Download Phys 214. Planets and Life - Department of Physics, Engineering

Phys 214. Planets and Life
Dr. Cristina Buzea
Department of Physics
Room 259
E-mail: [email protected]
(Please use PHYS214 in e-mail subject)
Lecture 2. The possibility of life beyond Earth
(Page 1-32)
January 9
Phys 214. Planets and life
Textbook required
“Life in the Universe” Second Edition 2007
By Jeffrey Bennett & Seth Shostak
Other reading resources:
1. Astrobiology: A Multi-Disciplinary
Approach (2004) by Jonathan Lunine
2. An Introduction to Astrobiology (2004)
by Iain Gilmour, Mark A. Sephton
3. Planets and Life: The Emerging Science
of Astrobiology (2007)
by Woodruff T. Sullivan & John Baross
Planets and Life
•
Multidisciplinary study of
the origin, distribution, and
evolution of life
(astrobiology).
The disciplines:
• Astrophysics
• Geology
• Planetary sciences
• Biology
Questionnaire to asses your interest levels for various topics. During the
next 34 lectures I will try to accommodate your preferences allowing
more time for the highest ranked subjects.
A Universe of Life
Textbook pages 1-32
•
•
•
The possibility of life beyond Earth
How does astronomy, planetary sciences, and biology help us understand the
possibilities for extraterrestrial life
Places to search
•
•
•
•
•
•
•
•
•
The ancient debate about life beyond Earth
The Geocentric Model
Explaining Retrograde Planetary Motion
Ptolemaic Model
Aristarchus (heliocentric) model
Kepler - a Successful model of Planetary Motion
Galileo – proving the Earth is not the centre of everything
Newton’s three laws of motion
The revolution in science
The possibility of life beyond Earth
The portrayal of most aliens in movies and
on TV as being humanoid is probably
unrealistic because the human form is
most likely a result of the particular
conditions and events that occurred on
planet Earth.
•
•
Extraterrestrial life could be similar
to life on Earth or might be
completely different.
Extraterrestrial life is defined to be
any kind of life beyond Earth
Most important branches in the study
of life in the Universe are:
Astronomy, Planetary science,
Biology.
Astronomy
Astronomy shows that the Earth is just
one planet orbiting an ordinary star
in a vast universe.
Astronomy shows that the fundamental
laws of physics are the same
everywhere in the universe.
Planetary science
•
Planetary science predicts that planets around
other stars should be common.
-
how planets are formed
how planets work
(why Venus is so much hotter than Earth, why the Moon
is barren even if it is at the same distance from the
Sun as Earth)
-
what is a habitable world
A habitable world is defined to be a world that has
conditions suitable for life.
The fact that the life on Earth seems to have
appeared quite rapidly suggests that life can
arise on most habitable worlds.
Biology
-
The laws of physics and chemistry
are universal
Could biology be also universal?
The molecular building blocks of
life have been found on the
Earth, in interstellar clouds, meteorites.
Biology tells us that life on Earth can survive over a
wide range of environmental conditions.
Places to search for life
-
On Earth
Our Solar System (8 planets, dwarf planets, >150 moons, asteroids, comets)
looking for a liquid (water, methane)
Eris (Xena) 2005 - distance from the Sun is 96.7 AU, roughly three times that of Pluto.
The recently discovered Eris (Xena) is slightly larger than Pluto.
Places to search for life
Among the planets, Mars is the most
likely place to find evidence for
life either now or in the past.
If life exists on Mars today it will
most likely be found beneath the
surface.
Places to search for life
On the moons of Jovian
planets.
Europa might have all the
conditions needed for both
life to arise and life to
survive.
Ganymede and Callisto show
some evidence of
subsurface oceans, with
less evidence for energy
availability.
Strongest evidence for the
existence of a subsurface
ocean of liquid water
points out towards Jupiter’s
moon Europa.
