Planet Formation

Topic:

Connecting to the solar system

Lecture by: C.P. Dullemond

Meteorites

Radioactive Dating

If we somehow know what the original abundances were at time t0,then by measuring the abundances now, we can calculate the time since t0.

Problems:• We often do not know original abundances• How do we define t0 anyway??

Radioactive DatingDefinition of time t0: The moment at which a rock has solidified (crystallized).

Key ingredient: We assume we have chemically heterogeneous patches of rock (if not, then we would only measure the time since element formation in previous generation of stars, which is nice, but not what we want; or if the system is not closed, we would not know initial abundances, so our measurement is useless).

Before t0: We assume that the material has been well-mixed between the above mentioned patches, so that all isotopes of the same element have the same abundance ratios between the patches. Only the chemistry can create heterogeneous patches: but this only creates variations in abundances of different elements, not of their isotopes.

Radioactive Dating

After t0: We assume that chemistry has stopped. No exchange of elements nor of isotopes between patches. From now on the radioactive decay can change isotope ratios. For different patches this occurs with different rate, because of different chemical compositions.

Example: U-Pb system: 238U 206Pb Halflife = 4.47 Gyr 235U 207Pb Halflife = 0.70 Gyr

If the U/Pb ratio in one patch is high, then 206Pb grows fast; if the U/Pb ratio is low, then 206Pb grows slow. Same for 207Pb but with 6x lower rates. By comparing two or more patches you get age since solidifying.

U-Pb system: Holmes-Houtermans method

The radioactive decay channels:

238U 206Pb Halflife = 4.47 Gyr 235U 207Pb Halflife = 0.70 Gyr

We measure the abundances of 206Pb and 207Pb in various chemically slightly distinct patches of rock. We take 204Pb as a reference.

U-Pb system: Holmes-Houtermans method

U total

238U

235U

U-Pb system: Holmes-Houtermans method

Increase of 206Pb for different patches

High initial U/Pb Ratio

Low initial U/Pb Ratio

U-Pb system: Holmes-Houtermans method

Increase of 207Pb for different patches

High initial U/Pb Ratio

Low initial U/Pb Ratio

U-Pb system: Holmes-Houtermans method

Increase of both 206Pb and 207Pb for different patches

U-Pb system: Holmes-Houtermans method

Increase of both 206Pb and 207Pb for different patches

U-Pb system: Holmes-Houtermans method

Isochrones for casewith t0=-4.56 Gyr

t=-4.56 Gyr

t=-3.04 Gyr

t=-1.52 Gyr

t=today

U-Pb system: Holmes-Houtermans method

Now a different solidifying date: 3.5 Gyrs ago

t=-3.50 Gyr

t=-2.23 Gyr

t=-1.17 Gyr

t=today

Clair Cameron Patterson

Clair Patterson was the first (1953) to use Pb-Pbdating to date the age of the Earth and the solarsystem bodies:

Age = 4.5 Gyr (Now: 4.567±0.001 Gyr)

He also discovered in the process that humanity was in the process ofpoisoning itself with leaded car fuel! His continued effort agains strong lobbies by the industry eventually led to the Environmental Protection Agency 1973, which stopped and eventually reversed the lead output into the food chain.

Late Heavy Bomardment?Samples from the Apollo moon landings all seem to date from between 3.92 and 3.85 Gyr ago (remember: age of the solar system = 4.5 Gyr). Conclusion: Something dramatic must have happened: A lunar cataclism.

But: could the Apollo samples all be „polluted“ by a single large impact?

Meteorites: Chondrules, matrix & CAIs

Meteorites: Chondrules, matrix & CAIs

Chondrules: A big mystery

• Chondrules are round once-molten droplets that have been flash-heated and very rapidly cooled (the whole process no longer than a few hours).

• Evidence for this quick process:– They still contain many volatile elements, which they

would not, if they were heated over a long time. – Their textures require quick cooling

• Ideas:– Colliding planetesimals?– Shock heating in the nebula?– Lightning in the nebula?– Reconnection events in the nebula?

Saturn‘s Rings: A dynamicist‘s paradise

Saturn‘s Rings: A dynamicist‘s paradise

From Cassini spacecraft

Saturn‘s Rings: A dynamicist‘s paradise

From Cassini spacecraft

The moonPrometheus

Saturn‘s Rings: A dynamicist‘s paradise

From Cassini spacecraft

Encke gap edge

Saturn‘s Rings: A dynamicist‘s paradise

From Cassini spacecraft

Resonances...

Saturn‘s Rings: A dynamicist‘s paradise

From Cassini spacecraft

Model of the formation of Saturn‘s rings

Crida & Charnoz (2010)Nature 468, 903