Mars's atmosphere

Because many similarities exist among the planetary atmospheres, the study of one planet may contribute to the understanding of others. The atmosphere of Mars, with its simple structure, can be used to model certain aspects of Earth’s atmosphere and is, therefore, a valuable aid in comprehending the past, present, and future of that atmosphere.

Overview

One goal of planetary scientists is to unravel the history of Earth in the context of the origin and evolution of the solar system. Because all data record what is presently observed, conditions and processes of the past must be inferred from current conditions and processes. Scientists construct a model of an atmosphere’s history from those inferences and use that model to project the future evolution of the atmosphere. The model is revised as better data are gathered.

Astronomers generally assume that the terrestrial planets (Mercury, Venus, Earth, and Mars) have had two atmospheres. Primitive atmospheres would have existed soon after the formation of the planets and would have been distinctly different in composition from those that exist in twenty-first century. These atmospheres contained hydrogen, helium, and other lightweight compounds with speeds near or above the escape velocities of terrestrial planets. Consequently, primitive atmospheres would be lost over a reasonably short time by a process of gases escaping into space, even as the planets cooled.

Secondary atmospheres developed as nitrogen, carbon dioxide, water, and argon were released from the planetary interiors as molten rock outgassed and volcanoes erupted. Volcanoes on Earth emit mostly carbon dioxide and water. Because scientists assume that the planets formed from the same cloud of protoplanetary material, they also assume that the gases coming from the interiors of different terrestrial planets would be similar. Therefore, the secondary atmospheres of Venus, Earth, and Mars should contain the same compounds, but the specific quantities of those compounds should vary as conditions on the planets vary. Venus should have mostly water vapor because it is nearer the Sun and has a higher surface temperature than Earth. Mars should have mostly solid water because its distance from the Sun makes it a colder planet than Earth. Of course, Earth has water in all three phases: gas, liquid, and solid.

Until the first spacecraft reached Mars, there was no accurate method for determining the pressure, composition, and temperature profile of its atmosphere. Mars was expected to have a thin atmosphere with its low escape velocity of 5.0 kilometers per second. Most estimates of Mars’s atmospheric pressure made during the first half of the twentieth century were between 1.2 and 1.8 pounds per square inch (Earth’s is 14.6 pounds per square inch at sea level)—high enough for liquid water to exist on the surface of Mars as long as the temperature is not above 310 kelvins (98 degrees Fahrenheit). Most scientists did not believe that Mars could retain much water, but if pressure estimates were accurate, the existence of water on Mars could not be eliminated from models of the planet and its atmosphere.

The composition of the Martian atmosphere was difficult to determine from Earth because the Earth’s own atmosphere obscured much of the information that came from the planet. Because the atmospheres of the terrestrial planets were expected to have similarities, astronomers looked for nitrogen, oxygen, water, and carbon dioxide. Nitrogen is very difficult to observe, so no one was surprised when it was not detected in visible light. Astronomers, however, expected to find a significant amount of nitrogen when a spacecraft arrived at the planet. Earthbound telescopes also failed to detect oxygen or water but found carbon dioxide. Astronomers used this information and their assumptions to predict that the Martian atmosphere was largely nitrogen with some carbon dioxide present.

During the late 1960s, a series of United States Mariner spacecraft flew past Mars and found that the average atmospheric pressure was less than 0.09 pounds per square inch. Roughly 95 percent of the atmosphere was carbon dioxide, with 1 to 3 percent nitrogen. Many astronomers concluded that Mars was dead and moon-like. Subsequent analysis of the Martian atmosphere has shown its composition to be 95.3 percent carbon dioxide, 2.7 percent nitrogen, 0.13 percent oxygen, 0.03 percent water, and 1.6 percent argon, with trace amounts of krypton, xenon, and other materials.

