Mars

Introduction

The possibility of life on Mars, whether extant or prospective, has long been the subject of heated debate in the scientific community. Organisms that can thrive under Mars’ harsh conditions must confront a number of environmental challenges, and possess a variety of unique abilities and characteristics. Restricting our analysis merely to the realm of bacteria, it appears highly unlikely that any terrestrial bacterial organism could (without considerable adaptation) survive on Mars, or that any potential denizens of the current Martian landscape are at all similar to their distant Earthly relatives. That said, Mars is host to a number of environments, or niches, in which members of Archaea or Bacteria could eventually thrive, given the opportunity to accumulate characteristics witnessed in some of Earth’s more exotic members of these families.

In order to assess what these characteristics may be, we must first look at the challenges posed by the environment. One of the primary differences between Earth and Mars is the latter’s lack of a global magnetic field (1). Lost sometime during the planet’s past (2), the magnetic field is unable to sustain the magnetosphere, Earth’s protection against solar wind, cosmic radiation, and asteroid impacts (3). Over time, these have thinned the Martian atmosphere to a fraction of Earth’s, resulting in a host of diverse consequences (4). The effect of a thin atmosphere is multiple-fold; first and foremost, the atmospheric pressure on the planet is far too low to allow for water to exist in the liquid phase. Additionally, Mars’ thin atmosphere is unable to trap heat from the sun, resulting in an extremely wide temperature range. What atmosphere there is, is composed of mostly CO2, and the high salinity and acidity of the soil further complicates the viability of bacterial life. Finally, recent findings reveal a highly oxidizing environment, which in itself is extremely damaging to most terrestrial organisms.

Location

Image courtesy of NASA/JPL/Malin Space Science Systems

Mars is the fourth planet from the Sun in the Solar System; about 230 million kilometers from the Sun. A Martian year is about 687 days, while a Martian year is about 24.5 hours (5). It's surface area is almost 145 million kilometers. From Earth, this 4.6 billion year old planet appears a bright reddish-range color due to the high iron content in the soil. (6)

The speculative Mars niche is relatively harsh with varying extreme conditions. Many scientists believe Mars to be outside the habitable zone, or distance from the sun. This zone, currently occupied by Earth, allows water to be present on the surface. Because Mars is outside of this zone, the lack of atmosphere causes water to freeze, and UV rays to reach the Martian surface. Mars was a believed to be more habitable planet 3.5 billion years ago, with water covering much of the surface as oceans lakes and rivers. If this theory holds true, there would be a much higher probability of life existing at some point in Martian history. To determine the possibility of life ever existing on Mars, we would need to investigate salt or mineral deposits deep under the surface, which would be protected from damaging cosmic rays. Although evidence exists to support past habitability of water, some scientists believe conditions to have been too salty and acidic for life. (7) Those who believe life exists or extant assume the most habitable environments to be the polar ice caps, subterrain, or crater gullies.

Physical Conditions

What are the conditions in your niche? Temperature, pressure, pH, moisture, etc.

Hydrogen radicals are produced in the Martian atmosphere by the photolysis of water vapour and subsequently initiate catalytic cycles that recycle carbon dioxide from its photolysis product carbon monoxide. These processes provide a qualitative explanation for the stability of the atmosphere of Mars, which contains 95 per cent carbon dioxide. Balancing carbon dioxide production and loss based on our current understanding of the gas-phase chemistry in the martian atmosphere has, however, proven to be difficult. Interactions between gaseous chemical species and ice cloud particles have been shown to be key factors in the loss of polar ozone observed in the Earth's stratosphere, and may significantly perturb the chemistry of the Earth's upper troposphere. Water-ice clouds are also commonly observed in the atmosphere of Mars and it has been suggested previously that heterogeneous chemistry could have an important impact on the composition of the Martian atmosphere. Here we use a state-of-the-art general circulation model together with new observations of the martian ozone layer to show that model simulations that include chemical reactions occurring on ice clouds lead to much improved quantitative agreement with observed martian ozone levels in comparison with model simulations based on gas-phase chemistry alone. Ozone is readily destroyed by hydrogen radicals and is therefore a sensitive tracer of the chemistry that regulates the atmosphere of Mars. Our results suggest that heterogeneous chemistry on ice clouds plays an important role in controlling the stability and composition of the martian atmosphere.

