Percival Lowell (1855-1916) in the period 1895-1906.[1-2] The possibility of intelligent life on Mars was the plot of H.G.Wells' (1866-1946) The War of the Worlds, published in 1897.[4]

An 1874 map of Mars by Dawes. This is fig. 22, Chart of Mars, on page 167 of Flowers in the Sky External by Richard Proctor.[6] Martian canals were mapped by Italian astronomer, Giovanni Schiaparelli (1835-1910) in 1877. His use of the Italian word, canali, for channel was mistranslated to mean a water canal, and this sparked speculation that intelligent Martians were using such canals to transport water from the polar regions. Subsequent observations by better telescopes in the early 20th century indicated that these features were optical illusions. (From the Library of Congress Digital Collections. Click for larger image.)[5-6]

Even technically-minded people found Lowell's arguments convincing. Inventor of the first practical telephone, Alexander Graham Bell (1847-1922), wrote an eight page letter to his wife in 1909 about life on Mars in which he states that "there is no escape from the conviction that Mars is inhabited by a highly civilized and intelligent race of beings carrying on a process of agriculture, and wringing subsistence."[1] However, there was strong scientific evidence that the Martian atmosphere was quite unlike our own. In 1874, astronomer and subsequent director of the Lick Observatory, William Wallace Campbell (1862-1938), used spectroscopy to confirm the absence of oxygen and water on Mars.

As they say, a picture is worth a thousand words. On July 15, 1965, NASA's Mariner 4 spacecraft acquired the first closeup images of the Martian surface. The crude technology of that time allowed for acquisition of just 21 images, each of which required six hours to be transmitted back to Earth, and the entire data stream of the mission was less than a megabyte. The images revealed a Moon-like, cratered surface. Other data indicated a surface atmospheric pressure of 4.1 to 7.0 millibars (410 to 700 pascals).

An image of the Martian surface, taken from a distance of 7,830 miles on July 14, 1965, by the Mariner 4 spacecraft.

(NASA/JPL image, via Wikimedia Commons. Click for larger image.)

Subsequent Mars missions, starting with the Viking orbiters, confirmed that Mars is a dry, desert planet. While the present atmospheric conditions will not allow liquid surface water, the vast Martian canyons indicate that water was abundant on Mars billions of years ago. The Martian polar regions do contain a mixture of water ice and carbon dioxide ice, and these caps vary in size with the Martian seasons. Water and carbon dioxide, sublimated from the polar regions in warmer weather, enter the atmosphere, and they can deposit as a frost at high latitudes, and elsewhere at high elevations. This frost causes color and luminance changes.

Observations at increased resolution have led to many scientific discoveries. As physicist, Floyd K. Richtmyer (1881-1939) wrote in his article, The Romance of the Next Decimal Place, "The whole history of physics proves that a new discovery is quite likely lurking at the next decimal place."[8] Subsequent observations of the Martian atmosphere have shown unexplained seasonal variation in oxygen and methane concentration (see figure). These phenomena have been reviewed in recent publications.[9-13]

Seasonal variation of atmospheric methane on Mars. These reading of atmospheric methane in Gale crater were taken by NASA's Curiosity rover for nearly three Martian years; that is, about six Earth years). There's a definite, and unexplained, peaking each Martian summer. Earth's atmospheric methane is nearly at the 2000 parts per billion level, so Martian methane concentration is very small. (Created using Inkscape from NASA/JPL-Caltech data. Click for larger image.)

Two international research teams have published observations of Martian methane in a 2018 paper in Science and a recent paper in the Journal of Geophysical Research.[9-11] The Tunable Laser Spectrometer on the Curiosity rover found that the background levels of methane have a mean value of 0.41 ± 0.16 parts per billion by volume (ppbv) at a 95% confidence interval, but the methane concentration also exhibits a strong, repeatable seasonal variation of 0.24 to 0.65 ppbv.[9] While a variation might be expected by ultraviolet light decomposition of meteor-delivered organic compounds and an annual surface pressure cycle, the variation is larger than can be accounted by these processes.[9, 12-13] Also appearing are higher temporary spikes of the order of 7 ppbv that might be associated with methane release from small, localized surface or subsurface reservoirs.[9] Since methane in Earth's atmosphere arises predominantly from biological processes, this might be evidence for Martian methanogenic bacteria.[9]

The recent study, published in the Journal of Geophysical Research, used the Sample Analysis at Mars instrument on the Curiosity rover to measure the concentrations of Martian carbon dioxide, nitrogen, argon-40, oxygen, and carbon monoxide.[11] Measurements have now been completed for multiple years, and they show that the annual average composition is 95.1% carbon dioxide, 2.59% nitrogen, 1.94% argon-40, 0.161% oxygen, and 0.058% carbon monoxide.[11] While nitrogen and argon show seasonal changes caused by pressure changes, oxygen has been observed to show a significant and unexpected year‐to‐year and seasonal variability caused by some unknown atmospheric or surface process.[11] The oxygen peaked by 30% in the spring and summer, then dropped in the fall.[11] Such changes suggests that oxygen is being created by by some process, and then removed.[11]

Martian Atmosphere

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