Mysteries of Mars: resolving the Shergottite Paradox

GeoSciences lecturer Dr Lara Kalnins, whose expertise stretches from marine geophysics to volcanism, recently worked alongside an international team of researchers to tackle a prevailing mystery in planetary science known as the "shergottite 40Ar/39Ar age paradox." 

Shergottites are one of three types of Martian meteorites and are considered relatively ‘young’ (200 to 500 million years old) compared to most of Mars’ surface (over 3 billion years old). This discrepancy has puzzled scientists for decades, particularly as one of the most reliable dating techniques has produced wildly varying ages, prompting a re-evaluation of the methods used to date these meteorites.

Using a technique known as Argon-Argon dating (40Ar/39Ar) and an innovative method of accounting for the influences of Mars, Earth, and outer space on the samples, the research team’s findings confirmed that the shergottites are indeed young, providing a coherent picture of their formation period.

The shergottite paradox highlights a gap in our understanding of Martian geology. Despite the ancient age of Mars' surface, most Martian meteorites that reach Earth – about three-quarters – are quite young. This raises questions about why meteorite samples – volcanic rocks that were ejected from the surface of Mars due to impact events and then traversed 140 million miles of space before landing on Earth – are not representative of the broader age distribution of the Mars’ surface.

According to Dr Kalnins, one theory suggests that asteroid impacts on Mars are more likely to dislodge and eject younger rocks from the planet, meaning that mainly younger materials reach Earth as meteorites.

To resolve this paradox, the researchers re-analysed seven shergottites using the 40Ar/39Ar dating method to test whether careful preparation and consideration of terrestrial and Martian influences could yield consistent ages that matched other dating methods. Argon-Argon dating is a method used to determine the age of rocks by measuring the ratio of Argon-40 (40Ar) that is produced via the radioactive decay of 40K (an isotope of potassium).

Collecting Martian meteorite samples is quite different from traditional fieldwork approaches on Earth, which mostly involve the researchers physically collecting their desired rock samples. 

Dr Kalnins described how the research team was able to get a hold of the Martian samples in the first place:

“For the meteorites, getting samples generally means either a trip to a museum – many of the ones in this study came from the Natural History Museum in London – or getting them sent by post, such as from NASA. We don’t have any control over what bits of Mars arrive as meteorites, and they don’t bring any details about where on Mars they came from.” 

The amount of material that can be analysed is also tiny compared to terrestrial samples on Earth. For the meteorites, the team had only fractions of grams of material to work with, whereas for terrestrial samples researchers might have a few hundred grams to a few kilograms. 

The new analysis approach worked better than the research team had dared hope.

Dr Kalnins described what the group discovered: 

“With careful sample preparation and analysis and considering all the different possible argon reservoirs in the meteorites, we calculated ages that agreed with the other methods – these shergottites are young.”

Here, Dr Kalnins conducted numerical modelling to show how rapidly meteorites of different sizes could cool in the vacuum of space. This was especially important for showing that the meteorites quickly cooled below the closure temperature – the point at which minerals stop losing Argon, which can affect the age – so it should be possible to get reliable ages from the samples after they’ve been ejected from Mars’ surface. 

According to Dr Benjamin Cohen, the paper’s principal author: 

“To be honest, the most surprising thing for me was how well it all worked. Shergottite meteorites have been analysed by 40Ar/39Ar for decades but with a very mixed bag of results. That contrasts sharply to terrestrial volcanoes, where 40Ar/39Ar is one of the most commonly applied and precise methods. 

Now we know that 40Ar/39Ar works for the shergottites (and more importantly, how it works, and why it didn't before) the technique can be applied to other samples. After all, we analysed seven shergottites in the 2023 study, and there are now over 300 of them recovered.” 

The findings from this research extend beyond just resolving the shergottite age paradox, offering new insights into the complex geological history of the Red Planet. 

Now equipped with a clearer understanding of the formation ages of these meteorites, scientists can use this knowledge to investigate Martian history during that era. As the shergottites are young, they likely originate from some of the youngest volcanoes on Mars, such as massive shield volcanoes like Olympus Mons. This offers a window into the geological processes that shaped these colossal structures over millions of years.