Transcript We especially need imagination in science. It is not all mathematics

We especially need imagination in science. It is not all mathematics,
nor all logic, but is somewhat beauty and poetry.
Maria Mitchell (1818-1889)
Astronomer and first woman elected to the
American Academy of Arts and Sciences
Sites of the Week
• NASA Astrobiology Institute:
• nai.arc.nasa.gov
• Class Web Page
• http://flash.uchicago.edu/~calder/teaching/life05/life
_spring05.htm
• Cassini mission to Saturn:
• http://saturn.jpl.nasa.gov
• Textbook
• www.astonomyplace.com
Class # 1
• Outline of Course/Class business
• Intro to Life in the Universe/SETI (Cosmos)
• Science, scientific discovery, and the
scientific method.
• A Universe of life: Habitable worlds, extrasolar planets, SETI
• History of the Universe
• Tour of Solar System
• Formation of Stars, planets, and Solar System
Outline of Course
• Scientific method, Solar System, Astronomy,
the greats- Kepler, Galileo, etc., habitability,
space exploration
• Life as we know it (DNA, Organic Chemistry)
• Geologic history of Earth
• Origin and evolution of Life on Earth
• The possibility of Life in the Solar System,
Mars, Jovian moons (Titan).
• Nature and evolution of Habitability
• Extra-solar planets
• SETI and the Drake Equation
• Interstellar communication and travel
• Fermi Paradox and implications of Contact.
Class Business
• Syllabus
• astronomyplace.com
• Reading:
• Chapt. 1,
• Chapt. 2
• Homework (Due Feb 11):
• Chapt. 1: Review questions 2, 6, 7, 10, 13,
Problems 3,5,7,14
• Chapt. 2: Review questions 1, 2, 4, 5, 9, 12
Class Business
• This is an exciting time to be studying life in the Universe!
All are encouraged to bring new items to class for
discussion. The instructor will smile!
NASA
ESA, NASA
Aside: Scientific Visualization: an Art
Which best describes coalescing neutron stars?
Scientific Visualization: an Art
Visualization: Dave Bock, NCSA
More info: books by Tufte
Definitions
• Astronomy- The study of the Universe and its contents
beyond the bounds of the Earth’s Atmosphere
• Astrophysics- The physical theory of astronomical objects
and phenomena
• Cosmology- The branch of astro concerned with the origins,
properties, and evolution of the Universe
• Astrobiology- The study of Life in the Universe
• SETI- Search for Extraterrestrial Intelligence
Science and the Scientific Method
• The first thing to note is that there is not agreement on
exactly what the scientific method is!
• The scientific method relies on testing and validation. Any
assertion is tested (or even attacked), and only after
reliable, repeatable experiments demonstrate the assertion
is the assertion accepted.
• Most everything presented on issues related to SETI is
really informed opinion.
• The key, however, is to base those opinions on scientific fact
as much as is possible.
Science and the Scientific Method
• A scientific “fact” begins as an observation. This may be a
discovery of some sort such as the recent Huygens data.
• A hypothesis is an educated guess at an explanation for the
observation.
• An experiment is a test of a hypothesis. Experiments can
easily disprove a hypothesis, but cannot prove a hypothesis.
• Once a hypothesis has been subjected to a battery of
experimental tests, none of which reliably disprove it, then
the hypothesis becomes accepted. We can then consider the
hypothesis a theory.
• Many theories are called laws, but these laws are not always
the most accepted. (Newton’s laws of gravity vs. Einstein’s
theory of general relativity)
Science v. Pseudoscience
• Can an assertion survive scientific scrutiny?
• ``... only provided further evidence that a technological
civilization once existed on Mars and that the artifacts of
that civilization are still visibly present. '' -- P. Gersten,
Formal Action Committee for Extra-Terrestrial Studies
Hallmarks of Science (Text)
• The Universe is inherently understandable, and we can
figure out how it works by observing it and examining the
processes that affect it.
• Science progresses through the creation and testing of
models (hypotheses), and the best models are the simplest.
• A scientific model should be testable so that we can indeed
test its validity and if necessary rethink the model.
Science in a nutshell (Calder)
• The Universe is inherently understandable.
• Scientific knowledge is durable.
• Scientific ideas may change.
• Science explains and predicts.
• Science demands evidence.
• Science may not be able to answer all questions, particularly
why.
A Universe of Life: Questions we want to address
• Are there other habitable worlds?
• Is biology universal?
• Should we expect other star systems to be like ours?
• NASA’s three fundamental questions (nai.arc.nasa.gov/):
• How does life begin and develop?
• Does life exist elsewhere in the Uni?
• What is life’s future on Earth and beyond?
• We begin by studying what we know about the Uni and life in
the Uni.
The Scientific History of the Universe
Address: Earth
•
How would the Universal
post office find us?
Star
A large, glowing ball of gas that generates heat and light through
nuclear fusion
Planet
A moderately large object which orbits a star; it shines by reflected
light. Planets may be rocky, icy, or gaseous in composition.
Moon
An object that orbits a planet.
Asteroid
A relatively small and rocky
object which orbits a star.
Comet
A relatively small and
icy object which
orbits a star.
Solar (Star) System
A star and all the material which orbits it, including its planets
and moons
Galaxy
A great island of stars in space, all held together by gravity
and orbiting a common center
Universe
The sum total of all matter and energy; that is, everything
within and between all galaxies
The Solar System
Distances not
to scale!
• The Sun and the objects that orbit it.
The Layout of the Solar System
• Large bodies in the Solar System have orderly motions
• planets orbit counterclockwise in same plane
• orbits are almost circular
• the Sun and most planets rotate counterclockwise
• most moons orbit counterclockwise
The Layout of the Solar System
•
Planets fall into two main categories
– Terrestrial (i.e. Earth-like)
– Jovian (i.e. Jupiter-like or gaseous)
Mars
Neptune
Terrestrial
Jovian
The Layout of the Solar System
• Swarms of asteroids and comets populate the Solar System
A Few Exceptions to the Rules…
•
•
•
•
Both Uranus & Pluto are tilted on their sides.
Venus rotates “backwards” (i.e. clockwise).
Triton orbits Neptune “backwards.”
Earth is the only terrestrial planet with a relatively large
moon.
The Sun – King of the Solar System
• How does the Sun influence the planets?
• Its gravity regulates the orbits of the planets.
• Its heat is the primary factor which determines the
temperature of the planets.
• It provides practically all of the visible light in the Solar
System.
• High-energy particles streaming out from the Sun
influence planetary atmospheres and magnetic fields.
This streaming of particles is known as the solar wind.
Apply the scientific method to formation of the
Solar system!
• Make observations
about the solar
system
• Hypothesize
• Test or experiment
• Draw conclusions
about the validity of
our hypothesis
• Repeat
What is density?
density = mass/volume
typical units: [ g/cm3]
Density of water is defined as 1 g/cm3.
Definitions
• meteor- a flash of light caused when a particle from space
burns up upon entering our atmosphere
• meteorite- a rock from space that lands on the Earth
• nebula- a cloud of gas in space (usually glowing)
• isotopes- elements with the same number of protons but
differing numbers of neutrons
• Asteroid- a small rocky body orbiting the Sun.
• Comet- a small icy body orbiting the Sun.
• Astronomical Unit (AU)- the average Earth-Sun distance,
about 15O million km (93 million miles)
Observation: Patterns of Motion
• All planets orbit the Sun
in the same direction:
counterclockwise when
seen from above the
Earth’s North Pole.
• All planetary orbits lie in
nearly the same plane.
• Almost all the planets
travel on nearly circular
orbits, with a spacing
that increases with
distance according to a
fairly regular trend.
Patterns of Motion
• Most planets rotate in the
same direction they orbit:
counterclockwise when
viewed from above the
Earth’s North Pole.
• The Sun rotates in the
same direction in which
the planets orbit.
• Almost all moons orbit
their planet in the same
direction as the planet’s
rotation and near the
planet’s equatorial plane.
Patterns of Motion
• Most planets have fairly small axis tilts, usually less than 25º.
Question #1:
Why are the observed motions in the solar system generally so orderly?
Observation: Categorizing Planets
Can we categorize the planets into groups? How many categories do we need?
Categorizing Planets
Terrestrial
Jovian
Smaller size and mass
Larger size and mass
Higher density (rocks, metals)
Lower density (light gases)
Solid surface
No solid surface
Closer to Sun (and closer together)
Farther from Sun (and farther apart)
Warmer
Cooler
Few if any moons and no rings
Many moons and all have rings
Question #2:
Why do the inner and outer planets divide so neatly into two classes?
Observation: Asteroids & Comets
• No formation theory would be complete without an explanation of the most
numerous objects in the solar system: asteroids and comets.
• Asteroids are small rocky bodies that orbit the Sun between the orbits of
Mars and Jupiter, primarily in the asteroid belt.
Observation: Asteroids & Comets
• Their orbits generally lie close to the plane of the planetary orbits, although
they are usually tilted a bit more. Some have quite large eccentricities.
