Planetary Settings for Life
Scientific investigations on the origin of life started on where it could be found: the earth. As one of the planets within the solar system, this pale blue dot has its own unique history that supports the emergence of life both in marine and terrestrial ecosystems. Examining geological evidence on earth and the moon, analyzing the dynamics of greenhouse gases in young earth’s atmosphere, and mathematically modeling the early history of the Solar system, are some of the efforts to know how earth could host the emergence of life from non-living materials.
What was the early earth’s composition?
To get the picture of the early earth, scientists are using geological evidence in the present time. At first, it was a quite challenging task due to the nature of the earth that changes the shape and composition of its crust over time. Plate-tectonics allow rocks and minerals to disappear from earth’s crust via subduction. While weathering by the wind and waters changes earth composition and hence transforms the landforms. These processes leave scientists with a very few evidence that could take them back in time. The first scientific attempt to measure the age of the earth was done by William Thompson in 1862. Relying on the physics at his time, he concluded that earth was 20–40 million years old, — a far missed prediction compared of our knowledge today.
The discovery of Zircon (ZrSiO4) as a “geochronometer” speeds up the geological approach to understand the origins of life. The mineral that is commonly found on earth survives erosion, metamorphosis, and other geological processes. It also contains high concentration of uranium trace and chemically “inert”, making it a reliable “time-machine”. [1]Grains of Zircon in Jack Hills, Western Australia, are estimated to be 4.4 billion years old, with the ratio of O18 implying that the earth already had a continent surrounded by oceans with “low temperature” [2]. This confirmed what Morse (1998) found that earth’s ocean temperature was around or below 100 degree Celcius in Hadean eon.[3]
The confirmation of ocean existence was crucial for the origin of life research. Liquid water is the key ingredient for life. Julien de Wit, an astrobiologist at MIT, says liquid water is a probe for a planet’s habitability as it acts as a universal solvent. He and his colleagues are developing methodology to detect liquid water on the exoplanets in TRAPPIST-1 system using carbon depletion measurement. While Greg Fournier at MIT explained that chemical feature of water as a polar molecule allows complex chemical element to fold, a characteristic of polymers that might lead to self-replicating molecules like RNA. It is a crucial pre-requisite of abiogenesis to occur.
But where did all the water on earth come from? The question pushes us further back in time. Some research suggest it existed before the earth was formed as a planet. During the formation of our Solar System, the swirling materials which would form the earth already contained water compound[4]. Once they were condensed, the earth outgassed them as water vapors[5]. How did it become liquid? It is still an open question. Sarafian et al (2014) offer an alternative story, that the earth water might come from comets and asteroids. [6]But new evidence from Rosetta mission suggests that most of the water on comets and asteroids have more deuterium than water on earth[7], a different kind of water than one we have on earth, suggesting water on earth comes from within.
What was the atmosphere like on early earth?
It is important to note that during the Hadean and Archaean eons, oxygen could not be found. This would be a suffocating and hazardous environment for us. But Andrew Knoll, a geobiologist at Harvard University, says it was good for life because if there was an oxygen, every volatile chemical on earth would be oxidized and might fail to create complex molecules. He also emphasizes the fact that is often neglected: that oxygen is not a primary key for life. The diversity of microbes’ metabolism on earth today has shown that life could survive without oxygen, he says. In fact, the earliest evidence of photothrophy came from a bacterium that did not require oxygen to survive and did not produce it either (Sleep 2010)[8].
With no oxygen, other gas chemicals filled the air of the young earth. Carbon-dioxide, which has become well-known in this century because of the capitalism-induced climate crisis, was the dominant gas in earth’s early atmosphere. But it was slowly removed to the earth’s mantle through the process of carbon sequestration in the ocean. Other gases such as ammonia (NH3) and methane (CH4) were also found in significant amount in the Hadean, but they were also easily transformed into other molecules due to their interaction with sunlight. These greenhouse gases, however, is considered as an important factor for the origins of life, as they created a shield on the earth’s atmosphere to trap a portion of sunlight and warming the earth.[9]
Confirming the temperature of the earth in the Hadean to Archaean eon to be supportive on life existence is important. This is because the mathematical modelling of the sun evolution showed that our sun was 25% dimmer at its young age (Feulner 2012[10]). In this model, with all other parameters on earth remain the same, the lower luminosity of the sun would froze the earth into a giant snowball in its first 2 billion years, but yet we found evidence of liquid water, warm temperature, and photosynthetic bacteria in those periods. We knew that earth’s atmosphere had to be warmer to support life because all these features could only be possible in warm temperature.
Is our earth special?
The answer: it depends. It depends on the definition of what we mean with the word “special”. If it is some characteristics that could only be found on earth. Then the answer is yes because the earth has life, — at least in NASA’s definition of life (a self-sustaining chemical system capable of Darwinian evolution). [11]It also has plate-tectonics, abundant waters, and unique atmospheric conditions. But other planets too, have their own unique characteristics. Recent exploration on the exoplanets have found planets that has four stars, or even doesn’t have one[12]. Characteristics that are special for these planets only.
