Mars' atmosphere is over 100 times thinner than Earth's.
Mars' atmosphere is more than 100 times thinner than Earth's and is mainly made up of carbon dioxide, nitrogen, and argon gases.
Oxidized dust particles from the Martian surface fill the atmosphere, giving Mars' skies a rusty tan hue, according to NASA.
Water is present on Mars, but the atmosphere is too thin for it to remain in a liquid state on the surface for long. Instead, water on Mars is located beneath the surface of the polar regions as water-ice and also appears as seasonal briny water flows down hillsides and crater walls.
Despite its thin atmosphere, Mars still experiences a dynamic climate and extreme weather events, including impressive dust storms and even snow! However, Mars wasn't always like this. Scientists from NASA's MAVEN mission reported that Mars once had a thick atmosphere that could have allowed liquid water to exist on the surface for extended periods.
As per ESA, the atmosphere of Mars consists of 95.32% carbon dioxide, 2.7% nitrogen, 1.6% argon, and 0.13% oxygen. The surface atmospheric pressure is 6.35 mbar, which is more than 100 times lower than Earth's. Consequently, humans cannot breathe Martian air.
For human exploration of Mars, it is essential to devise a method to produce oxygen from the thin, carbon dioxide-rich atmosphere. An experiment on NASA's Perseverance rover has shown this is feasible. On April 20, 2021, the rover used its MOXIE (short for "Mars Oxygen In-Situ Resource Utilization Experiment") to successfully transform carbon dioxide into oxygen on Mars. "MOXIE has more work to do, but the results from this technology demonstration are full of promise as we move toward our goal of one day seeing humans on Mars," stated Jim Reuter, associate administrator of NASA's Space Technology Mission Directorate.
Mars' atmosphere: Facts about composition and climate
In its early history, Mars possessed a sufficiently thick atmosphere for water to flow across its surface. NASA indicates that certain surface features imply Mars experienced massive floods approximately 3.5 billion years ago.
Images from orbit reveal extensive river plains and potential ocean boundaries, while several Mars rovers have discovered evidence of water-logged rocks on the surface, such as hematite or clay. Nevertheless, the Martian atmosphere became thinner for reasons that remain largely unknown.
Due to its thin atmosphere and greater distance from the sun, Mars is much colder than Earth. The average temperature on Mars is around minus 80 degrees Fahrenheit (minus 60 degrees Celsius), though it can range from minus 195 F (minus 125 C) near the poles in winter to a comfortable 70 F (20 C) at midday near the equator.
Similar to Earth, Mars experiences four seasons, but the duration of each season varies more significantly than on Earth because of the Red Planet's eccentric orbit, as noted by NASA science.
Length of seasons on Mars and Earth according to NASA Science. | ||
Season (Northern Hemisphere) | Length of Martian season (sols) | Length of Earth season (days) |
Spring | 194 | 93 |
Summer | 178 | 93 |
Autumn | 142 | 90 |
Winter | 154 | 89 |
Mars' ice caps, composed of water ice and carbon dioxide, expand and contract with the seasons. These seasonal variations impact Mars' atmosphere, functioning as a single interconnected system, as noted in a statement from ESA. "The lower and middle levels of Mars' atmosphere seem to be connected to the upper levels: a clear link exists between them throughout the Martian year," explains Beatriz Sánchez-Cano, a planetary scientist at the University of Leicester, UK.
"Every winter, up to a third of Mars' atmospheric mass condenses to create an icy layer at each of the planet's poles. In spring, some of this mass within the caps sublimates back into the atmosphere, causing the caps to visibly shrink," stated ESA.
Giant dust devils frequently stir up the oxidized iron dust that covers Mars' surface. Dust is a constant component of the atmosphere, with higher levels in the northern fall and winter, and lower levels in the northern spring and summer. Mars' dust storms are the largest in the solar system, capable of covering the entire planet and lasting for months. These typically occur in the spring or summer.
These dust storms can disrupt Mars exploration missions and even halt flights (yes, Earth isn't the only place where flights can be delayed due to bad weather!). NASA's Ingenuity Mars helicopter was scheduled for its 19th flight on the Red Planet on January 5, 2022, but a dust storm near Jezero Crater changed those plans.
"Most notably, there was a sharp decline in air density — about a 7% deviation below what was observed before the dust storm," stated Jonathan Bapst and Michael Mischna from Ingenuity's weather/environment team. "This decrease would have pushed the density below the safe flight threshold, posing undue risk to the spacecraft. We also noticed the impact of dust on the amount of sunlight absorbed by Ingenuity's solar array, which dropped significantly below normal 'clear sky' levels, by about 18%." Over a month passed before Ingenuity was cleared to fly again, successfully completing its 19th flight on February 8, 2022.