[*]Nature. 2008 Aug 21;454(7207):971-5.

Pressure ~600Pa (6 mbar)

Temperature -123°C to 25°C

UV Radiation (200nm - 400nm) 8.4 - 67 W/m²

Chemically oxidizing, as well as high salinity

[*]Astrobiology. 2006[6], November 2, 2006 [Diaz et al]

           Compare to Earth’s climate, atmosphere, etc.

The factors governing the amounts of CO, O2, and O3 in the martian atmosphere are investigated using a minimally constrained, one-dimensional photochemical model. We find that the incorporation of temperature-dependent CO2 absorption cross sections leads to an enhancement in the water photolysis rate, increasing the abundance of OH radicals to the point where the model CO abundance is smaller than observed. Good agreement between models and observations of CO, O2, O3, and the escape flux of atomic hydrogen can be achieved, using only gas-phase chemistry, by varying the recommended rate constants for the reactions CO + OH and OH + HO2 within their specified uncertainties. Similar revisions have been suggested to resolve discrepancies between models and observations of the terrestrial mesosphere. The oxygen escape flux plays a key role in the oxygen budget on Mars; as inferred from the observed atomic hydrogen escape, it is much larger than recent calculations of the exospheric escape rate for oxygen. Weathering of the surface may account for the imbalance. Quantification of the escape rates of oxygen and hydrogen from Mars is a worthwhile objective for an upcoming martian upper atmospheric mission. We also consider the possibility that HOx radicals may be catalytically destroyed on dust grains suspended in the atmosphere. Good agreement with the observed CO mixing ratio can be achieved via this mechanism, but the resulting ozone column is much higher than the observed quantity. We feel that there is no need at this time to invoke heterogeneous processes to reconcile models and observations.

[*]Icarus. 1994 Sep;111(1):124-50.

           Mars before; mars after

Individual Regions:

           Polar Caps (ice veins, etc.) / Permafrost
  

           Underneath the Soil (potentially water, etc.)

                       A habitat beneath the soil poses a particularly attractive option for a number of reasons:

1. As discussed in [Diaz et al, Astrobiology 11/06], both E. Coli & D. radiodurans viability in simulated Martian conditions (pressure, UV exposure, & temperature) improved dramatically when "protected in a microhabitat 5cm beneath the surface" (or in liquid water). The conditions "at or near" the surface, namely, the dessication stress (due to the extremely low pressure), and UV radiation make survival at the surface highly improbable.

2. Evidence of subsurface water article(s)

           One other place – maybe surface?

   The surface is unlikely to yield a hospitable environment for a number of reasons. First and foremost, water cannot exist in liquid form at the surface, due to the extremely low     pressure. While many of Mars' unconventional environments may still allow some type of living organism to exist, the absence of liquid water is an almost universally limiting factor.     Furthermore, life on the surface is additionally complicated by the barrage of solar radiation. Radiation damage is particularly harmful to DNA & RNA, and organisms that could survive     on the surface would need to have DNA-protective mechanisms in place.

Conditions under which the environment changes

Do any of the physical conditions change? Are there chemicals, other organisms, nutrients, etc. that might change the community of your niche.

           Solar Winds

           Radiation (no magnetosphere)

           Meteorite Impact

           Human Involvement

Evidence of Life

Evidence of water depicted by these gullies or channels in craters on the Martian surface. Image courtesy of NASA

The arid conditions or the Martian surface is believed to be too cold, dry, and oxidizing for life to survive. Therefore, if life is to survive, or have survived, organisms would most likely exist in the channel craters, subsurface, or polar ice caps.