It is also important for us to see the earth as a planet, which means it is a part of a bigger entity: the Solar system. What happened on earth, including the origins of life, is inseparable from the history of the Sun and other planets. Besides the Faint Young Sun phenomenon, another solar system history that contributed to provide habitability to the young earth was the moon formation. The moon was formed when a planet-size asteroid hit the earth and ejecting materials that made up the moon. Some researchers say that gravity interaction between the earth and the moon allows the creation of tides in the ocean which might be important for life.[13]
Another major event in our solar system history is the migration of gas planets Jupiter and Saturn to its current orbit. It is a crucial moment for the origins of life as the big planets disrupt the asteroids belt and throwing them to the planets of the inner solar system such as the moon and the earth itself.[1] This Late Heavy Bombardment event, although supported by astronomical modeling, is still debated within the scientific community. The major evidence for this are the specimens from moon craters collected during the lunar expedition in the 1960s. Scientists criticized the dating of these samples as bias and methodologically weak[2] as it measured the ratio between potassium and argon, the number (which is used to measure the age of the specimens) could be misleading because argon was evaporated as gas during the first episode of impact (liquid lava).
But even without this episode of heavy bombardment, the asteroid impacts (which are still happening right now above earth atmosphere) did have tremendous effects on earth’s habitability. Osinsiki et al confirmed that it impacts could create habitats favorable for life such as the hydrothermal vents, porous rocks, and subaerial clay. It could also bring “building blocks of life” such as the hydrogen, carbon, and nitrogen to the earth’s the atmosphere[16]
Perspective: Matter doesn’t matter.
Reviewing the planetary settings for the origins of life shows us the importance of interconnection between matters. This dynamic interaction happened both in the macro and micro-world. In the macroworld, the matters are all the planets and the stars, most especially our Solar System. The history of the sun evolution, the migration of Jupiter and Saturn, the disruption in the asteroid belt, the moon formation, and gravitational interaction between the earth and the moon, were events that were crucial to the origins of life. As in the microworld, the matters are atoms and molecules that were vulnerable for changes in the early earth. Chemical reactions between the volatiles in the earth were the starters of life on earth.
Both the macro and microworld also performed complex interactions. They both mingled at the time of the Late Heavy Bombardment. When asteroids hit the earth, it opened the crust, and it enabled interactions between reduced atmosphere with the oxidized environment within the earth’s crust. This interaction is the basis of making organic molecules, says Greg Fournier, an evolutionary biologist at MIT. Complex interactions of the two worlds could also be found in the process of greenhouse effects on the earth’s early atmosphere. It could be a solution the Faint Young Sun problem in the context of origins of life, but it could also be the driver of today’s disappearance of life.
Nevertheless, complex interactions between matters are part of what we call “life”. This is interesting given that some scientists declare themselves as “materialist”. Alan Lightman, science writer and theoretical physicist at MIT, wrote in a Nautilus feature that “everything is made of atoms, molecules, and nothing more,”[17]. While he mentions that these atoms and molecules are governed by small number of fundamental laws, he strongly argues that matters are the most important element of reality. But scientific research on the origins of life shows how matter is a second-tier player. The most important is the complex interactions between matters, and not the matters itself. “How many of these atoms of molecules must exist is not important, but the important one is the conditions that allow these molecules to create life,” says Fournier.
Michael Wong, a post-doctoral fellow at Carnegie Science Institute at Washington DC, says “You are not what your molecules are, but you are what your molecules do,”. Science has shown us that atoms and molecules are changing over time and these changes define life and other phenomenon in the observable universe. Apparently, atomism and reductionism seem to be an obsolete philosophy. Besides atoms and molecules, there is another important element of life. Jack Soztak, a major player in the origins of life research in the University of Chicago, says in Lightman’s Searching for Meaning episode that it is “the organization.” One could stop there and continue their scientific quest like Soztak. But a religious scientist like Jonathan Lunine , an astrobiologist at Cornell University, could say this organization means something else: an emanation from God. Which one would you choose? It is all up to you.
[1] Davis, Krogh, and Williams, “Historical Development of Zircon Geochronology.”
[2] Wilde et al., “Evidence from Detrital Zircons for the Existence of Continental Crust and Oceans on the Earth 4.4 Gyr Ago.”
[3] Morse and Mackenzie, “Hadean Ocean Carbonate Geochemistry.”
[4] Perotti et al., “Water in the Terrestrial Planet-Forming Zone of the PDS 70 Disk.”
[5] Bower et al., Retention of Water in Terrestrial Magma Oceans and Carbon-Rich Early Atmospheres.
[6] Sarafian et al., “Early Accretion of Water in the Inner Solar System from a Carbonaceous Chondrite–like Source.”
[7] “Rosetta Fuels Debate on Origin of Earth’s Oceans.”
[8] Sleep, “The Hadean-Archaean Environment.”
[9] Feulner, “The Faint Young Sun Problem.”
[10] Feulner.
[11] Benner, “Defining Life.”
[12] “Kepler-64b.”
[13] Dorminey, “Without the Moon, Would There Be Life on Earth?”
[14] Fassett and Minton, “Impact Bombardment of the Terrestrial Planets and the Early History of the Solar System.”
[15] Boehnke and Harrison, “Illusory Late Heavy Bombardments.”
[16] Osinski et al., “The Role of Meteorite Impacts in the Origin of Life.”
[17] Lightman, “The Spiritual Materialist.”