“I’m not picking up any cities or technology. Massive life form readings though. There’s something alive down there!”
- Luke Skywalker approaches Dagobah in his X-Wing, Star Wars: The Empire Strikes Back

The concept of a scanner - a remote sensing device that can identify evidence of life at a distance - is a common staple of science fiction. From Star Trek to Star Wars and beyond, it is common to assume that such signs of life on a planet can be identified by passive sensors. These reveal the presence of organic or other materials, short-cutting the otherwise laborious (and plot-dragging) process of reaching the new world and analysing samples. While some science fiction takes this to an extreme, it’s neither a new idea nor one that is entirely divorced from science fact.

A biosignature is the generic term for a remotely-detectable indication that life is present. In the case of advanced technological civilisations, this may include a directly received electromagnetic communications or radiation signatures that indicate nuclear processing (aka technosignatures). However in most cases, the term is used to describe chemicals (usually in the atmosphere of a planet) which are more likely to be associated with life processes than geological ones.

The Red Edge

One of the earliest biosignatures to attract attention is the compound responsible for both the green colouration of vegetation and its ability to convert sunlight into chemical energy: chlorophyll. On Earth, the evolution of chlorophyll was vital to the development of animal life, and energy production by photosynthesising vegetation lies at the base of almost all food chains.

The nature and use of this biosignature was explored in surprising detail in a science fiction comic story from the 1950s. Dan Dare - Pilot of the Future was the cover feature and primary draw of the weekly comic Eagle. The second adventure of Dare and his crew was The Red Moon Mystery, serialised between October 1951 and June 1952. The mysterious disappearance of most of Mars’ native biosphere millennia before proves relevant to a danger that threatens the contemporary human colony on the planet: the imminent return of a hive of Space Bees. Having stripped Mars of surface life for the second time, the space bees turn their attention towards Earth.

Both the most plausible explanation for the space bees’ activity and the best hope of a solution is explained in detail by Dare’s scientist crewmate, Professor Peabody, in a densely-lettered series of panels towards the end of the story. And it is here that biosignatures become a key plot driver.

As Peabody explains, an essential method for detecting biosignatures is through spectroscopic analysis. Different chemical compounds imprint a distinctive pattern, like a barcode, on the light from a source - whether that light is self-emitted as in the case of stars, or reflected as is the case for planets. In the case of chlorophyll the most distinctive feature is known as the vegetation red edge, and is characterised by a sharp drop in emission in the optical spectrum, relative to slightly longer wavelength light in the near-infrared. The use of this feature as a biosignature was proposed as early as 1895 by Lewis Jowell [1] as a way to test the idea that vegetation might be responsible for observed seasonal changes on the Martian surface. Intensive studies in the first few decades of the twentieth century completely failed to detect a chlorophyll signature, and it’s interesting to note that the response from some in the community appears to have been to explain that “the failure of the test is not conclusive, for many terrestrial plants do not show this peculiarity” (JRASC, 1925).

In fact, the early twentieth century study of Mars is a good example of confirmation bias - the tendency of humans to find an interpretation which fits with one’s preconceptions and discard contradictory evidence. Investigations continued well into the 1930s, with increasingly desperate attempts to explain away negative results for biosignatures, and even as late as the 1950s popular textbooks suggested lichens as an explanation for the seasonal colours now known to be associated with dust storms.

Dan Dare’s universe reflects some of this ambiguity in the contemporary 1950s literature, with Mars showing a weak chlorophyll signature in the story, attributable as much to introduced Earth vegetation as the remnants of a previously-stripped native biosphere. The story concludes with a plan to distract the space bees from Earth with a giant, chlorophyll-filtered lamp (calculating the power requirements for this can make for a fun undergraduate physics tutorial). It’s striking just how much thought and research went into the work of Dan Dare’s writer Frank Hampson, and the level of understanding and thought that was expected of the comic’s young readers of the time. And while the hope of detecting chlorophyll signatures on Mars has faded, it remains a key target for future observations of exoplanets.

Considering Cow Farts?

An alternative spectral biosignature was discussed in the work of Frederick Pohl, particularly his 1977 novel Gateway [2]. This novel describes the efforts of human space explorers to exploit the remnant technologies of an ancient race known as the Heechee. In particular, the titular space station is equipped with a large number of individual explorer ships which travel on preprogrammed courses the humans taking them out are unable to predict. Amongst the most valued prizes of such trips are worlds with signs of life.

