Dark Dune Spots: Possible current habitats on Mars

1. Dark Dune Spots: Possible current habitats on Mars Eörs Szathmáry Collegium Budapest szathmary@colbud.hu Eötvös University Budapest

2. Mars: a dead planet (?) • Very dry: no water • Very cold (minus 130 C) • High UV radiation (above 190 nm) • Thin atmosphere (6 mbar)

3. There may have been plenty of water in the past… • Water was lost due to radiation decay and loss of hydrogen to space • A large oxygen sink must be present • More than 3 billion years ago conditions for the origin of life seem to have been favourable • Could anybody have survived till today?

4. An ancient lake and outflow lake Loire Vallis

5. Cataclysmic flood channels

6. Cyclic and episodic changes

7. Viking lander: no evidence for life • It is easy to land in the equatorial desert, BUT • It is very hard to find even traces of life there • The soil seems oxidising • BUT some oxidized organic compounds may have escaped detection

8. Claims of past life remain controversial Is this magnetite biogenic?

9. Dark Dune Spots (DDSs): a candidate habitat (2001)?

10. Analyzed DDS sites in the south polar region (Mars Global Surveyor data) MOC images Dark dunes Malin Space Science Systems

11. DDSs stick to the dunes…

12. …and have inner structure

13. Mere frosting-defrosting of the CO2 ice cover • Albedo decreases as frost thins • Positive feedback: faster sublimation MUST mean: • Start spot formation at sites with strong exposure to insolation • Otherwise at random sites (wind, etc.)

14. Simple frosting-defrosting does NOT work, because… Spot formation begins at the bottom, not at the top!

15. Simple frosting-defrosting does NOT work, because… Spots do not develop on exposed sites!

16. Simple frosting-defrosting does NOT work, because… There is annual recurrence (>75%) at the same sites!

17. Simple frosting-defrosting does NOT work, because… • On slopes flows originate from the DDSs • Which always flow downwards • From elongated spots • Gravitation is a formative cause

18. Flows may be due to water runoff • The dry planet is much wetter than thought • There is plenty of water: • In both ice caps • In the upper layer of the polar region (permafrost) • In liquid form in the gullies

19. Mars Odyssey HEND measurements • High Energy Neutron Detector • Deficit of high-energy neutrons • Hydrogen is concentrated in the subsurface • Water-rich layers tens of centimetres thick

20. Frozen water in polar regions

21. Recent gullies by melting snow • Occur in clusters on slopes • Between 30 and 70 latitude in both hemispheres • Consist of alcoves several hundred metres wide, • Channels up to several kilometres long and several tens of metres deep • Typically originate within several hundred metres of the slope crest, • Can occur on crater walls that are raised above the surrounding terrain or near the summit of isolated knobs.  North

22. Christensen’s mechanism (2003) • (1) Water is transported from the poles to mid-latitudes during periods of high obliquity, forming a water-rich snow layer • (2) Melting occurs at low obliquity as mid-latitude temperatures increase, producing liquid water that is stable beneath an insulating layer of overlying snow. • (3) Gullies form on snow-covered slopes through erosion by melt water or as a result of melt water seeping into the loose slope materials and destabilizing them.

23. Christensen’s mechanism II • (4) Gullies incised into the substrate are observed where the snow layer has been completely removed. • (5) Patches of snow remain today where they are protected against sublimation by a layer of desiccated dust/sediment • (6) Melting could be occurring at present in favourable locations in these snowpacks.

24. Clow’s (1987) model for melting • Melting occurs beneath the surface at temperatures well below freezing, because sunlight is absorbed at depth rather than at the surface, and this absorption is substantially increased by the incorporation of minor amounts of dust. • Can occur for a wide range of snow properties and atmospheric pressures, and occurs under current conditions in mid-latitudes if dust abundances are greater than 1,000 parts per million by mass.

25. Clow’s model II • Meltwater moving downwards under gravity will encounter lower temperatures and refreeze. • Conduction and latent heat transfer will gradually warm the snow and substrate, allowing liquid water to accumulate and be available for erosion • Subsurface erosion and collapse of the snow mantle will occur, with liquid water potentially reaching and eroding the substrate as the snow layer continues to melt.

