Class 17 - TP Geology: Cratering

READING: We have pretty much finished Chapter 8, first dip back into Ch 7 (pp 178-179) and then we are moving on to Chapter 9.

Links to check out (added Oct 30, 2002):

Dan Durda (Southwest Research Institute, Boulder) has a great Apollo landing site website

Views of craters on Earth - from the Lunar and Planetary Institute

Clem entine explores the Moon - from the Lunar and Planetary Institute

Impact craters on Mars - from the Lunar and Planetary Institute

Imp act craters on Europa - from Galileo Imaging Team

The Richat Crater - the crater that supposedly isn't - the JSC Digital Collection.

Upheaval Dome, Canyonlands, Utah - the crater that supposedly wasn't - but is. Color version.

The Boltysh crater - related to the dinosaur-killing Chicxulub event 65 million years ago? Another Chicxulub site.

And, just for curiosity - check out the "new moon of Earth"

Here are some geological maps of terrestrial planets - the legend shows how the different colors correspond to different types of geological units - cratered areas, volanic areas, folded mountains, etc.

Limited coverage - but mostly impact terrain.

Sister planets - but note linear mountain chains and rifts on Earth.

Mars - volcanic in north, cratered terrain in south. Moon - cratered all over but large impact basins filled with lava on earth-facing side.

HOW DID THESE PLANETS GET TO BE SO DIFFERENT??

First.... remind yourself of Classes 15 + 16 - the internal thermal evolution of planets. The primary controling factor of the geological activity of the TPs is SIZE - BIG planets cool SLOWLY.

............

........ We'll start with the planets where there is mostly only primary crust - the smaller planets - where the main geological process is cratering.

Flow Charts for Geological Processes - these charts show CAUSE-AND-EFFECT RELATIONSHIPS between Formation Properties (top green row) and the Factors which control geology (brown lower row)

First - let's look at CRATERING..... and see how the Formation Properties and Geological controling factors are related to the GEOLOGICAL PROCESS (botom blue row) of Cratering (leftmost).

Interestingly, most of the arrows seem to be coming OUT of the blue circle labelled cratering - this means that few factors control the amount of cratering - just a (small) dependance of impact cratering on gravity (and therefore mass) of the body being hit (more massive bodies produces higher impact velocities). Otherwise, all planetary surfaces are equally likely to be hit (excepting that on planets with atmospheres, the smaller impactors vaporize on entry and are less likely to make it to the surface).

IMPACTS PROCESS:

• Impact speeds few-10s km/s - orbital speeds of objects in inner solar system.
• Does not really matter what direction the impactor comes in - it explodes to make circular, symmetric crater
• At these speeds, the kinetic energy delivered is so huge (Figure 8-9.6 shows 10¹⁵ to 10²⁵Joules)
• Impactor vaporizes (unless a large iron core), some of crater vaporizes, big hole excavated and deposited as ejecta
• Size of crater made by impactor of given size follows "rule of thumb" that Dcrater ~10 Dimpactor.

Figure 9-6 IMPACTOR-CRATER SIZE relationship

CRATER DENSITIES:

• More craters/area = older surface, less craters/area = newer surface
• For relative aging, this is kinda obvious. But what about absolute ages?
• Rocks brought back by Apollo astronauts from different areas on the Moon with different crater densities means that we now have an absolute CRATER DENSITY-AGE relationship

Note: There really probably was a burst of impacts - the Late Heavy Bombardment - about 4 billion years ago .... more about that later.

• Figure 9-19 shows that for areas of heavy cratering on the smaller bodies (too small for internal heat to have sustained geological activity), the CRATER DENSITY vs SIZE distribution is basically the same. This shows that everything got bashed about the same amount and by a similar population of impactors.
• Figure 9-20 for the Moon shows that the distribution of CRATER DENSITY vs SIZE follows the same slope at different locations - in fact, proportional to Diameter^-2. At different times the RATE changed (decreased with time) - but the distribution of crater sizes remains about the same. Since we just found that the crater size is pretty much linearly related to the size of the impactor, we can say that the IMPACTOR SIZE DISTRIBUTION must also have been proportional to Diameter^-2.

Figure 9-19 & 9-20 CRATER DENSITY - SIZE relationships

(Remind yourself how power-law functions produce straight lines on log-log plots)

IMPACTORS GENERATED BY COLLISIONAL PROCESSES

• Look at Figure 7-13 for Fragment size vs asteroid size - examples of populations with approximately 1/R^-2 size distributions - indicative of being produced by collisions with each other - these are the fragments that did not accrete but bashed into each other at high speed and broke up (the "non-conformist planetesimals"!). Read pages 178-179.

FORMATION OF THE MOON by GIANT IMPACT

Of course the ultimate impact was the original one (much earlier than 4 billion years, obviously), when a Mars-sized object hit the Earth - sending material into orbit which then accreted to form the Moon. Note in the movie how the core material from the impactor gets left with the Earth - making Earth's core large and the Moon's core small (and the Moon's depleted in siderophiles - iron-loving elements).

Movie - computer simulation of the formation of the Moon by Robin Canup of Boulder's Southwest Research Institute (and APS PhD graduate).

The BIG 5 Facts About Impact Cratering

1. Impactors hit at HYPERVELOCITIES (~escape speed of planet) so they explode when they hit the surface and make round craters with little indication of the direction they hit. When the impact <45 degrees from the surface there is some indication of "shadow zones" of little ejecta. VERY oblique trajectory is need to produce any asymmetry in the crater.
2. Kinetic Energy of impactor -> Excavation, ejection, dissipation (shock waves, heat, melting, etc).
3. D(crater) ~ 10 x D(impactor) - useful rule of thumb - Figure 9-6.
4. Crater Density vs. Age - profile of cratering rate provided by lunar rocks, applicable (more or less) across the solar system - critical for dating surface by crater density. See Figure 6-6.
5. Crater size distribution - N(craters) a D(crater)^-2 (see Figure 9-20 - interesting exceptions of Earth and Venus where there seem to be a lack of small craters mostly due to the small impactors "burning" up in the atmosphere, and to a lesser extent erosion)
6. (2)+(4) gives us the fact that N(impactors) a D(impactors)^-2 - a size distribution characteristic of a population produced by (mutual) collisions. See Figure 7-13.