Places to search for life
Saturn’s moon Titan
- the only moon with substantial atmosphere
- too cold for surface liquid water (may have
water underground)
- has liquid methane
Titan
Evidence of subsurface liquids (including liquid
water) on Saturn’s moon Enceladus and
Neptune’s moon Triton.
Triton
Enceladus
Places to search for life
Searching among the stars is more difficult incredible distances to the stars.
Pioneer 10 took 21 month to reach Jupiter
(628 million km from Earth).
The closest star - Proxima Centauri is
roughly 4.2 light years from Earth, is
70,000 times farther away than Jupiter.
A trip to the closest star would take more
than 100,000 years
Searching for life with telescopes (extrasolar
planets and their spectral signature).
Earth-size planets detectable by 2010.
Search for extraterrestrial intelligence SETI
(civilizations might broadcast signals we
could detect with radio telescopes).
The new science of astrobiology
The study of life in the universe is best described by the term
astrobiology (NASA).
Other names used: exobiology, bioastronomy
The goal of astrobiology is to:
1) discover the connection between life and the places it is
found
2) Look for such conditions on other planets and moons in our
solar system and around other stars
3) Look for the actual occurrence of life elsewhere.
According to modern views of our place in the universe, life
elsewhere may be common.
The ancient debate about life beyond Earth
The possibility of extraterrestrial life was first considered many thousands of years
ago during ancient times (at least 2300 years ago by the Greeks).
For many thousands of years the Earth was believed to be a flat, motionless disk
and the sky was a dome where heavenly objects moved.
In order to understand the possibility of life beyond Earth, our ancestors had first
to understand Earth as a planet, its place in the Solar system, and the Universe.
Many civilizations made detailed astronomical observations:
- The Chinese kept astronomical observations beginning 5,000 years ago
- Babylonians kept written records since 2,500 years ago, predict eclipses.
- Mayans observed the cyclical nature of time.
However were not interested in constructing physical models explaining their
observations.
Early Greek Science
The Greeks were the first to use methods we called today Modern Science.
The basis of Modern Science
1) The Greeks tried to understand nature without resorting to supernatural
explanations. The philosophers worked together, debating and testing each
other`s ideas – feature of the modern science of challenging every new idea.
2) Greeks developed mathematics in the form of geometry.
Today, mathematics is a tool in exploring the implications of a new idea.
3) Greeks understood that an explanation about the world is correct if it agrees
with the observed facts.
Greeks used all three above ideas and created MODELS of nature.
A scientific model is a conceptual representation for explaining and predicting
phenomena.
Even a failed model can be used in building a more accurate one.
The Geocentric |Model
Anaximander (610-547 B.C.)
The heavens form a complete sphere
(celestial sphere) around Earth –
Geocentric Model.
Greeks believed the Earth was round as early
as 500 B.C.
Pythagoras (560-480 B.C.)
Motivation for adopting a spherical Earth –
philosophical.
Sphere is geometrically perfect.
Aristotle – cited observations of Earth’s
curved shadow on the Moon during a
lunar eclipse as evidence for a spherical
Earth.
Explaining Retrograde Planetary Motion
While the patterns of constellations seem
not change, the Sun, Moon, and the 5
planets visible with the naked eye
(Mercury, Venus, Mars, Jupiter, and
Saturn) gradually move among the
stars.
While the planets usually move eastwards
compared to constellations, sometimes
they reverse course and go westwards
(backwards) – apparent retrograde
motion.
This observation was very difficult to
account by Greeks, who were ruled by
the idea of heavenly perfection (Plato)
– all heavenly objects move in perfect
circles.
Composite of photographs of Mars between
June and November 2003.
The Sun and Moon move in ways easier to
comprehend. Te planets (Mars, Jupiter)
have much more complicated motions.
Apparent Retrograde Motion
Ptolemaic Model
Claudius Ptolemy
(100-170 A.D.?)
Ptolemaic model – explains retrograde
motion by having all planets more
around Earth in small circles that
turned around larger circles.
The model worked so well that
remained valid for the next 1500
years.
Aristarchus (heliocentric) model
Aristarchus (310-230 B.C.)
