Meteorites A meteor that survives its fall through the atmosphere is called a meteorite Hundreds fall on the Earth every year Meteorites do not come from comets First documented case in modern times was recorded in 1803 Meteorites are discovered in two ways Observed meteorite falls Meteorite finds About 25 per year are found Antarctica is a fertile ground for finding meteorites Ice cap collects over a large area and preserves the meteorites Meteorite Classification Traditionally meteorites have been placed into three broad classes Irons Nearly pure nickel-iron Stones Silicate or rocky Stony-irons Mixture of stone and metallic iron Class Primitive stones Differentiated stones Irons Stony-irons Falls 88% 8% 3% 1% Finds 51% 1% 42% 5% Example of iron meteorite found in Antarctic Antarctic 85% 12% 2% 1% 1 2 A Variety of Meteorite Types Allende carbonaceous meteorite Imilac stony meteorite Fragment of iron meteorite that created Meteor Crater in Arizona Mern primitive stony meteorite Ages and Compositions of Meteorites Meteorites include the oldest and most primitive materials available for direct study Using radioactive dating, the average age of meteorites is between 4.54 ± 0.1 billion years Usually taken as the age of the solar system (4.5 billions years) Meteorites almost certainly originate from asteroids Two famous meteorites (both fell in 1969) Murchison (Australia) Carbonaceous. Contained complex organic molecules, amino acids Allende (Mexico) Contained material older than the solar system Material formed by previous generations of stars 3 4
Observational Constraints on the Formation of the Solar System There are several observation constraints that any theory of solar system formation must explain All the planets orbit the Sun in the same direction and in approximately the same plane Most of the planets rotate in the same direction that they orbit the Sun and most of the moons orbit in the same direction In general, the solar system looks like a giant frisbee There are also exceptions, such as the rotation of Venus that must be explained The giant planets have hydrogen and helium with the terrestrial planets do not There is a striking progression from the inner to the outer solar system from rocky metal dominated planets to ice-dominated planets all the way to the comets in the Oort cloud The Solar Nebula All of the constraints just discussed are consistent with the idea that the solar system formed 4.5 billion years ago out of a rotating cloud of vapor and hot dust called the solar nebula The terrestrial planets were formed from planetesimals Few km to a few 10s of km Still survive today as asteroids and comets Gravitational formation is called accretion Protoplanets were formed and were heated by collisions with planetesimals 5 6 Continued Evolution of the Solar System In the outer solar system, the protoplanets grew much larger Masses 10 times Earth The outer planets retained the gaseous composition of the solar nebula (and the Sun) Jupiter and Saturn especially because of their large size After the first few millions years, protoplanets ruled the solar system but many planetisimals still existed and many cataclysmic collisions occurred The comets were ejected by the gravitation of the large planets Earth may have gotten a large share of it water and organic compounds from comet impacts Stages in the Geological History of a Terrestrial Planet Stages in the Geological History of a Terrestrial Planet Time 7 8
Geological Activity The Moon and Mercury were once geologically active but have been geologically dead for 3.3 billion years Mars was once active but most activity ceased 3 billion years ago Earth and Venus are still active geologically Earth s surface appears to be 200 million years old Venus surface appears to be 500 million years old On the outer worlds we see low temperature volcanism Io is a prime example 9 Elevation Differences The mountains on Mars are higher than the mountains on Earth and Venus 10 km max on Earth and Venus 26 km max on Mars Due to Time to grow upward is different Constant evolution of the crust on Earth and Venus Mountains are erased Lack of evolution on Mars Mountains can grow Force of gravity is different Earth and Venus have three times the gravity of Mars Large mountains on Earth and Venus cannot sustain their own weight 10 Atmospheres The atmospheres of the planets were formed by a combination of gas escaping from their interiors and