The model
The DDS-MSO (Dark Dune Spots - Mars Surface Organisms) model synthesizes observations of Mars, theoretical modeling, and analysis of Earthly extremophiles, in order to elucidate what environmental conditions may exist in the micro-environments on the Martian polar dunes in springtime and whether these are permissible for the existence of living organisms. The first version of the model was published in 2001, and the latest version in 2007.
In the following the bases of the model is summarized divided into subtopics:
Mars observations
Challenges of the model
Earth observation of possible analog colonies
Recent results from other researchers
Summary of the model
Mars observations: the spots
The Dark Dune Spots (DDSs) are ephemeral seasonal albedo markings at high-latitude frost covered surface, and may be the indicator of favorable surface/shallow subsurface microhabitats on Mars. There are various seasonal albedo features on Mars, DDS are a special group of them with the following basic characteristics:
low albedo markings on the seasonal frost-covered surface, they are dark relative to the surrounding on optical images,
they appear late winter, are observable and grow in size only in spring and disappear with the disappearance of seasonal surface frost early summer,
present at the Southern hemisphere between 65-80 degrees of latitude (resembling seasonal dark albedo markings are present at the northern hemisphere too, but they are not discussed here),
occurrence on dark dunes, usually inside craters (various dark spots outside dune fields are not part of this research),
have internal structures: a darker (umbra-like) central part, surrounded by a lighter, outer (penumbra-like) ring,
diameter varies between 5 and 200 m,
show yearly reoccurrence to a degree of 50-65%, (about half of the spots appear very close to the location of spots from the previous year),
on slopes they give rise to elongated structures.
A Dark Dune Spot and slope streak emanating from it on HiRISE image no. PSP-3432-1115
Mars observations: the elongated slope streaks
 Two groups of elongated structures emanate form DDSs (Note: streaks mentioned here are on slopes but differ form the "classic" slope streaks observed at low latitude terrain on Mars):
fan-shaped streaks, with tho following characteristics: do not have sharp edges, widen and fade toward their end, present on slopes and horizontal surfaces as well, one Dark Dune Spot may give rise to several fan-shaped streaks oriented in different directions, probably formed by slope winds and the CO2 geyser hypothesis.
confined streaks with the following characteiristics: narrower toward the end, or sometimes they keep constant width, albedo does not change systematically toward their end, always on steep slopes and point downward, show curvatures according to local topography, sometimes show accumulation-like structures ("pond") at their end, the physical dimensions of the DDSs, their streaks and accumulated structures show correlation, one DDS gives rise to confined streaks elongated only in one direction, in some cases toward different directions, formed later under higher temperature than the fan shaped ones.
See examples for the differences of fan shaped (gray) and confined (black) slope streaks of HiRISE image no. 3175-1080 acquired at Ls=210.6 at 179.4E 72S at the right
Challenges of the model
Temperature models to produce warmer "spots", than those values, based on purely the global insolation:
heating by insolation: below ice dark absorbing dark dune surfaces, horizontal heat conduction from barren surfaces
maintenance and duration of high temperature: heat insulation by ice layer H2O of ~mm thickness, and low heat conductivity of water vapor formed by the volume decrease during the ice melting, "high" temperature is present probably around only noon for a few hours
maintenance of H2O during "warm" daytime: presence of materials with strong surface H2O absorption, diffusion barriers botween the dune grains
Earth observations of possible analog colonies
For the DDS-MSO model the best possible analogs are near surface communities and microenvironments populated by simple organisms, and called cryptobiotic crust. This small ecosystem is composed of cyanobacteria, algae and fungi inside the uppermost 2-4 mm of soil or weathered rock surfaces. They show high tolerance against extreme dryness, low and high temperature, and as a result are present at the wildest deserts of Earth.
Various Mars relevant extremophiles and survival strategies were analyzed. Our samples are from various desert environments, including Australia, Northern Kenya, India, Devon-island (Arctic Canada), King George-island (Antarctica). During the analysis, provision of UV-shielding with more pigmented taxa at the upper level, and less pigmented, UV-sensible organisms were observed below them. They have seasonally active life cycle according in wet periods, and similar taxa of cyanobacteria in hot and polar deserts (for example Microcoleus paludosus) suggests high level of tolerance.
Observed survival strategies:
UV-shield task-sharing: in granitic inselbergs in the "shade" of strongly pigmented Stigonemetaceae weakly pigmented coccoid cyanobacteria exists with large jelly sheath.
Crack colonizing: inside weathered dolomitic rocks from Devon-island, Haughton-crater, Gloeocapsa/Aphanocapsa type coccoid cyanobacteria, forming a semi-continuous layer on the rock surface, penetrating in the rock along cracks and crevices, partly shielded form the atmosphere, but the cracks let enough light to penetrate for photosynthesis. The cracks also help maintain the microenvironment wet.
 Dew trapping by the bundles of Microcoleus in Sahara: the vapor condensation on the bundles of bacterial filaments is helped probably by electrostatic forces, and the H2O is conducted downward in mucilaginous sheath to the assimilating part.
