November 10, 2016
AST 101: Professor Freeman
Lecture: The origin of the Solar System
Textbook Pages: 151-154
Lecture Tutorials: n/a
∙ How did it get to be this way?
∙ Where we’ve come from:
o How the Sun and the solar system formed
o How the planets formed, the history of Earth, and how we know it
o The special role of atmosphere – the greenhouse effect and climate change o The rest of the Solar System: what else is out there, and what might live there ∙ Where we’ve been and where we’ll go
o Travel to the moon: the current state of spaceflight
o How we might get to the stars
Don't forget about the age old question of How do you use conjunction in a sentence?
o …and what we might find living there once we do
∙ Deducing the origin of the Solar System: what do we have to work with?
∙ What patterns do we see?
o Think about understanding oddballs
o In the inner solar system:
▪ An enormous hydrogen/helium star, with trace elements, at the middle If you want to learn more check out What is a descriptive field study design called?
▪ Four small, rocky planets around it, including our own If you want to learn more check out How did this person, group, event, institution, or concept influence immediate historical events and longer-term developments?
∙ No large moons here, except Earth’s
∙ Mercury: 300K
∙ Venus: 700K, huge solar day, sidereal day is as long as a year
∙ Earth: cooler than both at about 287 K
∙ Mars: like Earth, but with no atmosphere, a little smaller
o Phobos and Deimos are moons (Mars), but they’re big asteroids
o In the outer solar system
▪ Large “gas giant”/Jovian planets
▪ Mostly hydrogen and helium, hydrogen compounds
▪ Thick atmospheres
▪ Get successively colder
▪ Many moons (the number of moons, respectively: 67, 62, 27, 14)
∙ Eris Don't forget about the age old question of What are the interest rate formulas?
o Even further out:
▪ The Kuiper belt
∙ Lots of small, icy bodies (Pluto and Eris among them)
∙ Orbit roughly along the plane of the solar system
▪ The Oort cloud
∙ Contains trillions of comets
∙ More distant than the Kuiper belt
∙ Roughly spherical
∙ Organized motion of the planets (nearly circular orbits, most in the same direction). Why? o The Solar System formed out of a cloud of gas that collapsed under its own gravity, forming a protoplanetary disc (friction)
o Collapsing proto-solar system wound up with a lot of gas in its center being squeezed together. Nuclear fusion, hey-presto, sun!
o Remember angular momentum: as the disc shrinks, the rotation of the disc speeds up o If the universe contained only hydrogen and helium, where do metals come from? ▪ All stars do contain metals (small amounts) and nuclear fusion in the Sun does build them out of hydrogen and helium, but this doesn’t answer the question entirely. We also discuss several other topics like What is the meaning of trnas in genetics?
▪ Nuclear fusion in earlier stars forged heavier elements out of lighter ones: these stars have since exploded (supernova)
∙ A spinning cloud of gas
o At the center, where the gas is most dense, hydrogen accumulated, until gravity was strong enough to kindle fusion. The Sun was born.
∙ “Why are there lumps in my Solar System?”/What about the planets?
▪ The “lumps”, or planetesimals, will become planets, but how?
∙ Gas and dust couldn’t pull itself together on its own, but rubbing Don't forget about the age old question of What is the meaning of ça va in english?
together gave rise to static cling, and thus, planetesimal, (or a lump that
may grow to become a planet someday) is formed
∙ These in turn form bigger and bigger chunks w/gravity of their own, and planetesimals w/eccentric orbits “crash” into each other, leaving only
circular (or mostly circular) orbits in place
∙ The planets condensed out of bits of dust that first formed by static
electricity, then grew large by gravity. Gas giants were large enough that
they accreted a great deal of gas as well, “stealing” gas from the Sun
o If Jupiter got really, really big, it would start nuclear fusion and a
star would literally be born
▪ This is really common: most stars “come in pairs” or are
∙ Why are there different sorts of planets?
o The primordial nebula contained different constituents which condense at different temperatures
▪ Hydrogen and helium: never condense in nebula (98%)
▪ Hydrogen compounds (water, methane, ammonia): condense at less than 150 K (1.4%)
▪ Rocks: condense at 500-1300 K (0.4%(
▪ Metals: condense at 1000-1600 K (0.2%)
o Further out it is colder, and those hydrogen compounds could condense to form the Jovian planets
o This explains the differences in core compilation.
