Copernicus and Eratosthenes


Avani Soares

Parsec Observatory, Canoas, Brazil

C14 Edge + ASI 290 + IR 685

Copernicus and Eratosthenes
The chart above may be considered one of the most important diagrams of lunar science. This graph is the basis for estimating the ages of the parts of the Moon where we do not have physical samples. In the Y-axis of the graph is represented the number N of impact craters in areas where the Apollo and Luna probes collected samples. In the laboratory the ages of these samples were determined and these data are shown on the X axis. The units for the X axis are easy to understand, the age of formation of rocks in giga years, ie, billions of years. Y-axis units represent the cumulative number of craters equal to or greater than 1 km in diameter per square kilometer. Remote sensing scientists use the best images of the Moon to count the number of primary impact craters in areas centered on the locations where the Apollo and Luna mission samples were collected. Normally this crater count covers areas of hundreds of square kilometers in order to ensure statistical significance for the value of N. This graph has been used for a few decades but, for example, Copernicus crater data did not fit with it , there appear to be also many subsequent craters comparing to the assumed age of 0.8 Ga based on the dates made on the Apollo 12 samples than may be a Copernicus radius. But now Harry Heisinger and his colleagues used high-resolution images of the LRO probe to retell the craters that formed on top of Copernicus and its ejected material, determining an N-value that exactly matches what was predicted. The other red markings show that the new N-values ​​determined for Tycho and the Apollo 16 North Ray Crater crater confirm previous estimates. The power of this chart is that you can count craters for many areas of geological interest on the Moon and extend a horizontal line from the calculated values ​​of N on the Y axis to the right until you intercept the curve. Arriving at the bend and down to the X axis you then have the estimated age based on the counts of calibrated craters. Scientists from the Kaguya mission used this technique when they determined that there were lavas with age of 1.2 Ga in the Oceanus Procellarum. This figure is also a story of the bombardment that the Moon suffered, with the tail of the growth being responsible for the steep fall in the value of N to around 3.5 Ga. Nobody knows exactly why the rate of crater formation has stabilized with only a small decrease from 3.5 to 1 Ga, or why it has since plunged. But there are considerable uncertainties in this curve, the lunar samples were dated to determine the ages for Nectaris, Copernicus and Tycho, but there is no clear evidence that the samples actually came from these features.
Source: LPOD / Cienctec
Adaptation: Avani Soares