Surface processes I: Impact cratering

Yours truly pleased to be in Chicxulub Pueblo.
Why was I so pumped up to be at Chicxulub Pueblo?

Although it looks placid and unremarkable on the surface, Chicxulub is the site of one of the great pivotal events in Earth History.

Lunar craters from Wikipedia
Ubiquitous impact cratering in early Solar System is implicit in our understanding of how the planets formed from the repeated collisions of smaller bodies.

These impacts occur as hypervelocity - up to:

(Psst! why should these be different?)

Lowest impact velocities would roughly equal planetary escape velocity

Gravitation anomaly image of Chicxulub impact.
Some significant impacts:

Whereas most geologic processes operate gradually, effects of impacts are essentially instantaneous.

Slow acceptance of the idea

The rise of geology as a science occurred in the late 18th and early 19th centuries through the work of people like James Hutton and Charles Lyell. Their big contribution was the concept of uniformitarianism. The uniformitarian philosophy holds that the processes that we see acting on Earth's surface today are the ones that have shaped it in the past. Summed up by the slogan:

"The present is the key to the past."

To geologists raised in this philosophy, the idea of instantaneous catastrophic changes seems intellectually ugly. Thus, the reality of impact cratering gained very slow acceptance. Into the 1960s, the prevailing view was that lunar craters were volcanic. Two misconceptions perpetuated this view:

The remedies:

Barringer Crater from Wikipedia
Barringer Crater (aka "Meteor Crater") A case study:

Lake Manicouagan Wikipedia
Today, Barringer crater is an excellent case study because it is young (50,000 years), relatively well preserved, and accessible to study. Indeed, more ancient impacts such as the roughly 200 ma site marked by Lake Manicouagan (right) are hard to identify on Earth due to the destructive effects of plate tectonics, erosion, and sedimentation. (Note, the Manicouagan impact seems increasingly likely to be the agent of the relatively minor Adamantian-Revueltian faunal turnover during the later Triassic.) Indeed, the Chicxulub impact site - a primary contributor to the extinction of the non-avian dinosaurs, is buried beneath later sediment and can only be imaged remotely.

The Impact

Terminal Free-Fall: A small enough impactor can be slowed to a terminal free-fall velocity by the same factors that impose terminal velocity on falling volcanic material. On Earth, objects smaller than 20 m can be slowed in this way. On Mars, much smaller objects might strike the surface at cosmic velocities, whereas on Venus, much larger impactors may be slowed significantly.

What would you expect to see on Titan, with a surface atmospheric pressure of 1.45 bars and escape velocity of 2.65 km s-1?

What actually occurs during an impact. Although instantaneous on a geologic time scale, impacts actually unfold over time. We conventionally divide an impact event into three stages:

Crater types:

Crater morphology typically varies according to size:

Schiller Crater on the Moon from The Thunderbolts Project
Low-angle impacts: Viewed form above, impact craters are roughly circular regardless of the path of the impactor. This only changes if the impactor approaches at an angle less than 10 degrees from horizontal. Before planetary scientists appreciated the role of the expanding shock wave in the creation of the crater, they expected that impactors many impact structures would be elongate because impactors don't necessarily approach from directly above. This misconception contributed to our slowness to recognize the impact origins of lunar craters. (E.G. Schiller crater, right)

Double-layered Martian crater from International Business Times
Double-layered craters: A Martian anomaly, these appear to have two distinct ejecta blankets. A 2013 study suggests that this occurs when the impact penetrates a thick layer of ice, depositing ejecta on top. When the underlying ice later melts, it lubricates some ejecta causing it to slide downhill to produce the second outer blanket.

Contrasting crater saturation of young and old terranes (which is which?) on Enceladus from

But what GOOD are craters? - Impact Chronology:

If impactors struck planetary bodies at a known constant rate, and each impact yielded a recognizable crater, then it should be easy to determine the age of a region from the concentration of craters. Thus, cratering provides a useful window into the relative ages of regions. On the image at right, two distinct terranes meet on Enceladus. Which is clearly older, the one on the left or right?

Impact chronology relies on knowledge of both the number and size of craters in a region. Plotted against one another, these yield distinct trends for regions of different ages. The example at right shows the cratering frequency of distinct lunar regions, whose names are the basis for periods of lunar history. Of course, this system breaks down in regions so saturated with craters that every new impact destroys the record of older ones.

Note: The y-axis shows cumulative frequencies of craters. Thus, the blue point at lower center is telling us the number of craters with a diameter => 100 km / square km. The blue point at the top of the blue line is telling us the number of craters with a diameter => 10 km / square km. Naturally, as the size of the craters counted gets smaller, the number of crater / square km increases.

We also note that for any given crater diameter, older terranes have more craters than younger ones. Thus, pre-Nectarian lunar landscapes are older than Copernican/Eratosthenian ones.

Caveats: Note that, for a given body:

Finally, different bodies have different susceptibilities to impacting. Mars, being closer to the asteroid belt than the moon, is more likely to be struck. Io, being more geologically active than the moon, is more likely to have its craters obliterated as soon as they form (Note: Io shows no impact craters!).

Record of Late Heavy Bombardment
The Late Heavy Bombardment: In most cases, crater counts yield relative ages only. But for the moon, we can calibrate these by means of radiometrically dated rocks. An interesting pattern emerges: The most intense lunar cratering occurred during the waning stages of the moons accretion (duh). There appears to be a second peak in intensity between 4.0 and 3.8 ga. This second peak has now also been recorded for Mercury according to Marchi et al., 2013, and probably effected the entire Solar System. We will return to the causes of this Late Heavy Bombardment later. For now, understand that a thumbnail sketch of Earth's history would include: Link to a good review by Fassett and Minton, 2012.

Recent Martian impact from NASA
Although the rate of impact has diminished since the Late Heavy Bombardment, impactors still exist and do their thing. The HIRISE images at right were taken of the same locality two years apart. In the interval, a new impact occurred. (Psst! Why is the impact scar black?) Indeed, the Mars Exploration Rovers have encountered meteorites sitting on the surface in their travels.

Key concepts and vocabulary.
Additional reading: