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Lecture Notes Review
6.2 The Origin of Life
The Miller-Urey experiment (1953) simulated early Earth conditions and demonstrated that organic molecules, like amino acids, could form from inorganic precursors such as water, methane, ammonia, and hydrogen gas when subjected to electrical discharges. This suggests life's building blocks could have arisen abiotically. However, the exact atmospheric composition of early Earth is still debated; some evidence suggests it was less reducing than assumed in the experiment.
Organic Building Blocks Abundant
Besides the Miller-Urey experiment, there are at least three other potential sources of organic molecules:
Deep-Sea Hydrothermal Vents: These vents release chemicals from Earth’s interior, providing energy and materials for forming organic compounds.
Extraterrestrial Delivery: Meteorites, such as the Murchison meteorite, contain amino acids, lipids, sugars, and nucleobases, indicating that these molecules could have been delivered to early Earth from space.
Volcanic Activity: Volcanoes release gases that can form organic molecules when exposed to UV radiation or lightning.
From Chemistry to Biology: RNA World
RNA Simplicity: RNA is structurally simpler than DNA, consisting of a single strand and using ribose sugar instead of deoxyribose.
Hereditary Information: RNA's ability to carry hereditary information is evident in many viruses that use RNA as their genetic material.
Catalytic Activity: Ribozymes are RNA molecules capable of catalyzing chemical reactions, including self-replication. The discovery of ribozymes supports the idea that RNA could have played a central role in the origin of life.
Clay Catalysis: Experiments have shown that clay minerals can catalyze the formation of RNA strands, providing a plausible mechanism for the polymerization of RNA monomers on early Earth.
The "RNA World" hypothesis suggests that RNA was the primary hereditary molecule before DNA evolved. RNA could both store genetic information and catalyze reactions necessary for replication.
Lipid "Pre-Cells"
Lipid "pre-cells" (or protocells) form spontaneously when lipids are mixed with water. These structures can encapsulate other molecules and may have served as early cellular compartments.
They provide a confined environment for early biochemical reactions.
Chemical Evolution and Natural Selection
Simple self-replicating RNA strands could have undergone a chemical form of natural selection, where variants with higher replication rates become more abundant.
This process would have been accelerated within pre-cells, which provide a contained environment and promote interactions between molecules.
Plausible Scenario for Origin of Life
Abundant Organic Molecules: Organic molecules were synthesized through various processes, including Miller-Urey-type reactions, deep-sea vents, and extraterrestrial delivery.
Pre-Cell Formation: Clay minerals catalyzed the production of RNA and the formation of lipid membranes, leading to the encapsulation of RNA and other organic molecules in pre-cells.
RNA Evolution: Within pre-cells, RNA molecules evolved through molecular natural selection, with some RNA molecules developing self-replication capabilities.
RNA World: In the RNA world, RNA and proteins co-evolved through molecular natural selection, eventually leading to the formation of true living cells capable of self-replication. Ribosomes, which are composed of RNA and proteins, may be remnants of this era.
Transition to DNA: Biological evolution in the RNA world eventually led to the first DNA molecules. DNA’s double-stranded structure and more stable sugar-phosphate backbone made it a more reliable hereditary molecule, allowing it to replace RNA as the primary genetic material.
Migration from Elsewhere (Panspermia)
Some life forms, such as certain bacteria and spores, can survive the harsh conditions of space, including radiation, vacuum, and extreme temperatures.
Impacts on planets can eject material into space, and these materials can travel between worlds. Martian meteorites found on Earth provide evidence of this process.
Venus, Earth, and Mars have exchanged material for billions of years, making it plausible that life could have been transferred between them.
However, even if life migrated to Earth, it still had to originate somewhere. Panspermia only shifts the problem of the origin of life to another location.
Interstellar origin seems highly unlikely due to the vast distances and extreme conditions involved.
A future test to determine if life on Mars shares a common origin with Earth life could involve comparing the genetic code and biochemistry of Martian organisms with those of Earth organisms.
Review: Origin of Life
How did life begin?
Miller-Urey experiment
Transition from chemistry to biology or transport from space.