26. Clow’s model III • Liquid water will begin to be generated 100 d after the spring equinox under current conditions for snow with a dust content of 1,000 p.p.m.m. • Will reach a depth of 20 cm approximately 25 d later. • Up to 0.33 mm of snowmelt runoff is produced each day for 50 d each martian year.

27. Compare HEND with DDS sites!

28. DDSs are different from gullies, because… • They reappear annually • Frost/snow cover above them is re-established each year and disappears by midsummer • Crater slopes and alcoves are NOT necessary • BUT the dune material IS • Channels from DDSs on slopes are also thought to form below the snow • Slow melting, NOT gas outbreak: complete lack of explosive formations • Salinity unknown, could be important

29. Layered frost on the dunes (2002) • Water ice, clathrate and CO2 ice are deposited in that order • Dunes are the first to frost and the last to defrost • Total frost between 0.2-1 m (laser altimeter)

30. Dark spots are transformed to summer grey spots

31. The biological hypothesis • Annual reactivation and growth of photosynthetic organisms • Ice: excellent shield against cold, UV and dryness • Organisms must go to dormancy before water ice shield melts through (‘adaptive sporulation’)

32. Looking for partial analogues on Earth (extremophiles) • The Dry Valleys of Antarctica • Cold, dry • High UV due to continuous solar exposure • Ozone depletion

33. Bacterial activity in lake ice These bacteria are permanently buried in ice!

34. Photosynthetic microorganisms At the centre of a rich consortium

35. Ice and snow lake covers in mountains (Alps, etc.)

36. UV protection by snow

37. Thermal tolerance on Earth • Temperatures go down to –70 °C in the Antarctic valleys • Spores can be cooled down arbitrarily • Photosynthesis is possible under the snow down to –20 °C, when the temperature above is a lot colder • You do not need liquid water, only positive water potential • Extremely fast rehydration in cyanobacteria • Many cyanobacteria are halophilic

38. Snow and ice UV protection

39. More on UV resistance • There are bacteria (e.g. Deinococcus radiodurans) on Earth, extremely resistant to radiation and dryness • Martian organisms must have undergone billions of years of adaptation • Dead cells in the upper layer efficiently protect viable cells in the lower layer • Viable stock may also endure as endoliths • Efficient external and internal shields (e.g. black in cyanobacteria: “sunglass”)

40. A Mars chamber simulation is being prepared… at Centro di Astrobiologia (CAB) in Madrid

41. Objective of the chamber experiments • To prove that there is a layered structure of the frost under simulated Martian conditions • To see whether spots form by simple frosting-defrosting or not • To introduce biological material (e.g. cyanobacteria) into the simulation

42. Summary • DDSs are a potential habitat for life on Mars today • They may be actual habitats • Earthly analogues are encouraging • Chamber simulations have to be carried out • Looking for pigments by appropriate spectroscopy (resolution, wavelengths!) • Sending landers to interesting sites!

43. Some further thoughts • Those who consider this story scandalous must also ultimately think that life is improbable! • If life is probable, this story is not against parsimony at all • We are faced with NON-abundant life • We are in a lucky period, because the phenomenon may disappear in the future

44. Main collaborators • András Horváth, astronomer and planetologist • Tibor Gánti chemical engineer and theoretical biologist (Principles of the Living State OUP, in a week) • Susanna Manrubia physicist (CAB) to coordinate the Spanish group (incl. chamber)

45. Reading Horváth, A., et al. (2001) Probable evidence of recent biological activity on Mars: Appearance and growing of dark dune spots in the South Polar Region, Lunar Planet Sci.XXXII, #1543, Horváth, A., et al. (2002) Morphological Analysis of the Dark Dune Spots on Mars: New Aspects in Biological Interpretation, Lunar Planet. Sci.XXXIII, #1108. Horváth, A., et al. (2002) The “Inca City” Region of Mars: Test field for Dark Dune Spots Origin,Lunar Planet. Sci.XXXIII, #1109. Gánti et al. (2003) Origins of Life and Evolution of the Biosphere in press