- 4561 - COMETS - seeding life on the planets? - Could comets have delivered the building blocks of life to ocean worlds like Europa, Enceladus and Titan? These are the moons of Saturn. Throughout Earth's history, the planet's surface has been regularly impacted by comets, meteors, and the occasional large asteroid.
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- COMETS -
seeding life on the planets?
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- While these impact events were often
destructive, sometimes to the point of triggering a mass extinction, they may
have also played an important role in the emergence of life on Earth. This is
especially true of the “Hadean Era” ( 4.1 to 3.8 billion years ago) and the
“Late Heavy Bombardment”, when Earth and other planets in the inner solar
system were impacted by a disproportionately high number of asteroids and
comets.
-
- These impactors are thought to have been how
water was delivered to the inner solar system and possibly the building blocks
of life. But what of the many icy bodies in the outer solar system, the natural
satellites that orbit gas giants and have liquid water oceans in their
interiors ( Europa, Enceladus, Titan, and others)?
-
- Impact events on these "ocean
worlds" could have significantly contributed to surface and subsurface
chemistry that could have led to the emergence of life. Impacts from asteroids, comets, and large
meteors are more often associated with destruction and extinction-level events.
However, multiple lines of evidence indicate that these same types of impacts
may have supported the emergence of life on Earth roughly 4 billion years ago.
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- These events not only delivered volatiles
(such as water, ammonia, and methane) and organic molecules, but modern
research indicates that they also created new substrates and compounds
essential to life. Moreover, they
created a variety of environments that were essential to the emergence and
sustainment of life on Earth.
-
- Exogenously delivered materials have been
estimated to be an important source of organics on early Earth. Shockwaves
could provide the energy for organic synthesis of important precursors like HCN
or amino acids. The iron and heat from
very large impactors can facilitate the reducing atmospheric conditions
necessary for abundant HCN production. Impacts fracture and, in typical
terrestrial events, melt the target: the more permeable substrates and
excavation of deeper rock layers promote hydrothermal activity and endolithic
habitats.
-
- The earliest life forms emerged on Earth
roughly 4.28 billion years ago. These fossils were recovered from hydrothermal
vent precipitates in the Nuvvuagittuq Greenstone Belt in northern Quebec,
Canada, confirming that hydrothermal activity played a vital role in the
emergence of life on Earth.
-
- But what about the many "ocean
worlds" that reside in the outer solar system? This includes bodies like
Europa, Ganymede, Enceladus, and Titan, as well as Uranus' moons Ariel and
Titania, Neptune's moon Triton, and trans-Neptunian bodies like Pluto, Charon,
and possibly more.
-
- Ocean worlds refers to bodies predominantly
composed of volatile elements such as water and differentiated between an icy
crust and a rocky and metallic core. At the core-mantle boundary, tidal flexing
(the result of gravitational interaction with another body) causes a buildup of
heat and energy released via hydrothermal vents into the ice.
-
- This allows these worlds to maintain oceans
of liquid water in their interiors.
These worlds have all the necessary ingredients for life: water, the
requisite chemical compounds, and energy.
The plumes regularly erupting
from Enceladus' southern polar region contain organic molecules. Last but not
least, the presence of surface craters indicates that these bodies have
experienced surface impacts throughout their history.
-
- The question naturally arises: Could impacts
have delivered the necessary building blocks of life to "ocean
worlds" the same way they delivered them to the inner solar system? And if
so, what does that mean about their potential habitability today?
-
- Impact processes are likely an important
part of the answers to these questions, as impacts can drive exchange through
the ice crust—either through direct seeding or flushing through the crust—and
therefore drive episodic influxes of organic and inorganic materials from the
surface and/or from the impactor itself. Impacts can also generate ephemeral
microcosms: any liquid water melted during impact freezes out over timescales
commensurate with the impact energy.
-
- The exciting potential for chemistry within
these pockets has been established, from concentrating salts to driving amino
acid synthesis. Furthermore, shock-driven chemistry of icy, sometimes
organic-rich (in the case of Titan especially) target materials may generate
new 'seed' compounds (e.g., amino acids or nucleotides) in the melt pool.