Average temperatures at the surface of Mars are about 210 kelvins (–81 degrees Fahrenheit). The warmest spot on the planet may reach 300 kelvins (80 degrees Fahrenheit) near local noon, but temperatures dropped to less than 192 kelvins (–114 degrees Fahrenheit) at the Viking 2 landing site. Polar regions are much colder, as confirmed in 2008 by the Phoenix lander after it touched down on the Red Planet’s northern polar region. Liquid water will not, of course, exist under these conditions. During the summer days, however, the surface temperature is high enough for liquid water to exist briefly. Because the vapor pressure of water in the atmosphere is very low, this liquid water evaporates quickly. As winter begins, the water molecules freeze to cold dust particles in the atmosphere. Carbon dioxide molecules also attach themselves during the cold nights, and when particles have enough mass, they fall to the surface. The temperatures during the day are high enough to vaporize carbon dioxide but not water. Pictures of the soil around the Viking 2 lander show a frost of these water-ice-coated dust grains. Clouds or ground fog of water ice crystals form about half an hour after dawn in areas heated by the Sun. Beyond latitudes of 65°, winter conditions cause carbon dioxide to freeze, and a hood of carbon dioxide clouds and haze hangs over the polar regions. The coldest regions on Mars are the poles during winter. Polar caps made of water ice and carbon dioxide ice (dry ice) change size as the seasons change, but a permanent cap of water ice remains at each pole, where the temperatures never get warm enough to allow water to melt or vaporize. Mars Phoenix dug trenches into the soil near the lander and found evidence of the permafrost layer not very far beneath the surface. The permanent cap at the South Pole is smaller than the one at the North Pole, because Mars is much nearer the Sun during the South Pole’s summer than during summer in the Northern Hemisphere. The total atmospheric pressure changes by 26 percent seasonally because of the vaporization/condensation cycle of carbon dioxide at the poles. Much of the carbon dioxide vaporized at one pole moves toward the other pole and precipitates there.

In the Northern and Southern Hemispheres, high-pressure systems form during the summer months, and low pressure develops during winter. The pressure difference is the greatest when it is summer in the Southern Hemisphere and winter in the Northern Hemisphere. Large-scale wind currents flow toward the North Pole during northern winter and toward the South Pole during southern winter. Dust particles picked up by these winds cause large-scale dust storms. These storms cause little erosion because the thin Martian atmosphere can carry only small dust particles. Viking 1 measured wind gusts up to twenty-six meters per second as a dust storm arrived. The most vigorous storms may involve wind velocities greater than fifty meters per second. The Mars Pathfinder landing stage provided wind speed and direction data in 1997. It picked up mostly light winds, with direction varying considerably over the mission. On a mission that began in 2003, the Mars Exploration Rovers imaged dust devils. Indeed, some of the dust devils performed a useful function for the rovers, as they blew off dust that had accumulated on solar panels. This cleaning effect boosted spacecraft power.

Some data imply that the Martian atmosphere was denser at one time than is presently observed. Channels (not canals) found by the Mariner spacecraft have the same appearance as channels formed on Earth by flowing water. Many craters show more erosion than is possible with the current atmosphere. Indirect evidence indicates that significant quantities of nitrogen, water, and carbon dioxide have been outgassed from the Martian interior. Many astronomers thus believe that the Martian atmosphere was once denser and warmer than now and that it became moist periodically as polar caps melted. Rivers may have flowed during these periods. As time passed, pressure gradually decreased as water vapor and carbon dioxide were lost from the atmosphere. Those losses reduced the capacity of the atmosphere to retain heat. A change in the tilt of Mars’s rotational axis and a change in Mars’s orbital path also reduced the temperature. As temperatures dropped, water from the atmosphere was permanently trapped in the polar caps. Eventually, much carbon dioxide was also deposited in the polar caps, and the current cycle of vaporization and condensation was established. Exposed solid water ice was seen to sublimate away during Phoenix investigations in 2008.

