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40 ASTRO PUBLISHING tion or isotope ratios of sediments, but we could not rule out natural causes behind such anomalies. How- ever, the existence of a previous ter- restrial civilization remains more of a thought experiment than a realis- tic hypothesis. Regardless of the relative abun- dances of the two markers of life in the cosmos, both the search for bio and techno are hampered by the problem of the ambiguity of their recognition. Wright and colleagues highlight two main forms of ambigu- ity: nature and the significance of the detected marker. Biosignatures suf- fer more from this because the po- tentially life-related molecules rec- ognizable today in the atmospheres of planets other than Earth can easily arise from natural processes. Let’s think about the recent cases of methane on Mars or phosphine on Venus. On the other hand, some technosignatures detectable even at great distances, such as very narrow- I nterstellar spacecraft can be de- tectable from hundreds to thou- sands of light years away through various forms of radiation. The aster- oid `Oumuamua (illustrated here) has shown that we can even receive vis- its from objects from other planetary systems. [ESA/Hubble, NASA, ESO, M. Kornmesser] A megastructure, such as the Dyson sphere pictured here, would be capable of generating tech- nosignatures visible at great distances in the galaxy, revealing the presence of an alien technological civilization even in the absence of radio commu- nications. [Capnhack.com] band radio signals, are decidedly less ambiguous, since no known natural source can generate them. At this point, it is easy to under- stand why astrobiologists prefer to look for an anomalous signal in a large star field that likely contains thousands or millions of planets, rather than struggling to isolate the spectrum of a single planet, looking for molecules that today could not be associated with certainty to the presence of life. In this astrobiolog- ical context, the variants proposed by Wright and colleagues will be useful in developing the discussion on the implications of some proper- ties of technological life that differ fundamentally from those of non- technological life. !