The search for evidence of life in the Solar System beyond the Earth used to have the mantra—find the water. The idea was that water is a key ingredient of life as we understand it, so a suitable habitat for life would be one that had liquid water readily available. A key step in the search for life would be to identify potential habitats and to focus the search there. Thus, the presence of liquid water would identify sites worth exploring. However, we now know of many Solar System bodies, which possess liquid water in such quantities, these are classed as ocean worlds [1
]. These bodies are typically icy satellites of outer planets, where the liquid water is not on the surface, but in the interior under a surface cap of ice. Liquid water alone is not sufficient. There also needs to be chemical disequilibria (or to put it another way a source of energy to drive chemical reactions) and the presence of organic molecules to provide the inputs to drive forward the prebiotic chemistry and then the increase in complexity characteristic of life. As discussed further below, the icy ocean worlds may meet these criteria so are considered of great interest in terms of the search for life elsewhere.
How to access these sites to explore their contents is thus an issue. Remote sensing of the surface ice can provide some clues as to the composition of the interior oceans. However, conveniently, some of these icy satellites naturally expel water into space via plumes. Europa and Enceladus are particularly good examples, and the plumes permit a more direct sampling of the contents of the oceans. A proper characterization of these plumes focusses on determining their content in detail. For example, what is the mineral content? In addition, is there any evidence for organics?
Organic materials are widespread in space, for example, a notable fraction of the carbon in the interstellar medium is in the form of polycyclic aromatic hydrocarbons, e.g., [2
]. Aliphatic organic compounds have also been observed, not just in the interstellar medium but also on asteroids [4
] and comets for example [5
]. We thus should expect to find organics when we visit places in the Solar System where chemistry has processed materials.
One simple question is: Can we recognise the organic material in an in situ analysis? Further, even if we cannot identify the exact nature of the material, can we do something as basic as distinguish aromatics from aliphatic? As ever with space missions, the answer depends not just on the analysis method, but also on the sample collection method, which may cause damage to the samples. This paper briefly considers one aspect of this by looking at Enceladus, an icy satellite of Saturn.
Enceladus and Sampling Its Plumes
There is now ample evidence that Enceladus contains an interior ocean, e.g., [6
]. Further, due to gravitational forces arising from its orbit around Saturn, water plumes are forced out of the icy surface of Enceladus near its southern polar regions. These plumes were first observed by the Cassini spacecraft orbiting Saturn e.g., [12
]. By examining the material from the plume, the Enceladan internal ocean has been shown to be salt rich [15
]. If we accept that the centre of Enceladus is a solid core with a rocky, mineral content, the ocean floor will be in contact with minerals. Due to heat added by the gravitational flexing processes during the orbit around Saturn, plus any internal heat in the core, the ocean will be warm. Water, heat, minerals and salts are all key ingredients for interesting chemistry, and maybe something more, biology perhaps? Conveniently, a visiting space mission can sample this fascinating ocean by flying through the ejected plume. This minimises the risk of contamination of Enceladus, thus preserving its isolated nature (unless the spacecraft unfortunately crashes into the ice) and so observing the necessary planetary protection protocols.
A visiting spacecraft has the option of orbiting the parent planet Saturn (easier, plus more science can be done elsewhere in the Saturnian system), or of entering orbit around the satellite itself. In both cases, it will have to pass close to the surface, at an altitude of much less than 100 km, in order to intercept the plume before the larger droplets of water (which may have frozen into ice) fall back to the surface. Indeed, the lower the altitude the better in terms of droplet size. Which approach is taken is important as it dictates the relative speed of the craft when it intercepts the plume. If it is in a Saturnian orbit the encounter speed will be many km s−1
, if it orbits the satellite it could be as low as several hundred m s−1
. Data has been obtained in situ from the Enceladus plumes by the Cassini space mission, which flew through the water vapour plumes as it passed by Enceladus. Results on the Enceladus plume composition are given in [16
], where the various Saturnian orbits followed by Cassini allowed Enceladus fly-by data to be collected at impact speeds of 5–15 km s−1
, with typical impacts being in the speed range 6–8 km s−1
. The data in [16
] show the presence of organic molecules in the brine.
