May comets even have delivered the constructing blocks of life to “ocean worlds” like Europa, Enceladus and Titan?

Throughout Earth's history, the planet's surface has been regularly struck by comets, meteorites, and occasionally large asteroids. These events were often destructive, sometimes leading to mass extinctions, but they may also have played an important role in the origin of life on Earth. This is particularly true during the Hadean (ca. 4.1 to 3.8 billion years ago) and the Late Heavy Bombardment, when Earth and other planets in the inner Solar System were struck by a disproportionate number of asteroids and comets.

These impact bodies are thought to have brought water to the inner solar system, and possibly the building blocks of life. But what about the many icy bodies in the outer solar system, the natural satellites orbiting gas giants that have oceans of liquid water inside them (e.g. Europa, Enceladus, Titan, and others)? According to a recent study led by researchers at Johns Hopkins University, impact events on these “ocean worlds” may have contributed significantly to the surface and subsurface chemistry that could have led to the emergence of life.

The team was led by planetary scientist Shannon M. MacKenzie and her colleagues at the Johns Hopkins University Applied Physics Laboratory (JHUAPL). They were joined by researchers from the Thayer School of Engineering at Dartmouth, the University of Western Ontario, the School of Earth and Planetary Sciences at Curtin University, the Planetary Habitability Laboratory (PHL) at the UPR in Arecibo, Jacobs Technology, NASA's Jet Propulsion Laboratory, and Astromaterials Research and Exploration Science (ARES) at NASA Johnson Space Center. The paper detailing their findings recently appeared in the Planetary Science Journal.

Voyager 1 image of Valhalla, a multi-ring impact structure 3,800 km (2,360 miles) in diameter.
Photo credit: NASA/JPL

Exogenesis

As outlined in their article, impacts of asteroids, comets, and large meteorites are more commonly associated with destruction and extinction events. However, several lines of evidence suggest that these types of impacts may have contributed to the emergence of life on Earth around 4 billion years ago. Not only did these events provide volatiles (such as water, ammonia, and methane) and organic molecules, but modern research suggests that they also created new substrates and compounds essential for life.

In addition, they created a variety of environments that were essential to the emergence and maintenance of life on Earth. They wrote:

“External input materials are considered an important source of organic matter on the early Earth. Shock waves could provide the energy for the organic synthesis of important precursors such as HCN or amino acids. The iron and heat from very large impact bodies can promote the reducing atmospheric conditions required for abundant HCN production. Impacts break and melt, in typical terrestrial events, the target: the more permeable substrates and the hollowing out of deeper rock layers promote hydrothermal activity and endolithic habitats.”

According to recent fossil discoveries, the first life forms on Earth arose about 4.28 billion years ago. These fossils were recovered from hydrothermal vent precipitation in the Nuvvuagittuq Greenstone Belt in northern Quebec, Canada, and confirm that hydrothermal activity played a crucial role in the origin of life on Earth. But what about the many “ocean worlds” found in the outer solar system? These include bodies such as Europa, Ganymede, Enceladus and Titan, as well as Uranus' moons Ariel and Titania, Neptune's moon Triton and trans-Neptunian bodies such as Pluto, Charon and possibly more.

Worlds of the Oceans

This term refers to bodies composed predominantly of volatile elements such as water, and which vary between an icy crust and a rocky and metallic core. At the core-mantle boundary, tidal motion (the result of gravitational interaction with another body) leads to a buildup of heat and energy, which is released into the ice via hydrothermal vents. This allows these worlds to maintain oceans of liquid water within their interiors. In short, these worlds have all the necessary ingredients for life: water, the necessary chemical compounds, and energy.

Estimates of impact velocity and first contact pressure for potential ice and rock bodies on Ocean Worlds. Source: Mackenzie, SM et al. (2024)

In addition, data from the NASA/ESA Cassini-Huygens mission confirmed that the plumes periodically emanating from the south polar region of Enceladus contain organic molecules. Last but not least, the presence of surface craters indicates that these bodies have been subjected to surface impacts throughout their history. The question naturally arises: Could impacts have delivered the necessary building blocks of life to the “ocean worlds” in the same way that they brought them to the inner solar system? And if so, what does that mean for their potential habitability today? As the team wrote in their paper:

“Impact processes are likely an important part of the answers to these questions, because impacts can drive exchange through the icy crust – either by direct seeding or by scouring the crust – and can therefore cause episodic influxes of organic and inorganic material from the surface and/or the impactor itself. Impacts can also create ephemeral microcosms: any liquid water that melted during the impact freezes over timescales corresponding to the impact energy.”

