1.1 ‘Are we alone?’
The question, ‘are we alone in the Universe?’, has been at the heart of scientific, philosophical and religious discourse for millennia. Greek and Roman philosophers, Renaissance astronomers with early telescopes, 20th century space agencies with robotic spacecraft and 21st century artificial intelligence-based technologies that search vast swathes of the cosmos, have all contributed to furthering this debate.
Since the dawn of the space age in the 1950s, this question has become the central tenet of the interdisciplinary science of astrobiology.
Although there are programmes searching for advanced, intelligent life, for example SETI – the Search for Extraterrestrial Intelligence – this is largely focused outside our own Solar System. In this section, and this wider course, we are considering the search for life as investigated by exploring our own Solar System in more detail, using robotic spacecraft to do the job of the scientist on or around other planets and moons.
Several space missions (Table 1) have had the search for life as an explicit objective and many more have contributed to our understanding of the environments beyond Earth that might be, or might once have been, habitable. Take a look at Table 1 and answer the questions that follow.
| Table 1 Space exploration missions that have had the search for life as an objective. Only those that reached their target destination and partially or fully deployed have been included. | |||||
|---|---|---|---|---|---|
| Mission Name | Mission type | Target | Launch Year | Country/Agency | Objective |
| Viking 1 & 2 | Orbiter-lander (x2) | Mars | 1975 | USA (NASA) | Direct life detection on Mars via biology experiments |
| Cassini-Huygens | Orbiter-lander | Titan and Enceladus | 1997 | NASA/ESA/ASI | Investigate Saturn's moon's Titan and Enceladus for organic chemistry and habitability, Titan lander |
| Beagle 2 / Mars Express | Lander | Mars | 2003 | UK/ESA | Search for biosignatures on Mars |
| Spirit & Opportunity (Mars Exploration Rovers) | Rover | Mars | 2003 | USA (NASA) | Investigate past water activity and habitability |
| Phoenix | Lander | Mars | 2007 | USA (NASA) | Search for water ice and potential habitability near martian poles |
| Curiosity (Mars Science Laboratory) | Rover | Mars | 2011 | USA (NASA) | Assess Mars' habitability, detect organic molecules |
| ExoMars TGO-Schiaparelli | Orbiter-lander | Mars | 2016 | ESA/Roscosmos | Search for trace gases related to biological processes |
| Perseverance | Rover | Mars | 2020 | USA (NASA) | Search for biosignatures and collect samples for return |
| Tianwen-1 / Zhurong Rover | Orbiter-lander-rover | Mars | 2020 | China (CNSA) | Study martian soil and water-related features to assess habitability |
| James Webb Space Telescope | Space telescope | Cosmos | 2021 | NASA/ESA/CSA | Analyse atmospheres of exoplanets for gaseous biosignatures |
| JUICE | Orbiter | Jupiter’s moons | 2023 | ESA | Explore Jupiter's moons for subsurface oceans and signs of habitability |
| Europa Clipper | Orbiter | Europa | 2024 | USA (NASA) | Investigate Europa, habitability and subsurface ocean |
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What does Table 1 tell you about where scientists think life is most likely?
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Over half of the missions deployed to date have been to Mars, which suggests this is thought to be the most likely place for life to be found. However, there are other factors to consider that lead to this focus, such as the cost and timescales of sending missions over longer distances and changing priorities of space agencies. For example, you might notice a hiatus in missions from the 1970s to the 1990s during which time NASA focused their efforts on the space shuttle programme and developing the use of low Earth orbit (e.g., the International Space Station).
Mars has been a focus for the search for life for over a hundred years, since Italian astronomer Giovanni Schiaparelli observed lines on the martian surface through a telescope in the 1870s. These ‘canali’ as he described them were mistranslated to be ‘canals’ - artificial waterways - and suggestions of the existence of martian civilisations emerged. Since then, even though the ‘canali’ features themselves have been shown to be nothing other than irregular features on the martian surface, Mars has captured the imagination of wider society as a potential home to extraterrestrial life and remains the target for much scientific attention.
If you want to learn more about the exploration of Mars, you can visit the OpenLearn interactive ‘15 minutes on Mars [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] ’, and we shall return to consider Mars in the context of planetary protection shortly. However, we first need to consider what it is we are looking for when searching for ‘life’.