Lesson 5: Introduction to Biosignatures
Key Learning Objectives:
Define the term ‘biosignature’
Explain why a biosignature is useful in science
Identify a biosignature graph (spectroscopic)
Identify what features of a biosignature allow for efficient identification of compounds in the exoplanet atmosphere
In this lesson you'll find...
What is a Biosignature?
Types of Biosignatures
Biosignatures and Life
What Does a Biosignature Look Like?
Additional Information
What is Biosignature?
The search for life outside our planet was once a topic of fiction but is now a serious enquiry. It is well understood that life, from the earliest plant to our bustling society today, effects the atmosphere in a strongly detectable way. The atmosphere and biosphere (zone of Earth where living organisms exist, encompassing the surface, atmosphere, and hydrosphere) are dependent on one another and astronomers analyse the atmosphere of potentially habitable planets to determine if there is an active biosphere. A biosignature is an important way of detecting life on other planets by observing the planet’s atmosphere.
Biosignature: Any substance (eg. a chemical, compound, molecule or set of molecules) that provides scientific evidence of past or present life. This information is usually gathered using spectroscopic methods.
Class Discussion
What compounds/chemicals do you think should be present in a biosignature to indicate life on a planet? Remember, scientists search for ALL water-based life, not just homo-sapiens.
Biosignature Types
We can categorise all biosignatures into three main classes: gaseous, surface, and temporal biosignatures. These classifications provide valuable insight into determining the most effective methods for the chemical species belonging to each category. The figure below presents an example of the three types of biosignatures:
Gaseous biosignatures: Volatile molecules in the atmosphere.
Surface biosignatures: Pigments such as chlorophyll.
Temporal biosignatures: Molecules whose concentration changes over time due to biological activity.
Gaseous biosignatures can either be directly generated through metabolic processes or result from environmental processing of primary metabolic products, leading to secondary compounds. Both of these sources can be understood by considering as an example the synergy between the production of oxygen (O2) and ozone (O3) in Earth’s atmosphere: whilst O2 is mainly produced by plants from photosynthesis on the ground (and so a direct product of metabolism), O3 forms in the stratosphere via the photochemical breakdown and rearrangement of O2 molecules, making it a secondary species. Hence, though O3 is not directly produced by life (in fact, it can be harmful for most forms of life when directly exposed to it), its presence in our atmosphere is a consequence of the presence of life on Earth.
Surface and temporal biosignatures both provide valuable insights into the planet’s potential of hosting life. Surface biosignatures primarily involve changes in the planetary surface spectrum resulting from the presence of life, e.g., pigments and bioluminisence. Temporal biosignatures, on the other hand, are timedependent modulations of gases, indicating the influence of a biosphere on the planetary environment, e.g., the seasonal variation in CO2 concentration observed on Earth.
However, studying these surface and temporal biosignatures poses greater challenges compared to their gaseous counterparts. Surface spectral signals, for instance, may be obscured by cloud decks and atmospheric haze, making their detection more difficult. Similarly, observing seasonal changes in exoplanet atmospheres requires a substantial investment in observations, further adding to the complexity of studying these biosignatures. Gaseous biosignautres are therefore the preferred choice when looking for signs of life in exoplanet atmospheres.
Biosignatures and Life
The atmosphere of a habitable exoplanet must contain certain elements to encourage the formation of life. Remember that for now, we only consider water-based life. Compounds such as carbon dioxide (CO2), nitrogen (N2), and methane (CH4) aid the beginnings of life by raising the surface temperature and reacting with each other to establish additional life-activating compounds.
Bacteria aid photosynthesis by absorbing carbon dioxide to release oxygen (O2) into the environment which then instigates aerobic life by encouraging the formation of atmospheric ozone (O3) and increasing the concentration of atmospheric O2. Hence, by analysing the spectra of the atmospheres of other planets, we are able to identify ozone and oxygen peaks which indicates that photosynthesis is taking place and an aerobic (oxygen-dependent) biosphere has, or will, form.
Class Discussion
With limited resources it is not possible to look at the biosignature for every planet/exoplanet so where should scientists be looking first?
Space agencies are trying to detect the atmospheres of “potentially habitable planets” (which has a set of criteria which will not be covered in this course) as well as those planets within our own solar system. By analysing both extra and intra-solar planets, astronomers engage in the discovery for “new Earths” while also discovering properties of planets both within and beyond our own Solar neighbourhood.
What Does a Biosignature Look Like?
A biosignature shows the compounds present in a planetary atmosphere as a series of peaks. This data is collected via spectroscopic methods that will be explored later in this course. Each compound gives a peak at a specific place in the spectrum and when an atmopshere is analysed as a whole, many peaks are shown in the plot.
Activity
In upcoming lessons, we'll learn that each biosignature has a distinctive spectrum, akin to a molecular fingerprint, enabling scientists to detect these signatures and other molecules within the atmospheres of planets, whether inside or beyond our solar system.
The figure on the left displays the comprehensive spectra of three solar system planets—Venus, Earth, and Mars. Disregarding factors such as solar distance and planetary size, why are Mars and Venus inhospitable for water-based life?
A biosignature can be thought of as a piano. Each key on the keyboard represents a particular compound in the atmosphere and when played together, they create a chord. A full chord would be the whole biosignature of a planet, where every peak is a note on the piano. The chords are not the same forever since the atmosphere evolves as the planet does. Earth’s biosignature musical would sound very different from Venus’ musical because different notes and different chords are being played at different times in history. If an exoplanet’s chord or musical sound similar to Earth’s, then it is possible to make some strong and exciting conclusions about life there!