“The exciting potential for chemistry in these pockets is well established, from salt concentration to amino acid synthesis. In addition, shock-driven chemistry of icy, sometimes organic (especially in the case of titanium) target materials can generate new 'seed' compounds (e.g. amino acids or nucleotides) in the melt pool.”

Investigation

The first step for MacKenzie and her team was to study the initial shock levels caused by the most common impacts at Ocean Worlds – comets, likely from the Kuiper Belt and Oort Cloud. To do this, the team calculated the speeds and maximum pressures that would be reached in impacts involving icy and rocky bodies. They also considered how these would vary depending on the type (primary or secondary impacts) and the systems involved – that is, Jupiter or Saturn. While primary impacts involve comets or asteroids, secondary impacts are caused by the ejecta they generate.

In the case of the Jupiter and Saturn systems, secondary impactors can be icy or rocky depending on their place of origin (an icy body like Europa, Enceladus and Titan, a rocky body like Io, and larger asteroids). While primary impacts have higher velocities and produce larger volumes of melt, secondary impacts are more common. To determine melt sizes, the team consulted observed crater sizes on Europa, Enceladus and Titan, as well as dynamic models that calculate the cumulative cratering rate over time. They then compared peak pressures at impact to thresholds for the survivability of vital elements, organic molecules, amino acids and even microbes identified in previous studies.

Cumulative cratering rates assuming heliocentric comet impacts. Image credit: Mackenzie, SM et al. (2024)

From this, they concluded that most impacts on Europa and Enceladus experienced peak pressures that were higher than what bacterial spores can survive. However, they also found that a significant amount of material survives these impacts and that higher pressures at first contact may also facilitate the synthesis of organic compounds in the meltwater that fills the craters. On average, Titan and Enceladus experienced impacts with lower impact velocities, creating peak pressures that are within the tolerance range for both bacterial spores and amino acids.

The next step was to study how long fresh craters would survive and whether that would be enough for the synthesis of biological materials. Based on the observed crater sizes on Enceladus and Europa, they found that the longest-lived craters last only a few hundred years, while on Titan it can take centuries to tens of thousands of years for fresh craters to freeze. While Europa and Enceladus experience more high-velocity impacts due to Titan's thick atmosphere, the longevity of Titan's craters means there is potential to conduct organic chemistry experiments on all three bodies.

They also looked at the surface renewal rates of Europa, Enceladus and Titan and how they would transport biological material into their interiors. In all three cases, the satellites have relatively “young” terrain, suggesting regular surface renewal.

Results

Based on these considerations, Mackenzie and her team concluded that melts from comet impacts on Europa, Enceladus and Titan were frequent and long enough to be of astrobiological interest. However, this varies depending on the composition of the comets and the surface ice involved. They summarized:

“On Europa and Enceladus, the survival and deposition of impact organics is more important because there is less organic matter at the surface within the ice crust to seed the meltwater. On Titan, the survival of elements such as phosphorus may be more important. Therefore, even the small, more frequent impact events contribute to the astrobiological potential by delivering fewer altered compounds to the surface, available either for immediate response if melt is generated, or for later processing (including in subsequent impact events).”

Total melt production for observed craters on Enceladus (cyan) and Titan (orange), sorted by observed crater diameter. Image credit: Mackenzie, SM et al. (2024)

For example, they found that a comet hitting Europa at average impact speed would leave a 15 km crater and deliver about 1 km3 of meltwater. Based on the amount of glycine (an essential amino acid) found on comet 67P Churyumov–Gerasimenko, they concluded that several ppm would survive – about three orders of magnitude more than the amount observed around hydrothermal vents here on Earth. “Thus, impact bodies seed the chemistry that occurs in the melt, delivering organic and other essential elements depending on the composition of the impact body,” they added.

While this does not necessarily mean that these and other “ocean worlds” are currently habitable or support active life, they do offer potential for future study. In the coming years, missions such as ESA's JUpiter ICy Moons Explorer (JUICE) and NASA's Europa Clipper and Dragonfly missions will reach Ganymede, Europa, and Titan (respectively). There are also plans to build an Enceladus orbiter to pick up where the Cassini-Huygens probe left off by studying the activity of the Enceladus plume in more detail.

Therefore, collecting and analyzing samples in situ on these moons could provide important insights into prebiotic chemical processes and the conditions under which life could emerge. These sample studies will also answer the larger question of whether life could exist inside the “ocean worlds” and provide a taste of what future missions exploring beneath the ice will find.

Further reading: The Planetary Science Journal

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