source : www.sciencealert.com
Humans have yet to set foot on Mars, but over time Mars has come to humans. Chunks of Martian rock ejected from their homeworld by processes such as violent impacts have worked their way through the solar system to eventually – smack! – crash to earth.
As we collect these samples from our neighboring planet, a curious pattern has emerged. Most of the samples appear to be rocks that formed quite recently on the red planet; a peculiarity, considering that most of the surface of Mars is so old.
It is possible that the age measurements are largely wrong. Different dating techniques have produced different results, meaning scientists are not completely confident in estimates of when these rocks formed on Mars.
A team of scientists from the US and Britain has now found a way to solve this problem. And to their surprise, many of these rocks are indeed quite young: only a few hundred million years old. This information could provide clues about how long it took for the meteorites to arrive here, as well as geological processes on Mars.
“We know from certain chemical signatures that these meteorites definitely come from Mars,” says volcanologist Ben Cohen of the University of Glasgow, who led the study.
‘They were blown away from the red planet by huge impacts and formed large craters. But there are tens of thousands of impact craters on Mars, so we don’t know exactly where on the planet the meteorites come from. The best clue you can use to determine their source crater is the age of the samples.”
About 360 meteorite samples have been found on Earth and have been determined to be from Mars. About 302 of these – the most – are classified at the time of writing as shergottite, a type of metal-rich Martian rock forged in the heat of volcanic activity.
Based on how heavily cratered Mars’ surface is, scientists estimate that surface is quite old. If the surface were younger, freshened by volcanic activity, many of the craters would be obliterated by volcanic currents. So any rocks ejected from the surface of Mars must also be old.
Not only are dating techniques on shergottite here on Earth complicated by their composition, but the little we have been able to gather of them has indicated that many of them are less than 200 million years old. This has led to what is known as the shergottite age paradox, and it has plagued scientists for decades.
Explanations for this surprisingly young possibility ranged from a single origin point for all younger shergottites, to the idea that the impact event could have heated and crushed the rock enough to more or less reset its age. But these theories didn’t match the evidence: the rocks themselves.
The method used to determine the age of shergottite is known as argon-argon dating, which is based on the decay of radioactive potassium into argon. Because this decay rate produces a known ratio of argon isotopes, scientists can look at that ratio to determine how long the radioactive decay has occurred, and thus date the rock sample.
The problem is that here on Earth we can easily account for different sources of argon that can enter a sample. For shergottite, which started on a completely different planet and spent God knows how long in space, this is more complicated. There are five potential sources of argon for shergottite, compared to only three for terrestrial rocks.
To compensate for this, Cohen and his colleagues developed a method to correct for argon contamination from Earth and space. “Once we did that, the argon-argon ages emerged as young and fit perfectly with other methods, such as uranium-lead,” he explains.
They dated seven samples of shergottite, with ages ranging between 161 million and 540 million years ago. The researchers say the reason for this could be that the frequent bombardment of Mars has broken up the older surface, exposing the younger rock beneath, which has been replenished by volcanic activity. Ultimately, that younger rock becomes more likely to be excavated and ejected.
Volcanic activity on Mars may still be ongoing, and Mars is constantly being bombarded. Scientists estimate that about 200 impacts create craters with a diameter of more than 4 meters each year. So it’s probably not surprising that younger rocks are occasionally flung toward Earth, in a sort of roundabout way in the solar system.
The research was published in Earth and planetary science letters.
source : www.sciencealert.com