1. Meteorites & Asteroids
23 October 2018

2. What is a meteorite?
A meteorite is a solid piece of debris from an object, such as a comet or asteroid, that survives its passage through the atmosphere to reach the surface of a planet. When the object enters the atmosphere, various factors such as friction, pressure, and chemical interactions with the atmospheric gases cause it to heat up and radiate that energy

3. What is a meteor?
It then becomes a meteor and forms a fireball, also known as a shooting star or falling star; astronomers call the brightest examples ”bolides". Meteorites vary greatly in size. For geologists, a bolide is a meteorite large enough to create a crater

4. As of 2018, there are more than 59,200
well-documented meteorite finds
Meteorites are always named for the places
they were found

5. Chondrite

6. Achondrite

7. Martian

8. Chixulub, Yucatan penninsula, Mexico
Gravity map of buried structure
180 miles across; 65 millions years old
Identified in early 1990s with seismic data, after 10 year ‘search’

9. Entry and Impact
As meteoroids are heated during atmospheric entry, their surfaces melt and experience ablation. 
Meteoroids that experience disruption in the atmosphere may fall as meteorite showers
The largest meteorites in a shower are found farthest down-range in the strewn field
Antarctic ice sheets act to concentrate meteorites in certain areas
The largest meteorites leave craters

10. Tunguska, Siberia, June 30, 1908
Black and white photos taken during field expedition in 1927; color photo taken in 1990

11. Jackson Hole Fireball, August 10, 1972

12. Potentially Hazardous Asteroid Threat Size-frequency diagram for impacting objects
~100 tons of meteroritic dust falls each day
50 m impactor once per 1000 yr (local effects)
500 m impactor once per million years (regional effects)
5 km. impactor once per 100 million years (global effects)

13. Hoba Iron 3m x 2m x 1m; 60+ tons Found 1920, Namibia
No crater, classified ataxite

14. The 60-ton, 2.7-metre long Hoba meteorite in Namibia 
is the largest known intact meteorite

15. when it hit the roof of a house on October 17, 2012
The 'crater' made by a 61.9-gram Novato meteorite 
when it hit the roof
of a house on October 17, 2012

16. NWA 859 iron meteorite showing effects of atmospheric ablation

17. Ordinary Chondrites (S Asteroids?)

18. Three Views of Vesta Hubble image, model and color-shaded topography
Largest member of V class of asteroids (vestoids)
Spectral variations consistent with HEDs
Three Views of Vesta

19. What were the processes and products in the early Solar System
Impact features on all planetary surfaces; planets formed by accretion of planetesimals from a turbulent solar nebula
Much mixing of components; completed in 5-10 million years
‘Residual’ debris forms asteroid belt; Kuiper belt, Oort cloud

20. Meteor showers Time exposure image, tracking stellar motion
Stars stay still, meteorites make trails

21. The Peekskill (NY) Fireball

22. P Jenniskens et al. Nature 458, 485-488 (2009)
Macroscopic features of the Almahata Sitta meteorite.
P Jenniskens et al. Nature 458, (2009)

23. Chondrites Rocky, inhomogeneous, contain round “chondrules”
Microscope image

24. Iron meteorites: from core of differentiated asteroids

25. Stony-Iron meteorites - the prettiest
Crystals of olivene (a rock mineral) embedded in iron
From boundary between core and mantle of large asteroids?

26. Sutter’s Mill meteorite 2012

29. The main points: Meteorites
Each year the Earth sweeps up ~80,000 tons of extraterrestrial matter
Some are identifiable pieces of the Moon, Mars, or Vesta; most are pieces of asteroids
Meteorites were broken off their parent bodies 10’s to 100’s of million years ago (recently compared to age of Solar System)
Oldest meteorites (chondrites) contain interstellar dust, tiny diamonds made in supernova explosions, organic molecules and amino acids (building blocks of life)
Direct insight into pre-solar system matter, solar system formation

30. Asteroids
Apollo
Trojans

31. What are Asteroids?
Asteroids, sometimes called minor planets, are rocky remnants left over from the early formation of our solar system about 4.6 billion years ago.
They are found orbiting the sun between Mars and Jupiter within the main asteroid belt. Asteroids range in size from Vesta—the largest at about 530 kilometers in diameter - to bodies that are less than 10 meters across. 

32. Asteroid Belt as viewed from Above
Over 100,000 objects greater than 10 km. now identified in the Main Belt
Total mass less than 1% of moon’s mass
Over 100 NEAs greater than 1 km. across are being tracked; probably part of a population of about 2000
Kirkwood gap (and others) occur in the belt where there are orbital resonances with Jupiter
Asteroids classified by ‘spectral group

33. Asteroid History
Jupiter's massive gravity and occasional close encounters with Mars or another object change the asteroids' orbits, knocking them out of the main belt and hurling them into space in all directions across the orbits of the other planets. Stray asteroids and asteroid fragments slammed into Earth and the other planets in the past, playing a major role in altering the geological history of the planets and in the evolution of life on Earth.

34. Kirkwood Gaps

35. Most asteroids are irregularly shaped, though a few are nearly spherical, and they are often pitted or cratered.
As they revolve around the sun in elliptical orbits, the asteroids also rotate, sometimes quite erratically, tumbling as they go.
More than 150 asteroids are known to have a small companion moon (some have two moons). There are also binary (double) asteroids, in which two rocky bodies of roughly equal size orbit each other, as well as triple asteroid systems
The three broad composition classes of asteroids are C-, S-, and M-types.

36. S Asteroids (‘silicaceous’)
951 Gaspra 433 Eros (true color) Ida (and Dactyl)
19 x 12 x 11 km x 13 x13 km 58 x 23 km (1km)
Galileo flyby, NEAR orbit/landing Galileo flyby, 1993
Grooves, curved near-Earth asteroid, member of Koronis
depressions, ridges space weathering family, first ID of
(Phobos-like) effects documented asteroid ‘moons’

37. C Asteroids (‘carbonaceous’)
253 Mathilde; 66 x 48 x 46 km, visited by NEAR Shoemaker
Surface as dark as charcoal; typical outer belt asteroid

38. Ida and Dactyl

39. Itokawa

40. Hyabusa samples Itokawa

41. Hyabusa
Returns
June 2010

42. Steins 2008

43. Toutatis

44. Vesta, Ceres, Moon

45. Dawn Mission at Vesta

47. Vesta Craters

48. What about M-type Asteroids?
16 Psyche is the largest known M-type asteroid, and is thought to be the exposed iron core of a protoplanet This asteroid may be the remnant of a violent collision with another object that stripped off the outer crust.

49. Artists' impression of the 
Psyche spacecraft

50. Psyche artist’s conception

52. Asteroids Summary
Solid objects mostly in a belt between Mars and Jupiter
Small bodies much more common than larger ones
Classes similar to meteorites: Stony (S), Carbonaceous (C), Metallic (M)
Bodies and belts shaped by collisions, resonances
Source of meteorites