Throughout the history of humankind, a seemingly-inherent penchant for exploration,
coupled with various climatic events, has driven the proliferation of the human species across
planet Earth. Whether nomadic bands of hunter-gatherers were searching for newer, more
abundant sources of food, or simply exploring for its own sake, homo sapiens successfully
populated the Earth through a journey that began thousands of years ago in south-central Africa and took several dozen
centuries. But, even with all that humankind has accomplished, and with all the introspective and
extrospective information accumulated in the past four centuries alone, there is much left to
explore both on the surface and in the oceans of this majestic world. Moreover, there is an entire
universe out there—unimaginably vast and just waiting to be discovered. But, humans tend to
get easily distracted by both forms of intergovernmental conflict (within and between
governments), wars, manufactured controversy, and a host of other trivialities, ultimately
veering the species off that road to cosmic discovery. Nevertheless, space programs the world
over have managed to send spacecraft on trajectories that have taken them from precariously
close to the Sun1 to beyond the heliosphere of the solar system2. Several rovers have successfully
descended through the Martian atmosphere34 and one on Saturn’s moon Titan5; an
orbiter/lander package was even sent to an asteroid6! And, while all of this exploration has been
done in the hopes of gaining a deeper understanding of how life arose on Earth and whether or
not it happened elsewhere in the cosmos, an extraordinary wealth of knowledge has been
amassed regarding the physical and chemical properties of the cosmos itself. But, before drawing
the lines of evidence for otherworldly life in this solar system, a few questions should first be answered: What is life? Are there certain elements that are considered crucial for life? How
common are these ingredients? And, what processes exist on Earth have been identified as
possible catalysts for the transition from inorganic to organic molecules?
A Definition of Life
For many centuries, philosophers the world over struggled to explain this experience
called ‘life’. The elements that formed everything around were simple: earth, water, air, and fire.
Life itself was viewed as something rather peculiar—especially human life. And somehow
humans were supposed to strive for the good life, as Aristotle put it in his Nicomachean Ethics.7
In retrospect, however, something always seemed to be missing from these arguments: hard
evidence. Thought experiments can take one but so far; it is only when unbiased physical,
chemical, and biological experiments bear out the facts of reality through the accumulation of
evidence that such claims should be accepted. And even then, uncertainties should always be
taken into account. Perhaps this perspective seems obvious in the highly technical, information-
driven world of today but it certainly wasn’t always the case. The concepts of innocent until
proven guilty and evidence-based reasoning are relatively new in human history. Before the
Renaissance began sometime in the 14th century, people largely took things at face value with
very little thought—burning ‘witches’ alive and massacring countless ‘others’ that dared to
question any form of assumed, typically divine, authority. In fact, life seemed to be defined in
terms of that authority: God > kings > other humans > animals > everything else in the universe.
But, this geocentric, authoritative perspective left much to be desired. Observations began to
contradict doctrine which eventually led to a revolution in thinking. Life, it turned out, wasn’t so
simply defined.
Although the debate rages on today, at least in philosophical circles, biologists and
chemists have in recent years joined it in an effort to standardize the definition of life. Among
the many that have been posited over the decades, the definition that perhaps best encompasses
humankind’s present understanding was proffered by Gerald F. Joyce in the 1990s and then
included in an anthology released in 2008 called Extraterrestrials: Where are They?: “In a very
broad sense, living organisms turn food into offspring. They metabolize food and use the energy derived from the food to produce offspring, that is, to produce more life. Among biologists and
biochemists a working definition of ‘life’ is: ‘a self-sustained chemical system capable of
undergoing Darwinian evolution’.”8 Objections to this definition remain but it suffices for the
purpose of this essay.
