Mars' atmosphere: Facts about composition and climate

The atmosphere of Mars changes over the course of a day because the ground gets extremely cold at night on Mars, down to around minus 160°C. At such cold temperatures, both major and minor constituents of the atmosphere might either condense (snow, frost) or just stick to the soil grains a lot more than they do at warmer temperatures. Because of differing condensation temperatures and "stickiness", the composition can change significantly with the temperature. 

During the day, the gases are released from the soil at varying rates as the ground warms, until the next night. It stands to reason that similar processes happen seasonally, as the water (H2O) and carbon dioxide (CO2) condense as frost and snow at the winter pole in large quantities while sublimating (evaporating directly from solid to gas) at the summer pole. 

It gets complicated because it can take quite a while for gas released at one pole to reach the other. Many species may be more sticky to soil grains than to ice of the same material, so for those chemicals, the diurnal change could be more significant than the seasonal change. 

The seasonal variation changes the global air pressure on Mars and changes which gases are in the air. On Mars, the air pressure can change globally by ±8%, depending on the season. The result is that there's no specific place, time of day, or season that fully represents THE composition of Mars' atmosphere. Finally, sunlight drives atmospheric chemistry by way of solar ultraviolet light breaking down CO2 and H2O molecules that are then free to react in new ways. As a result, the changing pressure and abundance of H2O and CO2 changes the abundance of carbon monoxide (CO), oxygen (O2), ozone (O3), and other trace species.

The most obvious reason to think that Mars once had a much more dense atmosphere is that there are clear signs all over Mars of erosion by water in processes that occur on Earth but could not occur on Mars as it is today. 

There are river channels, eroded valleys, rocks worn into round river cobbles and dumped at the end of valleys, consistent with the way that rivers deposit rocks. More has changed than just the amount of water. Under current conditions, a puddle of liquid water at the surface would quickly evaporate, or freeze and then sublimate, because of the extremely low air pressure. For water to have once persisted long enough to flow in large quantities and to pool, the air pressure had to be much greater. 

From the surface, rovers have detected salt minerals layered in patterns that are consistent with the way that salty water forms lakes on Earth that evaporate over long periods of time in places like the Great Salt Lake or the Dead Sea, and in fully dried lakes such as salt flats in the American West, in Africa, and numerous other places. From orbit, we have observed "bathtub rings" in large depressions in the surface and detected minerals that can only form when materials dissolve in water and have time to react with each other and accumulate in large quantities.

What we lack is a good estimate of how much more dense the atmosphere once was. An important clue is in the isotopes of atmospheric gas atoms. Atoms in gases exist in different versions, called isotopes, that each have the same number of protons and electrons, but different numbers of neutrons in the nucleus. There are two stable versions of carbon, and three stable versions of oxygen. 

The ratio that we measure now between the abundance of stable isotopes depends on the original ratio when the planet formed, and then how it changed over time as heavy or light isotopes (more or fewer neutrons) were removed from the atmosphere by chemistry to form solids, or escaped to space. Hydrogen, nitrogen, and argon atoms are all enriched in heavy isotopes, consistent with losing a lot of gas to space, which removes the light isotopes faster than the heavy ones. Carbon and oxygen, which make the main atmospheric gas, carbon dioxide (CO2), are much more confusing. 

Measurements from the Phoenix lander and Curiosity rover show the heavier isotopes are enriched, but they do not agree with each other, and neither agrees with Viking measurements from the 1970's that showed no enrichment. We need an estimate of what the isotope ratios actually are today, what they once were in the deep past, and how quickly each kind of isotope is lost from the atmosphere. With those things, we can estimate how much atmosphere has been lost and thus estimate how much atmosphere Mars once had. It will be a slow and messy process of gradually improving measurements, vigorous disagreements, improving understanding, and comparing results from entirely different methods to hone in on the truth.

We also have another published set of measurements, made by telescope from Earth, that suggests that the relative amount of heavy isotopes in CO2 changes with the ground temperature over the course of the day. These measurements were interpreted by me as due to the heavy versions of CO2 sticking to soil grains at low temperatures more effectively than the light isotopes, so that the isotope ratios change throughout the day and only get close to the real value of heavy isotope enrichment when the ground is warmest. That would suggest that the reason none of the previous measurements agree with each other is because they were all acquired at different times of day, under different climate conditions, at different seasons. However, my interpretation could be wrong. 

The telescopic measurements need to be tested with other measurements, from Earth and at the surface of Mars, to determine whether it is a real process, whether it is routine, or whether the interpretation is a misunderstanding of the data. A powerful argument against my interpretation is that laboratory measurements do not yet show any process that could be effective enough to make this happen to the extent that the measurements appear to show. Also, we don't have the data (yet) to show whether there is a significant seasonal effect in atmospheric isotope ratios on Mars. There probably is not, because lab measurements show that freezing pure ices of CO2 does not result in significantly changing isotope ratios in the remaining gas. But that could be incorrect in the complex environment of a planet's surface.

Mars once was a more Earth-like planet, but it was the ancient Earth that it resembled, a planet with a CO2-rich atmosphere and no free oxygen, with extensive oceans that may have been frozen-over much of the time. We don't know for certain what chemicals were in the atmosphere of Earth besides CO2, N2, and H2O prior to the rise of widespread oxygen-producing photosynthesis that radically altered Earth's atmosphere. There could have been gases like methane (CH4) and ammonia (NH3), sulfur-bearing gases, and other gases that are not common today. We don't see those gases in Mars' atmosphere today, either, but possibly they were there in the past. The Sun has also been slowly increasing in luminosity, so that Mars and Earth of a few billion years ago had less sunlight to warm them, but more CO2 in the atmosphere to hold in the heat. Mars of a few billion years ago probably had exposed lakes and oceans, and probably had rain.

Given how much uncertainty there is about how Earth's atmosphere may have been different when it was a younger and "less Earth-like" planet, it is awfully hard to make an educated guess as to what Mars' ancient atmosphere may have been like. Earth is particularly challenging because there has been constant chemistry and weathering as well as plate tectonics remaking the planet's surface over billions of years, minerals reacting with the atmosphere and sinking to the bottom of the oceans, then buried by tectonics. As we continue to gather new information about Mars, it may end up being our best guide to what Earth once was like, because the water chemistry on Mars, and plate tectonics, turned off a long time ago, leaving the rocks of that time intact for us to examine.

The rocks remember. Many types of rock can only form at the surface, where water dissolves both the minerals and the atmosphere so that they can react with each other to form new compounds that can condense to form new minerals. Examples on Earth include limestones (carbonate minerals) and many iron-bearing minerals. 

The goal of the Perseverance rover mission on Mars now is to obtain samples of surface rocks in the mouth of an ancient river by drilling into the interior of rocks in the streambed to extract pencils of material. The exterior of these rocks has been weathered by the modern Mars environment, but the interior represents the chemical conditions under which the rock formed. That includes the relative rate at which different chemical reactions happened (a thermometer), what was dissolved in the water (including the atmosphere), and what isotope ratios were present for different elements. 

Perseverance leaves "caches" of these samples in sealed containers. Sometime in the next decade, a contingent of spacecraft will be sent to retrieve the samples from the surface and launch them to Mars orbit, then take them from Mars orbit back to Earth. It is a tremendously ambitious and complex undertaking, involving many first-time events. Much of what we need to investigate to understand ancient Mars is far beyond the capability of instruments that can be made small, light, and efficient enough to fit onto a spacecraft. We need these samples from Mars to be returned to Earth where we can use the great variety of instruments we have here to study them completely.