1

Astr 1050 Wed., March. 22, 2017

Today: Chapter 12, Pluto and “Debris”

March 24: Exam #2, Ch. 5-12 (9:00-9:50) March 27: Mastering Astronomy HW Chapter 11 & 12

2

Chapter 12: Meteorites, Asteroids, Comets

•  Small bodies are not geologically active •  They provide “fossil” record of early solar system

–  Asteroids •  Mostly from region between Mars and Jupiter •  Left over small debris from accretion, never assembled into a large

planet •  Meteorites come mostly from asteroids

–  Comets •  “Stored” on large elliptical orbits beyond planets •  Thought to be “planetesimals” from Jovian planet region, almost

ejected from solar system in its early history

•  Meteorites provide only samples besides Apollo –  With sample in hand, can perform very detailed analysis:

detailed chemistry; radioisotope age; other isotope info

3

Asteroids •  Most located between

Mars and Jupiter •  Largest is Ceres

–  1/3 diameter of moon –  Most much smaller

•  >8,000 known •  Total mass << Earth •  A few make it to earth

–  source of the meteorites

The New Solar System, Beatty et al.

4

Meteorites from Asteroids •  If meteorite speed and

direction is observed as it enters Earth’s atmosphere, you can work backwards to find its orbit.

•  Almost all of the meteorites with well determined orbits have most distant part of orbit ellipse within the asteroid belt.

5

The larger asteroids

The New Solar System, Beatty et al.

6

Are Asteroids Primitive?

•  Ida (56 km diam.) and its moon Dactyl (1.5 km diam.) –  Colors have been “stretched” to show subtle differences

•  Imaged by Galileo on its way out to Jupiter •  Presence of craters indicates great age

–  Absolute age requires knowledge of cratering rate – uncertain –  Not spherical – gravity too weak to pull it into a sphere

7

Another Galileo Asteroid: Gaspra

8

Phobos & Deimos: Two “misplaced” asteroids?

•  Phobos and Diemos are small (~25 km and ~15 km diam.) moons of Mars •  Look like captured asteroids rather than moons formed in place •  Are “C” class – i.e. dark “Carbonaceous” type “asteroids”

Grooves seem related to the large crater called “Stickney”

9

Clues from Meteorites

•  Three main kinds of meteorites –  Carbonaceous chondrites: Most primitive material –

dark because of C –  Stones Similar to igneous rocks –  Irons Metallic iron – with peculiarities

•  Why do we have different kinds? –  How are the main types of meteorites related to the

asteroids?

10

Origin of different asteroid types

•  Carbonaceous = undifferentiated?

•  Stones and Metals from differentiated planetesimals? –  S = mantles –  M = cores

•  Try to sort out using meteorite samples

11

Meteors vs. Meteorites •  Meteor is seen as streak in sky •  Meteorite is a rock on the ground •  Meteoroid is a rock in space

•  Meteor showers (related to comet orbits) rarely

produce meteorites –  Apparently most comet debris is small and doesn’t

survive reentry

•  Meteorites can be “finds” or “falls” –  For a fall – descent actually observed and sometimes

orbit computed –  Most have orbits with aphelion in asteroid belt

12

Large Meteor over the Tetons (1972)

Aphelion distance 2.3 AU Diameter 3 to 10 m Seen at height of ~50 km – skipped out of atmosphere

13

The Leonids

2001 •  APOD site:

Picture by Chen Huang-Ming

14

Meteor Showers and Comets •  Meteor showers caused

by large amount of small debris spread out along comet orbits

•  Almost none makes it to the ground – no meteorites

•  Occur each year as earth passes through orbit of comet

•  Appears to come from “radiant point” in sky

•  Leonids: Mid November

From our text Horizons, by Seeds

15

Comets: Hale-Bopp in April 1997

16

Comet characteristics •  Most on long elliptical orbits

– Short period comets – go to outer solar system •  “Jupiter family” still ~ in plane of ecliptic •  “Halley family” are highly inclined to ecliptic

– Longer period ones go out thousands of AU •  Most of these are highly inclined to ecliptic

•  Become active only in inner solar system – Made of volatile ices and dust – Sun heats and vaporizes ice, releasing dust – “Dirty snowball” model

17

Comet structure •  Gas sublimates from nucleus •  Dense coma surrounds nucleus •  Ion tail is ionized gas points

directly away from sun –  shows emission spectrum –  ions swept up in solar wind

•  Dust tail curves slightly outward from orbit –  shows reflected sunlight –  solar radiation pressure gently

pushes dust out of orbit

18

Hale-Bopp clearly shows components

Ion Tail

Dust Tail

Coma (Nucleus too small to see)

19

Where do comets come from? Long period comets: The Oort Cloud

•  Most (original) orbits have aphelions of >1000 AU

•  Need ~6 trillion comets out there to produce number seen in here

•  Total mass of 38 MEarth

•  Passing stars deflect comets in from the cloud

The New Solar System, Beatty et al.

¼ to 1 parsec

20

Formation of Oort cloud comets

•  Composition indicates formation in region between Jupiter and Neptune

•  Ejected to the Oort cloud by near collisions as Jovian planets formed

•  Most probably lost from solar system – a few have just barely closed orbits

•  Occasional passing stars perturb more comets into orbits passing in close to sun

21

Where do the Jupiter family comets come from?: The recently discovered Kuiper Belt

•  Material beyond Neptune never ejected into the Oort cloud

•  Pluto and Charon the biggest members – now also Quarar, Sedna

•  Very hard to detect because very faint –  far from the sun so little

illumination –  comets not active at that

distance –  Hubble and new large

telescopes have recently detected ~100

22

Pluto and Charon

23

Importance of comets

•  Evidence of solar nebula •  Source of H2O and CO2 for earth •  Impacts continue

–  Impacts on Earth •  Extinction of the dinosaurs

–  SL-9 impact on Jupiter