Thermal Properties of the Martian Atmosphere
Mars has a unique atmosphere due to the planet’s mass and lack of a magnetic field. The atmosphere can be divided into four separate regions named the exosphere, thermosphere, mesosphere, and troposphere. The exosphere is the outer most region extending from two hundred kilometers until the vague radius at which it blends with space. Below the exosphere is the thermosphere or upper atmosphere where the sun bombards the gasses with thermal radiation causing layers of high temperature gas. Approaching the surface, the thermosphere merges with the mesosphere just above one hundred kilometers above the Martian surface. The mesosphere ranges from sixty to one hundred kilometers above the surface of mars. In this region of the atmosphere the temperature is more or less constant. There do exist variation in temperature due to thermal wave propagating vertically away from the planet. These waves cause adiabatic heating and cooling, leading to global thermal tides. The amplitude of the tides increases as the propagate upwards. These waves gain amplitude until the lapse rate becomes too high and they ‘break’ which leads to local mixing. Below the mesosphere exist the warmest and densest region, the troposphere. This region is relatively warm due to suspended dust particles which absorb radiation from the sun. The average lapse rate in the troposphere is 2.5 K km-1. This is less than the lapse rate of dry adiabatic air due to the heat from suspended dust. Below fifteen kilometers convection plays a large role in the temperature of the atmosphere. Above this altitude temperature is mostly dictated by solar radiation. The relationship of the Martian atmosphere’s temperature as a function of altitude can be seen in Figure 1. (Haberle) At the surface of the red planet the average temperature is 210 K.
Figure 1: Temperature of Martian Atmosphere calculated from deceleration of Viking 1 (blue), Viking 2 (green), Pathfinder (red)
Five gases account for over ninety-five percent of the atmosphere. Carbon Dioxide makes up much of the composition at 95.32%, followed by diatomic Nitrogen (2.7%), Argon (1.6%), diatomic Oxygen (0.13%), and Carbon Monoxide (0.08%). Present are trace amounts of other gases such as gaseous water (210ppm), Nitrogen Oxide (100 ppm), Hydrogen-Deuterium-Oxygen (0.85 ppm), Krypton (0.3 ppm), and Xenon (0.08 ppm).
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With a diameter of 6790 km compared to earth’s 12750 km the force of gravity is much weaker on mars (38% of earth’s gravitational field). This cause the atmosphere to be less dense than earths with an average surface density of 0.020 kg/m3. Known as the scale height the pressure of mars’ atmosphere drops by a factor of e 11.1 km above the surface. This is considerably higher than Earth’s scale height of 8.5 km. The carbon dioxide density is modelled in figure 2. The surface pressure is much lower on mars than earth with an average value of 636 Pa. This value is known to vary by around 250 Pa depending on the season. (Williams) This pressure is 0.01 of earth’s surface atmosphere at most. This pressure is well below the Armstrong limit, at which the pressure is low enough for water to boil at human body temperature.
Figure 2: Carbon Dioxide density throughout Martian atmosphere. The solid line shows data of the northern summer solstice (Ls=90-120 degrees) and the dashed line shows data of the northern winter solstice (240-270 degrees). Data was obtained from the Mars Express SPICAM experiment.
Unlike Earth mars is not a closed system, particles do escape. Mar is less massive than Earth, leading to a lower escape velocity (~5 Km/s). Jean’s escape is an atmospheric process that leads to particles exiting the top of the exosphere. A particle on the upper end of the Boltzmann distribution may have adequate kinetic energy to reach escape velocity and leave the atmosphere. Jean’s escape can be modeled by the equation,
Where B is a positive constant less than 1,
is the density at the bottom of the exosphere, U is the average speed of the Maxwell distribution, and
is the escape parameter defined as
(Lewis) This effect is exponentially dependent on the mass of the escaping particle and become negligible for elements heavier than helium. This process is magnified by the energy introduced to the system by solar winds. The effect of solar winds greatly impacts mars as it does not have a magnetic field. The atmosphere is bombarded with charged particles which ionize the gasses and increase their kinetic energy. Hydrogen is the atom that escapes the atmosphere most readily as it is the lightest. This phenomenon can be observed in Figure 2. (Jones)
Figure 3: Mars atmospheric escape obtained by MAVEN’s Imaging Ultraviolet Spectrograph.
