Life in the Solar System - Let's be reasonable

Hypothetical life in a gas giant from The Worlds of David Darling
The prospect of extraterrestrial life exercises an understandable fascination among the general public, and could be seen as an unhealthy preoccupation of the popular press and funding institutions. Our purpose today is quickly to review the prospects for extraterrestrial life in the Solar System. To do this, we focus on the three locations with the best practical prospects for the actual identification of potential life. I.e., the places where life might live in regions that we could conceivably get a spacecraft to to observe it. These are: We dismiss other more exotic locations that are immune to our scrutiny where life must remain speculative, such as the atmospheres of the giant planets (right).

Life on Earth

Definitions of life:

There is no single standard definition of life. Different definitions tend to reflect the priorities of their authors. One that Merck is fond of:

Living things are systems that tend to respond to changes in their environment, and inside themselves, in such a way as to promote their own continuation.

These responses may involve:

Cartoon by R. Grossman, 1962 from The Worlds of David Darling
Requirements for life: Any extraterrestrial life would have to have analogous components, but not necessarily the same ones (right).

Abiogenesis: The origin of life from non-living precursors. Establishing the manner and environment in which these components arose on Earth is a big research priority. For a clever review, see the summary of Dr. Jack Szostak's ideas, ay this video.

Black smoker from NOAA - Ocean Explorer
Energy source: The crucial fact in the foregoing is that the both cell membranes and proteins seem to have originated in environments that are at least intermittently hot.

The environment in which these conditions are routinely found is near deep sea hydrothermal vents. Currently, these seem like the most likely locations for the origin of life. These environments were:

Interestingly, phylogenetic analyses suggest that among the most primitive organisms are thermophylic prokaryotes known as Archaea. Their special features:

Location of LUCA (the Last Universal Common Ancestor):

Current thinking maintains that life probably originated in hydrothermal vents (right). These environments were:
  • Interestingly, phylogenetic analyses suggest that among the most primitive organisms are thermophylic prokaryotes known as Archaea. Their special features: The environment in which these conditions are routinely found is near deep sea hydrothermal vents. For some time the most likely locations for the origin of life. Weiss et al., 2016 have attempted to identify genes conserved across all major domains of life (Archaea and Prokaryota) in order to characterize LUCA's environment and ecology. Their conclusions:

    Bacterial mat Floreana, Galapagos Islands
    Criteria to apply to other worlds

    When we look in the Solar System outside of Earth, we have almost zero hope of finding animal or plant-grade multicellular life. We will have to be satisfied with simple microbial life. Indeed, the equivalent of a bacterial mat (right) would be very exciting. But what do we look for?

    Martian life from TOP50SF


    Ancient Martian environments:

    Assertions that Mars could currently support life as it is known on Earth are quite common, and are based on:

    Indeed, there seems little doubt that Mars during its first billion years had: In short, it was a place in which life might have been able to survive, but that doesn't address the true issue:

    Eridania Basin from Sci-Fi News

    Has Mars ever possessed an environment in which life could have originated?

    Hydrothermal Environments: Despite all of the hype about ancient liquid water on Mars, only one publication, Michalski, et al., 2017 has offered a possible environment in which life might have originated, a preserved ancient hydrothermal environment in the ancient Eridania basin (right). Only a beginning, but interesting.

    Nili Patera from NASA Photojournal
    Hydrothermal refugia: The discovery (right) of a "fossilized" fumarole-like hydrothermal environment consisting of a volcanic cone containing copious hydrated silica - typically the result of the alteration of volcanic material by hot groundwater. The cone represents a late stage in the volcanic life of Nili Patera in the Syrtis Major region. (Skok et al, 2010) This particular spot could function as a refuge for any life that might have existed 3.5 Ga when Mars was rapidly becoming a cold desert.

    Blueberries on Meridiani Planum from Daiy
    Neutral pH surface water: Since Mariner 9 (1971 - 1972) returned images of Martian channels from orbit, we have suspected that water once flowed over parts of Mars. In 2005, the rover Opportunity identified clear evidence of ancient Martian water in Meridiani planum in the form of: The cross-beds, however, were quite thin. Worse, the presence of minerals like jarosite indicated that the water was of very low pH. Apparently the ancient groundwater of Meridiani was like battery acid. Suspicions arose that this was typical for ancient Mars.

    Conglomerates at Glenelg in Gale Crater from SciTechDaily
    Discoveries by Curiosity in Gale Crater have changed that view: Thus, Gale Crater, at least, once possessed a surface environment in which life could have lived. Maybe Mars hosted a wide range of environments.

    Silicate crystals at Home Plate in Gusev Crater from PhysOrg
    At the end of its mission in 2010, Spirit encountered a feature called Home Plate in Gusev Crater. At it were finger-like formation representing hydrothermal silicate deposits. Interestingly, the formation of similar features on Earth is catalyzed by microbes! Could these be actual signs of life?