Deep-sea vents
Clay and other minerals
Catalysts to build RNA strands
Lipid pre-cells
RNA self-replication enabling natural selection
Improvement of replication until life emerges.
Could life have migrated to Earth?
Migration from Venus or Mars.
Impact blasts from one world to another.
Meteorites from Mars.
Chance of survival traveling through space.
Migration from other star systems highly unlikely.
Cells: The Basic Units of Life
Living cells are the fundamental units of life on Earth, capable of performing all essential life functions.
The cell membrane, composed of a lipid bilayer, separates the internal environment of the cell (organic material) from the external environment (inorganic material).
Molecular Components of Cells
Major components:
Carbohydrates: Provide energy and structural support.
Lipids: Form cell membranes, store energy, and act as signaling molecules.
Proteins: Perform a wide range of functions, including catalyzing reactions, transporting molecules, and providing structural support.
Enzymes: Biological catalysts that speed up biochemical reactions by lowering the activation energy.
Nucleic acids (DNA, RNA): Store and transmit genetic information.
Amino acids: The building blocks of proteins.
Handedness (chirality): All known life on Earth uses left-handed amino acids to build proteins and mostly right-handed carbohydrates. The preference for one enantiomer over the other is a unique characteristic of life.
Major Groupings of Life on Earth
Eukaryotes (cells with nuclei) and prokaryotes (cells without nuclei). Eukaryotic cells are generally larger and more complex than prokaryotic cells.
Super-kingdoms (Domains):
Bacteria: A diverse group of prokaryotic organisms.
Archaea: Another group of prokaryotic organisms that often inhabit extreme environments.
Eukarya: All eukaryotic organisms, including plants, animals, fungi, and protists.
Classification is based primarily on chemical properties (biochemistry) rather than appearance. Molecular data, such as DNA and RNA sequences, are used to classify organisms.
The tree of life, constructed from molecular data, indicates that all life on Earth shares a common ancestry. The three domains (Bacteria, Archaea, and Eukarya) diverged from a universal common ancestor.
Possibility of Life Beyond Earth
What are we searching for?
Extraterrestrial life is difficult to define precisely. Generally, it refers to any life form that did not originate on Earth.
The search is primarily focused on finding life similar to life on Earth, based on carbon-based chemistry and the presence of water.
Single-celled and multicellular life forms are both potential targets in the search for extraterrestrial life.
Is it reasonable to imagine life beyond Earth?
Examining the possibilities using modern science suggests that life may be possible on other habitable worlds.
As of now, there is no confirmed evidence of life beyond Earth, but ongoing and future missions aim to address this question.
Scientific Context of the Search
How does astronomy help us understand the possibilities?
The same physical laws operate everywhere in the universe, suggesting that the principles governing life on Earth could also apply elsewhere.
The universality of physics, chemistry, and biology provides a framework for understanding the conditions necessary for life to arise and evolve on other planets.
How does planetary science help?
Expect planets are common around other stars (extrasolar planets). Thousands of exoplanets have been discovered, and many more are expected to exist.
Habitable worlds are planets that have conditions suitable for liquid water to exist on their surface.
How does biology help?
Evidence suggesting life might be common on habitable worlds:
Life arose relatively quickly on Earth after the planet cooled and conditions became stable.
Organic molecules are common in meteorites and interstellar clouds, indicating that the building blocks of life are widespread.
Some life on Earth survives in extreme conditions (extremophiles), suggesting that life can adapt to a wide range of environments.
Places to Search
Where should we search?
Direct searches on other planets and moons:
Mars: Evidence of past liquid water and the potential for subsurface habitats make Mars a prime target.
Jupiter’s moons (Ganymede, Callisto, Europa): These moons have subsurface oceans that could potentially harbor life.
Saturn’s moons (Titan, Enceladus): Titan has a thick atmosphere and liquid methane seas, while Enceladus has a subsurface ocean and cryovolcanoes that eject water vapor and organic molecules into space.
Telescopic searches for life on other stars involve looking for biosignatures in the atmospheres of exoplanets.
Could aliens be searching for us?