Follow the Water
Perhaps the most essential substance in which countless reactions can take place is water
in its liquid form. Known as ‘the universal solvent’ for its ability to dissolve an array of other
substances, water is a polar molecule with a negatively charged oxygen atom opposite two
positively charged hydrogen atoms that are spaced 104.5° apart. This arrangement makes it
“sticky”, a characteristic ultimately derived from electromagnetic forces; it also means that water
is capable of conducting electricity when salts are dissolved in it. These properties prove essential
in the synthesis of other molecules that could lead to self-sustaining chemical systems. Thus, the
catchphrase for finding life throughout the astrobiology and space exploration community has
unsurprisingly become “follow the water”.9 Since Earth harbors life everywhere liquid water
exists, then a reasonable supposition would be that where there is water, there is life.
SPONCH for Life
The chemical systems that eventually gave rise to biological entities here on Earth can be
described by the acronym SPONCH. Sulfur, phosphorus, oxygen, nitrogen, carbon, and hydrogen
are the key elements to life on Earth, and, considering the relative abundances of these elements
throughout the universe, presumably life elsewhere. So, if a search for life in the universe is to
remain as objective as possible, a focus on these six elements and the many molecules that can
be made from them, as well as environments with temperatures and pressures conducive to
biological processes, can be considered reasonably unbiased. Carbon, the most chemically active
element on the periodic table, is the basis for millions of molecules and its ability to readily form
strong, stable bonds (even with other carbon atoms and some metals) makes it the most logical
choice for a molecule long-lived enough to build self-sustaining, evolving systems. Silicon is
chemically similar to carbon and has also been considered as a possible basis for life; however, its weaker bonds and completely different oxidation product—the solid silicon dioxide (SiO2)—
present problems with known biological processes. For example, can the polymers formed by
silicon remain stable in varying environments and for long enough to become biologically active?
How would the mechanism by which this solid waste product is readily removed function? And,
what kinds of silicon compounds could be used as energy sources? Of course, this line of
questioning could likely continue ad infinitum, but it is clear that in the prebiotic race toward
‘self-sustaining chemical systems’, carbon is the winner.
Organic molecules are divided into large (lipid collections, carbohydrates, proteins, and
nucleic acids) and small (fatty acids, sugars, amino acids, and nucleotides) varieties that perform
a number of important functions in living systems. Combining these organic molecules through a
process involving the loss of water is called polymerization and allows such ‘macromolecules’ to
form. Both lipids, such as fats and oils, and carbohydrates called polysaccharides store energy
while proteins perform a range of tasks. Nucleic acids, composed of smaller building blocks called
nucleotides, are essentially the replication factories of biological molecules; deoxyribonucleic
acid (DNA) and ribonucleic acid (RNA) being two of the most important (Rothery, Gilmour,
Sephton 2011). But, in order for living systems to perform the myriad functions necessary to
subsist and for evolutionary processes to proceed, lipids and proteins form membranes within
carbohydrate/amino acid ‘shells’ that maintain stable environments. Thus, it can be stated with
reasonable confidence that these cells are what can be considered life. And some of the places
thought to have been possible crucibles for certain forms of life are at the bottom of the ocean
floor near geothermal vents called black smokers that eject superheated mixtures of water and
mostly sulfides while providing an immediate environment capable of sustaining certain forms of
so-called extremophiles. So, having arrived at a reasonable definition of life (of course, several
details were glossed over and many others were not mentioned, such as chirality, the RNA world,
panspermia, the top-down and bottom-up approaches, and the details of boundary layers), some
of the most promising possible environments for its existence elsewhere in the solar system can
now be examined: Jupiter’s moon Europa and Saturn’s moons Enceladus and Titan.
To the Moons of Jupiter
At 5.2 AU, Jupiter is more than five times the distance from the Sun to the Earth and gets
around 25 times less solar irradiance than here on Earth. The magnetic field of this gas giant is
more than 20,000 times that of Earth, making life a much more difficult prospect on the surfaces
of bodies orbiting close to the planet. Dozens of moons are in orbit around the Jovian world but
only the inner Galilean moons—specifically Europa—are of present interest in this search for life.