∙ So here we are…
o But with one more process.
▪ There was a lot of gas left over, even despite absorption
▪ As the Sun began to burn, it began to emit the solar wind, propelled into space by magnetic wind
∙ This swept the solar system clean of gas and dust
∙ So much so, in fact, we could make a spacecraft propelled by a solar
∙ How do we know this is right?
▪ We can see nebulae, protoplanetary discs, and 11 planetesimals
▪ Other cases of this happening at different points in the life story of the
∙ The difference between this and the Big Bang
o The Big Bang was an immense amount of matter and energy when, while expanding and cooling, formed ions which absorbed and emitted photons, and the universe became atomically neutral and transparent, and light and matter could escape
▪ Short wave primordial radiation got longer, echoes of Big Bang still exist (at 2.73 K), as microwaves
▪ Thermal radiation curve is some of strongest evidence for the Big Bang
▪ 2.73 K: the temperature of space, but it varies a smidgen; not quite the same in all directions, a galaxy here, not there (400 millionths of a degree)
o What features of our solar system provide clues to how it formed?
▪ Patterns of motion among large bodies – The Sun, planets, and large moons generally orbit and rotate in a very organized way
∙ All planetary orbits are nearly circular and lie nearly on the same plane
∙ All planets orbit the Sun in the same direction; counterclockwise as
viewed from high above Earth’s North Pole
∙ Most planets rotate in the same direction in which they orbit, with fairly small axis tilts. The Sun also rotates in this direction.
∙ Most of the solar system’s large moons exhibit similar properties in
their orbits around their planet’s equatorial plane in the same direction
that the planet rotates.
∙ The Sun, planets, and large moons orbit and rotate in an organized way. ▪ Two major types of planets – The eight planets divide clearly into two groups: the small, rocky planets that are close together and close to the Sun, and the large, gas-rich planets that are farther apart and farther from the Sun
∙ Small, rocky planets close to the Sun: terrestrial planets
∙ Large, gas-rich planets far from the Sun: Jovian planets
▪ Asteroids and comets – Between and beyond the planets, vast numbers of asteroids and comets orbit the Sun; some are large enough to qualify as dwarf planets. The locations, orbits, and compositions of these asteroids and comets follow distinct patterns
∙ Asteroids – rocky bodies that orbit the Sun much like planets, but they
are much smaller. Even the largest asteroids are much smaller than our
Moon. Most known asteroids are found within the asteroid belt
between the orbits of Mars and Jupiter.
∙ Comets – small objects that orbit the Sun made largely of ices (water
ice, ammonia ice, and methane ice) mixed with rock with highly
∙ Kuiper belt – a donut-shaped region beyond the orbit of Neptune which
contains at least 100,000 icy objects that are more than 100 kilometers
in diameter, of which Pluto and Eris are the largest known
∙ Oort cloud – a second cometary region which is much farther from the
Sun and may contain a trillion comets
▪ Exceptions to the rules – The generally orderly solar system also has some
notable exceptions. For example, among the inner planets only Earth has a large moon, and Uranus is tipped on its side. Venus also rotates “backward”
(clockwise as viewed from high above Earth’s North Pole).
o Nebular theory – the theory that our solar system formed from the gravitational collapse of a great cloud of gas
“4 billion years and a lot of bacteria later, here we are in Stolkin.” – Professor Freeman
November 15, 2016
AST 101: Professor Freeman
Lecture: The history and ages of the planets/The ages of the planets: radiometric dating Textbook Pages: 164-165
Lecture Tutorials: 111-112
∙ How old is the Earth?
∙ Radioactive dating
∙ Summary of last time
o The solar system started from a spinning ball of gas, which collapsed and begins to spin quickly (angular motions/momentum), bits coalesce into planets, solar wind blew away little stuff, leaving only big stuff
o A lot of drastic, quick changes in the beginning, things are still changing (asteroids hitting us, a planet between Jupiter and Mars shattered and became the asteroid belt), but these are relatively small, as we are in a stable state
∙ …How long ago was this?
o The process used for the ages of the planets is the same as the process used for more recent objects
▪ Carbon dating: the use of radioactive decay of carbon to figure out how old things are.