In mid-January 2009, the National Aeronautics and Space Administration (NASA) announced that ground-based telescopes atop Hawaii’s Mauna Kea had detected the infrared signature of methane in the thin Martian atmosphere. Lacking protection from solar ultraviolet radiation, methane molecules in the atmosphere would be subject to breakdown. Thus, the detection of significant amounts of methane indicated that the gas was almost certainly being replenished. Methane production could have a geological origin, but it also could have a biological one—indicating the potential for the existence of microbial life on Mars. A geological source for Martian methane could involve the conversion of iron oxide into serpentine minerals, a process that would require water, carbon dioxide, and an internal heat source; this process does occur on Earth. A biological origin for Martian methane seen in the atmosphere would involve digestion processes inside microorganisms. The methane data NASA reported could not be used to distinguish between a biological or geological source for Martian methane production. It was hoped that the Mars Science Laboratory, whose Curiosity rover landed on the planet in August 2012, would be able to learn more about methane on Mars and its possible origin, but as of November 2012, the rover had come up empty-handed, detecting methane in only insignificantly small quantities. While Curiosity did detect large methane spikes in late 2014 that NASA and many others believed served as the necessary confirmation, others remained skeptical and questioned whether the rover itself was not the cause of the spikes. he puzzle of Martian methane was further fueled in 2021 when the European Space Agency’s Trace Gas Orbiter appeared to pick up high levels of methane gas only days after the Curiosity recorded low levels. Understanding the accuracy of both reporting vessels, scientists have come to study the fact that Mars’ methane levels must fluctuate regularly and frequently.

In late 2015, the orbiting spacecraft Maven gathered information that gave scientists a better understanding of what has contributed to the rather severe depletion of Mars's atmosphere over time. It was reported that Maven's findings suggest that solar winds and storms created by spurts of gas and magnetic energy from the Sun have been steadily stripping away Mars's atmospheric gases. Because Mars does not have a magnetic field like Earth, the planet does not have a means of shielding itself from these powerful events that occur several times per year. From this data, scientists estimated that atoms escape into space at about a quarter of a pound per second during these phenomena.

Methods of Study

Astronomers face a great challenge in collecting data on objects that are millions of miles away. Earth-based telescopes collect light that is analyzed for relevant information. This technique, however, often does not provide the precision needed for the study of planets. The advent of the space age gave astronomers new opportunities to gather data as sophisticated spacecraft traveled to the remote parts of the solar system. Mariner 4 took the first close-up spacecraft photos of Mars on July 15, 1965. It was followed by Mariner 6 and 7 flybys in 1969, the Mariner 9 orbiter in 1971, and the Viking orbiters and landers in 1976.

Although the Mariner spacecraft had determined that carbon dioxide was the main component of the Martian atmosphere, concentrations of other compounds present in small amounts were still unknown. As the Viking landers descended through the atmosphere, they looked for nitrogen, argon, and other elements whose molecular weight was less than fifty. A mass spectrometer, an instrument specifically designed to find compounds and identify them according to their masses, analyzed the atmosphere at altitudes above 100 kilometers. Nitrogen was discovered with an abundance of roughly 3 percent. Argon, an inert gas, was found with an abundance of 1.6 percent. Another instrument, the retarding potential analyzer, showed that many hydrogen and oxygen ions were escaping from the atmosphere. Because these two elements combine to form water, their loss can be expressed in terms of water loss. Roughly 240,000 liters of water were lost each day.

