Seeking evidence of life in our solar system by drawing on what is known of terrestrial biology, NASA scientists have followed the water. The good news is that there are places with plenty of it around today or once had large amounts of it in the past.
Recognizing evidence of life forms, even types that are functionally analogous to terrestrial organisms, is not necessarily a simple task. The Viking Project (1) featured two landers which executed several experiments to reveal evidence of living microorganisms. Although some unusual chemical reactivity was observed in the soil samples, the consensus is that the Viking lander results cannot be taken as definitive evidence of living organisms.
Powerful New Techniques – Definitive Answers?
The Viking Project landers melded impressive technology with a well-executed mission plan that was completed under extremely challenging conditions. Advances in genetic analysis technology will enable future missions to probe for life on Mars and elsewhere with more robust methods than anything available in 1976. In principle the new generation of explorers will no longer be limited to detecting the metabolic activities of living microbes. Genetic-based approaches may reveal the hereditary material of cells even if they happen to be inactive or are dead.
Culture-independent methods also have significant limitations of their own and may be confounded by minute levels of terrestrial contamination. Notwithstanding their new capabilities, investigators may once again be confronted with samples that fail to yield clear evidence of the presence of microbial life. However, if experiments can be devised and conducted such that contamination is improbable, systematic surveillance efforts using genetic methods may make it possible to be conclude that microorganisms like those now found on Earth were absent or present at extremely low amounts in the samples tested.
“Viruses are astronomically abundant” (2)
Noting viral particles are the most numerous structures on Earth derived from biological activities and their intimate involvement in fundamental ecological and evolutionary processes, a group of scientists is proposing efforts to employ them in projects to detect extraterrestrial life (2). The total number of viruses on our planet is vast and a new study reveals that uncounted billions of them are blown from ocean waters into the air to travel far and wide. Hundreds of millions or more settle out harmlessly around us every day (3). On planet Earth at this point in time, wherever life is present virus particles are pervasive. Perhaps finding viruses or their molecular traces in extraterrestrial samples would provide solid evidence something living is – or was – in them.
Because it is possible to recognize many viruses by virtue of their odd shapes, in one sense looking for virus particles represents the ultimate culture-independent biology search method. Samples can be examined without concern as to whether any viable organisms are present or the conditions/nutrients required for them to be metabolically active. Whether the viruses harbor genetic materials composed of RNA, DNA, or something else, are viable, or have been inviable for years will not matter. All methods have intrinsic limitations and one inescapable problem with looking for virus particles is that fact that most are tiny. Still, although systematic direct searches for viruses will be laborious, including them or bio-signature proxies (2) in future missions to find evidence for life in our solar system seems well worth the effort. It could turn out that describing viruses as “astronomically abundant” (2) could be quite accurate.
Where to Look for Extraterrestrial Viruses?
A few places in our solar system may harbor water oceans, but a small moon of Saturn, Enceladus, may be the go-to place for life-seekers (4). This moon has an orbital friction energy source, liquid water and precursor molecules (5). But it also has something special – cryovolcanic geysers that spray the deep ocean materials far out into space – where it would be (comparatively) easy to sample and analyze them. A probe would not have to land, it would only need to position itself in the outflow to collect samples from deep within the hidden ocean. Experiments have revealed that at certain microbes are able to be active under conditions mimicking those on Enceladus (6).
Maybe We Have Already Seen Them
The spectacular rings of Saturn harbor substantial quantities of water ice. Embedded in the E Ring, Enceladus seems to be the source of the microscopic water ice particles composing this structure. When we look at the rings of Saturn maybe we are seeing the outer space analogue of viruses tossed about on an extremely long voyage.
Many thanks to Dr. Curtis Suttle for bringing the PRI.org story to my attention. To find out more about Dr. Suttle’s research visit his University of British Columbia web page. http://www.ocgy.ubc.ca/~suttle/?p=home
(1) Viking 1 & 2. https://mars.nasa.gov/programmissions/missions/past/viking/
(2) Aaron J. Berliner et al. Astrovirology: Viruses at Large in the Universe. Astrobiology 18(2), February 1, 2018. https://www.liebertpub.com/doi/full/10.1089/ast.2017.1649
(3) Adam Wernick. It’s Raining Viruses, But Don’t Panic. PRI.org, 9 March 2018. https://www.pri.org/stories/2018-03-09/its-raining-viruses-dont-panic
(5) Annie Sneed. Excitement Builds for the Possibility of Life on Enceladus. Scientific American, 28 June 2016. https://www.scientificamerican.com/article/excitement-builds-for-the-possibility-of-life-on-enceladus/
(6) Meriame Berboucha. Is There Life on Enceladus? Forbes, 28 February 2018. https://www.forbes.com/sites/meriameberboucha/2018/02/28/is-there-life-on-enceladus/#2539deca59fa