The book alternates the main text with interspersed newspaper articles, technical reports and interview transcripts. In one (reproduced in the image from the 1999 Gollancz edition) a Dr. Asmenion explains that methane gas on Earth comes predominantly from the digestion tracks of ruminants - hence its use as a spectral biosignature to indicate animal life.

In fact, this is perhaps over-optimistic. Unlike chlorophyll, methane gas is generated by a range of geological as well as biological processes, although it is also broken down readily by chemical reactions. The main natural source of methane in Earth’s atmosphere is the breakdown of organic matter by microorganisms - this does indeed include the action of gut bacteria in ruminants, but the majority is associated with vegetative decay in wetlands. Increasingly, this has been matched or even exceeded by anthropogenically generated methane, notably from microbial decay processes in farming and waste-processing. As such concentrations of methane comparable to those in Earth’s atmosphere would indeed be an indicator of life, albeit not necessarily complex or animal life. It is an example of non-equilibrium chemistry - life is needed to prevent the establishment of a status-quo in which the methane would be chemically locked up.

As a result, methane has certainly been investigated as a potential biosignature, and continues to be an area of active investigation, particularly in combination with other signatures. It has also attracted attention in recent years due to the detection of methane signatures in the atmospheres of Mars and Saturn’s moon Enceladus. In both cases the concentration of the gas in the atmosphere, and the way it has varied over time, is difficult to explain through geological processes and may hint at subterranean microbial life (although this is far from definitive at this point).

All Purpose Pebbles

Atmospheric spectral biosignatures can be seen at large distances, detectable not only from planetary orbit but at interstellar distances, but are not specific - they indicate the presence of large volume biological processes rather than identifying individual types of life forms, let alone specific individuals. Other spectroscopic signatures include the presence of free oxygen (which would usually react chemically and be bound to surface rock unless replenished by life), argon and other rarer compounds such as phosphine.

Such spectroscopic remote sensing can be complemented with other techniques including those which involve more active measurements. An example of a general-purpose remote sensing device, building on searches for specific gas signatures, can be found in the novel Revelation Space (2000) by (astrophysics-trained) author Alistair Reynolds. His protagonists use a cloud of “pebbles” - compact detectors which can be scattered through a solar system, and pass through the upper atmosphere of interesting planets:

The dayside pebbles were looking away from Delta Pavonis, so they snooped for neutrino leakage from fusion and antimatter power units on the surface. The nightside pebbles snooped for the heat signatures of population centres and orbital facilities. Other sensors sniffed the atmosphere, measuring oxygen, ozone and nitrogen levels; sensing the extent to which the colonists had tampered with the native biome.
- Revelation Space, Reynolds (2000).

Here spectroscopic investigation is combined with a direct chemical analysis (“sniffing” the atmosphere). Interestingly, this example also introduces a second category of life signs - while chemical markers may indicate life of any form from microbes upwards, technological biosignatures are specific to intelligent and scientifically advanced life. These can include the electromagnetic signals which are often shown spilling out into space in science fiction movies (notably in the opening sequence of Contact, 1997), radio-isotope signatures or even the hypothetical signs of alien megastructures which distort the light from their host stars.

Remote Sensing

“Life-forms! You tiny little life-forms! You precious little life-forms! Where are you?”
- Data, scanning for life forms from the bridge of the Enterprise, Star Trek: Generations (1994) [watching the video on this link is much recommended]

The examples I’ve focussed on above demonstrate an interesting, but perhaps unsurprising, engagement of science fiction writers and readers with contemporary research. The academic search for biosignatures on alien worlds has extended from study of our neighbouring planets to the most newly-discovered exoplanets, and from microbial gas to the largest artificial structures. Short-distance sensor technology, including the ability to detect heat, metals or chemical traces, has also advanced to levels which might have appeared science-fictional until relatively recently. A number of devices which detect biological signals without physical contact have been either inspired by, or described as, a Star Trek tricorder when first introduced.

However science fact nonetheless falls a good distance short of the scanners which are often portrayed in science fiction. Current solar system probes have employed a variety of remote sensing devices, as well as performing analysis in-situ on planetary surfaces. Exoplanet studies have explored key spectral biosignatures outside our Solar System, and are complemented by increasingly complex modelling which should improve the confidence in any future detection. But unlike Star Trek officers such as Data (quoted above) we are unable to peer through ionospheres to identify individuals, and given our current scientific understanding it seems unlikely we will ever do so. And as much as we’d love to find “precious little life forms” outside of Earth’s biosphere, it remains to be seen whether this will ever happen.