the impacts of debris from outer space The terrestrial planets must have had similar atmospheres Mercury was too small and too hot to retain its atmosphere The dominant gas is now CO 2 but there was originally CO, NH 3, and CH 2 UV disassociated the hydrogen based gases and the hydrogen escaped Venus and Mars lost their water while Earth kept its water With hydrogen gases and water gone, Mars and Venus were left with CO 2 Life removed the CO 2 from Earth s atmosphere leaving mainly N 2 and O 2 In the outer solar system, only Titan retains its atmosphere Our Sun Our Sun is a rather ordinary star It is not unusually hot or cold It is not unusually young or old It is not unusually large or small The Sun has been shining for 5 billion years and expected to shine for another 5 billion years However, the Sun goes through various cycles Solar activity varies with a period of about 11 years These x-ray images show the change in solar activity from 1991 to 1995 N 2 11 12
Outer Layers of the Sun The Sun is a huge ball of hot gas shining under its own power We can only see the atmosphere of the Sun The Abundance of Elements in the Sun Element Hydrogen (Z=1) Helium (Z=2) Carbon (Z=6) Percentage by Number of Atoms 92.0 7.8 0.02 Percentage by Weight 73.4 25.0 0.20 Nitrogen (Z=7) 0.008 0.09 Oxygen (Z=8) 0.06 0.8 Neon (Z=10) 0.01 0.16 Magnesium (Z=12) 0.003 0.06 Silicon (Z=14) Sulfur (Z=16) 0.004 0.002 0.09 0.05 Iron (Z=26) 0.003 0.14 13 14 The Solar Photosphere The Solar Chromosphere The photosphere is the boundary in the Sun s atmosphere where the Sun becomes opaque Beneath the photosphere, photons are absorbed and re-emitted The photosphere goes from transparent to opaque over a depth of 400 km The temperature of the gases vary from 4500 K to 6000 K and the pressure and density increase by a factor of 10 as the photosphere is traversed The surface of the Sun has imperfections Sunspots Sunspots activity varies with a period of 11 years The Sun s gases extend out far beyond the photosphere The region of the Sun s atmosphere just above the photosphere is termed the chromosphere Until the 20th century, the chromosphere could only be studied during total solar eclipses The chromosphere consists of bright emission lights indicated that it is a hot, thin gas The reddish color is created by the presence of hydrogen In 1868, new discrete lines were seen in the chromosphere Helium had been discovered Helium was not found on Earth until 1895 15 16
The Solar Atmosphere The temperature of the gases of the Sun increase dramatically in the transition region between the chromosphere and the corona The Solar Corona The outermost part of the Sun s atmosphere is called the corona The corona is very hot Millions of K Observe elements such as iron (Z=26) with 16 electrons ionized (removed) 17 Photos of the Sun s corona taken by NASA Marshall Space Flight Center 18 The Solar Wind The Sun produces a stream of charged particles (mainly electrons and protons) called the solar wind The Sun loses 10 million tons of material per year in the form of solar wind X-ray pictures of the Sun show coronal holes that are dark areas on photographs The solar wind is thought to mainly arise from these magnetic anomalies X-ray photograph of the Sun s corona showing coronal holes 19 Aurora The charged particles from the Sun are trapped by the Earth s magnetic field and spiral down along the field lines Sometimes these charged particles hit molecules and atoms in the air and cause them to glow Germany, April 6, 2000 20
The Active Sun The Sun is in a perpetual state of change It s surface is a seething cauldron of hot gas Occasionally there are large solar flares that disrupt communications on Earth The surface of the photosphere has a mottled look resembling grains of rice Each small bright spot is a rising column of hot gas Sunspots Sunspots are cooler than the surrounding solar gases Sunspots can last from a few hours to a few months The number of sunspots varies with a cycle of about 11 eleven years During the maximum activity, there can be 100 visible sunspots During the minimum activity, there can be no sunspots Sunspots can be seen to rotate with the surface of the Sun Photosphere Sunspot Chromosphere 21 22 Portrait of a Sunspot Portrait of a Sunspot The sunspot has a dark central region called the umbra The umbra is surrounded by a less dark region called the penumbra The granulation of the Sun s surface is also visible 23