Chronology workshop

Over the past few days I attended an interesting meeting hosted here at the LPI entitled "Workshop on Surface Ages and Histories: Issues in Planetary Chronology." One of the main ways we tell time in planetary geology is to look at the density of impact craters on a surface. Basically, the longer a given surface is exposed, the more impact crater it accumulates. If you measure the number and size of craters on a surface, you can determine its relative age - meaning you can tell if it is older or younger than other surfaces depending on if it has more or less craters.

There are, of course, subtle complexities to this method that makes something that sounds easy and trivial (counting the number of circular holes you see on a surface) rather difficult in practice. But this meeting was about more than crater counting: topics addresses included radiometric dating of lunar and meteorite samples, observation and modeling constraints on the population of impactors (i.e., asteroidal and cometary bodies), and much discussion about the early cratering history of the solar system.

One of the most interesting things I learned was from a presentation by Hal Levinson of the Southwestern Research Institute. Hal and colleagues have developed and modeled an interesting and very plausible scenario for the so late "Late Heavy Bombardment" or LHB. The LHB was a period of enhanced cratering when the rates of things striking the surfaces of the planets, including the Earth and our Moon, was thousands of times what the flux is today. This heavy bombardment occured around 3.8 to 4.0 billion years ago. Since the solar system is over 4.5 billion years old, it has always been something of a puzzle as to why there why a delay (500 million years) between the formation of the solar system and the onset of this heavy bombardment. Some have argued that the LHB was simply the terminal end of the accretion processes, which is the process of forming the planets through the accrection (via collision) of smaller bodies. Other, such as Grahm Ryder (formerly of the LPI), argued that this event could have represented a spike of intense activity.

In the model Hal presented, rapid migration of the outer planets (Jupiter, Saturn, Uranus, Nepture) after a long peroid of quiescence triggered a disruption of the Kupier belt. The Kupier belt is a zone of icy bodies that currently exisits outside the orbit on Neptune. In the model, this Kupier belt once a) held many more objects and b) was located closer to the sun. When it was disrupted, it sent a hail of large impactors all over the solar system that absolutely pounded the terrestrial planets. Gerhart Nuekum, a German scientists long involved in planetary research since the Apollo era, noted that if this cataclysm is true, it means ~90% (!) of the craters we see today on the Moon and other bodies date back to this peroid.

Hal showed a simulation of this event that was really eye-opening. The planets just sit there happily orbiting for 700 million years, and then boom - all hell literally breaks loose. It's the first time I've even seem the lunar cataclysm put on a firm theoretical basis.