• Almost 10,000 asteroids have been identified; these are probably only the
largest ones. The largest asteroids have a radius of about 200 km - much less
than half of the Moon’s radius.
Asteroids & Comets
• Comets are small, icy bodies residing in one of two
regions, the Kuiper Belt and the Oort Cloud.
Asteroids & Comets
Question #3: Why are
there a large number of
asteroids & comets in two
different locations?
Observation: Exceptions to the patterns
• Some objects don’t fit the general patterns:
• Mercury and Pluto have much larger eccentricities and inclinations.
• The rotational axes of Uranus and Pluto are substantially tilted.
Exceptions
• Venus rotates backwards - clockwise, rather than counterclockwise,
as viewed from above Earth’s North Pole.
• Earth has an exceptionally large moon.
Pluto’s moon is almost as big as Pluto.
• While most jovian moons orbit with
the same orientation as the planet’s
rotation, a few orbit in the opposite
direction.
Exceptions
Question #4: Why are there exceptions to the general patterns?
Four principal characteristics of the Solar System
• Patterns of motion
• Two types of planets
• Asteroids and comets
• Exceptions to the patterns
A theory of the formation of the Solar
System must include these.
Solar nebula theory
• In the last 20 years, a lot of
evidence has accumulated in
support of a model called
“solar nebula theory”.
• Originally proposed by Kant
(1755) and Laplace (~1795)
• This model holds that our solar
system formed from a giant
swirling interstellar cloud of
gas and dust. “Nebula” is the
Latin word for cloud.
Orion Nebula : an active
star-forming region.
Solar nebula theory
• Star systems are born within
interstellar clouds where the
gas is somewhat denser than
1 atom/sugarcube.
• Typical star-forming clouds
contain enough material to
form millions of stars.
• Triggered by a cataclysmic
event such as a nearby
supernova?
Collapse
• An individual star system forms from a small part of a giant interstellar cloud.
We call the piece of cloud that formed our solar system the solar nebula.
• At its start, the solar nebula was probably a few light-years in diameter.
It then began to collapse under its own gravity.
Collapse
• As it collapsed to a diameter of about 200 AU, about twice the diameter of
Pluto’s orbit, three processes gave form to our solar system:
• Heating
• Spinning
• Collisions
Heating
• Heating represents
energy conservation
in action.
• As the cloud shrank, its gravitational energy was converted into the energy of
motion of gas particles falling inwards. These particles crashed into one another,
converting energy of motion into the random motions of thermal energy.
Heating
• The solar nebula became hottest at the center, where much of the mass
collected to form the protosun. The protosun eventually became so hot that
fusion ignited at its core - at which point the Sun became a full-fledged star.
Spinning
• Spinning represents conservation of angular momentum.
mass x distance x speed = constant
• Like an ice skater pulling in her arms as she spins, the solar nebula rotated
faster and faster as it shrank in radius. The spinning helped ensure that not all
of the material of the solar nebula collapsed onto the protosun.
Spinning
• The greater the angular momentum of a rotating cloud,
the more spread out it will be.
Collisions
• Flattening is a natural consequence
of collisions, which is why disks are
so common in the universe.
• A cloud may start with any size or
shape, and different clumps may be
moving in random directions with
random speeds.
• As the cloud collapses these clumps
collide and merge, giving the new
clumps the average of the old speeds.
Collisions
• Thus, random motions in the cloud become more orderly as the cloud collapses,
changing the cloud’s original lumpy shape into a rotating, flattened disk.
• Collisions also reduce their ellipticities, making the orbits more circular.
Collapse of the Solar nebula
• Flattening of the disk explains why all the planets orbit in nearly the
same plane.
• Spinning explains why planets orbit in the same direction, and also plays
a role in making most of the planets rotate in the same direction.
• Collisions explains why most planets have nearly circular orbits.
• Heating, spinning, flattening, and collisions explain the tidy layout of
our solar system, thus answering “why the orderliness”, question #1.
Evidence of nebular collapse
HST
Four dusty protoplanetary disks around young stars in the Orion nebula. The red
glow in the center of each disk is a newly formed star, roughly a million years old.
Each image is a composite of emission lines from ionized oxygen (blue), hydrogen
(green), and nitrogen (red).
Sowing the seeds of planets
• The churning and mixing of the gas in the solar nebula ensured that its
composition was about the same everywhere: 98% hydrogen and helium,
and 2% heavier elements.
• How did the planets end up with such a
wide variety of compositions when they
came from such uniform material?
Sowing the seeds
• The formation of a solid or
liquid particles from a gas
is called condensation.
