Friday, January 13, 2006Friday, January 13, 2006

Planetological Foundations for Planetological Foundations for thethe

Origin of LifeOrigin of Life

Formation of the Solar SystemFormation of the Solar System

Formation of the Solar SystemFormation of the Solar System______________________________________________________________________________

Essentially two categories of theories explaining the formation of the Solar System:

1. catastrophic theories - invoke an accidental catastrophic encounter event such as near collision between the Sun and a star or a comet

2. non-catastrophic theories - involve a natural, non-catastrophic event such as might occur in conjunction with the birth of a star, e.g., Nebular Hypothesis or Protoplanet Theory and Condensation Model

Formation of StarsFormation of Stars______________________________________________________________________________

• galaxies are host to molecular clouds, which contain elevated densities of hydrogen (H2), helium (He) and heavier elements, and substantial amounts of dust

• stars form from these mixtures of primordial hydrogen and helium and the products of dying stars by collapse of interstellar gas as a result of intrinsic gravitational attraction

• because molecular clouds contain much more matter than for average interstellar space, higher density means greater attractive potential

Formation of the Solar SystemFormation of the Solar System______________________________________________________________________________

Any model of Solar System formation must explain the

following observations:1. all the orbits of the planets are prograde (i.e.,

if seen from above the North pole of the Sun, they all revolve in a counter-clockwise direction)

2. all the planets (except Pluto) have orbital planes that are inclined by less than 7 degrees with respect to each other (i.e., all in the same plane)

3. planets exhibit a chemical gradation, i.e., the inner terrestrial planets (Mercury, Mars, Earth, Venus) are dense, rocky and small, with few satellites, while Jovian or outer planets (Jupiter, Saturn, Uranus, Neptune) are gaseous and large with many satellites

4. the asteroid belt between Mars and Jupiter

5. beyond the planets, (greater than ~ 30 AU), lies of belt of rocky and icy debris (comets), i.e., the Kuiper Belt

6. Pluto, once considered the 9th planet, fits better as the largest known member of the Kuiper Belt, the source of short-period comets

7. at distance of thousands of AU from the Sun is a loose cloud of icy material, i.e., the Oort cloud

8. the distances of the planets from the Sun follow a simple arithmetic progression:

• R = a + b x 2n (Bode’s Law, 1772)

Formation of the Solar SystemFormation of the Solar System______________________________________________________________________________

These properties suggest that whatever process formed our planetary system generated:

• a geometry of raw material in the form of a

flattened disc• that the disc was hotter close to the centre

and cooler farther out• the existence of debris belts and clouds

suggests that planet formation truncated after some time

• the large amount of remaining material was scattered by the gravitational fields of the planets themselves

Condensation Model of Solar System Condensation Model of Solar System FormationFormation

______________________________________________________________________________

• planets originate from gravitationally contracting and chemically condensing eddies as a natural by-product of the formation of a star

• the condensation model essentially is an advanced solar nebula model that recognizes the importance of interplanetary dust particles (IDP’s) to the formation of the solar system

The story of the first 600 million years from the verybeginning of the solar system to the time when life can establish a foothold on this planet we call home—Earth.

The story begins about 5 billion (5,000,000,000) yearsago, 5 x 109 a or 5 Ga, with a supernova in a nearby starsystem . . .

Where Did the Solar System Come From?Where Did the Solar System Come From?______________________________________________________________________________

Supernova SN 1998S in NGC 3877Supernova SN 1998S in NGC 3877

http://www.info.waseda.ac.jp/muraoka/members/seiichi/nova/sn1998s.html

• the story of new solar systems begins with the death of a previous one

• a star becomes a nova or a supernova, destroying itself and its solar system

• but at the same time elements are made and the seeds of a new solar system created

Galactic RecyclingGalactic Recycling______________________________________________________________________________

Remnants of a Destroyed Solar System:Remnants of a Destroyed Solar System:The Crab Nebula in the Constellation TaurusThe Crab Nebula in the Constellation Taurus

• observational evidence supports the idea that our solar system was born from an interstellar cloud of gas, because stars that appear to be in the process of formation today are always found within interstellar clouds

• over the next few million years, thousands of stars will be born in this gas cloud – some may form their own planetary systems

SupernovaSupernova______________________________________________________________________________

(1997 DigitaLight Pictures)

Compression of Nebular Dust and Gas by Compression of Nebular Dust and Gas by Supernova Shock WaveSupernova Shock Wave

• the solar nebula likely began as the result of a supernova, which would trigger the collapse of the interstellar gas cloud

After Passage of Supernova Shock Wave--After Passage of Supernova Shock Wave--Beginning of Solar SystemBeginning of Solar System

Entire Life-Cycleof Stars

Clouds of gas, dust and ice being compressedby shock wavefrom supernova

Orion Nebula--1,500 light years from Earth (in the dagger of the constellation Orion)

Star and Planet Birth in the Winter SkiesStar and Planet Birth in the Winter Skies

• initially the interstellar cloud was about several light years across

• a small overdensity in the cloud caused the contraction to begin and the overdensity to grow, thus producing a faster contraction - run away or collapse process