If life is common, then intelligent life could also be common.
The Search for Extraterrestrial Intelligence (SETI) is an ongoing effort to detect signals from other civilizations.
New Science of Astrobiology
How do we study the possibility of life beyond Earth?
Three major areas:
Study conditions conducive for the origin and ongoing existence of life.
Finding habitable conditions within our solar system and around other stars.
Finding actual occurrences of life beyond Earth.
Developing connections between living organisms and the places they live (ecology) helps us understand how life can adapt to different environments.
Ancient Debate About Life Beyond Earth
How did attempts to understand the sky start us on the road to science?
The sky was observed by many ancient civilizations, including the Ancient Chinese, Babylonians, and Mayans. These observations led to the development of early forms of astronomy and mathematics.
Early Greek Science:
Thales (624 - 546 B.C.): Considered the founder of Greek science and philosophy.
Anaximander (610 - 547 B.C.): Proposed that the universe originated from an indefinite substance called apeiron.
Pythagoras (560 - 480 B.C.): Emphasized the importance of mathematics in understanding the universe and believed in mathematical perfection.
Plato (428 - 348 B.C.): Developed a philosophical system based on ideal forms and believed in a structured universe.
Aristotle (384 - 322 B.C.): A student of Plato and tutor to Alexander the Great, Aristotle developed a comprehensive system of philosophy and science that dominated Western thought for centuries.
Eratosthenes (around 240 B.C.): Accurately calculated the circumference of the Earth using geometry.
Geometry Models: Conceptual representations used to explain and predict observed phenomena.
Led to Development of Modern Science
The Geocentric Model
Celestial Sphere - Anaximander
Geocentric Model (supporting arguments):
Ptolemy (A.D. 100 - 170): Developed a detailed geocentric model of the universe that was widely accepted for over 1400 years.
Epicycles: Small circles within larger circles used to explain the retrograde motion of planets in the geocentric model.
The Mystery of Planetary Motion:
Planet: Means "wanderer" in Greek. Planets generally move eastward relative to the fixed stars but occasionally undergo retrograde motion.
Visible with the naked eye: Mercury, Venus, Mars, Jupiter, and Saturn.
Seven Days in a Week: Named after the seven celestial bodies visible to the naked eye (Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn).
Explanation for Apparent Retrograde Motion:
Ptolemaic Model: Geocentric model with epicycles (1500 years) - Almagest
Heavenly perfection: The belief that the heavens were perfect and unchanging.
Still no evidence of life beyond Earth
An Alternate Model
Aristarchus (310 - 230 B.C.): Proposed a Sun-Centered Model with the Sun Much Larger Than Earth.
Arguments Against:
Inconsistent with observation of stellar positions: The absence of observable stellar parallax was used as an argument against the heliocentric model.
Stars lie at different distances rather than all on the same spheres.
Stellar Parallax: The apparent shift in the position of a nearby star relative to distant stars as Earth orbits the Sun.
Stars too far away to be detectable with the naked eye.
The Roots of Modern Science:
Greeks reject the idea of a Sun-centered Model (Heliocentric Model).
Modern Science: Emphasizes reason, models consistent with observations, and empirical evidence.
Nicholas Copernicus (1473 - 1543): Re-visits Aristarchus’ Model
Led to the development of Modern Science.
Why did the Greeks argue about the possibility of life beyond Earth?
Atomists (Democritus - 460 - 370 B.C.):
Believed that the world is formed by an infinite number of indivisible atoms, implying the existence of other worlds.
More associated with Atheism
Aristotelians:
Believed that all Earth must fall to the center of the universe.
They supported a geocentric universe and believed that the Earth is unique.
Heavens fundamentally different from the Earth
No Other Worlds
Four Elements:
Earth, Water, Air, Fire
Fifth Element: Aether (or Ether) or Quintessence
Thomas Aquinas (1225 - 1274):
Connected Aristotelian philosophy with Christian Theology
The Copernican Revolution
Europe in Dark Ages
Baghdad, Iraq - Intellectual Center of World (Middle Ages):
Byzantine Empire (Eastern Roman Empire) - sought knowledge to better understand the wisdom of Allah.