    Chryse Planitia from Viking I from NASA

    Viking Lander Data:

    In 1976, NASA landed Viking 1 and Viking 2 on Mars. Each carried three life-detection experiments:

    Viking I labelled release results from Gilbert Levin
    Results: The GEX and PR experiments were negative, and the GCMS detected no organic compounds (very strange considering their presence in other planetary bodies), but the LR was strongly positive. The result has been enduring ambiguity, with advocacy for two possible sources of the reactions:

    Some aspects are consistent with either source:

    Some seemed to favor a biological origin: But there were causes for skepticism: But finally, where were the organic molecules to go along with this "metabolic" activity? Ultimately NASA adopted the official position that the Viking biology experiments had not detected clear evidence of life. Instead, it was concluded that some strong oxidizing chemical is present in Martian soil. One consequence of this strong oxidizer's presence would be the sterilization of martian soil. Subsequent debate has take the form of:

    Martian soils and the mystery oxidizer:

    Two major candidates have arisen for the mysterious strong oxidizer that explains the Viking LR data:

    Artist's martian dust devil from
    Hydrogen peroxide (H2O2): The familiar household disinfectant was discovered in the martian atmosphere in 2004 (Encrenaz et al., 2004). A mechanism for its synthesis was proposed by Atreya et al., 2006 - Static electricity generated during martian dust storms could split CO2 and H2O molecules, allowing them to recombine as H2O2. (We see similar electrical discharges when plinian volcanic eruptions create similar aerosols of fine particles.) These authors speculated that during intense dust storms, hydrogen peroxide could literally snow out of the atmosphere and into the soil.

    Vastitas Borealis from Phoenix lander from Astro Bob
    Perchlorate ion (ClO4-): A major result of the 2008 Phoenix lander mission was the discovery of this strong oxidizing agent in the soil of the martian boreal plain (Hecht et al, 2009). It was found to have a patchy distribution consistent with its having been transported and deposited in solution by groundwater. (It was later detected by Curiosity in Gale crater, indicating a global distribution.) Water - good for life, but the perchlorate, itself makes Mars seem less hospitable. But note, on Earth, the highest concentrations of perchlorate are in the Atacama Desert, which is often used as a stand-in for Mars. What do Mars and the Atacama have in common? They are dry and rain-free. On both Mars and Earth, perchlorate seems to be synthesized in the atmosphere and rain out onto the surface. On Earth, however, it is typically removed from the soil by water.

    One objection: At Martian temperatures, perchlorate is non-reactive. Quinn et al., 2013 demonstrated that the irradiation of perchlorate with UV such as would occur on Mars' surface can transform it into more reactive substances like chlorine dioxide (ClO2) or hypochlorite (ClO).

    The presence of H2O2 and ClO4- seem to tie up the two big Viking mysteries:


    Not dead yet:

    But neither of these oxidizers are fatal to the notion of a living Mars:

    The H2O2 * H2O hypothesis: Biological interpretation proponents Schulze-Makuch and Houtkooper, 2006, point out that the Viking LR results' enigmas go away if we suppose that - instead of employing a saline solution as the medium for life, martian organisms use a hydrogen peroxide solution. The idea of living in disinfectant seems strange, but many Earth microbes have methods of tolerating or employing it metabolically, using chemical stabilizers that inhibit its corrosive effects. Some felicitous properties of hydrogen peroxide solution:

    Moreover, the H2O2 * H2O hypothesis explains some anomalous aspects of the Viking data: Not crazy but sounds a little like a just-so story.

    Further suggestive patterns:

    Atmospheric methane concentration from Science Daily
    Martian Methane enigma:

    Methane enters Earth's atmosphere from two sources:

    But it does not last long, as it is quickly oxidized to CO2. The Mars Global Surveyor detected significant quantities of methane in the martian atmosphere. A given molecule of the stuff seems to last, on average, less than a year.

    The map at right shows regions of high methane concentrations in yellow. Interestingly, they are the Elysium, Tharsis, and Arabia Terra volcanic regions - home to many giant volcanoes. As a news story, this finding is revealing. The methane could be from microbial activity and, because <sarcasm> we absolutely know that Mars is volcanically dead </sarcasm> microbes are the likely source of the gas.

    Alas, Curiosity has detected methane only briefly (and *that* might have come from the rover, itself.)

    An oxygen atmosphere? Tuff et al., 2013 infer from chemical differences in martian meteorites (originating deep in the crust) and surface rocks observed by the Spirit rover that 4 ga, Mars had an oxygen rich atmosphere. (!!!) In combination with other evidence of a wetter warmer early Mars, this is a provocative idea.

    SEM image of the Allan Hills meteorite from NASA
    The Allan Hills Meteorite

    In 1996, the meteorite ALH84001, (a nakhalite that we have previously discussed), was reported to contain the possible fossil remains of martian organisms (McKay et al., 1996). Over time, the credibility of this claim has waxed and waned several times, and is now regarded skeptically by most researchers.

    My last word on martian life: We obviously don't know whether Mars has ever harbored life, but sufficient evidence exists to fuel continued interest. The wonder and glory is that Mars is close enough, and getting spacecraft there sufficiently within our technical grasp, that within our lifetimes, we might actually have an unambiguous answer. That's probably much more than we can say about the remaining cases. Indeed, the Curiosity rover could transmit key evidence any time.

    Key concepts and vocabulary:
    Additional reading: