Differentiation, Radioactive core metals, Iron, 7 October 2013!Differentiation occurs when material in space heats up to the point that it 1.melts into a liquid, and all the different materials separate into layers based on their relative densities. !Differentiation is important because it tells how almost everything in the A.universe is structured. It is also a straightforward process that you already understand, though probably not in the context of astronomy. !The past lectures have been about objects that did not differentiate, but a.most objects do.!How objects heat up: condensation, sunshine, impact, radioactive decay, tidal B.stress, magnetic induction!Condensation and Sunshine happen everywhere, all the time and in a a.large amount. !Impact has a large impact, but it happens sporadically. !b.Impacts bring in a lot of heat suddenly, which can dissipate over days, 1.months, or years. !Though another impact may happen in a million years, a lot of heat A.can be generated from sporadic impacts. !Radioactive decay definitely happens, but it is rarely mentioned because c.its rate has not been firmly established. !Radioactive decay is when a substance changes into a more stable 1.element by releasing particles of energy. !The half life of a radioactive substance is the amount of time for A.that substance to be reduced to half its original amount due to radioactive decay. !Since 30 years ago, most isotopes have been well rated (their half lives 2.are well known).!There are continuing disagreements about the decay rate of aluminum 3.26.!Everyone agrees this starts out abundant, decays, and creates heat A.as it decays. Everyone will also tell you what the decay rate is, but you will get different answers from everyone. !In historical astronomy, puzzles that persist for this long mean a.that there must be some unknown factor that is affecting the results. !A lot of results in the coming unit depend on amount of heat, and a B.significant factor in amount of heat is how fast or how slow aluminum 26 cools off.!The rates of all of the other categories have good numbers (they are 4.well estimated). !Note: Like radioactive decay, insulation as it applies to the A.temperature of objects also lacks good numbers:!The more insulation an object has, the slower it will cool off.!a.Numbers about the insulative properties of surfaces or near b.surfaces of celestial objects are mostly guess work and arm waving. !Tidal stress and Magnetic induction both have negligible affects, so they d.are rarely invoked. !These occur mostly in the moons of Jupiter which are embedded in 1.Jupiter's magnetic field. !The melting point for the components in most objects in the solar system C.is around 1100 degrees celsius, although there are some substances that don't melt until around 4000 degrees celsius. !These numbers vary based on other factors, so it is really more like a.1100 degrees celsius plus or minus 80 degrees. !For example, internal components will melt faster than external 1.components because they are under greater pressure.!As soon as materials melt, they undergo differentiation.!b.The key principles of differentiation: Dense stuff sinks. Light stuff floats. !D.The reason for differentiation happening is gravity. !a.Light stuff is at the top of the period table (gases) and dense stuff is at b.the bottom of the periodic table (metals).!An example of differentiation that almost everyone encounters is the c.differentiation of herbs, peppers, oils, and cheeses in italian salad dressing that is left to settle for a while. !Oils are less dense than the other ingredients and float to form a 1.layer at the top. Pepper forms a layer at the bottom. !Density is mass divided by volume.!d.It is given in grams per milliliter.!1.Water has 1 gram per milliliter density.!2.The process of differentiation goes at different rates for different E.substances at different temperatures. !Pressure could also be a factor if the liquids are compressible, but a.astronomers don't know if any liquids in planets are compressible yet.!Typical (relative) rates of differentiation: !b.Water, CO2, Methane, and Ammonia all migrate upwards quickly in 1.the form of steam. !Rocks which have densities between 2 and 4 will migrate upwards, but 2.slower than metal migrates downwards. !Iron and nickel who have densities of 7.8 migrate downwards quickly.!3.There are probably some heavy metals with densities as high as 22 4.which would plunge to the center of a planet even faster than iron and nickel. !Rates do not matter too much because objects in the solar system stay c.hot for a millions of years. They have plenty of time to differentiate. !Differentiation occurs in every object in the entire universe that ever F.melted: !Even if an object later freezes, it will still be frozen with its differentiated a.layers intact. !This means that every object that melted has a core of its densest b.materials surrounded by layers of increasingly less dense material.!The proportions of each type of material may be different though:!1.For example:!A.The structure of the moon is off centered by about 3 a.kilometers, but it has progressively thinning layers around a denser core. !Mercury has a proportionally larger core than the earth, but the b.layered structure is still the same.!The differentiated structure of the earth: !G.The inner core is made of heavy metals.!a.Some astronomers suggest that the inner core is solid, but no one 1.knows that.!The outer core is iron and nickel.!b.This iron core prevents the radioactive core metals at the center of 1.the earth from irradiating its surface:! The most radioactive elements are also the densest elements (they A.are at the bottom of the periodic table). They sunk to the center of the earth. Though they generate heat and dangerous ionizing radiation, they are wrapped in a less dense iron core which lets the heat through, but absorbs all the ionizing radiation. This means that organisms on earth aren't subject to this type of radiation. !There is some natural radiation on the surface, but it comes a.from lighter elements like aluminum 26. !The cores are surrounded by a mantle made of olivine rock!c.The crust is the outermost layer. !d.This is the only layer of this planet that is provably solid, though it is 1.the least dense. !The crust is generally formed of basalt and granite type rocks: !2.Basalt type rocks have an average density of 3.6 and are found on A.volcanic zones and the ocean bottoms.!Granite
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