Small Bodies in Solar System

Lecture 14

Small chunks of rocks in orbits around the SunAsteroids : 2-3.5AU. main belt of asteroid at 2.7AUAsteroids at the outer region have lots of ice Major source of Earth’s water?

Ceres (900km), Pallas and Vesta (500km) “minor planets” > 100,000 objects with size larger than 1 km.

Asteroids ≠ broken up planet

total mass of asteroids ≈ Moon’s mass

asteroids = debris (remnant) of planet formation

Near Earth Objects (NEOs) : Solar System object whose orbit brings it into close proximity with the Earth.

Congressional mandate to catalog all NEOs that are larger than 1km.As of 2006, about 20% are still missing…

433 Eros : 33×13km

Trans-Neptunian Objects : small bodies (minor planets and smaller) with orbits lie beyond of Neptune’s (30AU)

35,000+ objects with size larger than 100km total mass is comparable to Jupiter’s

Largest Trans-Neptunian Objects

Oort CloudShort period comets are originated

from Kuiper belt objects and they are more or less close to the Ecliptic plane.

However, long period comets have random inclinations and their orbital sizes are ~10,000 AUs.

Cratering on Planets and Satellites Impact crators = leave scars on the surface of rocky objects. Some objects (Mercury, Venus) are heavily cratered than others (e.g., Earth). Why?

Geological activity erased old impact craters! Geological activity depends on the size of planets!

Cratering and Geologic activity Smaller objects lose heat faster…

Why? For a given radius, smaller objects have larger ratios of surface area to volume.

CPU coolerincreased surface area

for more efficient radiative cooling

€

Volume =4πR3

3Surface = 4πR2

Ratio = Surface /Volume = 3R

Mercury smallest most crateredMars next smallest, cratering is about halfway b/w Earth and MercuryVenus slightly smaller than Earth slightly less cratered than Earth Earth geological activity (and small effect from erosion) erased most impact

craters (only about 200 are known).

Mercury Moon Mars Venus

Flow of charged particles create a magnetic field Dynamo theory

Stronger flow stronger magnetic field

To have a flow, interior of planets has to be non-solid

Magnetic Field

Magnetic Fields Shield against strong solar winds!

Relic Magnetism on Mars

In the past, when Mars still had some internal heat, the interior of Mars was molten and created a global magnetism relic magnetic field on Mars

Mercury.. hosts a weak magnetic field (1% strength of Earth’s) a small fraction of interior is in liquid state.

Venus.. although it may still have a molten interior, its slow rotation (243 days) cannot generate strong dynamo effect.

Strong magnetic fields in Jupiter and Saturn In the solar system, Jupiter has the

strongest magnetic field Jupiter’s rotation is fast (10 hours).

from liquid metallic hydrogen acting like a metal (under 1.4 millon bars of pressure!)

Diversity of the Solar System

Diversity of the Solar System

In summary…

Important ConceptsSmall objects lose heat faster than

larger objects

Dynamo theory

Global magnetic field internal structure!

Important Termsasteroid belt• near-Earth object

Kuiper belt : Trans-Neptunian objects

Oort cloud

Liquid metallic hydrogen

Chapter/sections covered in this lecture : sections 7-5 through 7-8