Mars has been the most extensively studied planet in the Solar System, except of course Earth. For the last 25 years, these missions have focused on the search for life by “following the water.” Although we have acquired compelling evidence of flowing liquid water on early Mars, the fundamental question about how water could be stable under Martian atmospheric conditions remains unsolved. Everything we have learned about Mars points towards a freezing cold Martian climate that would be incapable of stabilizing liquid water throughout Mars’ history.
The two ideas that suggest liquid water could not be stable on early Mars are the “Faint Young Sun Paradox” and the Martian orbit. The following is a summary of two recent papers about the problem of Mars’ early climate: “The climate of early Mars,” by Robin Wordsworth, and a book chapter by Robert Haberle and coauthors, “The Early Mars Climate System.”
Water, water everywhere…
Mars today as we know it is a cold and dry desert with a thin atmosphere not capable of stabilizing liquid water on its surface. However, there is ample evidence that Mars had flowing liquid water on its surface about 4 to 3.7 billion years ago (named as the Noachian Period). The evidence gathered by Mars orbiters, rovers, and landers is geomorphological; (valley networks, crater lakes, purported Northern ocean, glacial landforms, etc.); mineralogical (iron- and magnesium-rich clay minerals, sulfates, chlorides, iron oxides, and oxyhydroxides, etc.); and isotopic (noble gases, nitrogen, hydrogen, oxygen and carbon).
Challenge #1: The Faint Young Sun
The source of energy in the sun is nuclear fusion, a process in which two or more atomic nuclei combine (fuse) to form one or more different atomic nuclei and other subatomic particles. Stellar Nucleosynthesis, a special nuclear fusion process, is the creation of new atomic nuclei from pre-existing nuclei occurring in stars like our Sun. Main Sequence Stars like our Sun evolve, changing composition over time. This increases the average mass of the star that leads to contraction of the star’s core causing subsequent heating up and brightening (increase in luminosity) of the star. The rate of fusion increases with the star temperature and density and results in increasing the star’s luminosity over time. Therefore, the Sun must have been less bright than the present day and the luminosity of the Sun about 4 billion years ago (when Mars had flowing liquid water) was only 75 percent of the present value. With less heat from the Sun, it would be more difficult for liquid water to be stable on Mars than it is today. Some scientists have hypothesized that the Sun may have lost a lot of mass during its youth (2% in the first 2 billion years), and an earlier, more massive star could compensate for its relative faintness. However, based on the study of nearby Sun-like stars, we believe the probability of this event occurring in our Sun’s history is extremely slim.
Challenge #2: The Martian Orbit
Mars is about 50 percent farther away from the Sun than Earth, with a semi-major axis of 1.524 AU, and therefore receives about 40 percent less solar insolation than Earth. The mean distance of Mars from the Sun should have remained relatively constant during the last 4 billion years. However, other aspects of the orbit have experienced greater variations. The orbital eccentricity has ranged between 0 (circular) and 0.125 (slightly more elliptical than its present value of 0.094). Its axial tilt has ranged between 10 and 60 degrees or more. These variations can play an important role in determining peak summertime temperatures on Mars. However, these changes have little effect on the total annual solar energy received by Mars.
Preliminary calculations considering standard Martian conditions predict an equilibrium temperature (Te) of 210 Kelvin (minus 63 degrees Celsius or minus 81 Fahrenheit) if Mars were so dark that it absorbed all incoming solar radiation, a perfect blackbody. (It is not a perfect blackbody, but it only reflects about 15 percent of the light that strikes it, so it is not a bad simplifying assumption). For liquid water to be stable at Earth atmospheric pressure, the surface temperature of Mars must be 63 Kelvin (63 degrees Celsius or 113 degrees Fahrenheit) warmer. What could warm early Mars by so much?
The most obvious solution to this problem seems to be greenhouse warming. In order for greenhouse warming to make a water cycle happen on Mars, it needs to fulfill the following requirements:
Can Carbon Dioxide Do It?
