Periodic Reporting for period 3 - Habitat-OASIS (Habitability of Oceans and Aqueous Systems on Icy Satellites)
Reporting period: 2020-04-01 to 2021-09-30
Habitat-OASIS addresses the question of habitability of the outer solar system by looking at space craft data and preparing future space missions using novel approaches. The classical view held that a habitable planet or moon requires liquid water at or near its surface. This view has been challenged by the discovery of numerous subsurface oceans below the icy crusts of ice moons orbiting Jupiter and Saturn in the outer solar system. The amount of water detected there is several times higher than on Earth and is kept liquid not by solar heat but mostly by tidal heating. Among them, the cryo-volcanic moons Enceladus at Saturn, and Europa which orbits Jupiter, are considered to have the largest astrobiological potential. On these two moons, the ocean floor is on top of a rocky core and there are indications for hydrothermal activities where hot water flows out from the rocky sea floor into the ocean. On Earth, these kinds of hydrothermal vents are places where life developed independently of sunlight.
It is apparent that the search for habitable places on our doorstep, inside our solar system, is important beyond Europa’s space science community and is relevant for the society in a more general sense. By understanding the habitability of icy ocean worlds, the ambitious ERC projects will translate to the habitability of the Universe in general, because icy moons orbiting giant planets should be fairly common in our galaxy. Consequently, this work will inspire astronomy and exoplanet-research opening up fundamental perspectives: if habitable conditions can occur at remote places like on ‘tiny’ Enceladus, we may have drastically underestimated the life-friendliness of our universe. Habitat-OASIS aims to explore the habitability of these worlds using in situ compositional data from current and future space missions.
On Enceladus (and probably also on Europa), the ice grains expelled by active plumes carry matter previously dissolved and suspended in the subsurface oceans, allowing constraining their geochemistry. The mass spectrometers aboard the Cassini-Huygens spacecraft orbiting Saturn until fall 2017 analyzed this material and already delivered spectacular science results. Project 1 of this proposal is the refined data analysis of the Enceladus plume material using novel techniques and is the first ever opportunity to explore in detail a potential ocean habitat outside Earth. Newly developed laser-assisted dispersion experiments are used to acquire mass spectra on a wide variety of analogue materials, enabling the identification and quantification of inorganic, organic and possibly biogenic compounds embedded in the ice grains. Geochemical aqueous alteration experiments and numerical modeling help to further constrain the habitability of Enceladus and extrapolate the results to other ocean moons. Project 2 will leverage the laboratory capabilities from Project 1 to create a comprehensive library of mass spectra in preparation of the upcoming missions visiting Jupiter’s icy moons: ESA’s JUICE Mission and NASA’s Europa Clipper Mission. Having analogue measurements available early in the missions will be critical for exploiting their full potential.
It is apparent that the search for habitable places on our doorstep, inside our solar system, is important beyond Europa’s space science community and is relevant for the society in a more general sense. By understanding the habitability of icy ocean worlds, the ambitious ERC projects will translate to the habitability of the Universe in general, because icy moons orbiting giant planets should be fairly common in our galaxy. Consequently, this work will inspire astronomy and exoplanet-research opening up fundamental perspectives: if habitable conditions can occur at remote places like on ‘tiny’ Enceladus, we may have drastically underestimated the life-friendliness of our universe. Habitat-OASIS aims to explore the habitability of these worlds using in situ compositional data from current and future space missions.
On Enceladus (and probably also on Europa), the ice grains expelled by active plumes carry matter previously dissolved and suspended in the subsurface oceans, allowing constraining their geochemistry. The mass spectrometers aboard the Cassini-Huygens spacecraft orbiting Saturn until fall 2017 analyzed this material and already delivered spectacular science results. Project 1 of this proposal is the refined data analysis of the Enceladus plume material using novel techniques and is the first ever opportunity to explore in detail a potential ocean habitat outside Earth. Newly developed laser-assisted dispersion experiments are used to acquire mass spectra on a wide variety of analogue materials, enabling the identification and quantification of inorganic, organic and possibly biogenic compounds embedded in the ice grains. Geochemical aqueous alteration experiments and numerical modeling help to further constrain the habitability of Enceladus and extrapolate the results to other ocean moons. Project 2 will leverage the laboratory capabilities from Project 1 to create a comprehensive library of mass spectra in preparation of the upcoming missions visiting Jupiter’s icy moons: ESA’s JUICE Mission and NASA’s Europa Clipper Mission. Having analogue measurements available early in the missions will be critical for exploiting their full potential.
Our scientific focus during the first half of the funding period was on Project 1: the compositional interpretation of ice grains emerging from the subsurface ocean of Saturn’s small ice moon Enceladus. Here, the main focus over the course of the first two years was on the analysis of organic material. It became clear early in the project that a major breakthrough could be achieved by identifying complex organic material in about 1% of the ice grains emerging from Enceladus. To better understand the organic composition, we used data from Cassini’s Cosmic Dust Analyser (CDA) and the Ion and Neutral Mass Spectrometer (INMS) to decipher the composition. Our laser-assisted laboratory experiments were used to acquire mass spectra from a wide variety of analogue materials, and to mimic the spectral signature from space. We identified large organic fragments that show structures typical for very large molecules. The results indicated molecular masses in excess of 200u, maybe up to the macromolecular regime of several 1000u. These huge molecules contain a complex network often built from hundreds of atoms of carbon, hydrogen, oxygen and likely nitrogen (Postberg et al., Nature 2018).
