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=Role of Jupiter in Human Evolution=

Intro to Jupiter


Jupiter is the largest planet in the solar system, with a mass of 10^24kg. It is a gas giant, made of mostly hydrogen and helium, but it is hypothesized to have a solid core. This is similar to the sun, however Jupiter did not form large enough to ignite as a star. It is estimated that if it were eight times more massive, it would have been large enough. Jupiter has 50 known moons, with more potentially awaiting confirmation. The four largest moons, initially discovered by Galileo, are Io, Europa, Ganymede and Callisto. Jupiter’s surface appears as cloud stripes with a distinctive red spot, which is a gigantic storm that has been going on for hundreds of years. These bands are a result of Jupiter’s very fast rotation, spinning once every 10 hours. This rotation is theorized to create electrical currents in Jupiter’s region, leading to a powerful magnetic field, called the Jovian magnetosphere. It has been discovered that Jupiter also has rings, two of which are composed of small, dark particles and one composed of microscopic debris. Jupiter plays an important role in the evolution of life on Earth for many reasons, which are to be discussed in later sections of this page.

What is needed for life?
The development of life is a complex process that is still somewhat poorly understood. It is well recognized that for life to develop, it needs to be within a suitable environment. The definition of a "habitable planet" is still somewhat contested, due to assumptions regarding what constitutes life. One assumption is that complex life would be carbon based and require water, and thus a habitable planet should provide access to them. Therefore, a habitable planet should be a certain distance from its parent star to allow water to be sustained. The atmosphere of a planet also contributes to its habitability, as its greenhouse effect cannot be too strong, allowing the planet to get too hot and lose water, or too cool, so that water in a liquid state would not exist. The atmosphere is also important in that it should contain molecular oxygen, at least when it comes to survival of many organisms on earth. Additionally, an ozone also shields life from harmful UV radiation. Overall there are many factors that contribute to the success of life on a planet, many of which can be traced back to an early period in the planet's evolutionary history.

Solar System Formation
Our solar system is nearly five billion years old, and within its origin are many key facts for understanding why complex life on Earth was able to arise. Stars, such as our sun, are born within nebulas, which are large clouds of dust and gas throughout galaxies. It is estimated that stars likely do not form in isolation and instead form within groups or clusters. It is also common for stars to exist in binary systems, however our sun remained single. It is possible that the sun had a companion much earlier in its history, although this is difficult to determine. In regard to formation, a trigger, such as a shock from a supernova explosion, causes dust particles within the nebula to draw together. This forms an accumulation of dust, which attracts more dust due to gravitational pull, and eventually collapses in on itself. At the same time the dust cloud begins rotating, increasing in speed, and flattens into a disk surrounding a dense core. This disk keeps increasing in mass until there is enough energy to initiate nuclear reactions, fusing hydrogen to make helium and creating a star. Planets and asteroids are formed from the outer regions of the disk, as electrostatic forces cause dust to accrete and form clusters. These clusters form rocks, which collide to form the rocky planets. It is not guaranteed that planets will form with the formation of a star, and it is estimated that only 20% of solar-like stars, like the sun, have giant planets, like Jupiter. Also, the solar system formed well after the sun had left its birthplace in nebular gas. Since the sun most likely formed in a cluster of stars, and the formation of solar systems involves the ejection of many rocks, it is possible that our solar system contains rocks from many other solar systems.



The gas giants are made of mostly helium and hydrogen. These gases do not condense under the conditions of star formation, so these planets must have accumulated them as gases within the first few million years of the solar system creation. In a similar way to terrestrial planets, dust and small particles form into larger particles and aggregate to form planetesimals. As they grow, they form what is called a planetary embryo, which can grow to consume other planetesimals until it becomes a protoplanet. Eventually when a protoplanet is large enough, an envelope of gas forms around the inner solid core. As the protoplanet continues to grow, the envelope undergoes rapid contraction, attracting more gas, known as the runaway gas accretion. Once it reaches a certain mass, depending on the conditions, the gas accretion rate declines, but the planet continues to grow until gas can no longer be accreted within the planet’s gravitational reach.