Io, Europa, and Ganymede are in a 4:2:1 orbital resonance, which keeps their orbits eccentric,
while Callisto has an orbital period of just under 17 days with no resonance. (Orbital resonances
tend to cause tidal heating as these satellites make their way around their orbits. Moons that are
mostly rocky become geologically active when presented with the stresses of tidal forces.) In fact,
Io is the most geologically active body in the entire solar system with many confirmed volcanoes
currently erupting. But, in cases where moons have rocky cores surrounded by thick layers of ice,
the rock-ice interface presents a situation where the tidal heating causes at least some of the ice
to melt and form either a localized or a global ocean. Since the surface temperature of Europa
ranges from around -160 degrees Celsius at the equator to less than -220 degrees Celsius at the
poles, no known life can exist there. However, if the situation is similar to that of at least portions
of the Earth’s oceans, black smokers could also exist in the subsurface ocean of Europa—
confirmed by the Galileo spacecraft10—providing a possible energy source on which
extremophiles of some sort might thrive. Plumes ejecting material from the surface have been
observed by the Hubble Space Telescope:
"The perspective that plumes on Europa are fed by a subsurface ocean offers an
opportunity to study the ocean composition and investigate its habitability.
Europa is widely considered one of the likeliest candidate bodies in our solar
system able to support biological activity [Shapiro and Schulze-Makuch, 2009].
Due to the association of liquid water with life, biological activity would most likely
be found in an ocean beneath Europa’s ice crust. The plumes offer a unique
chance to directly sample subsurface material in situ from space. (Southworth,
Kempf, and Schmidt, 2005)"
The Galileo mission delivered some of the most spectacular images of Jupiter and, related
specifically to this discussion, of Europa. These data revealed a moon made of water ice and
silicate rock with a tenuous atmosphere of mainly oxygen and a likely iron-nickel core that,
through tidal heating, is responsible for the situation of either warm ice or liquid water beneath
the icy, mostly crater-free surface. Features resembling icebergs and tilted blocks on Earth are
visible while a so-called “brown gunk” covers much of the crust, possibly the result of radiolysis
of salts—a reasonable guess considering the thin oxygen atmosphere is almost certainly
produced by the radiolysis of water. Some sort of process similar to tectonics exists on Europa,
as evidenced by the stretched and cracked surface:
A future mission with multiple flybys and perhaps several passes through the plumes
themselves to collect particles for analysis could further confirm the existence of biogenic
material. However, any spacecraft that orbits Jupiter must be overbuilt to some degree in order
to withstand the effects of such a massive magnetic field. While electromagnetic shielding is
certainly required, one failure of any component of such shielding could spell disaster for
missions costing billions of dollars. And such an unfortunate event would mean that it would be
several years—perhaps 10 or more—before another mission (assuming approval) could be at the
Jovian system for similar investigations. Thus far, Pioneer 11 and 12, Voyager 1 and 2, Cassini-
Huygens, and the Galileo missions have visited or flown past Jupiter giving us new perspectives
and new insights each time as successive spacecraft were equipped with better instrumentation.
NASA’s proposed Europa Clipper Mission is supposed to launch sometime in the 2020s and is
host to a suite of scientific instrumentation focused on studying this possible abode for life12,
including an ice-penetrating radar to accurately determine the thickness of Europa’s crust.13
Landing a rover on the surface of Europa would be a herculean undertaking filled with serious
risks involving electromagnetic shielding and choosing the proper landing location. However, it is
well within the realm of possibilities given the current state of technology. And, finding life
anywhere outside of Earth would perhaps be the most important human discovery of all time.
So, the possible reward seems to far outweigh the risks involved in such an important mission.
Other possibilities where life might have developed separately are Saturn’s icy moon Enceladus
and its large moon Titan.
The Jewel of the Solar System
Figure 2: The first colored image from the
Huygens lander of the surface of Titan. The
Cobbles are water ice. (NASA - Dr. David R.
