Transcript Design of a Locomotive Engine for Dalian Locomotive & Rolling

Cratering in the Solar System
William Bottke
Southwest Research Institute
Boulder, Colorado
Craters
 Craters are found on
nearly every solid body
in the solar system.
 If properly interpreted,
craters can help us
understand how these
bodies have evolved
over the last 4.5 Gy.
Craters are Formed by
Impacting Comets and Asteroids
“Killer Asteroid,” National Geographic Television, 2004
The Physics of Impact Cratering
Impact into “Rubble-Pile” Asteroid
 Impact parameters.
– Projectile/target composition and
porosity, impact energy, etc.
 Additional effects.
– Slumping of crater walls.
– Formation of ejecta blankets,
secondary craters, spall, etc.
– Post-impact effects on the target
body (e.g., K-T impact).
Durda, Bottke et al. (2006)
Planetary Chronology from Crater Counts
 Relative surface ages can be derived from crater counts.
 Absolute ages of various surfaces can be estimated if we
understand the impact flux over time (and vice versa).
Focus on the Moon
 The Moon holds the most complete and clear history
available of the last 4.5 Gy of Solar System history.
The Lunar Impact Rate
 Lunar impact rate
has been variable
with time.
Hartmann et al. (1981); Horz et al. (1991)
The Lunar Impact Rate
 Lunar impact rate
has been variable
with time.
 Crater production
rates >100 times
higher >3.8 Gy ago.
Hartmann et al. (1981); Horz et al. (1991)
The Lunar Impact Rate
 Lunar impact rate
has been variable
with time.
 Crater production
rates >100 times
higher >3.8 Gy ago.
 Relatively constant
crater rate since
~3.7 Ga.
Hartmann et al. (1981); Horz et al. (1991)
Lunar Basins and the Moon’s Early History
 More than 40 basins (D > 300 km) formed on the Moon
between ~3.8-4.6 Gy ago (Wilhelms 1987).
Lunar Basins and the Moon’s Early History
Imbrium Basin
(3.91-3.82 Ga)
Orientale Basin
(3.82-3.75 Ga)
Stoffler and Ryder (2001);
Gnos et al. (2004)
 The two largest and latest-forming basins with solid age
constraints are Imbrium (1160 km) and Orientale (930 km).
Lunar Late Heavy Bombardment
Koeberl (2003)
 Were these two large basins produced by a spike of
impactors near ~ 3.8 Ga, creating a terminal cataclysm?
Lunar Late Heavy Bombardment
Koeberl (2003)
 Or were they produced by a declining bombardment of
leftover planetesimals from terrestrial planet formation?
Declining Bombardment Model
Code tracks collisional & dynamical
evolution of planetesimals
Bottke et al. (2006), Icarus, in press.
 Model lunar impact
rate for 1 Gy after
Moon-forming
event.
Declining Bombardment Model
 Model lunar impact
Imbrium &
Orientale
Formation Time
rate for 1 Gy after
Moon-forming
event.
 Imbrium/Orientale
formed between
631-821 My.
Bottke et al. (2006), Icarus, in press.
Declining Bombardment Model
 Model lunar impact
Imbrium &
Orientale
Formation Time
rate for 1 Gy after
Moon-forming
event.
 Imbrium/Orientale
formed between
631-821 My.
 Tests indicate we
cannot make these
basins at the 3σ
confidence level!
Bottke et al. (2006), Icarus, in press.
The Terminal Cataclysm
 If the declining bombardment model cannot work, most
lunar basins formed in an impact spike ~3.8 Gy ago.
 To produce a system-wide cataclysm, we need to
destabilize a large reservoir of asteroids and/or comets.
 The only known way to do this is modify the architecture
of the solar system!
New Solar System Formation Model
TNOs
 Old view. Gas giants/TNOs formed near present
locations and reached current orbits ~4.5 Gy ago.
New Solar System Formation Model
TNOs
 Old view. Gas giants/TNOs formed near present
locations and reached current orbits ~4.5 Gy ago.
Primordial TNOs
 New view. Gas giants formed in a more compact
configuration between 5-15 AU. Massive TNO
population existed between 15-30 AU.
Destabilizing the Outer Solar System
Tsiganis et al. (2005); Morbidelli et al. (2005);
Gomes et al. (2005)
Watch what happens after 850 My!
Destabilizing the Outer Solar System
Tsiganis et al. (2005)
Jupiter/Saturn enter 1:2
mean motion resonance
 Gravitational interactions with planetesimals cause
migration. Over time, Jupiter/Saturn enter 1:2 MMR.
 This destabilizes orbits of Uranus and Neptune.
Uranus and Neptune May Switch Positions
 A “close up” view
of the instability.
 Uranus/Neptune:
– Go unstable and
scatter off Saturn.
– Migrate through disk.
 Dynamical friction
causes orbits to
“cool down”.
Orbits of Giant Planets
 Model reproduces orbital
elements of giant planets.
 Model sensitive to one
parameter: disk mass.
 A ~35 Earth mass disk
produces long delay and
orbits of planets.
Tsiganis et al. (2005)
The Lunar Late Heavy Bombardment
 The 1:2 MMR crossing
causes secular
resonances to sweep
across the main belt.
 The asteroid belt loses
~90% of its population.
 Comet spikes comes
first; asteroids last.
 The Moon accretes
Gomes et al. (2005)
61021 g, consistent
with mass flux
estimates from basins.
This Model Also Explains…
 The formation of Uranus and Neptune over reasonable
Solar System timescales.
 The formation and orbital distribution of the Kuiper belt.
 The capture and orbital distribution of the Trojan and
Hilda populations (which are captured KBOs).
 The shape of the oldest crater size-frequency
distributions on the Moon, Mercury, and Mars.
Possible Implications for Mars
 Many buried basins found by MOLA
may be ~3.8 Gy old.
 Like the Moon, no Martian surface
may be older than ~3.8 Gy old!
– No surfaces survived from accretion.
– Rocks older than 3.8 Gy can exist and are
not a surprise.
 The earliest Martian events (Early
Noachian) may have taken place
over a much more compressed
timescale than previously thought.
Conclusions
 The study of lunar craters has led us to new planet
formation models that suggest the Solar System
dramatically rearranged itself ~3.8 Gy ago.
 Further crater research on the Moon and other bodies
may hold the key to properly interpreting the evolution
history of many Solar System objects.