Greek works translated into Islam
Mathematics & astronomy
Algebra developed
Capital Constantinople (Modern Day Istanbul, Turkey):
Falls (1453)
Scholars Leave East & go West (Europe) - Start of European Renaissance
How did the Copernican revolution further the develop the science?
Copernican Revolution:
Nicholas Copernicus (1473 - 1543)
Book published after his death: “Concerning the Revolutions of the Heavenly Spheres”
Heliocentric Model of Universe
Debated for 50 years / Not any better at prediction than Ptolemaic Model - Ideas and Debates - formed foundations of modern science
Moved Earth away from the center of the universe & replaced it with the Sun - Kept the concept of perfectly circular orbits
People did not want to replace 2000 year old (Ptolemaic Model) - that produced similar inaccuracies
Old Data was not revised for 200 years
Tycho - A New Standard in Observational Data
Lack of Data - could not validate the Ptolemaic Model vs. Copernican Model- Telescope Not Yet Invented
Tycho Brahe (1546 - 1601) - From Dutch Aristocratic Family:
Built the largest naked-eye observatory - giant protractors
30 years of observations (w/in 1 arc minute - less than the thickness of a thin part of a fingernail) - Most accurate data at the time
Kepler - A Successful Model of Planetary Motion
Johannes Kepler (1571 - 1630) - German Astronomer & Mathematician:
Used Tycho Brahe’s data to validate Ptolemaic Model vs. Copernican Model- Deeply Religious
Understanding the Geometry of the Heavens - Initially used circular orbits to model Tycho’ data
Mars data did not match Kepler’s predictions by 8 arc minutes (with circular orbits)
Did not ignore the discrepancy or blame the data - Abandoned perfectly circular orbits
Used a special shape - called an ellipse (type of oval)
Kepler’s Laws of Planetary Motion
Kepler’s 1st Law of Planetary Motion
Each planet orbits the sun in an ellipse with the sun at one focus.
Terms to Know:
Perihelion: The point in a planet's orbit where it is closest to the Sun.
Aphelion: The point in a planet's orbit where it is farthest from the Sun.
Semi-major axis: Half of the longest diameter of the ellipse.
Center-to-focus distance: The distance from the center of the ellipse to one of its foci.
Eccentricity: A measure of how much an ellipse deviates from a perfect circle.
Eccentricity of 1 (straight line) & Eccentricity of 0 (perfect circle)
Calculation of Eccentricity: e = \frac{\text{center-to-focus distance}}{\text{semi-major axis}}
Kepler’s 2nd Law of Planetary Motion
A planet moves faster in the part of its orbit nearer the Sun and slower when farther from the Sun.
Sweeping our equal areas (A) in equal times (t): \frac{A}{t} = \text{constant}
Kepler’s 3rd Law of Planetary Motion
Planets farther from the Sun orbit at slower average speeds than planets closer to the sun.
Obeying the mathematical relationship:
p^2 = a^3
p is the orbital period of the planet (time to complete one orbit)
a is the semi-major axis (or average distance from the sun)
Kepler’s Laws of Planetary Motion are far more accurate than the Ptolemaic Model. Describes the motion but does not explain why the planets behave that way.
Galileo - Answering the Remaining Objections
There were still strong objections to the heliocentric model regardless of Kepler’s Success.
Three Basic Objections:
Earth cannot move because objects would be left behind as Earth moved.
Like feeling a continuous wind on Earth
Non-circular orbits contradicted the idea of heavens being perfect.
Realm of heavens: Sun, Moon, Planets, and Stars
Must be perfect and unchanging
No detection of Stellar Parallax
Should be observed if Earth revolved around the Sun
And the fixed stars were not that far away
Galileo Galilei (1564 - 1642) - Italian Mathematical and Astronomer
Answers to Three Objections:
Overturned Aristotelian view of physics (heavier objects fall to the ground faster than lighter objects).