We further evolved our results by linking them to geochemical and geophysical processes occurring on subsurface Enceladus. We inferred that the detected complex organics with high probability originate from Enceladus’ hydrothermal core and can be efficiently transported to the oceanic surface by large-scale thermal convection and bubbles of volatile gases emerging from depth (Postberg et al., Nature 2018). It is quite likely that the organic material has been processed or produced inside the hydrothermally active core of Enceladus. Furthermore, we developed a model to demonstrate how these solid and poorly soluble organics are incorporated into ice grains and how they are accelerated into space. In analogy to ice cloud formation on Earth we proposed an formation from an organic rich oceanic surface layer. In Earth’s oceans, organic substances are dispersed along with sea spray when air bubbles bursts. On Enceladus, bubbles of volatile gases, rising through tens of kilometres of ocean, would burst at the surface and help to disperse some of the organics. Tiny droplets of the dispersed organic material become ice-coated when water vapour freezes on their surfaces and are ejected into the plumes and then detected by Cassini (Postberg et al., Nature 2018).
At the same time, we investigated the more numerous CDA mass spectra showing organic signatures of low mass (< 100u). Again, we extensively used the laser-based analogue experiment to decipher the spectral signatures. In contrast to the previously mentioned complex organic material, these low mass organics have to be volatile in conditions in which liquid water can exist (Khawaja et al., MNRAS 2019). Our analysis found oxygen-bearing, nitrogen-bearing and aromatic compounds (carrying a single benzene ring). These molecules are mostly soluble in water. Interestingly, the identified molecules are known to be highly reactive under hydrothermal conditions on Earth: Under the presence of catalysing minerals in the rock they produce prebiotic molecules like amino acids (Barge et al, PNAS 2019; Menez et al., Nature 2018).
We further evolved our results by linking them to geochemical and geophysical processes occurring on subsurface Enceladus. We inferred that the detected complex organics with high probability originate from Enceladus’ hydrothermal core and can be efficiently transported to the oceanic surface by large-scale thermal convection and bubbles of volatile gases emerging from depth (Postberg et al., Nature 2018). It is quite likely that the organic material has been processed or produced inside the hydrothermally active core of Enceladus. Furthermore, we developed a model to demonstrate how these solid and poorly soluble organics are incorporated into ice grains and how they are accelerated into space. In analogy to ice cloud formation on Earth we proposed an formation from an organic rich oceanic surface layer. In Earth’s oceans, organic substances are dispersed along with sea spray when air bubbles bursts. On Enceladus, bubbles of volatile gases, rising through tens of kilometres of ocean, would burst at the surface and help to disperse some of the organics. Tiny droplets of the dispersed organic material become ice-coated when water vapour freezes on their surfaces and are ejected into the plumes and then detected by Cassini (Postberg et al., Nature 2018).
At the same time, we investigated the more numerous CDA mass spectra showing organic signatures of low mass (< 100u). Again, we extensively used the laser-based analogue experiment to decipher the spectral signatures. In contrast to the previously mentioned complex organic material, these low mass organics have to be volatile in conditions in which liquid water can exist (Khawaja et al., MNRAS 2019). Our analysis found oxygen-bearing, nitrogen-bearing and aromatic compounds (carrying a single benzene ring). These molecules are mostly soluble in water. Interestingly, the identified molecules are known to be highly reactive under hydrothermal conditions on Earth: Under the presence of catalysing minerals in the rock they produce prebiotic molecules like amino acids (Barge et al, PNAS 2019; Menez et al., Nature 2018).
Our results mark the first ever detection of complex organics from a contemporary, extraterrestrial water-world. We severely constrained the molecular structure and composition of this organic material emerging from depth, and could link these to hydrothermal processes occurring at the interface of the subsurface ocean with the moon’s rocky core. Therefore, in summary, both our findings of complex and simple molecules (Postberg et al., Nature 2018; Khawaja et al., MNRAS 2019) substantially enhance the astrobiological potential of the moon. However, although conditions for prebiotic chemistry seem favourable, no unmistakable evidence for these processes could be found yet.
In the next phase of our ERC projects we will further explore spectrometric fingerprints which biotic or prebiotic material would leave if encapsulated in ice grains that hit our space-born detectors. Furthermore, we will analyze Cassini data to look specify salts and minerals that are present in the frozen ocean droplets emerging through Enceladus’ plume into space. Both investigations will then also be used for the preparation of the upcoming ESA and NASA missions to the Jovian ocean moons Europa and Ganymede. Due to the much harsher radiation in the Jovian system compared to the Saturnian system, the radiolysis of all analogue materials is of particular importance for these moons. This is especially relevant for organic and biogenic compounds. For that reason, the signals of these materials in space detectors at different stages of radiolytic alteration will be investigated.
In the next phase of our ERC projects we will further explore spectrometric fingerprints which biotic or prebiotic material would leave if encapsulated in ice grains that hit our space-born detectors. Furthermore, we will analyze Cassini data to look specify salts and minerals that are present in the frozen ocean droplets emerging through Enceladus’ plume into space. Both investigations will then also be used for the preparation of the upcoming ESA and NASA missions to the Jovian ocean moons Europa and Ganymede. Due to the much harsher radiation in the Jovian system compared to the Saturnian system, the radiolysis of all analogue materials is of particular importance for these moons. This is especially relevant for organic and biogenic compounds. For that reason, the signals of these materials in space detectors at different stages of radiolytic alteration will be investigated.