-
- Researcher calculated the velocities and
maximum pressure that would be achieved by impacts involving icy and rocky
bodies. They also considered how this
would vary based on different families (primary or secondary impacts) and which
systems were involved, Jupiter or Saturn. Whereas primary impacts involve
comets or asteroids, secondary impacts are caused by the ejecta they create.
-
- In the case of the Jupiter and Saturn
systems, secondary impactors may be icy or rocky depending on where they
originated (an icy body like Europa, Enceladus, and Titan, a rocky body like Io
and larger asteroids). Whereas primary impacts have higher velocities and
produce larger melt volumes), secondary impacts are more frequent.
-
- To determine melt sizes, the team consulted
observed crater sizes on Europa, Enceladus, and Titan, and dynamic models that
calculate the cumulative rate of cratering over time. They then compared the
peak pressures at impact to thresholds for the survivability of elements
essential to life, organic molecules, amino acids, and even microbes identified
in previous studies.
-
- From this, they determined that most
impacts on Europa and Enceladus experience peak pressures greater than what
bacterial spores can survive. However, they also determined that a significant
amount of material still survives these impacts and that higher first-contact
pressures could also facilitate the synthesis of organic compounds in the
meltwater that fills the craters.
-
- Titan and Enceladus experienced impacts
with lower impact velocities, creating peak pressures that fall within the
tolerance range for both bacterial spores and amino acids.
-
- The next step was to consider how long fresh
craters would survive and whether this would be sufficient for synthesizing
biological materials. Based on the observed crater sizes on Enceladus and
Europa, they determined that the longest-lived craters last only a few hundred
years, whereas Titan could 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 dense atmosphere), the long-lived nature
of Titan's craters means that all three bodies have a chance for organic
chemistry experiments to occur.
-
- They also considered resurfacing rates on
Europa, Enceladus, and Titan and how these would cycle biological material to
their interiors. In all three cases, the satellites have relatively
"young" terrain, implying regular resurfacing events.
-
- Based on these considerations, reseachers
determined that melts produced by comet impacts on Europa, Enceladus, and Titan
have been frequent and long-lived enough to be of astrobiological interest.
However, this varies based on the composition of the comets and the surface ice
in question.
-
- At Europa and Enceladus, the survival and
deposition of impactor organics is more important as there are fewer surface
organics within the ice crust to seed the melt pool. On Titan, the survival of
elements like phosphorous may be more important.
-
- Thus, even the small, more frequent impact
events contribute to the astrobiological potential by delivering less modified
compounds to the surface that are available either for immediate reaction if
melt is produced or for future processing (including in subsequent impact
events).
They found that a comet
impacting Europa at the average impact velocity would create a 15 km (9.3 mi)
crater and provide 1 km3 (0.24 mi3) of meltwater.
-
- Based on the abundance of glycine (an
essential amino acid) found on the comet 67P Churyumov–Gerasimenko, they
determined that several parts per million would survive—roughly three orders of
magnitude higher than what has been observed forming around hydrothermal vents
here on Earth.
-
- Thus, impactors seed whatever chemistry
happens in the melt, providing organic and other essential elements depending
on the impactor composition. While this
does not necessarily mean that these and other "ocean worlds" are
currently habitable or actively support life, they demonstrate potential for
future study.
-
- In the coming years, missions like the 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 create an Enceladus Orbiter to pick up where the Cassini-Huygens probe
left off by examining Enceladus' plume activity more closely.
-
- Therefore, conducting in-situ sampling and
analysis on these moons could provide powerful insight into prebiotic chemical
pathways and determine under what conditions life can emerge. These sample
studies will also address the larger question of whether or not life could
exist in the interiors of "ocean worlds," providing a preview of what
future missions prepared to explore beneath the ice will find.
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-
September 21, 2024 COMETS
- seeding life on the
planets? 4561
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--------------------- --- Saturday, September 21,
2024
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