One way to intensify the greenhouse effect is to increase the density of the atmosphere. Carbon dioxide (CO2) is the most abundant gas in the present-day atmosphere of Mars. It is also what makes Venus so hot. An estimated 70 to 13 bar of CO2 is calculated to have degassed from the interior of the planet as Mars’ crust formed. Later large-scale volcanic and tectonic activity, like the formation of the huge Tharsis province of volcanoes, could have added as much as 1.4 bar of CO2 to the existing atmosphere.
If Mars once had such a dense CO2 rich atmosphere, where did it go? We need to find a way to account for its loss and its evolution to the thin atmosphere left today. Escape processes like sputtering, ion escape, etc. are considered highly ineffective ways of atmospheric loss and can account to a maximum of about 100 mbar CO2 pressure loss over the last 3.8 billion years. The remaining CO2 could be buried in the Marian crust in the form of carbonates. Surface carbonates are not commonly observed on Mars, at least from orbit, although a large carbonate reservoir could be hiding where our orbiters cannot see it.
A unique approach to estimating the ancient atmospheric pressure is by measuring crater size distributions in ancient terrain. A thicker atmosphere burns up more small impactors before they hit the ground, so the diameters of the smallest craters can lead to an estimate of the density of the atmosphere. A study based on this principle suggested an upper limit of 0.9 to 3.8 bar atmospheric pressure about 3.8 billion years ago. To find out the exact atmospheric pressure that existed in the past is a difficult task. However, based on studies conducted to date, Mars is expected to have an atmosphere of about 1-2 bars during its most ancient Noachian era.
There is a problem with a thick CO2 atmosphere, though. A groundbreaking study on CO2 phase changes demonstrated that CO2 could significantly increase the planetary albedo (the reflectivity) and condense out as dry ice (solid CO2), which effectively reduces the greenhouse warming of the planet. This result has been tested and confirmed by more recent, robust methods.
Help From Other Gases
However, CO2 could increase the atmospheric temperature more if it has some help. Greenhouse warming from CO2 is ineffective at some wavelengths, predominantly around 10 to 25 microns, in the thermal infrared, where carbon dioxide is transparent. Therefore, the next step would be to find out other minor greenhouse gases which can absorb effectively in the CO2 “windows.” Water vapor seems like a good option at first, but its effectiveness turns out to be limited at lower temperatures. Help from water vapor only yields an estimated equilibrium temperature of 225 Kelvin (minus 48 degrees Celsius or minus 55 Fahrenheit), which is not sufficiently warm to sustain liquid water on the surface of early Mars.
A lot of work has been done to search for the right combination of greenhouse gases in order to achieve warming of the early Martian climate. Methane (CH4) is an extremely potent greenhouse gas on Earth today but does not help much on Mars because it is most effective outside the CO2 “window” discussed earlier. On the other hand, hydrogen (H2) has been found to cause a significant increase in terrestrial atmosphere temperature when present with gases like nitrogen and carbon dioxide. The combination of these gases with H2 can allow energy absorption by H2 in the CO2 window. However, there is a small problem to address. The Martian mantle is thought to be oxidizing and therefore should not degas a lot of H2, which requires the mantle to be reducing. Research is being conducted to find out alternative ways of H2 production that can solve this puzzle. The effectiveness of water vapor and H2O clouds as Martian greenhouse gases is currently under debate and therefore requires future research.
The Martian crust is enriched in sulfur, which motivates the study of sulfur-bearing gases like sulfur dioxide (SO2) and hydrogen sulfide (H2S) as greenhouse gases on Mars. SO2, if present in the Martian atmosphere at about 10 parts per million, can act as a moderately effective greenhouse gas, while H2S is found to be a much less powerful warming agent. However, SO2 can break down in the Martian atmosphere and studies have found its lifetime limited to a few hundred years. Additionally, SO2 can form sulfate aerosols that reflect more light by increasing the albedo and thereby effectively cool the planet.
Our current understanding of the Early Mars climate is incomplete. Evidence for the presence of liquid water on the surface is staring us in the face. However, we cannot explain how it flowed. Further research is required in order to explain the climate conditions that allowed the stabilization of liquid water on Early Mars.