The Age of Planetary Surfaces: How Do We Discover It?
One of the pressing questions about meteorites is what their Parent Planets were like - how large they were, and how evolved internally and externally. Part of answering this question involves knowing about the Ages of meteorites and (ideally) the planetary objects they came from.
Ages of meteorites can be determined directly via Radiometric Dating - certain radioactive Parent Elements (such as the Uranium isotopes 238U and 235U) undergo Radioactive Decay (essentially, their nuclei lose protons and neutrons, generally as a particles (2 protons + 2 neutrons in a package) or as b particles (a neutron releases an electron, becoming a proton) and become specific Daughter Elements (in this case, the lead isotopes 206Pb and 207Pb), over a certain Half Life (t1/2, defined as the amount of time it takes for 50% of the radioactive parent atoms to decay and become daughter atoms). We do the measurement by chemically separating the parent and daughter elements from our sample meteorites, and examining the ratios of the Uranium and Lead isotopes in the sample via a technique called Mass Spectrometry.
When we do this, we discover that all but a few meteorites are very old - 4.55 Billion Years Old, in fact, as old as the Solar System itself! A few, however, (particularly the SNC meteorites, which we think are from Mars, and the "Lunar" meteorites recently found in Antarctica) give us younger ages (in some cases very much younger - the SNC rocks range in age from 150 Million years to 4.45 billion years old). That means at least some meteorites could actually have been knocked off existing planetary bodies. But how can we be sure?
A first step toward answering this question is finding out how old the Surfaces of existing planets are. While we think all planets originated with the Solar System, it's clear that the surfaces of planets change with time, both due to internal processes (on Earth, Plate Tectonics and the internal planetary convection that drives it recycles surficial rocks on timescales as short as 100 Ma), and due to one particular externally derived process: Meteor Impacts. How do we recognize the effects of meteor impacts? We look for Craters!
As we'll discuss this term, impact cratering is and has been the most common "geologic" process which occurs on planetary bodies in our Solar system. It has gone on since the planets formed by a process called Accretion (essentially impacting and piling up of small bodies via gravitational effects), and continues today. Basically, there is a whole lot of planetary debris floating around out there (most of it now coming from the Asteroid Belt), and over time it is drawn into the gravity fields of planets, where it lands explosively, making craters. The rates of impacts have declined predictably thorugh time as a function of meteor density (see Figure 1), and, most importantly, if a planet's surface has been undisturbed by geologic forces within the planet, that surface will accumulate impact craters as a function of is age! That means we can infer the ages of planetary surfaces from crater density!
So, what we're going to do is look at planetary surfaces, and make some inferences about their ages (actually, about their Age Distributions, as determining a real age via Crater Stratigraphy requires many hours of work!)
We will break up into groups. Each group will be given 10 regional-scale pictures of planetary bodies (the Moon, Mars, Europa and Venus). For about 30 minutes, each group is to look at their pictures and group them based on Crater Density. Distinguish the groups as follows:
•Craters on top of craters on top of craters (high density)
•Craters abundant, but not overlapping,
•Craters present but not abundant (>20 in the picture)
•No craters at all!
Every group will NOT have pictures that fall in every category - the images may only fall in one or two of these slots.
After about 40 minutes, your instructor will call for your data, which she/he will tally on the board according to planet. Using this data, you should INDIVIDUALLY answer the following questions:
1) Which planetary body of those examined is the most heavily cratered, generally?
2) Which planetary body shows the most diversity in cratered surfaces, and which one shows the least?
3) SNC meteorites range in age from 150 Ma to 4.45 Ga, while the purported "lunar" meteorites show two age clusters, one at 3.0 to 3.5 Ga, and another around 4.4 Ga. Based on the crater distributions of the planets you have looked at, which of these bodies is the most likely to have generated these meteorite families?
4) New Galileo data on the surface of Europa suggests that its icy surface gets extensively reworked by tidal forces from Jupiter, so that on average its surface is not older than about 30 million years. Based on the class cratering data, are there other bodies in the Solar system with surfaces or regions that young?
Hand these answers in for credit on the exercise!!
Links to images of the Moon
Links to images of Venus
Links to images of Europa
Link to GLY 4045 Homepage