It is thought that Jupiter migrated inward from its initial formation site and locked into a motion resonance with Saturn. It was pulled inward by gas currents that surrounded the sun at that time, in a location near to where Mars currently resides, before moving back outward to where it is now. It is hypothesized to be a result of Saturn that Jupiter did not continue this migration inward, and end up colliding with the sun. This event helps to explain the lack of planets very close to the sun, which allowed for the terrestrial planets to form further away, such as Earth. It is thought that there were a first generation of smaller planets that formed very near to the sun initially, but as Jupiter migrated inward it pushed these planets into the sun to be demolished. The movement of Jupiter also disrupted many icy objects, sending them to the asteroid belt, and spread other material in high concentrations where Earth and Mars would form. By the time Earth, Mars and Venus formed as a second generation of planets, the hydrogen and helium levels would also have dropped significantly, resulting in Earth’s lack of hydrogen in the atmosphere, leading to its hospitable nature. Jupiter also has a stable circular orbit, and models have predicted that if it were elliptical or decaying it would have disrupted the other planetary orbits to send them into the sun or out into interstellar space.

Asteroids and Comets
It has been hypothesized that Jupiter’s large mass has played a protective role in the flourishing of complex life on Earth. This is through shielding the Earth from frequent asteroid impacts, reducing the likelihood of mass extinctions. Collisions can destroy the planetary landscape, as well as induce climate change. Therefore, frequent collisions could slow the possibility of evolution, or render it theoretically impossible. On the other hand, without the existence of mass extinctions, ecological niches may not have become available for new species to flourish. Of course there appears to be a fine line between what may be beneficial in promoting evolution and what is far too detrimental to the conditions. Comets could also be a greater concern for Earth without Jupiter’s interception. The Centaurs, large ice-rich bodies orbiting between Jupiter and Neptune, are a main source of comets. The Oort Cloud and Edgeworth-Kuiper belt are others source of comets. The Oort Cloud provides long-period comets that have orbits that can take from thousands to millions of years to complete. Gravitational interactions from passing stars can disturb these orbits, sending them Earth’s way.



There is some evidence in the opposition of the idea of Jupiter as the Earth-protector. Near-earth asteroids, in which Jupiter does not play a role, are proving to be a greater threat to Earth than comets from the Oort Cloud. It is estimated that these asteroids make up around 75% of impact threat. However, asteroids from further distances may have greater collision velocities. Even the idea that Jupiter acts as a shield for the Earth originated most likely in the 1960s, but has little evidence in support. Simulation studies reveal complex results about the impact of Jupiter on collision potentials for Earth. Surprisingly, the studies have supported the idea that Jupiter may actually promote collisions of short-range asteroids and comets, only acting as a protector for long-range objects. At the very least, the studies do support the idea that if Jupiter was even as small as Saturn, it would have detrimental effects on the collision rates of asteroids on Earth.

Liquid Water
For complex life to develop, it is well hypothesized that a planet must be within a habitable zone, or an appropriate distance from a star to sustain liquid water. But a planet could develop in this zone without the building blocks that contain water. In this case, it would not really be habitable, despite being in the right location. Theories indicate that the dust particles that formed Earth were in an area too hot to maintain any volatiles, as they were probably over 1000K, so Earth must have acquired water by a different means. An idea in regards to Earth’s water was that Earth formed from primary material in the area of the habitable zone, and acquired water at a later time via comet impacts. However, studies have found that at most 10% of Earth’s water is of comet origin. Other studies have suggested that Earth acquired water via absorbing other planetary embryos from different regions of the developing solar system, during its creation. As previously mentioned, Jupiter played a large role in the formation of the solar system, and so it can only be expected to also play a role in the origin of water on Earth.

The water content of plantesimals depends on many factors including the characteristics of the protoplanetary disk, the molecular cloud from which the star formed, and the masses and formation timing of giant planets, such as Jupiter. Jupiter’s eccentricity also may play a prominent role in delivery of water content to terrestrial planets, as models predict a drastic reduction in water with only a slight change. Additionally, scientists argue that the plantesimals that collided to form Earth must have come from outer-belt water rich asteroids, and it was gravitational perturbations caused by Jupiter that disrupted their orbits to allow them to collide with Earth’s. For example, research suggests that when giant planets reached a mass of 10-20 terrestrial masses during development, they disturbed the orbits of billions of icy bodies, sending a selection of them to the inner solar system.

