Seeking Life on Mars

1. Seeking Life on Mars Topics to examine Three lines of evidence Gas emissions; Martian meteorites; Viking. Dangers of interplanetary contamination

2. Evidence: Methane on Mars In 2004, the Mars Express mission (ESA) announced the detection of methane (CH4) in Mars’ atmosphere. Because of the natural break-up of CO2 in the Martian atmosphere, oxygen would always be present in tiny amounts, and it would oxidize methane rapidly (in about 300-400 years) via: CH4 +O2  2H2O + CO2 Ultraviolet radiation would also contribute to destroying the methane. Something must be replenishing the methane! And whatever the source—the concentration of methane fluctuates with 1-4 year time scales.

3. Evidence: Methane on Mars In 2014, the Mars Curiosity Rover detected methane. On four separate occasions over a period of two months, methane was reported to spike to levels about ten times the normal background level, We expect the background methane (ultraviolet photons striking carbon from asteroids, etc), but do not know the origins of these whiffs of concentrated methane.

4. Evidence: Methane on Mars Five sources for this gas: Cometary impact—extremely unlikely for methane. Human contamination—ruled out by spatial resolution, and the timing of the discovery. Volcanism—not seen, but such energy sources would be fabulous for life. Weird geological or chemical reactions? Hot water running over specific types of rocks, etc. Life itself!

5. Evidence: Martian Meteorites In 1984, an expedition to the Allan Hills region of Antarctica picked up a 1.9 kg rock, thereafter called ALH84001. Isotopic ratios of 16O, 17O, 18O show this is not an Earth rock. These isotopic ratios strongly imply it is not an asteroidal chunk, or a piece of the Moon, either. Gas samples trapped in the rock are consistent with it being a chunk of Mars. Only 34 Martian meteorites have been found.

6. ALH84001 history 4.5 b.y.a.The parent rock solidified from molten material. (The southern highlands are suspected as the meteorite’s origin—spectroscopy matches with Eos Chasma in Valles Marineris.) 4-4.5 b.y.a.Heavy bombardment shocks occurred near the parent rock. (The meteorite contains characteristic shock structure). 3.9 b.y.a.Liquid water seeped into the rock, leaving behind carbonate grains 0.1-0.2mm in diameter. (From radiometry of minerals in the fractures.) 16 m.y.a.The rock was blasted into space to drift. (Based upon cosmic ray exposure.) 13,000 y.a.The meteorite impacted Antarctica. (Based on the age of ice layers at Allan Hills.) 1984ALH84001 was found by humans during a meteor-hunting expedition.

7. ALH84001: four lines of evidence of life The carbonate grains from 3.9BYA have alternating layers of Mg-rich, Fe-rich, Ca-rich carbonates. On Earth, such layers would be considered symptomatic of signatures of life. Counter-argumentSuch chondrule layers could be produced by multiple episodes of inundation by mineral-enriched water—each episode producing a different layer.

8. ALH84001: four lines of evidence of life The carbonate grains contain polycyclic aromatic hydrocarbons (PAHs) in much higher levels than is normally found when produced by non-biological processes. Counter-argumentPAHs are certainly not exclusive to biological processes. The PAH level in ALH84001 could be due to terrestrial bacterial contamination.

9. ALH84001: four lines of evidence of life Microscopic crystals of the iron mineral magnetite occur in the rock, and these are similar to crystals made by certain species of mud-dwelling bacteria (that are not found in Antarctica). Counter-argumentThe evidence is not very compelling, and may even simply be coincidental.

10. ALH84001: four lines of evidence of life Tiny, egg-shaped and rod-shaped structures in the rock look like fossil nanobacteria. Counter-argumentThe shapes could be a result of how the samples were produced. Even if they are true forms, they could simply be non-biological. These fossils are only about 100 nm across, even smaller than tiny nanobacteria.

11. Richard Hoover’s work A NASA scientist has been analyzing meteorites, and since 2004 has been trying to convince people he is finding remains of fossil microbes in carbonaceous meteorites. Strangely, while these “microbes” contain C, Mg, S, Si, as expected, nitrogen is lacking. Hoover argues that nitrogen outgases over very long times, indicating these fossils are very old, and are not modern contaminants. Counter-argument These meteorites contain calcium sulfide. Combined with humidity, this turns into calcium sulfate which has a lower density. As it forms, it oozes out of the cracks like toothpaste squirted from a tube.

12. Viking Viking results In 1976, two probes to Mars carried basic biological labs to look for life on Mars. The Viking probes were equipped with robotic arms that could scoop out Martian soil and bring it on board for analysis. The number one priority: search for evidence of life!

13. Evidence: Viking Labeled release experiment Hypothesis Martian soil was filled with microbes that would consume organic compounds. Prediction If given nutrients, Martian microbes would absorb them. Metabolic by-products would be released. (This is very much like the gas exchange experiment.). Experiment 1a Martian soil was given nutrients. The 32S and 12C in the nutrients were enriched by 35S and 14C. Any gaseous metabolic byproducts resulting from the added nutrients would be rich in these radioactive compounds. Experiment 1a results The soil sample released these radioactively-tagged gases, as predicted!

14. Evidence: Viking Labeled release experiment (continued) Hypothesis The proposed microbes that were consuming organic compounds would be affected by temperature. Moderately high temperatures would hurt them, while very high temperatures would kill them. Prediction If heated to higher and higher temperatures before being given nutrients, Martian soils would release fewer gases of metabolic origin. Experiment 1b Experiment 1a was reproduced. But first, in one case the soil was heated to 50ºC (122ºF) and in another case to 160ºC (320ºF) before being given nutrients. Experiment 1b results The 50ºC soil sample released fewer gases, and the 160ºC sample released no gases at all, as predicted!