• as a solar nebula shrinks in size, three processes occur to alter its density, temperature and shape:

• heating• spinning• flattening

Nebula to Solar SystemNebula to Solar System

• gravitational collapse was much more efficient along the spin axis, so the rotating ball collapsed into a thin disk with a diameter of ~200 AU (0.003 light years; twice Pluto's orbit), aka solar nebula with most of the mass concentrated near the center

• gravitational energy is converted to heat as the cloud collapses

• vaporization occurs

• most heat at centre--Sun “lights” up at about10 x 106 K by nuclear fusion

• cooling occurs and vapour recondenses as dust that grows into planetesimals then into planets

HeatingHeating

Hubble's Wide Field and Planetary Camera 2 (WFPC2) image shows a large part of the heart of the giant Orion molecular cloud, OMC-1, as it appears in visible light.

The infrared photograph (right) reveals a chaotic, active star birth region. Here, stars and glowing interstellar dust, heated by and scattering the intense starlight, appear yellow-orange. Emission by excited hydrogen molecules appears blue.

SpinningSpinning______________________________________________________________________________

• most of the motions of the interstellar cloud particles were random, yet the nebula had a net rotation

• as collapse proceeded, the rotation speed of the cloud was gradually increasing due to conservation of angular momentum, which ensures that not all material in the solar nebula collapsed into the centre

• conservation of angular momentum demands that a contracting, rotating cloud gradually speed up its rate of rotation and eventually the primitive Solar System resembles a giant pancake

When a figure skater draws his arms and a leg inward, he reduces the distance between the axis of rotation and some of his mass, reducing his moment of inertia. Since angular momentum is conserved, his rotational velocity must increase to compensate.

FlatteningFlattening______________________________________________________________________________

• flattening is the consequence of collisions between particles in a spinning cloud

• this simple idea of flattening as the solar nebula collapses leads to a natural explanation for several of the dynamical regularities of the planets

• namely, the orbits are in the same sense, the orbits are roughly co-planer, the rotations of the planets are the same sense as the orbital motions, and the orbits of moons are in the same sense as the planet's orbits

Image of the Milky Way from an edge-on perspective with the galactic north pole at the top, south pole at the bottom and galactic centre at the centre, was taken with NASA's Cosmic Background Explorer (COBE)'s Diffuse Infrared Background Experiment.

• around the Sun a thin disk gives birth to the planets, moons, asteroids and comets

• over recent years we have gathered observational evidence in support of this theory

Protoplanetary DisksProtoplanetary Disks

Images obtained with HST, revealing what seem to be disks of dust and gas surrounding newly formed stars in the Orion Nebula. These protoplanetary disks span ~0.14 ly and are probably similar to the Solar Nebula.

Condensation of Planetesimals from Dust and GasCondensation of Planetesimals from Dust and GasFirst by Electrostatic Forces then later by GravityFirst by Electrostatic Forces then later by Gravity

Formation of the PlanetsFormation of the Planets____________________________________________________________________________

• the first solid particles in the nebula were microscopic in size

• when the temperature is low enough, some atoms or molecules in a gas may bond and solidify – i.e., particles condensed out of the gas as they orbited the Sun in nearly circular orbits right next to each other

• these particles grew larger with time

• different materials condense at different temperatures

Composition of the Early Solar Nebula Composition of the Early Solar Nebula

• the churning and mixing of gas in the solar nebula should have ensured that the nebula had the same composition throughout – 98% hydrogen and helium plus 2% heavier elements

• yet the solar system ended up with two very different major types of planets

Composition of the Solar SystemComposition of the Solar System

center. The inner nebula was rich in heavy solid grains and deficient in ices and gases. The outskirts are rich in ice, H and He.

Condensation of Different ChemicalsThe vast temperature differences between the hot inner regions and the cool outer regions of the disk determined whatcondensates wereavailable for planetformation at eachlocation from the

Hydrogen compounds could condense into ices only beyond the frost line (between Mars and Jupiter)

Frost LineFrost Line

How Did the Terrestrial Planets Form?How Did the Terrestrial Planets Form?____________________________________________________________________________

• gentle collisions enabled the flakes to stick together and make larger particles which, in turn, attracted more solid particles

• this process is called accretion

Accretion of PlanetesimalsAccretion of Planetesimals____________________________________________________________________________

• objects formed by accretion are called planetesimals (small planets): they act as seeds for planet formation. At first, planetesimals were closely packed. They coalesced into larger objects, forming clumps of up to a few kilometres across in a few million years, a small time compared to the age of the solar system

Early in the accretion process, there are many large planetesimals on crisscrossing orbits

As time passes, a few planetesimals grow larger by accreting smaller ones, while others shatter in collisions

Ultimately, only the largest planetesimals avoid shattering and grow into full-fledged planets

• as planetesimals grow in size, collisions became destructive, making further growth more difficult. Only the biggest planetesimals survived this fragmentation process and continued to slowly grow into protoplanets by accretion of planetesimals of similar composition

Planetesimals to ProtoplanetsPlanetesimals to Protoplanets____________________________________________________________________________