Gravity
Another important consideration for the development of complex life on Earth is gravity. Gravity has played a role in the development of life from sea to land to sky. Gravity is one of the four fundamental forces of nature. It is a weak force, which causes massively large objects to attract other objects to them. In the universe, zero gravity is only a theoretical term, since there are masses everywhere. On Earth, gravity causes an object of any mass at the surface to accelerate towards the Earth’s centre at 9.8m/s^2. The strength of gravitational attraction depends on the mass of the large object, how far away the other object is and its mass. The connection of Earth’s gravity to Jupiter may not seem obvious at first, but going back to planetary development and the formation of the solar system it becomes clearer. Recall, that large planets such as Jupiter and Saturn were first to begin forming in the solar system, and accreted gas and solids at a rapid pace. Due to their rapid formation, some other planets that attempted to form at the same time were aborted and sent to the asteroid belt. Therefore, if Jupiter had a larger mass, or formed closer to Earth, it could have resulted in Earth’s ejection, or smaller size. This would have altered the conditions available for life, especially in regards to gravity.

The acceleration of gravity has been present for all 4 billion years of biological evolution on Earth. No vertebrate species has undergone a complete life cycle in space, where there is reduced gravitational pull, although this is not the same for plants. Research has also shown that certain biological structures have evolved to detect and combat gravity, at both the organismal and cellular level, further strengthening its importance in evolution. Gravity acts as a physical restriction, driving the development of structures and methods to counteract it, such as bone and a skeleton. It also serves as indication of orientation due to its fixed direction, which is helpful for many organisms. It affects circulation throughout the body, the development of bipedalism and even sensory reception, especially in relation to statolithic or otolithic organs, such as the inner ear. It is evident that gravity has played a large role in evolution, but studies are still not conclusive on the necessity of gravity for the development of life, so more research into organisms in weightless environments is needed.

Conclusion
There is a large body of evidence in support of the prominent role Jupiter has played in the evolution of complex life on Earth. However, it is not to say that Jupiter alone was responsible for this development, or that it was the most important. Other factors such as Earth’s position in the habitable zone, its large moon, plate tectonics and even stochastic events are also thought to have contributed to the flourishing of life on the planet. Nevertheless, without Jupiter’s role in the formation of the solar system, its influence on Earth’s size and position, and its protective nature regarding comets and asteroids, Earth and all of its life would not look the same as it does today. The search for extraterrestrial life may provide more clues into the importance of Jupiter, or Jupiter-like planets. If any life can develop in conditions lacking a “Jupiter”, this may indicate key details whether life is a common or unique occurrence in the vast universe.



References (alphabetized and annotated)
Adams, F. C. (2010). The birth environment of the Solar System. arXiv preprint arXiv:1001.5444.

This article by Adams provided information in regards to solar system formation in general and in regards to our own. It was important in the understanding of the conditions in which Jupiter formed, and how it influenced the formation of terrestrial planets.

Anken, R. H., & Rahmann, H. (2002). Gravitational zoology: how animals use and cope with gravity (pp. 315-333). Springer Berlin Heidelberg.

This article provided a detailed account of what is known about the relationship between animals (vertebrates and invertebrates) and gravity. It gave examples of structures and mechanisms that have evolved in relation to gravity, which strengthened the argument that they were heavily dependent on one another.

Batygin, K., & Laughlin, G. (2015). Jupiter’s decisive role in the inner Solar System’s early evolution. Proceedings of the National Academy of Sciences, 201423252.

This article helped to detail the development of the early solar system, especially in relation to Jupiter and its resonance with Saturn.

D’Angelo, G., Durisen, R. H., & Lissauer, J. J. (2010). Giant planet formation. Exoplanets, 319.

