1. Formation of the solar system
Utep planetary science
January 28, 2019

3. THREE PHASES OF GROWTH
PLANETESIMALS FORMED IN PROTOPLANETARY DISK OF GAS AND DUST
COLLISIONAL EVOLUTION OF PLANETESIMALS LEADS TO FORMATION OF PLANETARY EMBRYOS; GIANT PLANETS ALSO FORM
GAS DISAPPEARS FROM PROTOPLANETARY DISK, EMBRYO ORBITS BECOME UNSTABLE AND COLLIDE, FORMING TERRESTRIAL PLANETS

4. PHASE 1: DUST TO PLANETESIMALS
MOLECULAR CLOUD COLLAPSES GRAVITATIONALLY TO FORM A STAR
DISK-LIKE STRUCTURE FORMS AROUND NEW STAR TO CONSERVE ANGULAR MOMENTUM
DUST IN PROTOPLANETARY DISK SETTLES INTO THIN LAYER AT DISK MIDPLANE (TIMESCALE AND FINAL DENSITY DEPENDENT ON TURBULENCE IN DISK, KELVIN-HELMHOLTZ INSTABILITY)
RADIAL THERMAL GRADIENT EXISTED IN THE DISK, CAUSING INNER PART TO BEOME DRY AND GAS/VOLATILE-POOR
SOMEHOW DUST GRAINS GROW INTO PLANETESIMALS…

5. HOW DO YOU GROW KILOMETER-SIZED PLANETESIMALS FROM MICRON-SIZED DUST GRAINS?
ORDERED GROWTH
Grains will stick together… up to a point
…cm-sized grains are too small for gravity and too big for electrostatic forces to be effective at sticking together (centimeter-size barrier)
… and small grains will continually lose momentum due to drag and fall towards star at different, size- dependent velocities, causing collisions with enough Dv to cause breaking rather than sticking together (meter-size barrier)
GRAVOTURBULENT MODEL
The collective gravity of swarms of small particles concentrated at specific locations in the protoplanetary disk allows planetesimals up to 100-km size to form rapidly without cm- and m-barrier bottlenecks
Explains observed size distributions of objects in asteroid and Kuiper belts
Predicts planetesimal growth is asynchronous (doesn’t start at same time everywhere in the disk), but instead starts when the solid/gas density ratio is big enough (a threshold that will increase over time as gas leaves disk)
Predicts potential pattern of where planetesimals might have differentiated (stayed hot enough inside because they kept short-lived radionuclides) and where they did not (no short-lived radionuclides left)
Predicts ”less clean” radial gradient in condensation sequence because of sporadic, asynchronous formation of planetesimals
Explains observed radial overlap in distributions of E, S, and C asteroids, but so does dynamic mixing of asteroids after they formed.
May explain apparent continuum between comets and carbonaceous asteroids.

7. PHASE 2: PLANETESIMALS TO EMBRYOS
WHEN DISK CONTAINS ENOUGH PLANETESIMALS, DYNAMICS BECOMES DOMINATED BY GRAVITATIONAL ATTRACTION BETWEEN PAIRS OF OBJECTS
POPULATION OF PLANETESIMALS CONTAINS A SUBGROUP OF A FEW BIG BODIES AND MANY SMALL BODIES SUCH THAT MOST OF THE MASS IS IN THE SMALL BODIES
GROWTH OF BIG BODIES IS STRONGLY FAVORED BY GRAVITATIONAL FOCUSING FACTOR THAT IS MUCH LARGER FOR THE BIG BODIES THAN IT IS FOR THE SMALL BODIES (BIG BODIES WILL COLLIDE MORE OFTEN)
COLLISIONS TEND TO OCCUR AT SMALL ENOUGH RELATIVE VELCOITIES (I.E. PHYSICS IS DOMINATED BY SMALL BODIES, NOT BIG BODIES) SUCH THAT THERE IS TENDENCY TO ACCRETE RATHER THAN DISRUPT
LEADS TO RUNAWAY GROWTH IN WHICH BIG BODIES GROW MUCH FASTER THAN THE SMALL ONES, INCREASING THE MASS DIFFERENCE BETWEEN POPULATIONS, IN A POSITIVE FEEDBACK LOOP
RUNAWAY GROWTH ENDS WHEN LARGE BODIES BECOME NUMEROUS ENOUGH SO THAT THEIR MASS DOMINATES THE PHYSICS, RATHER THAN THE SMALL ONES, I.E. EQUAL MASS IS IN THE BIG BODIES AS IN THE SMALL BODIES
BIG BODIES THAT REMAIN ARE EMBRYOS
SMALL BODIES THAT REMAIN WILL NOT GROW TO EMBRYO SIZE BECAUSE AS PROCESS CONTINUES, RELATIVE VELOCITIES BETWEEN OBJECTS INCREASES TO THE POINT WHERE COLLISIONS CAUSE DISRUPTION RATHER THAN ACCRETION (OLIGARCHIC GROWTH)