Past

Halobacterium halobium would be one of the leading candidates to survive speculative past Martian conditions. H. halobium is a facultatitve anaerobe that uses photosynthesis for energy production. It thrives in extremely saline solutions, and can hide in salt deposits when the water freezes or evaporates. H. Halobium can then re-enter water if it ever reappears, allowing it to survive a daily change in water phases. Being dormant in the salt crystals also serves the purpose of protecting the microbe from the sun's UV radiation. The opaque characteristics of the salt crusts block the shorter wavelengths below 200 nm. The longer UV wavelengths are blocked by rhodopsinlike pigments produced within the microbe. If conditions are unfavorable, H. halobium can travel by wind gusts to other locations thousands of miles away, which could be better suited to its survival. (18)

Present

Recent discoveries of the Martian surface has exposed that methane currently exists in the Martian atmosphere. It has been suggested that methanogens are reasons for the existing methane levels. Methanogens are microorganisms belonging to the Archaea domain and can metabolize hydrogen as a source for energy to incorporate carbon from carbon dioxide into methane. Because life on the arid surface of Mars is unlikely, it must be assumed life to exist below the surface. Possible presence of water under the Martian soil for varying seasonal time periods presents a possibly habitable environment for methanogens. Current research has been able to simulate the Martian dessication and rehydration cycle by adding varying amounts of carbon dioxide, hydrogen, and varying amounts of water. (18) Through this simulant, research found that Methanosarcina barkeri was able to survive dessication over 10 days, while Methanothermobacter wolfeii and Methanobacterium formicium were able to survive for 25 days. (20) Equating the loss rate of methane recently discovered in the Martian atmosphere to the similar climate in the deep ice of Greenland, the presence of methanogens can be extrapolated to be 1 cell per milliliter in a 10 meter thick layer Martian habitat. (21)

References

[Sample reference] Takai, K., Sugai, A., Itoh, T., and Horikoshi, K. "Palaeococcus ferrophilus gen. nov., sp. nov., a barophilic, hyperthermophilic archaeon from a deep-sea hydrothermal vent chimney". International Journal of Systematic and Evolutionary Microbiology. 2000. Volume 50. p. 489-500.

(1) "MGS/MAGeR" http://mgs-mager.gsfc.nasa.gov/ Accessed Aug 27, 2008.

(2) B.Ouladdiaf, R.Ballou, G.Fillion ,P.Rochette, L.Hood, "How Mars Lost its Magnetism"

(3) Phillips, Tony PhD., "The Solar Winds at Mars" Accessed Aug 27, 2008.

(4) Lundin R., Barabash, S., "Solar Wind-Induced Atmospheric Erosion at Mars: First Results from ASPERA-3 on Mars Express" Science 24 September 2004. Vol. 305. no. 5692, pp. 1933 - 1936. DOI: 10.1126/science.1101860

(6) "Mars: Facts & Figures". NASA. Retrieved on 2008-8-27

(7) Briggs, Helen "Early Mars 'too salty' for life" BBC News Friday, 15 February 2008, 22:27 GMT. Accessed Aug 27, 2008.

(18) Landis, Geoffrey A., "Martian Water: Are There Extant Halobacteria on Mars?" Astrobiology. 2001. Volume 1. p. 161-64.

(19) Kral, Timothy A., Bekkum, Curtis R., McKay, Christopher P., "Growth of Methanogens on a Mars Soil Simulant" Origins of Life and Evolution of Biospheres. Dec 2004. Volume 34.

(20) Kendrick, Michael G., Kral, Timothy A."Survival of Methanogens During Desiccation: Implications for Life on Mars" Astrobiology. August 1, 2006, 6(4): 546-551. doi:10.1089/ast.2006.6.546.

(21) Tung, H. C., Bramall, N. E., Price, P.B., "Microbial origin of excess methane in glacial ice and implications for life on Mars" December 7, 2005, doi: 10.1073/pnas.0507601102.

Edited by Hank Hoang, Herman Davidovics, Grigoriy Shekhtman, students of Rachel Larsen