In 2012 and 2013, the Mars rover Curiosity began analyzing atmospheric samples to determine the concentrations of various isotopes of argon. Argon-36 and argon-38 were found in a ratio of approximately four to one, respectively—a much smaller ratio than is known to have existed in the solar system originally, confirming the hypothesis that the lighter isotope, and lighter gases generally, have been leaving the Martian atmosphere over billions of years. Atmospheric measurements are being recorded by Curiosity's Rover Environmental Monitoring Station (REMS), including wind speed and direction, ground and air temperature, relative humidity, and atmospheric pressure. Although atmospheric phenomena like dust devils have not been directly observed on this mission—partly because of Curiosity's location in Mars's Gale Crater—the rover's Chemistry and Camera Complex (ChemCam) is analyzing dust patterns on the surface left by whirlwinds. Also, various instruments are studying the transfer of water from the ground to the atmosphere and vice versa. Although Curiosity is studying evidence of environmental conditions that could possibly have supported microbial life at one time, conclusive evidence of past or present life on Mars has yet to be discovered. Although the focus of the Mars Perseverance rover, launched in 2021, has been mainly on the surface of Mars, scientists have used that data to also look for clues about the planet’s atmosphere.

Context

Planetary exploration has as its broad goals the search for life and for clues concerning the origin of the solar system. Water and nitrogen, among other atmospheric conditions, must be present for life to exist. The dense, hostile atmosphere of Venus precludes any hope of finding life there. The Martian atmosphere, in some ways, resembles Venus’s atmosphere more than Earth’s; in fact, Venus and Mars each have about 95 percent carbon dioxide and 3 percent nitrogen, but atmospheric pressure on Venus is more than nine thousand times that on Mars. Although there is evidence that water once flowed on the Martian surface and that the atmosphere was once thicker than it is now, there is no absolutely conclusive evidence that there has been enough water in the atmosphere to cause rain. Conditions for the development of life do not seem to have existed in the Martian atmosphere, but much more investigation is necessary to discount the possibility.

Further exploration of the planet will continue to be conducted by robotic spacecraft. Astronomers would also like to send scientists to explore Mars and set up a long-term research base, much like international scientists have established in Antarctica on Earth. Because the Martian atmosphere has a simple structure and because there are no bodies of water to affect the airflow, the atmospheric movement follows predictable patterns. A study of this simple system could lead to a better understanding of the more complex atmospheres of Earth and perhaps Venus. On Earth, this understanding could lead to better prediction of weather patterns, increased agricultural production, and decreased danger from natural disasters. Because carbon dioxide plays an important role in the greenhouse effect, the study of the Martian atmosphere could reveal important information about how to deal with the increasing concentration of carbon dioxide in Earth’s atmosphere. Scientists at NASA have announced they hope to send manned expeditions to Mars by the 2030s.

The beginning of the twenty-first century saw a tremendous growth in public concern about the warming of Earth's atmosphere, whether from natural causes or from human impact on the planet. However, the understanding of atmospheric physics was insufficient to predict with any certainty the future course of changes in Earth’s climate. A more basic issue than global climate change needs to be investigated before a better model of Earth’s total climate system can be developed: the conditions that influenced the evolution of the terrestrial planets’ atmospheres. Venus, Earth, and Mars have a great deal in common. Essentially, they started with similar early conditions. Why did Venus develop a thick carbon dioxide atmosphere and a runaway greenhouse effect, which left the planet’s surface tremendously hot, whereas Mars became arid and cold, with a thin carbon dioxide atmosphere? Why did Earth’s atmosphere develop in a way that led to the production of life as we know it?

Astronomers also believe that a better understanding of Earth’s atmosphere will help them to draw conclusions about the possible existence of other planets like Earth in other solar systems. If life can form from inorganic matter, a careful study of the atmospheres on Earth and Mars could set limits on the range of conditions suitable for life to exist. Such a study may even conclude that life cannot spontaneously erupt from nonliving matter. Such a conclusion would require a total revamping of modern scientific thought.

The major question to be answered in understanding the climate and atmospheric evolution on Mars involves where its water went. If that water remains on the planet in large quantities, it could have huge impact on future exploration of the planet. The discovery of flowing, though temporary and briny, liquid water on the planet's surface in September 2015 did boost hope that the planet may be more habitable for microbial life than originally thought, but the source of the water was still unclear. Further, in 2021, the Mars Reconnaissance Orbiter sent back images of craters believed to be formed in that year, leading to important data about the planet’s subsurface water ice.

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