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One theory for why dust storms can become so large on Mars begins with airborne dust particles absorbing sunlight, warming the Martian atmosphere around them. Warm air moves toward cooler areas, creating winds. Strong winds lift more dust from the ground, further heating the atmosphere, increasing wind, and raising more dust. A 2015 study also suggested that Mars' momentum — influenced by other planets — triggers planet-wide dust storms when its momentum peaks early in the dust storm season.
Occasionally, it even snows on Mars. Martian snowflakes, composed of carbon dioxide rather than water, are believed to be tiny particles that create a fog effect rather than falling snow. The north and south polar regions of Mars are covered by ice, mostly made from carbon dioxide, not water.
At a certain point in its history, Mars lost a significant portion of its atmosphere, changing from a warm, wet planet to the cold, dry plains we observe today, ESA stated.
Mars' atmosphere continues to "leak" into space, but how does this occur?
Mars' atmosphere: Facts about composition and climate
The prevailing theory is that Mars' weak gravity, combined with its absence of a global magnetic field, made the atmosphere susceptible to the solar wind's pressure, which is the continuous stream of particles emitted by the sun. Over millions of years, this solar pressure removed the lighter molecules from the atmosphere, causing it to thin. This process is being studied by NASA's MAVEN (Mars Atmosphere and Volatile Evolution) mission. Some researchers suggest that a massive impact by a small body might have also stripped away the atmosphere.
How and why might the composition of Mars's atmosphere change periodically (for instance, over the course of a day, seasonally, etc.)?
Mars's atmosphere undergoes changes throughout the day because the ground becomes extremely cold at night, reaching temperatures as low as minus 160°C. At these frigid temperatures, both major and minor atmospheric components may condense (as snow or frost) or adhere to soil particles more than they do at warmer temperatures. Due to varying condensation temperatures and "stickiness," the composition can alter significantly with temperature changes.
During the day, as the ground heats up, gases are released from the soil at different rates until the following night. It is logical to assume that similar processes occur seasonally, as water (H2O) and carbon dioxide (CO2) condense as frost and snow in large quantities at the winter pole while sublimating (evaporating directly from solid to gas) at the summer pole.
The situation becomes complex because it can take a significant amount of time for gas released at one pole to reach the opposite pole. Many species may adhere more strongly to soil grains than to ice of the same composition, making diurnal changes potentially more impactful than seasonal changes for those chemicals.
Seasonal variations alter the global air pressure on Mars and affect which gases are present in the atmosphere. On Mars, air pressure can vary globally by ±8% depending on the season. Consequently, there isn't a specific location, time of day, or season that accurately represents the complete composition of Mars' atmosphere. Additionally, sunlight influences atmospheric chemistry through solar ultraviolet light breaking down CO2 and H2O molecules, which then react in new ways. As a result, the fluctuating pressure and amounts of H2O and CO2 affect the abundance of carbon monoxide (CO), oxygen (O2), ozone (O3), and other trace species.
Why do scientists think that Mars used to have a much thicker atmosphere?
The most evident reason to believe that Mars once had a much denser atmosphere is the presence of clear signs of water erosion across the planet, indicative of processes that occur on Earth but are not possible on Mars in its current state.
There are river channels, eroded valleys, and rocks shaped into round river cobbles deposited at valley ends, consistent with how rivers lay down rocks. More has changed than just the water amount. Currently, a surface puddle of liquid water would quickly evaporate, or freeze and then sublimate, due to the extremely low air pressure. For water to have once flowed abundantly and pooled, the air pressure had to be significantly higher.
Rovers have found salt minerals on the surface layered in patterns similar to how salty water forms lakes on Earth, which evaporate over long periods in areas like the Great Salt Lake or the Dead Sea, and in completely dried lakes such as salt flats in the American West, Africa, and many other locations. From orbit, we have observed "bathtub rings" in large surface depressions and identified minerals that only form when materials dissolve in water and have time to react and accumulate in substantial quantities.
We lack a precise estimate of how much denser the atmosphere once was. A significant clue lies in the isotopes of atmospheric gas atoms. Atoms in gases have different versions, known as isotopes, which share the same number of protons and electrons but differ in the number of neutrons in the nucleus. Carbon has two stable isotopes, while oxygen has three stable isotopes.
