Life from Comet Collisions

The biosphere covering Earth remains a deep mystery. Where did life originate?

Bioluminescent dinoflagellates lighting a breaking wave at midnight

Earth might well have coalesced from cosmic dust and gas. Billions of years later, a thin raft surrounding Earth teems with co-dependent organisms.

The question of the origin of life is essentially speculative. We have to construct, by straightforward thinking on the basis of very few factual observations, a plausible and self-consistent picture of a process which must have occurred before any of the forms which are known to us in the fossil record could have existed.
— John Desmond Bernal

Where did the ingredients for the simplest unicellular life forms come from? Life's ingredients might have come from comets.

Early Earth was not very hospitable when it came to jump-starting life. In fact, new research shows that life on Earth may have come from out of this world.
― Astrobiology Magazine

Comet Hale Bopp

Comets are vestiges from the formation of our solar system some 4.5 billion years ago. Comets that move into range of space travel, arrive well-preserved from the freeze locker of the outer solar system.

As it travels near the Sun and Earth, a disintegrating comet represents a treasure-trove of hard data about conditions present during the early stages of the solar system.

The composition of comets interests us because scientists posit that life-building organic compounds might have come from icy comets that collided with Earth millions of years ago.

Organic compounds form the basis of proteins and nucleobases (the building blocks of DNA and RNA). Oceans might have been formed from icy comets and asteroids crashing into Earth.

The first comet landing of a robotic spacecraft touched down on Comet 67P on November 12, 2014. Until then, our knowledge of comets came from a handful of flyby space missions

Comet 67P

Comet 67P

The robotic lander Philae bounced to rest on Comet 67P more than 10 years after being launched from Earth by the European Space Agency.

The Philae landing was a historical touchdown on a comet nucleus.

Philae transmitted in-situ data that will enable scientists to determine the composition of the comet's surface material.

Our Quest

Comets were once believed to be harbingers of human destruction by anthropomorphic gods. Our understanding of our habitat and origin accrues through scientific discovery.

Scientific discovery yields physical explanations to mysterious and once feared phenomena. Our appreciation of the far-fetched phenomena of life grows with every scientific revelation.

Our quest to solve the existential puzzle continues.

The origin of life is a complex narrative of improbable events. The lineage of events stretch back perhaps 13.8 billion years to the Big Bang or beyond.


REFERENCES

• How Did Water Come to Earth? Brian Greene, Smithsonian Magazine, May 2013.
• Mission to Land on a Comet, NASA Science News, 31 January 2012.
• The Shocking Source for Life's Origin, Astrobiology Magazine, 27 August 2013.

It Goes On

Model of L-serine molecule

Moments after the Big Bang, the universe contained hydrogen, helium, and trace amounts of lithium. Once stars coalesced, carbon and oxygen were produced.

Carbon and oxygen are synthesized when helium burns inside red giant stars.

Carbon in the universe is traceable to stars

Carbon-based Life

Carbon is the essential ingredient of life-sustaining organic molecules: carbohydrates; proteins; nucleic acids; and lipids.

Carbon allows for bonds that form three-dimensional molecular structures likened to, and modeled by, rings and chains. Such complex molecular structures provide the building blocks of life.

“In three words I can sum up everything I've learned about life: it goes on.”
― Robert Frost

Earth's Carbon

Earth is largely comprised of oxygen and silicon with comparatively little carbon.

Earth exhibits an observable and measurable carbon cycle analogous to a bookkeeper's columnar ledger of debits and credits. Carbon is exchanged between these reservoirs:

• Biosphere - sum of all ecosystems, or life zone;
• Pedosphere - outermost layer of Earth comprised of soil and soil formation;
• Geosphere - solid parts of Earth including, but not limited to, the lithosphere;
• Hydrosphere - combined mass of water on and under the surface of Earth; and
• Atmosphere - layer of gases surrounding Earth retained by Earth's gravity.

Earth's carbon cycle is a model for the ongoing flows between each of the reservoirs. Some carbon flows are slow (e.g., on the order of hundreds of millions of years), while others are fast:

• Slow Carbon Cycle - Over the span of geologic time (100 - 200 million years), chemical reactions, tectonic events, and volcanic events occur that transfers carbon between rocks, soil, ocean, and atmosphere.
• Fast Carbon Cycle - Plants, and phytoplankton in the ocean, absorb carbon dioxide (CO₂) from the atmosphere into their cells via photosynthesis, then return CO₂ via cellular respiration and microbial decomposition.

Fast carbon cycle depicting exchange of carbon between reservoirs

“Originally, the atoms of carbon from which we’re made were floating in the air, part of a carbon dioxide molecule. The only way to recruit these carbon atoms for the molecules necessary to support life—the carbohydrates, amino acids, proteins, and lipids—is by means of photosynthesis. Using sunlight as a catalyst the green cells of plants combine carbon atoms taken from the air with water and elements drawn from the soil to form the simple organic compounds that stand at the base of every food chain. It is more than a figure of speech to say that plants create life out of thin air.”
― Michael Pollan, The Omnivore's Dilemma: A Natural History of Four Meals

Monitoring Earth's Carbon

NASA will launch the Orbiting Carbon Observatory-2 satellite in July 2014. It will record atmospheric CO₂ from space. The data will be used to map CO₂ sources and sinks and to provide insight into its variability over seasonal cycles.

You will die but the carbon will not; its career does not end with you. It will return to the soil, and there a plant may take it up again in time, sending it once more on a cycle of plant and animal life.
― Jacob Bronowski

REFERENCES

• Carbon Cycle, Wikipedia.
• Foundations of Carbon-Based Life Leave Little Room for Error, NC State University Press Release, 13 March 2013.
• Orbiting Carbon Observatory, NASA.
• The Carbon Cycle, Holli Riebeek, Earth Observatory, 16 June 2011.
• What is the Carbon Cycle? What is the science behind it?, United States Carbon Cycle Science Program.