1. Chapter 6: Meteorites and Meteoritics Estimating sizes, orbits Fate: breakup or impact

2. Review: States of Matter Matter can be in different states, depending on how tightly bound the atoms are. Changes in phase require the breaking of a binding force For our purposes, we are mostly concerned with gases, solids and (to a lesser extent) liquids.

3. States of Matter Matter can coexist in different phases. At the triple point, gas, solid and liquid coexist. Phase diagram for water

4. States of Matter The phase diagram for different elements tells us what phase they will be found in under given conditions. Knowing the triple point and critical point alone allow a rough estimate of the phase diagram. Phase diagram for waterPhase diagram for hydrogen

5. Gases Ideal Gas Law: relates pressure, density and temperature Where n is the number density and  is the mass density of the gas.  is the mean molecular weight. Such an equation, relating pressure, density and temperature, is known as an equation of state. The equation of state for solids and liquids is generally much more complex and/or poorly known. i.e. this is the average mass of a free particle, in units of the mass of hydrogen

6. Solids Minerals are substances that occur naturally and include no organic (animal or vegetable) compounds.  The most commonly occurring minerals are made of the most commonly occurring elements  In the inner SS these are dominantly O, Si, Mg, and Fe with lesser amounts of things like Na, Al, Ca, and Ni.  The minerals we find are vastly dominated by SiO 4 – these are called silicates. Rocks are solids made of more than one mineral and the mix of minerals in rocks varies from one part of the SS to another and well as within a given body. Ices are solids whose composition consists of the abundant elements C,N,O in combination with H.  These compounds (water, carbon dioxide, methane, ammonia etc.) freeze at different temperatures; strictly speaking these are also minerals but are referred to as ices because of their low solidification temperatures.  Most common in the outer SS beyond ~3AU from the Sun.

7. Silicates Feldspars SiO 2 + K, Al, Na, Ca… Least dense silicate Low melting point (~600 C) Pyroxene chains of SiO 4 + Fe, Mg, Al, Ca… Olivine simplest silicate SiO 4 + Fe and/or Mg Most dense silicate High melting point (>1000 C) The main silicate families are olivines, pyroxenes and feldspars. They are distinguished from each other by which elements are present and how complex are their crystalline structures.

8. Rocks Igneous  Formed directly from cooling of molten magma Sedimentary  Deposition or cementing of small particles Metamorphic rocks  Originally formed as igneous or sedimentary, but changed to a new form by high pressure, high temperature or addition of new chemicals

9. Ices solids that contain C,N,O – which are gaseous at T≥200K CNO overall more abundant than Fe,Mg,Ca,Al,Na…  but Fe,Mg,…condense into grains at much higher T ices are more abundant in outer SS objects – i.e. satellites, comets, some asteroids, Kuiper Belt… commonest ices: CH 4 (methane), NH 3 (ammonia), H 2 O (water) vapourous “at the least excuse” Core of Halley’s comet shown outgassing as Sun heats the ice

10. Meteoritic Complex Meteoritic complex: interplanetary objects, such as dust particles, asteroids and comets. Among terrestrial planets, the largest are ~30 km across Among giant planets, the largest are at least ~500 km across In the asteroid belt, between the terrestrials and giants, the largest object is Ceres (1000 km).

11. Meteors Occasionally one of these bodies intersects with Earth’s orbit, entering the atmosphere and burning up as a brief flash of light. This space debris is quickly slowed by frictional interaction with the atmosphere to speeds of ≤1km/s and the smallest ones burn up before reaching the surface. Some, normally the larger and more massive ones, will reach the Earth’s surface; they are called meteorites.

12. Meteorite Falls Supersonic velocities create sonic boom Larger objects are brighter May leave dusty train of debris Fall is so swift that only outer ~1mm is heated Fireball observed over Wales Sept 29, 2003?  Maybe not, actually - but still a cool picture

13. twisted meteor train – how? non-spherical? non-uniform? “wobbling”?