Optical fibre strategy: organisms forming a pigmented and photosynthesising upper, and an UV-sensitive and photosynthesising lower layer. The lower organisms are covered by opaque grains, which filter all the light. But the mucilaginous sheaths of upper bacterium filaments conduct light down to the protected layer, where the species can survive. These "optical fibres" are working only in wet phase, during the dry period the fiber is also opaque (see examples on the right image: the bacteria (top left), the light conducting fiber (top right), the conducted light observed from below (bottom).
Seasonal up and down shifting of the filamentous cyanobacterium Microcoleus paludosus. During the short wet season they thrive on the soil surface in the Australian deserts and in the Sahara. During the long dry season they leave their mucilaginous sheath empty on the surface, move downwards and build new sheath, surviving in the more protected depth. In this phase the grains and the previous sheath works as UV filter. Then, after the first rain, they move upwards again.
Recent results from other researchers:
Possibility of ephemeral water on Mars today: favorable micro-environments out of thermodynamic equilibrium (MEPAG MLFO-SAG 2006) are the best locations for liquid water today, like warming surface structures with ice cover. Based on computations and modeling, undercooled interfacial liquid water film exists with various thickness on Mars (Möhlmann D. (2004) Icarus 168, 318-323.).
Analogies from the Antarctic Dry Valleys suggest that some of the Martian slope streaks may be formed by subsurface water seepage (Head J.W. et al. (2007), Seventh International Conference on Mars #3114.)
Possibility of UV shielding: Thin grain layer at the top of the dunes gives enough shielding: "...under 1 mm of rock, Chroococcidiopsis sp. could survive (and potentially grow) under the high Martian UV flux if water and nutrient requirements for growth were met." (Cockell C.S. et al. (2005) Astrobiology 5, No. 2: 127-140.)
Shielding effect of rocks by their shadows may also substantially decrease the cumulative UV flux on the surface. (Moores J.E. et al. (2007) Icarus 192 417-433.)
Hypolithic colonies at the underside of stones in arctic deserts produce shielding form UV radiation, wind scour, and dew trapping – while between the opaque rocks enough light can penetrate for photosynthesis (Charles S. C. and Stokes M.D. (2006) Arctic, Antarctic, and Alpine Research, 38, No. 3, pp. 335-342.)
Theoretical proposals for the favorable microenvironments on Mars are similar to the cryptobiotic crust analyzed by our group: "...Desert crusts have been proposed as a potential mechanism to provide a diffusion barrier, and were considered in this study. Although crusts on Mars have been observed at the past landing sites, and other crust types are hypothetically possible elsewhere, experience with desert crusts on Earth shows that the effect of a semi-permeable crust is to retard, not prevent, the achievement of equilibrium...." (MEPAG MLFO-SAG 2006)
"... On Mars, even with the early start, the harsh conditions kept life from evolving beyond a relatively simple stage, although it has certainly had enough time for complex development, and Martian life may be more complex than is commonly assumed." Development of Life on Early Mars. Everett K. Gibson, David S. McKay, Kathie L. Thomas-Keprta, Simon J. Clemett and Susan J. Wentworth, 40th Lunar and Planetary Science Conference (2009) 1175.
Summary
The DDS-MSO model composed of the events in the following sequence:
layered frost formation in autumn on the Martian surface, with H2O at the bottom and CO2 at the top,
CO2 gas jet outburst by springtime sunshine,
formation of thin, ephemeral water layer around solar-heated dune grains,
formation of ephemeral favorable micro-environments between the dune grains
liquid water, solar radiation plus UV shielding together and temperature high enough for the survival of possible extremophile-like organisms,
realization of seepage-like process with the help of the lubricated grains,
disappearance of the surface ice by the end of spring, and the termination of the ephemeral life cycle of the possible MSOs.
 The model that syntetises Mars observations, modelling of environmental parameters, undercooled interfacial liquid water layers, and observed survival strategies of Earthly extremophyles. It explains the observations and compatible with the present knowledge of Martian surface and shalow subsurface conditions.
The model can be useful to estimate astrobiology potential of the Martian surface, above all high latitude dark dunes. Elements of the model is visible on the right image.
From left (Mars) to right (Earth): top: crater with dark dunes and Ayers rock surrounded by biogen spots, middle: DDSs on a dune field and cryptobiotic crust on a rock, bottom: signs of seepages and wet cyanobacteria.
|
|
Members of the ESA Mars Astrobiology Group at Collegium Budapest (Institute for Advanced Study) have been analyzing seasonally changing features and seepages on polar dunes of Mars since 2001. Interfacial water-covered grains with radiation shielding against UV may provide a tolerable microenvironment there.
Cryptobiotic crust communities were analyzed as partial analogues on Earth, showing adaptation to dryness, cold and a long dormant state. The DDS-MSO (Dark Dune Spot - Mars Surface Organism) hypothesis was developed, according to which ephemeral conditions are favourable for living organisms on Mars.
This hypothesis is compatible with all the recent discoveries of H2O and layered polar frost on Mars, and low-temperature metabolism on Earth. Click here for our publications and brief description of the model.
|
|
