martian atmosphere

7/23/2019 Martian Atmosphere

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Marcus Casillas

Professor Hughes

AS 311

12/9/14

Fate of the Martian Atmosphere

As our nearest neighbor, Mars has been a primary subject of humanity’s curiosity, fantasy, and

study for generations. Around the turn of the 19th century, Percival Lowell turned his telescope

towards the red planet and discovered channels carved into the surface. His discovery and writings on

the possibility of water on the surface, sparked the imagination of the public as these Martian waters

may harbor life. In the decades that followed many science fiction writers such as HG Wells and Ray

Bradbury described encounters with intelligent life forms from Mars. But observations from the

Mariner missions starting in the 1960’s revealed a bleaker truth of a cold, dead planet. However more

data from the Viking orbiters revealed that the dusty barren landscape that we see today may not have

always been the case.

Figure 1: Arabia and Deuteronilus shorelines

outlined by Clifford and Parker [2001] [Carr and

Head, 2003]



Closer inspection of the Martian surface revealed what appear to be ancient shorelines where

oceans were once present in the northern hemisphere. More observations in the 1990’s by the Mars

Global Surveyor and Mars Orbiter Laser Altimeter continued to show features on the Martian surface

which appeared to have been eroded by liquid. It became a leading theory then that Mars did in fact

have large bodies of liquid water sometime in its past. However there are a number of issues with this

claim when we observe Mars’s current state.

The leading issue is that the Martian atmosphere is extremely thin. With an average surface

pressure of only 0.6 kilopascals (6 mbar; compared to Earth’s 101 kPa), liquid water would quickly

boil away from the surface. In addition Mars’ average temperature is 238 K (-35︒C), well below the

freezing point of water. Mars must have had a warmer climate and a much thicker atmosphere in the

past to maintain liquid water on the surface. Which leads into our primary questions of: How did those

conditions exist before and why are they no longer present today?

The problems of heat and pressure go hand in hand when discussing the Martian atmosphere. In

order for the planet to have sufficiently high temperatures to hold liquid water on the surface the planet

would require a strong greenhouse effect, which can only be achieved with a thick enough atmosphere.

This is particularly challenging when it’s taken into account that geologic records seem to indicate that

the oceans existed around 4 billion years ago during heavy bombardment. The frequency of impacts

during this period would have posed a threat to the Martian atmosphere as large impacts carry with

them the potential to blast away parts of the atmosphere.

One way that this issue can be dealt with is to take into consideration that weathering can cause

CO2 to be sequestered into rocks in the form of carbonates then later recycled back into the atmosphere.



This temporary sequestering of gases may have protected them from impact erosion, significantly

reducing their effects.

Using this idea as a baseline Carr [1999] modeled a possible solution which would greatly

increase the Martian atmosphere by assuming some favorable conditions. He suggested that if impacts

during heavy bombardment were half as frequent as were normally expected, this would reduce

weathering to 20% of what had been previously suggested. This, combined with 75% of heat

conduction away from the planet, created a model which left a 100 kPa (1 bar) CO2 atmosphere at the

end of heavy bombardment.

However, early models, such as those below, showed that it would be extremely difficult for a

CO2. atmosphere alone to raise the surface temperature to the necessary 273 K required to maintain

water on the surface.

Table 1: Table of necessary mass and composition of atmosphere to reach an average annual surface temperature of 273 K

[Haberle, 1998]



This was due in part to the fact that the Sun was not as bright as it is now during this period and showed

that there must be something more in the Martian atmosphere to create an even more significant

greenhouse effect to make up for the weaker Sun.

One proposed model is that of clouds of carbon dioxide with large crystals of ice. If large

enough, these crystals can increase the scattering of photons trying to pass through the clouds and

significantly increase their greenhouse effectiveness. Initial models of these effects [Haberle, 1998]

showed that temperatures of around 273 K could be reached in atmospheres as thin as 0.5 atm. But

more recent attempts [Forget et al., 2013] have shown that these clouds would have only changed the

temperature by around 15 degrees in the best of circumstances.