Create experiments with rolling balls down a ramp (or incline)
Argued that moving objects would remain in motion unless a force acted to stop it
Also known as Newton’s 1st Law of Motion (or the Law of Inertia)
Therefore, all objects on Earth share Earth’s motion
And stays with the Earth rather than falling behind (or feeling a continuous wind)
Heavenly Perfection
Tycho Brahe: Observed a new star (supernova explosion) - Proved comets lie beyond the moon (not earthly objects but belonged as heavenly objects) - Heavens could undergo changes
Galileo: Built telescope (late 1609) - more powerful than the naked-eye - Observed sunspots on the sun (imperfections) - The Moon had mountains & valleys - Noticed shadows near the moon’s Terminator (line separating illuminated and dark sides of the moon)
Kepler: Elliptical orbits and not perfect circles
Absence of Observable Stellar Parallax
Tycho’s data (naked-eye observations) - precision not enough to observe
Needed to prove distances were much, much farther away
Galileo’s telescope: Resolved individual stars of the Milky Way - The number of stars in the Milky Way were distant stars - Further than Tycho anticipated - Fours moons (companion stars) orbiting Jupiter and not the Earth (Earth Is Not the Center of Everything) - Phases of Venus: Galileo observes phases just like the moon:
New Moons, Crescents, Quarter Moons, Gibbous, and Full Moons
The Ptolemaic Model only allows for Crescents and New Moons
The Heliocentric Model allows for all phases as observed by Galileo - Conclusive evidence supporting the heliocentric model
Newton - The Revolution Concludes
Kepler’s Laws Described the Motion of the Planets with Elliptic orbits w/ varying speeds - Accepted - But did not know why
Issac Newton (1642 - 1727) - English Physicist:
Invented Calculus
Published Book (1687): “Principia”
Precise mathematical description of motion - Explained and discovered many fundamental physics principles
Newton’s Three Laws of Motion:
1st Law - Law of Inertia (Galileo)
2nd Law - Net Unbalanced Force on an Object Causes Acceleration of that Object
3rd Law - Action-Reaction Law - Forces Exist in Pairs - Kepler’s Laws describe only the motion of planets around the sun - Newton’s Laws explain the motion of all objects
Newton’s Law of Universal Gravitation
Newton’s Law of Universal Gravitation - explains why Kepler’s Laws Describing the motion of planets works.
Created a new model for the inner workings of the universe
Motion governed by clear laws & force of gravity
Explained the motion of objects on Earth and the Heavens
Combined Earth and Heavens as a Universe (One-verse)
Destroyed the Geocentric Model of the Universe
Looking Back at Revolutionary Science:
Copernicus - De Revolutionibus (1543)
Newton - Principia (1687)
Radical Change in Human Perspective:
The Earth shifted its central role to just another planet.
Changed perspective on how to acquire knowledge
No longer tolerating inaccuracies in predictions
Models of nature reproducing observations
Importance to ask why nature works (not just how)
Past Thinking:
Focused on cultural aesthetics
Acceptance even w/o evidence
After Newton:
Guessing is no longer good enough
Hard evidence / Consistent w/ observations
Rigorous mathematics
Convincing colleagues through clear logic and debate
Closer to understanding the nature of the universe
How did the Copernican revolution alter the ancient debate on extraterrestrial life?
Recall Ancient Greek Debate:
Atomists (many worlds) vs. Aristotelians (one unique world / central everything falls toward center)
Copernican Revolution
Aristotle was wrong / Heliocentric view of the universe
Heavens are just other worlds, not just dots in the sky
Does not necessarily mean Atomists are right
Galileo: Speculated the moon was covered with land and water like Earth
Kepler: Suggested the moon had an atmosphere & was inhabited by intelligent beings - Science fiction story - called Somnium (The Dream”)
Giordano Bruno (Dominican Friar & Philosopher):
Convinced of Extraterrestrial Life - Battles with Authorities - Burned at the Stake
William Herschel (1738 - 1822) w/ sister Caroline:
Discovered Uranus - Assumed all planets were inhabited
Percival Lowell (1855 - 1916):
Discovered Pluto (PL may be his initials) - Believed he saw canals on Mars - Popularized the idea that Martians might want to invade Earth
It is possible to argue endlessly as long as there is no evidence to support your arguments.