Williams.)
|
On October 15th, 1997, the Cassini-Huygens spacecraft launched from Cape Canaveral Air Force station in Florida with the goal of studying the Saturnian system. A collaborative mission between the European Space Agency, the Italian Space Agency, and NASA, the spacecraft reached Saturn June 30th, 2004.14 The Cassini orbiter and Huygens lander were both successful; the former having now delivered nearly 20 years’ worth of data; so much that it is still being analyzed over a year after making its dramatic “Grand Finale” plunge into Saturn to avoid contaminating any moons that might harbor life. The Huygens lander was delivered to the moon Titan—the second largest moon in the solar system—and successfully landed January 14th, 2005.15 Through radar imaging, Titan showed signs of prebiotic chemistry, cryovolcanism, impact cratering, and dune field formation. Among the most fascinating discoveries have been the presence of methane lakes and that methane functions similarly to the way water functions on Earth in the hydrologic cycle; there might even be an underground ocean of liquid water with
hydrothermal vents. The atmosphere is composed of nitrogen (about 96%), methane, and
ethane, and has a pressure of 1.5 bar at the surface, similar to that on the surface of Earth. As
well, Titan has a thick photochemical smog layer in the upper-middle part of the atmosphere. In
short, all of ingredients necessary for life exist on Titan.16 Extending far beyond its original mission, the Cassini orbiter has provided some of the
most spectacular images ever taken of another planetary body. It is no wonder that Saturn is
known as The Jewel of the Solar System:
At around 300 miles across, the small, icy moon Enceladus is about the size of Colorado
and has been identified as a second possibility for life, due to the likely existence of a subsurface
ocean. By 2008, a set of geologic features dubbed tiger stripes were identified in the southern
hemisphere. These so-called tiger stripes are at least 100 degrees Celsius warmer than the
surrounding surface, indicating some type of geologic heating (likely tidal). In December of 2011,
a series of high resolution images were taken of Enceladus. But, one of the most breathtaking
views of this Saturnian satellite was taken over a year before on November 30th, 2010 and clearly
shows geysers erupting from the surface:
Future missions to the Saturnian system would likely put significant focus on this small
but promising world. Investigations of a next-generation orbiter could include a dust collector for
proper analysis of plume ejecta—responsible for Saturn’s E ring—and several high-resolution
imaging systems across the range of spectra. A lander would make exploration of the surface and
possible subsurface ocean much more fruitful in its returns since only so much can be learned
from orbit. However, finding a landing site free from debris or giant, spikey ridges will doubtless
be extremely challenging. It might require including instrumentation to analyze imaging data in
real-time and then choosing a landing site. Next, the challenge of driving a rover on a moon that
is almost 750 million miles from Earth would perhaps be just as exciting as successfully landing
one there. But, the insight gained from such an endeavor would certainly prove invaluable—a mission well worth the financial undertaking. For example, a seismometer can be utilized to
determine the internal structure as the moon is stressed under tidal forces, possibly answering
the question of the source of Enceladus’ geysers definitively. And the rover could drill into the icy
surface or release a tethered, shielded piece of radioactive material onto it to melt through;
perhaps to eventually reach an underground ocean and explore via a tethered submersible. The
possibilities are only limited by the resources involved in conducting such missions.
A Brief Comparison of Europa and Enceladus
With all that has been discovered about each of these moons, Europa and Enceladus
present very different challenges that must be taken into account—especially if a budgeting issue
forces a decision for one mission over the other. Jupiter is closer and Europa has been officially
considered the most likely place in the solar system for life, but Jupiter’s huge magnetic field
presents a significant engineering problem for probes and rovers moving through it. Saturn is
almost twice the distance of Jupiter so it receives far less solar radiation but has a much smaller
magnetic field requiring far less shielding, as evidenced by the Cassini-Huygens mission. Europa
is also around six times the diameter and has an escape velocity nine times that of Enceladus. So,
it is clear that many factors must be considered in a massive cost-benefit analysis if such a choice
must be made.
Final Thoughts
In any case, one thing is certain: In the search for life in the universe, a return to both
Europa and Enceladus is necessary, while a return to Titan would likely prove useful. It is
absolutely amazing what has been accomplished by the space program in the United States
considering the budget constraints over the past 50 years. To put it into perspective, the
proposed defense budget for FY2018 was around $640-billion21 while NASA’s proposed budget
for the same fiscal year sits at just over $19-billion22. It appears plenty of funding exists to drop
bombs on people around the world that are different but the coffers apparently come up short
when budgeting for planetary exploration that could prove just how similar humans are at a
fundamental level. But, with the advent of the privatization of space, perhaps in just a few years some of the companies that have established themselves in the past decade or so, such as SpaceX
and Blue Origin, will see the value in such exploration and aim for these possible abodes for life.