The increased greenhouse effect could be due to greenhouse gases other than CO2 existing to a

larger degree in Mars’s atmosphere. Although current measurements have shown that the trace amounts

of methane in the Martian atmosphere have little effect on its current climate, a large amount could

have had a larger impact by both increasing the greenhouse effect and lowering the planet’s albedo.

However to reach the desired temperatures would require what are likely to be unrealistic amounts of

methane [Kasting, 1997].

Methane could have played a less direct role, creating a haze which shields the air from UV

radiation allowing ammonia to stabilize and act as the primary greenhouse gas [Sagan and Chyba,

1997]. This was shown to be a particularly effective solution because of ammonia’s strong absorption

of wavelengths from 700 to 1300 cm making it a particularly strong greenhouse gas [Forget et al.,

2013].



Gases like SO2 and H

2S produced by volcanic activity could also have an effect on the surface

temperature. However as with the other gases, significant amounts would be required to reach 273 K.

Johnson et al. [2008] found that a concentration of around 10 parts per million in a 0.5 bar atmosphere

could raise the temperatures above freezing. This is the only modern model able to produce these

temperatures using the greenhouse effect alone (as of Forget et al., 2013).

As often is the case, we can suppose that the most likely scenario is a combination of the

possible conditions stated before which allowed Mars to have water on its surface. However these

conditions did not last long as the planet evolved into the extremely dry planet that we see today,

leaving us to question what could have happened to its atmosphere.

A common way for a planet to lose its atmosphere is to have chunks blasted away by impacts.

Using observations from MGS we are able to get an impact record by studying craters on the surface.

With this record Brain and Jakosky [1998] were able to derive a formula for atmospheric loss as a

function of crater density. However this model only accounted for a portion of the loss that would have

been expected if the atmosphere was once much thicker. They proposed that the other losses can be

attributed to a separate process called sputtering.

When the solar wind comes into contact with an atmosphere, photons can run into gas particles

in the upper atmosphere, knocking loose an electron and ionizing the gas. On Earth these ionized gases

travel around the ionosphere but tend to remain bound to Earth thanks to its magnetic field. Mars

however does not have a strong magnetic field like the Earth and the ionized particles can be further

accelerated by the magnetic field in the solar wind causing them to collide with other gas molecules at

high velocities and “sputtering” them out into space.



According to the models of Brain and Jakosky [1998] if the combination of impact erosion and

sputtering accounted for 95-99% of the atmosphere’s loss, a 3 bar atmosphere 4 billion years ago would

have now have 30-150 mbar, higher still than the current value of about 6 mbar. Thus they suggested

that the remaining gases could have been sequestered into the surface and remained there to possibly

make up the difference. However using magnetic readings from MGS, Brain et al. [2010] found that the

characteristics of the Martian magnetic field may have an even more dramatic effect on the

atmosphere’s loss.

Although Mars doesn’t possess a magnetic field Earth’s it likely once did and the remnants of

that field are still active in plumes of magnetic activity over the Martian crust. If these plumes form at

the correct angle towards the Sun, the solar wind may pull on the magnetic field, stretching it out until

it breaks off in the process shown below.

Figure 2: Possible field line geometries of bulk ejection seen by MGS. (a)

Crustal field lines are still attached to the planet, and have been stretched

tailward long distances by the solar wind; (b) Loops of crustal magnetic field

have detached, carrying ionospheric plasma away from Mars. The Sun is to

the left and the dotted line indicates the path of MGS. [Brain et al., 2010]

The bulk ejection of plasma carries with it remaining portions of the Martian atmosphere.

These ejections appear to be fairly common and if so they can account for around 5-10% of the ions



lost from the Martian atmosphere. This extra parameter could make up for the losses unaccounted for in

the previous Brain and Jakosky models.