15. Evidence: Viking Labeled release experiment (continued) Analysis The soil released gases, consistent with life. Furthermore, increasing temperatures were increasingly hostile to the chemical reactions involved, suggesting further that it was life. Conclusion Whatever released the gases is unknown, but it is certainly consistent with microbial action.

16. Evidence: Viking Gas chromatograph/mass spectrometer experiment Hypothesis Martian soil contains organic compounds. Prediction If chemically analyzed, Martian soil will be shown to contain organic compounds. Experiment 2 Martian soil was subjected to standard laboratory methods. The soil was heated to high temperatures (650°C). Any organic compounds would be vaporized (including little microbes!). Gas chromatography separates the gases, and the mass spectrometer would determine their masses. Experiment 2 results The soil sample contained no organic compounds. Therefore, life could not be present in any form we would recognize.

17. Viking summary Overall, the results are inconclusive On the whole, the evidence argues against life, although the results from the labelled release experiment is consistent with life. The results that there are no organic compounds on Mars (from the Gas chromatograph/mass spectrometer experiment) is a surprise, since organic compounds occur so frequently on asteroids, comets, and satellites of the Jovian planets. It was since found (by the Phoenix lander) that Martian soils are filled with highly oxidizing perchlorate ions. When heated, these would combine with carbon molecules and mask their detection. So the final conclusion is…. We don’t know.

18. Mission wrap-ups Viking Mars is not teeming with life, and seems very unlike the Earth. Spirit & Opportunity Hematite, ancient shorelines, moreevidence of long-standing acrid/salty water. Orbiters (Mars Recon Orbiter, etc) H2O ice in fresh (2008) craters, chloride deposits (formed from evaporation) seen. Recurring slope lineae observed. Phoenix Calcium carbonate and ice observed. Perchlorate (ClO4) verified in soil. Curiosity Its primary mission objective is to learnabout the current and past habitability ofMars. Has found clay, mudstone, sulfates, all indicating “drinkable” water.

19. Accidental life exchange between Mars and the Earth Current Martian exploration has an exceedingly (but nonzero) chance of introducing terrestrial organisms to Mars. But what about human visitation—Martian contamination is almost certainly inevitable. What are the ethics of possible planetary destruction of a native ecology? And even more generally, do we have the right to terraform another planet entirely? Note: Mars has a land surface area=Earth’s surface area. This doubling of land area would give us only one population doubling time of new growing space, i.e., about 40-50 years. Finally, note that while microbial Earth life has difficulty reaching Mars, microbial Martian life could relatively easily reach Earth.

20. Appendix: more Viking More experiments conducted by the Viking probes, for those really interested in the topic.

21. Evidence: Viking Carbon assimilation experiment Hypothesis Martian soil was filled with autotrophic microbes. Prediction Microbes in the Martian soil, brought into the Viking lander and given CO2 or CO, would assimilate the gases. Experiment 3a Martian soil was combined with CO2 and CO (and in some cases water). The carbon in the CO2 and CO, was in the form of the rare isotope 14C, so it would be easy to detect. Experiment 3a results The soil sample assimilated the carbon!

22. Evidence: Viking Carbon assimilation experiment (continued) Hypothesis The assimilation of 14C by the Martian soil was due to microbial action. Prediction Microbes in the Martian soil, first heated to high temperatures, would not assimilate the carbon. Experiment 3b Experiment 1a was repeated, but first the Martian soil was cooked at 175ºC (347ºF) for three hours before being given CO2 and CO. Experiment 3b results The soil sample still assimilated the carbon!

23. Evidence: Viking Carbon assimilation experiment (continued) Analysis It is remarkable that the soil assimilated the 14C. This was unexpected, and seemed to indicate microbes. However, organisms should have been killed by the extremely high temperatures in second experiment. It is difficult to explain how life could survive such conditions. Conclusion What absorbed the carbon is uncertain, but it does not seem to be life!

24. Evidence: Viking Gas exchange experiment Hypothesis Martian soil was filled with heterotrophic or autotrophic microbes. Prediction Given a rich nutrient broth, Martian microbes would feed and release waste products. Experiment 4a Martian soil was combined with a nutrient broth. The soil was watched for the release of H2, N2, O2, CH4, or CO2 (suggesting respiration, photosynthesis, or something similar). (This is very much like the labeled release experiment.) Experiment 4a results The soil released O2. However, unlike photosynthesis, the oxygen was released even in complete darkness!

25. Evidence: Viking Gas exchange experiment (continued) Hypothesis The O2 released by the Martian soil was microbial activity stimulated because the microbes had fed on the nutrients. Prediction Given only water, instead of a rich nutrient broth, the Martian soil would not release O2. Experiment 4b Experiment 2a was repeated, but this time the Martian soil was combined with sterile H2O instead of a nutrient broth. Experiment 4b results The soil still released O2!

26. Evidence: Viking Gas exchange experiment (continued) Hypothesis The O2 released by the Martian soil was microbial activity and not some strange chemistry. Prediction If cooked to high temperatures first, the soil would not release O2 if given nutrients. Experiment 4c Experiment 2a was repeated, but this time the Martian soil was heated to sterilizing temperatures before it was given the nutrient broth. Experiment 4c results The soil still released O2!

27. Evidence: Viking Gas exchange experiment (continued) Analysis It is remarkable that the soil produced O2. This seemed to indicate microbes engaged in photosynthesis (plants photosynthesize, and produce O2). However, photosynthesizing organisms should not have produced O2 in the dark. Also, they responded to both nutrients and sterile water the same way. Finally, they did not mind being cooked to sterilizing temperatures. Conclusion What emitted the O2 is uncertain, but does not seem to be life.