• Pressures in the solar nebula were so low that liquid droplets rarely formed,
but solid particles could condense like snowflakes condense from water
vapor in our atmosphere. Such solid particles are called condensates.
Sowing the seeds
• The ingredients of the solar nebula condense at different temperatures:
Category
Ingredients
Condensation
Temperature
Amount in Nebula
Metals
Iron, nickel, aluminum
1600 K
0.2%
Rocks
Silicon-based minerals
500 –1300 K
0.4%
Hydrogen
compounds
CH4, NH3, H2O
150 K
1.4%
Light gases
Hydrogen, helium
Never condensed
98.0%
Sowing the seeds
• The temperature differences between the hot inner regions and the cool outer
regions determined what kinds of condensates were available to form planets.
• Near Mercury’s orbit, metal started to condense.
Moving outwards to Venus and Earth, more rock condensed.
• Only beyond the frost line, which lay between the present orbits of Mars and
Jupiter, were temperatures low enough for hydrogen compounds to condense.
Sowing the seeds
• So, the outer solar system contained
condensates of all kinds. Since ice was
nearly three times more abundant, ice
flakes dominated the mixture.
Assembling planetesimals
• Since rocks and metals only made up 0.6% of the material in the solar nebula,
the planetesimals in the inner solar system could not grow very large, which
explains why the terrestrial planets are relatively small.
Assembling planetesimals
• Beyond the frost line planetesimals could be built from ice flakes. Since ice
flakes were more abundant, these planetesimals could grow much larger.
• These large icy
planetesimals
became the cores of
the jovian planets,
the composition of
which is mostly gas.
Solar nebula theory
• Our ideas of the formation of the solar system answer question #2: the general
differences between the terrestrial and jovian planets.
Competing hypothesis: a close encounter
• Early 20th century
• Planets formed from gas pulled from sun in a close encounter
with another star
• Lost the competition! Couldn’t explain observed motion of the
planets or the two classes of planets, Also, a close encounter
with another star is unlikely to have happened.
Leftover planetesimals
• After about 10 million years, there were hundreds of protoplanets in the inner
solar system and a few large ones in the outer solar system.
• About this time our
protosun ignited
hydrogen in its core to
become a real star.
• Young stars tend to
blow strong winds - a
flow of hot hydrogen
and helium gas ejected
in all directions.
Leftover planetesimals
• That wind is much weaker
today, but still with us.
• The strong wind from the
young Sun blew away the
excess gas, but many
planetesimals remained
scattered between the
newly formed planets.
ulysses.jpl.nasa.gov
Leftover planetesimals
• These leftovers became asteroids and comets. Rock and metal ones are inside
the frost line and icy ones outside the frost line.
Eros
Hale-Bopp
Juan Carlos Casado
Solar nebula theory
• The solar nebula theory thus
explains why there are numerous
asteroids and comets and why they
have different compositions, i.e.,
question #3.
May 1996,
Hyakutake
Era of bombardment
• After the young Sun cleared
out most of the leftover gas
from the solar nebula, a
period of consolidation took
place.
• This era of collisions
between planetesimals and
protoplanets would have
resembled a rain of rock
and ice.
Era of bombardment
• These three drawings show the results of a super
computer simulation of the formation of the inner planets.
a) The simulation begins with 100 planetesimals.
b) After 30 million years, the planetesimals have coalesced into 22 protoplanets.
c) After 150 million years, the inner planets are essentially complete.
Era of bombardment
• Impacts were extremely common in the young solar system. They are a normal
part of the accretion process during the late stages of planet formation.
This process has slowed down considerable, but still continues today.
HST 1994
Solar nebula theory
• Random, giant impacts during the era of bombardment are the most promising
explanation for meeting the “why the exceptions?” , question 4.
• Mercury and Pluto have larger eccentricities and inclinations.
• The rotational axes of Uranus and Pluto are substantially tilted.
• Venus rotates backwards.
• Earth has an exceptionally large moon.
• Unfortunately, this idea is hard to prove.
But no other idea so effectively explains the oddities we’ve discussed.
Note: extra-solar planets challenge the theory!
Matt Petersen
Jason Tranchida
Alison McGeary
• The extra-solar planets observed thus far do not seem to
match the pattern of the Solar System. Scientists are trying
to explain this.
Summary of Class #1
•
•
•
•
Class Business
Intro to SETI (Cosmos)
Scientific method
“Observed” the Solar System
• Patterns of motion
• Two types of planets
• Asteroids and comets
• Exceptions to the trends
• Solar Nebula theory
• Collapse
• Types of planets (temperature)
• Solar wind
• Heavy bomardment
• Extrasolar planets- may lead to some re-thinking!