15. Meteor sizes Consider a meteoroid (radius R, density ρ M, mass M=4/3 (πR 3 ρ M ) entering the atmosphere (density ρ a ) at some speed v  the meteor must clear a path through the atmosphere to move forward – push air out of the way. vdt πR 2 (Typically this relation gives results which are correct to about a factor of 2.) Observe v, dv/dt; know ρ a vs. altitude => this gives R 2 /M if we assume a value for ρ M, can solve for R, M

16. Example: meteor sizes From film footage of a fireball you estimate a meteor’s initial velocity to be 20 km/s. When it disappears from view, 3 s later, it is traveling at a velocity of about 10 km/s. Assume it is traveling roughly horizontally, at an altitude where the air density is 5x10 -5 kg/m 3. Estimate its mass.

17. Meteoroid Breakup  E K ~0.1v 2 (km/s) eV/atom  interaction with atmosphere, with sufficient E K, means meteoroid is destroyed  energy to detach 1 atom ~ 2-3eV  if v~10km/s -> E K ~10 eV – more than enough!

18. Ablation  The slowing of the meteor also means an energy loss which results in heating of both the meteor and the air around it.  ablation efficiency is higher for lower ρ a  can relate ΔE K and mass loss to meteor luminosity  so L → M (photometric mass)  Although the fall through the atmosphere heats a meteor, this heating penetrates only the outer mm- cm and leaves the interior very cool.

19. Example Calculate the final mass of a meteor, assuming its final velocity is much smaller than its initial velocity. Meteors with high initial velocities will tend to be completely destroyed; only the slowest survive.

20. Break

21. History of Meteorite Studies Read interesting history in textbook, p. 131 “Stones from the sky” observed by ancient Chinese, Greeks, Romans Only became accepted in early 1800s Huge collections of meteorites found in Antarctic ice fields in 1969  Falling on ice less damaging then on rock  Easier to find (contrast with surrounding terrain)  Less subject to erosion

22. Classification Some meteorites have clearly been altered by heat pressure, mechanical shock, or other processes  Sometimes the heating is gentle, so they are only slightly altered  Others have completely melted, allowing the separation of heavy and light materials: differentiation Differentiation forms the main basis of classification Many are breccias: formed of broken fragments cemented together  Breccias and fractures suggest violent collisions (a) A stony meteorite often has a dark crust, created when its surface is melted by the tremendous heat generated during passage through the atmosphere. (b) Iron meteorites usually contain some nickel as well. Most show characteristic crystalline patterns when their surfaces are cut, polished, and etched with acid.

23. Differentiation How does differentiation occur?  Imagine heating a solid mixture of primordial material so that it melts  High density metals such as iron and nickel, along with elements with a chemical affinity for iron (e.g., Co, Ni, Ru, Rh, Pd, Os Ir, and Pt) drain to the centre to form a metal core  Lighter, silica-associated elements float to the surface to form stony mantle.  Collisions then produce fragments with different compositions.  Later collisions can weld fragments together, forming breccias.

25. Stone meteorites Ordinary chondrites are the most common type of stone meteorite,chondrites Carbonaceous (C) chondrites are some of the most complex of all meteorites. chondrites Enstatite (E) chondrites are a rare and unusual type of meteorite. chondrites AchondriteAchondrite meteorites are very similar in appearance to terrestrial igneous rocks.igneous Stony meteorites are the most common (95%) Include the least differentiated meteorites Most numerous are the chondrites: a class that contain small spherules called chondrules

26. Chondrules Glassy spherules embedded in meteorites Composed primarily of olivine and pyroxene, with moderately high melting temperatures. Form during rapid cooling of droplets of molten material Formation?  Possibly formed in shock waves within original solar nebula  Or during impacts on surfaces of planetesimals.

27. Chondrites Chondritic meteors have nearly solar abundances of elements – except for volatiles  the oldest SS samples  formed from early solar nebula  most are unchanged since formation

28. Next lecture More meteors…  Finish classification  Ages  Origins