∙ Useful for things up to about 50,000 years old
o We can use the decay of other isotopes to age much older things, though—like planets! ∙ Nuclear physics: neutrons
o Atoms can have varying numbers of neutrons despite the numbers of protons and electrons staying the same
o A “default” carbon has 6 protons and 6 neutrons, or 8 neutrons
∙ Radioactive decay
o Fixed number of protons and electrons
o Chemical properties don’t depend on the number of neutrons
o “Ordinary” carbon, carbon-12
▪ It has six protons and six neutrons, for a total of twelve nucleons in the nucleus o Carbon-14: 6 protons and eight neutrons, for a total of fourteen nucleons in the nucleus o These different forms of elements with different numbers of neutrons are called isotopes
o Many isotopes are radioactive: decay into other isotopes of other elements after some time, eventually reaching stability
o Potassium 40 ???? Argon 40
o Carbon 14 ???? Nitrogen 14
o Uranium 235 ???? Lead 207
o We can characterize how fast they decay by “the half-life”
▪ One half-life: how long it takes for half of the substance to decay. Also a game in The Orange Box.
▪ Carbon-14’s half life: 5,730 years
▪ We can use these decays as a clock
∙ The penny example
o 10,000 pennies, every hour half of the pennies come up heads and are removed, after 1 hour there are 5,000 pennies, after 2 hours there are 2,500 pennies
▪ An average of a lot of random things ???? close to average
▪ Her pennies have a half life of 1 hour
▪ The more pennies she started with, the more accurately we can tell time this way
▪ Important: radioisotopes don’t decay every hour (or year, or whatever); they decay continuously
o There aren’t many of these unstable isotopes around
▪ Some of them, like carbon-14, are continually produced
▪ Some of them, like uranium-235 and potassium-40, are left-over from the supernova that produced them
▪ If we can figure out what fraction of the original amount of a radioisotope is left in an object, we can figure out how long ago it formed
∙ Carbon-14 has a half-life of 5,730 years and is continually produced. Historically, this has been constant, now, it has changed, because CO2 emissions has added only carbon-12, not carbon-14 o Carbon isotopes treated the same by our body
o Nuclear weapons made of plutonium or uranium
o Cosmic rays/solar sycle doesn’t really change
o Will be a problem for people trying to carbon-date us, however
∙ Carbon dating
o Living things constantly recycle their carbon, so their 14C fraction is the same as the atmosphere
o Once they stop breathing and die, 14C is replaced by 14N
o This lets us use the amount of 14C as a clock to see how long ago they died ▪ 1 half-life: 50%
▪ 2 half-lives: 25%
▪ 3 half-lives: 12.5%
▪ 4 half-lives: 6%
o We can use this procedure on things up to 50,000 years old. Past that, the 14C fraction is too small to give an accurate picture. We need some older process to date the planets!
∙ Other radioisotopes
o Longer-lived isotopes could work better
o Potassium-40: half-life of 1.251 Gyr (“gigayears” – billion years). Decays into argon-40 o Uranium-235: half-life of 0.7038 Gyr. Decays into lead-207
∙ Uranium-lead dating
o Crystals of the mineral zircon readily incorporate uranium into their structure, but not lead, while they are forming
o Thus, any lead present in zircon got there through the decay of uranium-235. ∙ Potassium-argon dating
o Argon is a noble gas: doesn’t bond readily
o Any argon-40 present in zircon got there through decay of 40K.
∙ Both rely on different assumptions, so it is good to use both to cross-check. ∙ Let’s date some rocks!
o Oldest Earth rocks: 4 Gyr (a few grains are slightly older)
o Oldest Moon rocks: 4.4 Gyr
o ….can we get anything older than that? What are the most primordial things in the Solar System?
o Some meteorites found date to 4.55 Gyr; the age of the condensation fo the first rocks in the Solar System
o Mars comes out to 3.86-4.56 Gyr b/c of Curiosity
o How come the Moon is older than Earth?
o What happened after everything formed? What different histories led to these differences?
o Radiometric dating – the method by which we measure a rock’s age
▪ Relies on careful measurement of the proportions of various atoms and isotopes in the rock. The method works because some atoms undergo changes with time that allow us to determine how long they have been held in place within that rock’s solid structure.
▪ The age of a rock is the time since its atoms became locked together in their present arrangement, which in most cases means the time since the rock last solidified.
o Radioactive isotope – a nucleus prone to spontaneous change, or decay, such as breaking apart or having one of its protons turn into a neutron. Always happens at the same, measurable rate for any particular radioactive isotope, usually stated in terms of a half-life.