There is no doubt that such a discovery will be considered the greatest achievement of
humankind: It would mean we definitely are not alone. From there it is easy to follow the lines of
logic to begin asking questions about intelligence. With thousands of exoplanets already
confirmed, and an entire universe presumably filled with innumerable others, the likelihood of
intelligent life existing elsewhere in the cosmos might just be inevitable.
Resources
Aristotle. (2009). The Nichomachean Ethics (D. Ross, Trans.). London: Oxford University Press.
Diamond, J. M. (1999). Guns, germs, and steel: The fates of human societies. New York: W.W. Norton & Company.
Joyce, Gerald F. (1993), The RNA World: Life before DNA and Protein, Website, https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19980211165.pdf.
Joyce, Gerald F. (1993), The RNA World: Life before DNA and Protein, Website, https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19980211165.pdf.
Rothery, D. A., Gilmour, I., Sephton, M. A., & Anand, M. (2011). An Introduction to
Astrobiology. Cambridge: Cambridge University Press.
Southworth, B. S., S. Kempf, and J. Schmidt (2015), Modeling Europa’s dust plumes, Geophys. Res. Lett.,42, 10,541–10,548, doi:10.1002/2015GL066502
Shapiro, R., and D. Schulze-Makuch (2009), The search for alien life in our solar system: Strategies and priorities, Astrobiology, 9, 335-343, doi: 10.1089/ast.2008.0281.
Southworth, B. S., S. Kempf, and J. Schmidt (2015), Modeling Europa’s dust plumes, Geophys. Res. Lett.,42, 10,541–10,548, doi:10.1002/2015GL066502
Shapiro, R., and D. Schulze-Makuch (2009), The search for alien life in our solar system: Strategies and priorities, Astrobiology, 9, 335-343, doi: 10.1089/ast.2008.0281.
Zuckerman, B., & Hart, M. H. (Eds.). (2008). Extraterrestrials: Where are they? Cambridge: Cambridge University Press.
Web, Online Journal, and Other Sources
-
http://parkersolarprobe.jhuapl.edu/
-
https://voyager.jpl.nasa.gov/mission/status/
-
https://mars.nasa.gov/mer/home/
-
https://solarsystem.nasa.gov/missions/cassini/overview/
-
http://sci.esa.int/rosetta/
-
http://classics.mit.edu/Aristotle/nicomachaen.html
-
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19980211165.pdf
-
https://www.nasa.gov/vision/earth/everydaylife/jamestown-water-fs.html
-
https://solarsystem.nasa.gov/missions/galileo/overview/
-
https://photojournal.jpl.nasa.gov/catalog/PIA19048
-
https://solarsystem.nasa.gov/moons/jupiter-moons/europa/in-depth/
-
https://www.jpl.nasa.gov/missions/europa-clipper/
-
https://www.jpl.nasa.gov/missions/cassini-huygens/
-
https://nssdc.gsfc.nasa.gov/planetary/titan_images.html
-
Notes from ASTR2040, Fall 2016.
-
https://solarsystem.nasa.gov/resources/15868/the-day-the-earth-smiled-sneak-
preview-annotated/
-
https://solarsystem.nasa.gov/news/13020/the-moon-with-the-plume/
-
https://solarsystem.nasa.gov/resources/16074/encroaching-shadow/
-
https://agupubs-onlinelibrary-wiley-
com.colorado.idm.oclc.org/doi/epdf/10.1002/2015GL066502
-
https://csbaonline.org/research/publications/senator-mccain-and-outlining-the-fy18-
defense-budget
-
https://www.nasa.gov/sites/default/files/atoms/files/fy_2018_budget_estimates.pdf
No comments:
Post a Comment