10. PHASE 2: PLANETESIMALS TO EMBRYOS
PREDICTIONS
10s TO 100s OF EMBRYOS
FORM WITHIN YEARS
SEPARATED BY ~0.1 AU DISTANCE FROM EACH OTHER
LUNAR TO MARTIAN IN MASS
NOT EXPECTED TO BE RADIALLY MIXED (DISK RADIAL GRADIENT SHOULD BE PRESERVED)
COULD HAVE DIFFERENTIATED IF FORMED RAPIDLY ENOUGH (BUT TIMESCALE OF PLANETESIMAL FORMATION IS LOWER LIMIT)

11. PHASE 2: PLANETESIMALS TO EMBRYOS
OUTER PLANETS
SNOW LINE AT 2-4 AU – DISTANCE BEYOND WHICH TEMPERATURE IN DISK IS LOW ENOUGH FOR WATER ICE TO CONDENSE
BEYOND SNOW LINE, DENSITY OF SOLID MATERIAL INCREASES TO POINT WHERE EMBRYOS CAN BE 1-10 EARTH MASSES
LARGE EMBRYOS BEYOND SNOW LINE WILL RAPIDLY ACCUMULATE H AND He ATMOSPHERES, FORMING GAS GIANT PLANETS (SATURN, JUPITER)
GIANT PLANETS PREDICTED TO FORM COMPLETELY IN SHORT TIME BEFORE FINAL STEP IN TERRESTRIAL PLANET FORMATION STARTS
GRAVITY OF GIANT PLANETS IS KEY IN FINAL STEP AS WELL AS IN OVERALL DYNAMICAL AND GEOLOGIC EVOLUTION OF THE EARLY SOLAR SYSTEM

13. PHASE 3: EMBRYOS TO TERRESTRIAL PLANETS
STARTING CONDITION
INNER SOLAR SYSTEM IS DISK OF PLANETESIMALS AND EMBRYOS WITH ABOUT EQUAL MASS IN EACH POPULATION
CENTRAL SOLAR SYSTEM HAS FULLY FORMED GIANT PLANETS WITH ORBITS INHERITED FROM EMBRYOS THEY FORMED FROM AND INTERACTIONS WITH DISK (I.E. ORBITS UNLIKE WHAT THEY ARE NOW)
OUTER SOLAR SYSTEM IS ANOTHER DISK OF PLANETESIMALS

14. PHASE 3: EMBRYOS TO TERRESTRIAL PLANETS
AS GAS DISSAPATES FROM THE DISK, THE DAMPING/STABILIZING EFFECT IT HAD ALSO GOES AWAY
Some amount of damping has to persist (dynamical friction due to interaction between populations of bodies with different masses) in order to account for observed angular momentum deficit (AMD)
ECCENTRICITIES OF EMBRYO ORBITS CAN THEREFORE GROW RAPIDLY, LEADING TO INTERSECTING ORBITS AND COLLISIONS
Not all collisions need be accretionary – perfect merging is rare and only 50% of collisions result in accretion at all – sometimes they bounce off each other with fragmentation, sometimes they merge with fragmentation
Fragments can either reaccrete with no net loss but they can also be ground to dust and effectively lost, leading to a type of chemical fractionation when fragmentation occurs preferentially from mantles of differentiated embryos
BIG EFFECT AT RADIUS OF ASTEROID BELT BUT SMALL EFFECT IN INNER SOLAR SYSTEM
HIGHLY CHAOTIC STAGE LEADS TO EJECTION OF PLANETESIMALS AND EMBRYOS FROM MIDPLANE OF SOLAR SYSTEM (SOME ENTIRELY)
PREDICTS
Right number (2-4), size (~Earth mass), spacing (all between au), and orbital characteristics (small eccentricities and inclinations) of observed terrestrial planets
Type of Mars-size collisions that formed our Moon
Myr timescale observed geochronologically for formation of terrestrial planets (e.g. Earth)
Observed mass depletion of asteroid belt and orbital characteristics of objects there, and, as a result, observed mixing of asteroid types
Mars is a problem… it is too small and formed too fast (in just a few million years, not 10s-100s)!

17. The Grand tack scenario
KEY TO GETTING A SMALL MARS IS HOW SOLID MATERIAL IS DISTRIBUTED IN PROTOPLANETARY DISK.
IF OUTER EDGE OF DISK OF EMBRYOS AND PLANETESIMALS IS AT 1 AU WITH NO SOLIDS OUTSIDE, YOU GET A SMALL MARS AND BIG EARTH
BUT THIS IS NOT CONSISTENT WITH ASTERIOD BELT BETWEEN 2-4 AU WITH SMALL MASS TODAY BUT (BY NECESSITY) 103 TIMES MORE MASS WHEN ASTEROIDS FORMED
SOLUTION SEEMS TO ALLOW GIANT PLANETS TO MIGRATE, SCULPTING CHARACTER OF INNER SOLAR SYSTEM AS THEY DO SO

18. The Grand tack scenario
IF JUPITER WAS THE ONLY GIANT PLANET, IT WOULD MIGRATE INWARD
WHEN JUPITER IS PAIRED WITH SATURN, THEY GET LOCKED INTO A 2:3 ORBITAL RESONANCE THAT ALLOWS THEM TO MIGRATE OUTWARD
SO…
JUPITER FORMED FIRST AT 3-4 AU
JUPITER MIGRATES INWARD WHILE SATURN STILL FORMING
SATURN REACHES CURRENT MASS AND IT MIGRATES INWARD FASTER THAN JUPTER
SATURN AND JUPITER GET LOCKED IN 2:3 RESONANCE
JUPITER GETS TO ABOUT 1.5 AU
INWARD MIGRATION STOPS AND JUPITER AND SATURN MIGRATE BACK OUT TOGETHER
MIGRATION STOPS WHEN GAS IN DISK COMPLETELY DISAPPEARS (TORQUES THAT CAUSE MIGRATION IN AND OUT VANISH)
JUPITER AND SATURN ARRIVE AT CURRENT ORBITAL DISTANCES FROM SUN
AS JUPITER PASSES THROUGH ASTEROID BELT (NOT ONCE BUT TWICE) IT CLEARS OUT OBJECTS, ACCOUNTING FOR ASTEROID BELT MASS DEFICIT, FOR “MIXING UP” OF ASTEROIDS OF DIFFERENT TYPES, AND FOR SIMILARITIES BETWEEN COMETS AND C ASTEROIDS
JUPITER’S CLOSEST APPROACH CAUSES OUTER EDGE OF INNER DISK TO BE AT 1 AU, CAUSING SMALL MARS AND STOPPING IT S ACCRETION PREMATURELY

23. NICE MODEL
DYNAMICAL MODEL FOR HOW ORBITS OF OUTER PLANETS HAVE CHANGED OVER TIME.
AT END OF GRAND TACK, SATURN AND JUPITER WERE LOCKED IN A 3:2 RESONANCE, URANUS AND NEPTUNE WERE LOCKED TO SATURN AND TO EACH OTHER, AND ORBITS WERE REALTIVELY CLOSELY-PACKED, CIRCULAR, AND CO-PLANAR…
THIS IS NOT AT ALL WHAT THEIR ORBITS ARE TODAY! OUTER PLANETS ARE NOT IN RESONANCES, ARE MORE SPREAD OUT, AND HAVE NON-NEGLIGIBLE ECCENTRICITIES AND INCLINATIONS!
MODEL ASSUMES THAT DISK OUTBOARD OF GIANT PLANETS (OUT TO 30 AU). AS GIANT PLANETS INTERACTED WITH THIS DISK, THEY CEASED TO BE IN RESONANCES, AND THEIR COMPACT ORBITS BECAME UNSTABLE.
NEPTUNE AND URANUS WERE SENT OUTWARD BY ENCOUNTERS WITH SATURN AND JUPITER, AND, AS THEY INTERACTED WITH DISK THEIR ORBITS RESTABILIZED.
RESULTED IN SCUPLTING OF THE OUTER DISK INTO THE KUIPER BELT, EJECTION OF OBJECTS FROM DISK TOWARD JUPITERS (TROJAN ASTEROIDS), CAPTURE OF SATELLITES OF OUTER PLANETS, AND…
RECONFIGURATION OF ORBITS OF OUTER PLANETS ACCOUNTS FOR LATE HEAVY BOMBARDMENT IN THE INNER SOLAR SYSTEM IMPACT HISTORY…

25. ORIGIN OF TERRESTRIAL WATER
PLANETESIMALS IN THE INNER SOLAR SYSTEM SHOULD BE VERY DRY
EARTH HAS A LOT OF WATER. WHY?
Water from the disk: At end of Earth formation, there somehow as a little H left in the disk that was captured in an early H-rich, dense atmosphere. This H was oxidized by FeO in the early Earth magma ocean to produce water. Problem: requires a lot of hydrodynamic escape of light H over a long time to get the 6x increase of D/H that is observed for Earth compared to disk.
Water from comets: LHB (or earlier) bombardment of Earth by comets brought the water. Problem: D/H of comets may/may not be right; not enough comets predicted to have hit us (only enough to account for 10% of water)
Water from outer asteroid belt: during later part of accretion (last 50-20%), C asteroids from outer asteroid belt were implanted into inner solar system by Grand Tack, providing enough material with correct D/H ratio to account for Earth’s water