This article described in detail the formation of the gaseous planets. This was essential in the understanding of the formation of Jupiter and in providing background information.

Delsemme, A. H. (1991). Cometary origin of carbon, nitrogen and water on the Earth. Origins of Life and Evolution of the Biosphere, 21(5-6), 279-298.

This article provided information about the early earth, in particular that it was unable to sustain water in its place of formation. This provided support to the argument that water had to be acquired by Earth in a different manner.

Horner, J., & Jones, B. W. (2010). Jupiter: friend or foe? An answer. Astronomy & Geophysics, 51(6), 6-16. Horner, J., & Jones, B. W. (2009). Jupiter–friend or foe? II: the Centaurs. International Journal of Astrobiology, 8(02), 75-80. Horner, J., & Jones, B. W. (2008). Jupiter–friend or foe? I: the asteroids. International Journal of Astrobiology, 7(3-4), 251-261.

These three articles were part of a series investigating the role of Jupiter as a protector of Earth. It served as an excellent starting point for this page and detailed many mathematical models for predicting the exact impact of Jupiter on Earth.

How did the solar system form? (n.d.). Retrieved April 6, 2015, from http://www.nhm.ac.uk/nature-online/space/planets-solar-system/formation/

This website article was created through the National History Museum and provided an easy-to-understand account of the formation of the solar system. Accompanied by other research, it served to introduce basics of solar system formation.

Kasting, J. F. (2003). The origins of water on Earth. Scientific American, 13, 28-33.

This short article in Scientific American provided information into a method of how Earth may have acquired water from asteroids, due to Jupiter.

Kasting, J. F., & Catling, D. (2003). Evolution of a habitable planet. Annual Review of Astronomy and Astrophysics, 41(1), 429-463

This article provided background information about the qualities of a planet that make it habitable for life. It was useful to provide context to relate back to later in the sections.

Morey-Holton, E. R. (2003). The impact of gravity on life. Evolution on planet earth: the impact of the physical environment, 143-159.

This article provided information for the section of gravity, particularly how life is affected by it. It also gave some limited information about gravity in general.

NASA (n.d). Jupiter: Overview. Retrieved April 6, 2015 from https://solarsystem.nasa.gov/planets/profile.cfm?Object=Jupiter&Display=OverviewLong.

This webpage by NASA was extremely helpful in providing background information on Jupiter in an easily accessible manner. NASA is a very reputable organization when it comes to information about the solar system and space, so it was appropriate to use despite it not being a primary or secondary source.

National Geographic (n.d). Jupiter, Jupiter Information, Facts, News, Photos -- National Geographic. Retrieved April 6, 2015 from http://science.nationalgeographic.com/science/space/solar-system/jupiter-article/.

This webpage by National Geographic gave additional information about the planet Jupiter, in regards to its size, make-up and moons.

Raymond, S. N., Quinn, T., & Lunine, J. I. (2004). Making other earths: dynamical simulations of terrestrial planet formation and water delivery. Icarus, 168(1), 1-17.

This article provided extensive information about water on Earth, including the methods that it could have been acquired from and how Jupiter may have played a role.

Walsh, K. J., Morbidelli, A., Raymond, S. N., O'Brien, D. P., & Mandell, A. M. (2011). A low mass for Mars from Jupiter's early gas-driven migration. Nature, 475(7355), 206-209

This article in Nature described how the movement of Jupiter may have changed during its development. It indicated the effects this movement may have had on the size of other planets and positioning of asteroids.

Ward, P. D., Brownlee, D., & Krauss, L. (2007). Rare Earth: Why Complex Life Is Uncommon in the Universe. Physics Today, 53(9), 62-63.

This article provided an overview of the conditions needed for life to develop in the universe. A section was dedicated to Jupiter, and how various aspects of the gas giant have made life on earth a possibility.

Van Loon, J. J. W. A. (2007). The gravity environment in space experiments. Biology in space and life on earth: effects of spaceflight on biological systems, 17-32.

This article provided basic information about gravity, such as what the force is and what it is dependent on. This was essential to make the connection between Jupiter's creation and Earth's mass, and why this would affect gravity.