The current ratio of stable isotopes depends on the initial ratio when the planet formed and how it changed over time as heavy or light isotopes (with more or fewer neutrons) were removed from the atmosphere by chemical processes to form solids or escaped into space. Hydrogen, nitrogen, and argon atoms are enriched in heavy isotopes, suggesting significant gas loss to space, which removes light isotopes faster than heavy ones. Carbon and oxygen, which constitute the main atmospheric gas, carbon dioxide (CO2), present a more complex picture.
Data from the Phoenix lander and Curiosity rover indicate an enrichment of heavier isotopes, but these findings do not align with each other or with Viking measurements from the 1970s, which showed no enrichment. We need to determine the current isotope ratios, what they were in the distant past, and the rate at which each isotope type is lost from the atmosphere. With this information, we can estimate the amount of atmosphere lost and, consequently, how much atmosphere Mars once had. This will be a gradual and challenging process of refining measurements, engaging in debates, enhancing understanding, and comparing results from various methods to uncover the truth.
Another set of measurements, made by telescope from Earth, suggests that the relative amount of heavy isotopes in CO2 varies with ground temperature throughout the day. I interpreted these measurements as indicating that heavy versions of CO2 adhere more effectively to soil grains at lower temperatures than light isotopes, causing isotope ratios to fluctuate during the day, nearing the true value of heavy isotope enrichment when the ground is warmest. This might explain why previous measurements do not agree, as they were taken at different times, under varying climate conditions, and in different seasons. However, my interpretation could be incorrect.
The telescopic measurements need validation through other measurements, both from Earth and on Mars's surface, to determine if this is a genuine process, a routine occurrence, or a misinterpretation of the data. A strong argument against my interpretation is that laboratory measurements have yet to show any process effective enough to cause the observed changes. Additionally, we lack data to confirm a significant seasonal effect on atmospheric isotope ratios on Mars. There likely isn't one, as lab measurements indicate that freezing pure CO2 ices does not significantly alter isotope ratios in the remaining gas. However, this might not hold true in the complex environment of a planet's surface.
Besides thickness, how might Mars's atmosphere differ from when it was a more Earth-like planet?
Mars once resembled a more Earth-like planet, similar to ancient Earth, with a CO2-rich atmosphere and no free oxygen, featuring extensive oceans that might have been frozen much of the time. We do not know exactly which chemicals were present in Earth's atmosphere besides CO2, N2, and H2O before the rise of oxygen-producing photosynthesis that drastically changed Earth's atmosphere. Gases like methane (CH4), ammonia (NH3), sulfur-bearing gases, and others uncommon today might have been present. These gases are not seen in Mars's atmosphere today, but they could have existed in the past. The Sun has gradually increased in brightness, meaning Mars and Earth had less sunlight a few billion years ago but more CO2 to retain heat. Mars likely had exposed lakes and oceans and experienced rain.
Given the uncertainty about Earth's atmosphere when it was younger and "less Earth-like," it is challenging to speculate on Mars's ancient atmosphere. Earth is particularly difficult to study due to constant chemical and weathering processes and plate tectonics reshaping its surface over billions of years, with minerals reacting with the atmosphere, sinking to the ocean floor, and being buried by tectonics. As we gather more information about Mars, it might become our best reference for what Earth was once like, as Mars's water chemistry and plate tectonics ceased long ago, preserving the rocks for examination.
How can scientists learn more about what the Martian atmosphere used to be like?
Rocks hold the memory. Many rock types form only at the surface, where water dissolves minerals and the atmosphere, allowing them to react and form new compounds that condense into new minerals. On Earth, examples include limestones (carbonate minerals) and many iron-bearing minerals.
The Perseverance rover mission aims to collect surface rock samples at the mouth of an ancient river by drilling into streambed rocks to extract core samples. The exterior of these rocks has been altered by the modern Martian environment, but the interior reflects the chemical conditions during formation, including the rate of chemical reactions (a thermometer), dissolved substances in the water (including the atmosphere), and isotope ratios for different elements.
Perseverance deposits these samples in sealed containers. In the next decade, a series of spacecraft will retrieve the samples from Mars's surface, launch them into Mars orbit, and bring them back to Earth. This ambitious and complex mission involves many unprecedented events. Much of what we need to explore ancient Mars exceeds the capabilities of instruments small, light, and efficient enough for spacecraft. We need these samples returned to Earth to utilize the wide array of instruments available here for comprehensive study.
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