Humans have not yet explored Mars, but interestingly, Mars has made its way to us. Through various processes such as violent impacts, chunks of Martian rock have been ejected from Mars and traveled across the Solar System, ultimately crashing into Earth.
As scientists collect these samples from our neighboring planet, they have noticed a peculiar pattern. Most of these samples appear to be relatively recent rocks, which is surprising considering that the majority of Mars’ surface is ancient.
One possible explanation for this discrepancy is that the age estimates are inaccurate. Different dating techniques have yielded different results, making it difficult for scientists to confidently determine when these rocks formed on Mars.
However, a team of scientists from the US and UK have now found a solution to this problem. To their surprise, they have discovered that many of these rocks are indeed quite young, only a few hundred million years old. This information not only provides insights into the duration of the meteorites’ journey to Earth, but also offers valuable clues about geological processes on Mars.
“We can confirm from certain chemical characteristics that these meteorites are indeed from Mars,” said volcanologist Ben Cohen of the University of Glasgow, who led the research.
“These rocks were expelled from Mars due to massive impact events, creating large craters on the planet’s surface. However, with tens of thousands of impact craters on Mars, it is challenging to determine the exact source crater of these meteorites. The age of the samples is one of the best clues we can use to pinpoint their origin.”
Currently, there are approximately 360 meteorite samples on Earth that have been identified as originating from Mars. At the moment, most of these, about 302, are classified as shergottite, a type of metal-rich Mars rock formed in volcanic activity.
Scientists have estimated that Mars’ surface is ancient based on its high number of impact craters. If the surface were younger and subject to volcanic activities, many of these craters would have been erased. Therefore, the rocks ejected from Mars should also be old.
However, the dating techniques used to determine the age of shergottite on Earth are complicated due to the rocks’ composition. The limited information obtained so far has suggested that many of these rocks are less than 200 million years old, creating the shergottite age paradox, which has puzzled scientists for decades.
Various explanations have been proposed to account for this unexpectedly young age, ranging from a singular origin point for the younger shergottite rocks to the possibility that the impact events could have reset the rocks’ age through intense heating and compression. However, none of these theories align with the evidence presented by the rocks themselves.
The method employed to determine the age of shergottite is called argon-argon dating, which relies on the decay of radioactive potassium into argon. By examining the ratio of certain argon isotopes, scientists can measure the duration of radioactive decay and establish the age of the rock sample.
However, determining the age of shergottite on Earth is more complex because scientists must consider different potential sources of argon that could contaminate the samples. For shergottite, which originated from another planet and traveled through space for an unknown period, the contamination sources are more diverse. There are five potential sources of argon for shergottite, compared to three for Earth rocks.
To address this challenge, Cohen and his colleagues developed a method to compensate for argon contamination from both Earth and space. Once they made the necessary adjustments, the argon-argon ages aligned with other dating methods, such as Uranium-Lead.
The researchers analyzed seven shergottite samples and obtained ages ranging from 161 million to 540 million years ago. They propose that the younger rocks result from the continuous bombardment of Mars, which has eroded the older surface and exposed the younger rocks underneath. These younger rocks have subsequently been replenished by ongoing volcanic activity and are more likely to be excavated and ejected into space.
Mars may still be experiencing volcanic activity today, and the planet endures constant impacts. Scientists estimate that more than 200 impacts, resulting in craters over 4 meters in diameter, occur on Mars each year. Therefore, it is not surprising that younger rocks occasionally find their way to Earth through the intricacies of the Solar System.
The research has been published in Earth and Planetary Science Letters.