A series of reactions occur with the result of water being converted in to diatomic Hydrogen and Oxygen. This Hydrogen escapes leaving ionized Oxygen to recombine with electrons into single atoms, light enough to reach escape velocity. If the water loss is extrapolated over the life span of the planet it would correspond to 2.5m of water covering mars’ surface. (Haberle)
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Both scientists and science fiction fans have posed the question whether it is possible to terraform Mars into a more habitable planet. To achieve this an engineering project of massive scale would be required over decades. Numerous scientists have hypothesized this question and determined the most feasible routes to do so. The consensus is if the atmosphere could be warmed by a large enough temperature the polar ice caps would melt. Melting the icecaps would increase the density and pressure on the surface of mars. With a thicker atmosphere of greenhouse gasses the planet would continue to warm to a point it may be possible for resilient plant life to grow. Proposed methods of warming mars include orbiting mirrors, moving ammonia asteroids, utilizing low albedo material, and developing an artificial magnetic shield.
Dr. Robert Zubrin and Christopher Mckay of NASA’s Ames Research center theorize that a thin mylar mirror could be set in orbit focused on mars’ poles. A mirror with a radius of 125 km would be sufficient to raise the temperature of the south pole by 5 degrees. It is believed that a temperature increase of this order would be enough to melt the majority of the polar ice caps. By melting the ice caps enough carbon dioxide would be released to produce a runaway greenhouse effect. A mirror of this size would have a mass of 200,000 tonne which is not feasible to send from earth. The same researchers proposed importing ammonia asteroids from the outer solar system. The impact of a 2.6 km asteroid would release enough energy to melt 1 trillion tonnes of water. The ammonia would act as a powerful greenhouse gas raising the temperature by 3 degrees. This method would require sustained impacts over decades to make the climate temperate. This method could be utilized to start the process but would have to be stopped once there are human missions to mars as each impact would be very destructive. Another drawback of this method is the lack of data on solar system ammonia objects.
Famous scientist Carl Sagan was proponent of utilizing the albedo effect as a method of warming the Martian climate. He proposed that transporting 109 tons of dark material to the polar ice caps over a century could increase the absorption of solar radiation to melt the ice caps. He suggested multiple materials that could be used from the dark dust of Phobos or Deimos to dark lichens that could survive on snow. Although simple in theory the process of transporting that much material in space in not feasible with current technology. A collaborative team headed by NASA published a paper proposing the possibility of a magnetic shield to protect mars’ atmosphere from solar wind. By creating a magnetic dipole field at mars’ L1 Lagrange point the planet would be within an artificial magnetosphere. With advancing technologies it is not improbable to build an inflatable structure capable of producing a 2 Tesla field which could shield mars from solar wind. The induced magnetosphere would reduce the escape of gasses increasing pressure and temperature. It is hypothesized that a four degree increase in temperature could lead to melting of water in the polar ice caps which is nearly 1/7th of the ancient oceans. The team believes that any effort to terraform mars without reducing the atmospheric escape caused by solar wind is infeasible in the long term.
The Martian atmosphere is well studied with our knowledge growing daily as new data comes in from mars experiments. Mars is within the solar systems “Goldilocks” zone but still has a very inhospitable atmosphere for human space missions. With an average surface temperature of 210 K and pressure 0.636 kPa the atmosphere is not survivable without protection. Many methods have been proposed to terraform the planet’s atmosphere through induced global warming. Although theoretically possible no proposed methods are feasible with current engineering practices and technology.
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