2.4 The Fact and Theory of Gravity
Important to like in the universe
If there were no gravity - life would float away - Planets could not have formed - Life couldn’t form
How does the fact of gravity differ from the theory of gravity?
Fact of Gravity:
Objects do fall to the ground - Planets really do orbit the Sun
Theory of Gravity:
Used to explain why gravity acts as it does
Many Theories Improved Over Time:
Aristotle’s Theory of Gravity (claimed heavier objects fell to ground faster that lighter objects)
Galileo’s Theory of Gravity (all objects fall at same rate - if no air resistance)
Newton’s Law of Universal Gravitation
The cause of an apple falling from a tree is the same cause as the moon orbiting earth (recall orbital cannon thought experiment).
Earth and Heavens governed by a single principle - a universe
Newton’s Law of Universal Gravitation
Every mass attracts every other mass through the force of gravity.
The strength of the gravitational force is directly proportional to the product of the masses.- FG \propto m1 \text{ and } m_2
The strength of the gravitational force between two objects increases or decreases as the square of the distances between them- F_G \propto \frac{1}{d^2} (Inverse Square Law)
All three statements combined:
FG = G \frac{m1 m_2}{d^2}
G = 6.67 \times 10^{-11} \frac{Nm^2}{kg^2}
Explained Galileo’s observations of falling objects - Explained Kepler’s Law of Planetary Motion
Edmond Halley (British Astronomer):
Predicted when a specific comet would return to earth - Returns every 76 years (Halley’s comet) using Newton’s Law
Urbain Leverrier (French Astronomer):
1846 - strange behavior of Uranus’ orbit - Hypothesized an 8th planet - Neptune
Johann Galle (German - Berlin Observatory):
Observes Neptune w/in 1 degree of where Leverrier predicted (using Newton’s Law)
Applies to Not just Sun and Planets, also applies to:
Orbits of extrasolar planets around their respective stars
Stars around the Milky Way Galaxy
Galaxies orbiting each other
Moons orbiting planets
Artificial satellites orbiting the Earth, etc,…
Problems with Newton’s Law of Universal Gravitation:
Discrepancy with observations of Mercury’s orbit (only observed with Mercury)
Leverrier hypothesized another planet closer to the sun - he called Vulcan (never found)
Newton never could explain spooky "action at a distance”
Distant masses never in contact w/ each other - how is a force then applied?
Einstein’s General Theory of Relativity
Einstein’s Solution:
Action at a Distance - assumed all objects reside in something called spacetime
4-Dimensional Spacetime (x, y, z, ct) - “Fabric”:
Where x, y, and z are space dimensions
And t is a time dimension, but ct can also become a spatial dimension.
Recall c is the speed of light (\approx 3.00 \times 10^8 m/s) & nothing can travel faster than c.
2-D analogy:
Bowling ball on a mattress - bowling ball causes an indentation on the mattress
Called a Gravitational Well - an object moving near the indentation will be deflected or trapped inside (like a planet orbiting the sun)
Action at a Distance - gravity arises from the way mass affects the structure of the spacetime “fabric”
Gravity is “curvature of spacetime”
Newton’s Law of Universal Gravitation still works in general but not with extreme conditions - when gravity is very strong (like being too close to the sun). Otherwise, one can still just use Newton’s Laws.
In the future, a better theory than Einstein’s Theory of General Relativity may be discovered - people are currently working to do this - Need to find out what happens with:
high-density blackholes
infinitely small spaces (quantum mechanics)
Cosmic Calculations 2.1: Kepler’s Third Law (Examples 1 & 2)
The Universe is Really Big (Worksheet) - Conversion Factors and Scale Factors
Stellar Motion
Apparent Motion
Circumpolar Stars, Polaris (North Star)
Seasons, Path of the Sun in Sky, Defining the Calendar - Summer Solstice, Winter Solstice, Equinox
Nightly Motion (Rotation of Earth)/Annual Motion (Orbit Around Sun)
Absolute Motion
Doppler Effect/Red Shift/Blue Shift
Distance to Stars
Light-year/Parallax