- 4556 -
PLANETS HAVE LIFE - delivered by comets? -
Could comets have delivered the building blocks of life to ocean worlds
like Europa, Enceladus and Titan?
Throughout Earth's history, the planet's surface has been regularly
impacted by comets, meteors, and the occasional large asteroid. While these
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.
-
---------------------------- 4556
- PLANETS HAVE
LIFE - delivered by comets?
-
- 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.
-
- 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.
-
- According to the latest fossilized
evidence, 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.
-
- 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.
-
- Data from the NASA/ESA Cassini–Huygens
mission confirmed that the plumes regularly erupting from Enceladus' southern
polar region contain organic molecules.
The presence of surface craters indicates that these bodies have
experienced surface impacts throughout their history.
-
- 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 (amino acids or nucleotides) in the melt pool.
-
- The initial shock levels created by the most
common impacts for ocean worlds—comets that likely originated from the Kuiper
Belt and Oort Cloud. Astronomers
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—i.e., 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.
-
- Meanwhile, on average, 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.
-
- 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, they
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.
-
- 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).
-
- For instance, they found that a comet
impacting Europa at the average impact velocity would create a 15 km (9.3 mi)
crater and provide 1 km^3 (0.24 mi^3) 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.
-
- 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.
-
-
September 16, 2024 PLANETS
HAVE LIFE -
delivered by comets? 4556
------------------------------------------------------------------------------------------
-------- Comments appreciated and Pass it on to
whomever is interested. ---
--- Some reviews are at: -------------- http://jdetrick.blogspot.com -----
-- email feedback, corrections, request for
copies or Index of all reviews
--- to:
------
jamesdetrick@comcast.net
------ “Jim Detrick” -----------
--------------------- --- Monday, September 16,
2024
---------------------------------
No comments:
Post a Comment