The surface of Mars has a rich geological record, and the Curiosity rover is reading that record by sampling the rocks and soils at different locations along its route to Mount Sharp. This record covers almost the entire history of the planet.
But the collection of information about each of these locations is like a mixed-up book with unnumbered and missing pages; page is very interesting, but without knowing the order of the pages it is hard to see the big picture.
What we need are page numbers. In the geological record, those are provided by dates.
With solar system bodies like Mars, the most commonly used dating method is “impact crater densities.” The early solar system was dominated by impacts, including some very large ones. As the solar system aged, the number of impacts decreased, but even today they continue.
This means that if a body has little or no atmosphere and no resurfacing processes, the older surfaces should have more craters in a given area.
The moon and Mars are examples of this rule. On Mars, for example, areas with more than 400 craters over 3 miles in diameter per 621,371 square miles (1 million square kilometers) are very old, whereas areas with 25 to 67 craters in a similar area are relatively young.
But impact crater density dating gives us only a relative date — older or younger. It would be much more useful to know an absolute date.
In the 1970s NASA solved this problem for the moon by sending astronauts who collected rocks and brought them back to labs on Earth for dating. This gave us a relationship between impact crater dates and absolute dates found on Earth. There are differences between the moon and Mars, but the lunar relationships can be used to give us rough absolute dates for impact crater density ages on Mars.
In labs on Earth, a common way to determine absolute dates is isotopic dating. For example, in rocks that contain potassium, part of the potassium will be an isotope, potassium-40. This isotope decays into argon-40, with a half-life of 1.25 billion years.
If we assume that the rock started in a molten state, it then contained no argon-40. Once the rock was in solid form, the argon-40 produced by the decay process was trapped in the rock. Later, if the rock was heated, the created argon-40 would be released, and the amount could be measured. Using the known half-life rate, we could determine the time needed to produce that much argon-40, telling us the age of the rock.
After Curiosity had landed, Kenneth Farley of California Institute of Technology proposed a bold new use for an instrument that was now on Mars: using Curiosity’s sample analysis instrument to test Martian rocks. This would be the first time that this type of analysis had been done anywhere besides Earth.
Last fall NASA took a sample of the rock powder from the second hole drilled at Yellowknife Bay and heated it within the sample analyzer. The rock powder released a large amount of argon-40. The detected argon-40 released from the powder yielded an age of 4.2 billion years.
This result was not a surprise; it is about the same as the 3.6- to 4.1- billion-year age predicted by the impact density method.
If NASA builds instruments on a future rover that are designed specifically for this measurement, we could expect even more precise results.
In understanding Mars, there is another important dating process: how long has a rock been exposed near the surface? In the next column I will explain how this age is determined and what the results tell us about ongoing processes such as erosion, as well as where organic molecules are most likely to be found.
Marty Scott is the astronomy instructor at Walla Walla University. He can be reached at firstname.lastname@example.org.