Something old or something new

A recent paper on the lunar impact craters has shown that meteorites crash into its surface of the Moon more frequently than previously estimated, this not only has implications for future exploration of the lunar surface but also may adjust when we think the events on other planets and moons in the solar system happened.


A new ~34  m diameter crater from Nasa’s LROC, image is 1000 m wide NASA/GSFC/Arizona State University

In order to understand how geological events area dated in the solar system, lets look at how we date events in on Earth; geologists have two styles of dating: absolute & relative. Relative dating is done by looking at the relationships between two different geological features to work out which is older and which is younger. For example, a layer of sandstone on top of limestone, we know the limestone came first and the sandstone was deposited on top of this. A volcanic magma is younger than the existing rocks it cuts through on its way to the surface. This form of dating enables working out the order of events but does not give an actual date, amount of time or how long has passed between the two events.


A volcanic dyke (yellow) cuts across preexisting volcanic ash layers (blue) in Tenerife

In contrast, absolute dating uses the radioactive decay of elements in to give a range of dates when something could have formed, based on the time that has passed. Measuring the amount of the parent (the initial element) and the daughter (what it decays into) allows the calculation of the age of the rock using the decay rate. Many different elements such as lead or potassium can also be used, as elements with vary in abundance  in a sample and have different decay rates vary, different methods have to be used for different situations.

These methods, alone or as a combination can be used to estimate when and in what order something happened on Earth, however, for other bodies in our solar system we have only undertaken limited exploration and whilst it is possible to do relative dating from a distance, absolute dating requires clean labs and high tech equipment, so how do we get dates for the events on other worlds?

Most of the exploration of the rest of the solar system has been done remotely by satellite, the features are not directly sampled. Where we have sent probes to the surface of planets so far no lander or rover has been built with the ability to carry out dating as it is done in an earth based lab (limited dating has been done on Mars but as the equipment was not designed for this so gives a wide range of possible dates and so the data is of limited use). The few meteorites found on Earth which come from other planets, can be used to get the date those samples, however, it is often unclear of the context (where on the planet they originated from) and so can’t be used to date regions areas and features.

In order to date features on other planets, a modified form of relative dating is used to provide dates. Meteorite impacts have occurred throughout the history of the solar system and continue to happen, the older the geological feature, the more impact craters are likely to have impacted this surface. Larger impacts happen less often than smaller impacts so surfaces with larger craters tend to be older than younger ones.

In order to give a date to this, the rate of cratering has estimated, whilst each impact event is random, a curve based on the predicted impacting rate for a certain size crates can be calculated for a given surface since the late heavy bombardment 4.1 billion years ago. Count the number of craters of a certain size in a given area and an estimation can be made of the age of that feature.

These curves are then calibrated to dates using the impact craters on the Moon lined tied to the limited number of moon rock samples retrieved by the Apollo astronauts. So all planetary bodies within the solar system are linked to the craters and the cratering rate of the moon.

The paper takes data from the  Nasa’s Lunar Reconnaissance Orbiter which has been orbiting the moon for 9 years and shows that the frequency of impact craters is higher than expected, so craters are forming more often, suggesting that surfaces with craters on them may be younger than previously thought.

This has implications for the possibility of previous life on Mars  which is linked to the presence of liquid water on mars, evidence for this is given by floodplains dated through impact craters, if these are slightly younger than previously estimated it then Mars may have had longer time period where mars had liquid water, the longer the time, the greater the opportunity for life to start.

The exceptions to this are Earth and Io. The rocks on Earth are easily available to get to labs for analysis, the surface is often very active with geological activity causing many smaller impacts to be destroyed. Io is so volcanically active that it its surface is covered by 1 cm of new material each year, which is faster than impact craters form.

It will be interesting to see how the new findings adjust the impact rates else where in the solar system and what effects that will have on the timing of events on our planetary neighbours.


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