Aristarchus suggested that the Earth goes
around the Sun, and not viceversa.
The heliocentric model was rejected because it
did not account for the Greeks
experimental observations.
If the Sun is the centre, Earth would be closer
to different parts of the celestial sphere at
different times of the year.
This would create annual shifts in the position
of the stars – not experimentally observed
by the Greeks.
This meant that either Earth is at the centre of
the Universe, or the stars are very far
away.
Stellar parallax – apparent shift in
position of nearby stars as the Earth
moves around the Sun.
Greeks argue about life beyond earth
Thales (423-348 B.C.) – the world consists of water, Earth
floating on an infinite ocean.
His student Anaximander, suggested a mystical element
“apeiron”, meaning infinite. All materials come and return
to apeiron, all world are born and die repeatedly. Through
this idea he suggested that other Earths and other beings
might exist at other times.
Other Greeks stated that the world is build from four elements:
fire, water, earth, and air.
-
Atomists – argued that the heavens are made of an infinite
number of indivisible atoms of the each of the four
elements;
-
Aristotelians believed that the four elements were found on
Earth while the heavens were made of a fifth element aether.
Greeks ideas around the world
Greeks ideas gained influence in the ancient world due to politics and war.
Around 330 B.C., Alexander the Great expanded the Greek Empire through
the Middle East and built the Library of Alexandria, destroyed in the fifth
century.
During Dark ages of Europe, building on the knowledge of the Greek
manuscripts, scholars in the new intellectual centre in Baghdad developed
mathematics, algebra, instruments and techniques for astronomical
observations.
When the Byzantine empire fell in 1453, many scholars headed west to
Europe, leading to Europe’s Renaissance.
Copernican Revolution
Nicholas Copernicus (1473-1543)
Copernicus revived Aristarchus’ idea of a Sun-centred solar
system and described it mathematically, starting the
Copernican Revolution.
Copernican model did not become popular within the next 50 years,
because it had many flaws, among which the perfect circular
motion of heavenly bodies, forcing him to use incorrect
assumptions (circles on circles motions, much like those of
Ptolemy)
Tycho’s observational data
The lack of experimental quality data was necessary to improve
either the Ptolemeic or Copernican model.
During that time the telescope had not yet been invented, and the
existing naked-eye observations were not accurate.
Tycho Brahe (1546-1601) danish nobleman and eccentric genius
built large naked-eye observatories.
Over three decades Tycho made detailed measurements of the
motions of the planets within a minute of 1 arc (1/60 of one
degree).
The discovery of a supernova by Tycho in 1572 contradicted the
commonly held belief that the universe was unchanging.
Tycho couldn’t explained the observed data, so he hired a German
astronomer Johannes Kepler to explain it.
Kepler- a Successful model of Planetary Motion
Johannes Kepler (1571-1630) believed that planetary orbits
should be circles, so he created a model able to explain
Tycho observations. For planet Mars, Kepler’s predicted
position differed from Tycho’s observations by 8
arcminutes (one fourth of the angular diameter of the full
Moon).
“If I had believed that we could ignore these eight minutes, I
would have patched up my hypothesis accordingly. But
since it was not permissible to ignore, those eight minutes
pointed the road to a complete reformation in astronomy.”
Kepler abandoned the idea of perfect circles and suggested
planetary orbits as ellipses.
He later developed Kepler’s laws of planetary motion.
Kepler’s laws of planetary motion
Kepler’
Kepler’s first law: The orbit of each planet about the Sun is an ellipse
with the Sun at one focus.
The planet is closest to the Sun at perihelion, and farthest at aphelion.
Planet’s average distance from the Sun is called semimajor axis.
Kepler’s laws of planetary motion
Kepler’s second law: As a planet moves around its orbit, it
sweeps out equal areas in equal times.
The planet moves fastest in its orbit when it is at perihelion.
Kepler’s laws of planetary motion
Kepler’s third law: More distant planets orbit the Sun at slower
average speeds, obeying the precise mathematical
relationship
p2 = a 3
where p is the planet’s orbital period in years, and a is the
average distance (semimajor axis) from the Sun in
astronomical units (AU).
!
1 AU = Earth’s average distance
from the Sun ~ 149.6 million km
Kepler published his laws
between 1610-1618.
His model predicted accurately
planetary motion.
Galileo – answering remaining objections
Galileo Galilei (1564-1642)
Galileo Galilei built a telescope and obtained the
first observational evidence suggesting the
Earth moved about the Sun.
He showed imperfections in heavens: spots on the
Sun, mountains on the Moon, contradicting
the common belief that the Heavens were
perfect.
He showed imperfections in heavens: spots on the
Sun, mountains on the Moon, contradicting
the common belief that the Heavens were
perfect. Galileo observed 4 moons orbiting
Jupiter, not Earth.
Galileo observed 4 moons orbiting Jupiter, not
Earth.
Galileo – proving the Earth is not the centre of everything
Galileo observed Venus goes through phases in a way that proved its orbits
the Sun and not the Earth.
With the proof from Galileo, Kepler’s model of planetary motion was
unanimously accepted by 1630, however no one understood why planets
moved in elliptical orbits with varying speeds, until Sir Isaac Newton.
Newton’s three laws of motion
Sir Isaac Newton (1642-1727) invented calculus and used
it to explain many fundamental principles of physics.
He also proved Kepler’s laws are natural consequences
of the laws of motion and gravity.
Newton’s laws are general and apply to any motion, while
Kepler’s laws apply only to planetary motion in the
solar system.
The revolution in science
The scientific revolution started by Copernicus in 1543
and continued by Kepler and Newton caused a
radical change in human perspective on our place
in the universe.
Earth is not regarded anymore as the centre of the
universe, but just one of the many words.
Copernicus
Newton
The science was not led by aesthetics anymore, perfect
circles, indivisible atoms, and guessing was no
longer good enough.
Experimental evidence backed up by rigorous
mathematics was required for a model to be
accepted.
Kepler
The ancient debate about extraterrestrial life?
Democritus (470-380 B.C.) argued that both Earth and
Heaven were created by random motion of infinite
atoms. Because of the infinite number of atoms,
one can assume other worlds similar to ours exist.
A later atomist, Epicur (341-270 B.C.) writes:
“There are infinite worlds both like and unlike this
world of ours… we must believe that in all worlds
there are living creatures and plants and other
things we see in this world.”
Aristotle differed in his opinions, rejecting the atomist
idea of different worlds. Each of the four elements
had its own motion and place (Earth moves
towards the centre, fire moves away from the
centre. It there was more than one world it should
be more than one natural place for the elements to
go, being a logical contradiction. “The world must
be unique… There cannot be several worlds”.
The ancient debate about extraterrestrial life?
Galileo suggested than the lunar features he saw with
his telescope might be land and water, much like
on Earth.
Kepler went further, suggesting the Moon had an
atmosphere and was inhabited by intelligent
beings. He even wrote a science fiction story,
“Somnium” (The dream) describing lunar
inhabitants.
Giordano Bruno (1548-1600) was convinced in the
existence of extraterrestrial life. In his book “On
the Infinite Universe and Worlds” he wrote
“[It] is impossible that a rational being … can imagine
that these innumerable worlds, manifest as like to
our own or yet more magnificent, should be
destitute of similar or even superior inhabitants”.
The ancient debate about extraterrestrial life?
William Herschel (1738-1822) and sister Caroline, codiscoverer of planet Uranus, assumed that all
planets were inhabited.
Percival Lowell (1855-1916), a rich Harvard graduate
documented canals on Mars from his observatory,
believing they are constructed by Martian
civilization.
Next Lecture
•
Movie + Lecture 3. Exercise on Kepler’s laws (Page 28-30)
Part III: Carl Sagan’s Cosmos - Keplers laws - The harmony of the world.
Episode 8 (35 min)