To investigate further we could mount a sample return mission to Enceladus. This is discussed for example in [18
], where it is noted that planetary protection would impose enormous cost and complexity into such a mission. Indeed, as noted in [19
], planetary protection regarding Enceladus would require that there is no direct physical link between the source material and the general environment here on Earth. One way to avoid this is to sample and analyse in situ.
Here, let us imagine a mission, which flies past Enceladus, collects samples and analyses them in situ. We will consider the worst case of the higher speed impact as being that which occurs, i.e., sample collection occurs when a spacecraft passes Enceladus whilst orbiting Saturn. The data in [16
] from Cassini were obtained by impact ionization during such fly-bys. In such a method, small (sub-micrometre) grains are vapourised during the impact, and the ionic plasma that formed was measured in a time of flight system. As noted in the supplemental material to [16
], and previously in papers such as [20
], at the encounter speeds during the Cassini fly-bys of Enceladus (<20 km s−1
), the impacting materials are not reduced to their elemental composition. Instead, molecular fragments are formed. These fragments have mass numbers (assuming single ionization) which show regular spacings in mass whose nature differs depending on the chemistry of the sample, and, for example, if the sample was originally an aromatic or aliphatic compound. We can thus say that it is possible to differentiate between different types of organic compounds. Nevertheless, this does not positively identify the sample, it just gives mass numbers of the fragments formed in the impact process. However, an ideal analysis would go further. We would want to determine what compound was it originally, and know more about its structure. The normal way to do this is to collect macroscopic residue from the impacting particle, and to analyse that.
For a macroscopic dust sample (and here micrometre scale is macroscopic), there is a wealth of data concerning how mineral grains behave in impacts at speeds up to 6 km s−1
, e.g., [24
]. A metal plate or foil, can act as a target. An impact crater is formed when the dust grain strikes the target, and impact residue lines the crater. The shock pressures in the sample and target can be in the tens of GPa, e.g., [29
]. There will be some heating as the sample releases from its shocked state and indeed some melt may form, but some samples may retain their original crystalline structure (e.g., see [30
]). Analysis of the resulting residue then reveals information about the impactor. A suitable analysis method has to be found, the common ones used in the laboratory include scanning electron microscopy with elemental analysis via dispersive X-rays (EDX-SEM). Structural information in the laboratory can come from either Raman spectroscopy or TEM work on samples. All this equipment tends to be bulky so is not ideally suited for a space mission where size, mass and power are major constraints. We can however suppose that a suitable technique will be found. For example, Raman spectrometers have been made robust enough to be deployed on space missions and two will soon be sent to the surface of Mars (on NASA’s Mars 2020 mission and ESA’s future ExoMars rover mission).
So can we now imagine that we can analyse the samples fully? Unfortunately, some issues remain. If thermally robust mineral grains are all that is in the water, the analysis is relatively straight-forward, in that the impact process may break particles apart, but will often leave their basic nature intact. Unfortunately, some of the more hydrated minerals are the ones that suffer the most in such impacts, so we need to allow for this in the analysis. Worse, organic materials will suffer thermally, and do so depending on their nature. It has previously been shown, in laboratory experiments, that various organic molecules frozen in ice can survive impacts on a variety of targets including water, sand and ice at impact shock pressure up to 10 GPa [33
]. However, the results in [33
] showed that not all organic compounds survive in equal quantities.
For a future space mission we would likely use metal targets as collecting surfaces, so this needs to be investigated. Accordingly, we present here results for impacts of small organic grains on metal targets, at speeds close to 5 km s−1 (the minimum speed in the Cassini fly past of Enceladus). We look at both polystyrene (aromatic-rich) and polymethlymethacrylate (solely aliphatic) projectiles, to see if we can find residues and distinguish between an aromatic and an aliphatic organic impactor respectively.