Very soon we will have more information on the present day evolution of the Martian

atmosphere as MAVEN (Mars Atmosphere Volatile and EvolutioN Mission) begins transmitting data

this month on the current state of the Martian atmosphere as it examines the interactions between the

solar wind and the upper atmosphere. These observations will help us to further determine the rate at

which the atmosphere is lost to space and hopefully allow us to work backwards to find even better

models of the early Martian atmosphere and determine the fate of the Martian oceans.

References

Brain, D. A., and B. M. Jakosky, Atmospheric loss since the onset of the Martian geologic record: Combined

role of impact erosion and sputtering, J. Geophys. Res., 103, E10, 22689-22694, 1998

Brain, D. A., A. H. Baker, J. Briggs, J. P. Eastwood, J. S. Halekas, T. -D, Phan, Episodic detachment of

Martian crustal magnetic fields leading to bulk atmospheric escape, Geophys. Res. Letters, 37, 14, 2010

Carr, M. H., Retention of an atmosphere on early Mars, Journal of Geophysical Research, 104, E9,

21897-21909, 1999

Carr, M. H., and J. W. Head, Oceans on Mars: An assessment of the observational evidence and possible fate,

Journal of Geophysical Research, 108, E5, 2003

Cess, R.D., V Ramanathan, and T. Owen, The Martian paleoclimate and enhanced carbon dioxide, Icarus, 41,

159-165, 1980

Clifford, S. M., and T. J. Parker, The evolution of the Martian hydrosphere: Implications for the fate of a

primordial ocean and the current state of the northern plains, Icarus, 154, 40–79,2001

Forget F., and R. T. Pierrehumbert, Warming early Mars with carbon dioxide clouds that scatter infrared

radiation, Science, 278, 1273-1276, 1997

Forget, F., R. Wordsworth, E. Millour, J.-B. Madeleine, L. Kerber, Leconte, E. Marcq, R.M. Haberle, 3D

modelling of early Martian climate under a denser CO2 atmosphere: Temperatures and CO2 ice clouds, Icarus, 222,

81-99, 2013



Haberle, R. M., Early Mars climate models, J. Geophys. Res., 130, E12, 28467-28479, 1998

Hoffert, M. I., A. J. Calegari, C. T. Hsieh, and W. Ziegler, An energy balance climate model for CO 2/H2O

atmosphere, Icarus, 47, 112-129, 1981

Johnson, S.S., Mischna, M.A., Grove, T.L., Zuber, M.T., Sulfur-induced greenhouse warming on early Mars,

J. Geophys. Res., 113, E8, 2008

Kasting, J. F., CO2 condensation and the climate of early Mars, Icarus, 94 , 1-13, 1991

NASA. "MAVEN Fact Sheet." (n.d.): n. pag. nasa.gov, 2013. Web

Pollack, J. B., J. F. Kasting, S. M. Richardson, and K. Poliakoff, The case for a warm wet climate on Mars,

Icarus, 71, 203-224, 1987

Postawko, S. E., and W. R. Kuhn, Effect of greenhouse gases (CO 2, H2O, SO2) on Martian paleoclimates,

Proc. Lunar Planet. Scie. Conf. 16th, Part 2, J. Geophys. Res., 91, suppl., D431-D438, 1986

Sagan C. and C. Chyba, The early faint sun paradox: Organic shielding of ultra-labile greenhouse gases,

Science, 177, 52-56, 1972.

Pollack, J. B., Climate change on the terrestrial planets, Icarus, 37, 479-553, 1979

Sagan, C., Reducing greenhouses and the temperature history of Earth and Mars, Nature, 269, 224-226, 1997

Yung, Y. L., H. Nair, M. F. Gerstell, CO 2 greenhouse in the the early Martian atmosphere: SO2 inhibits

condensation, Icarus, 130, 222-224, 1997



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