How did the largest volcanoes in the solar system grow on Mars?
Martian volcanoes are the largest in the solar system, but how fast they grow has long stumped planetary geologists.
The authors of a new paper are using meteorites that were blasted off the surface of Mars to find out more about how volcanic activity on the red planet has changed.
The research shows that some of these huge volcanoes grew at different rates over the planet's history. They took their time to grow – more than a billion years.
Mars's volcanoes are far larger than those on Earth. The giant Olympus Mons is 22 kilometres tall, two and half times the height of Mount Everest.
On Earth, the maximum lifespan of an active volcano is a few million years. But on Mars, a volcano can produce lava for over a billion years, although at a much slower pace than on Earth.
When a Martian volcano erupts the lava spreads out on the planet's surface and cools to form a layer of rock. Subsequent eruptions cause further layers to build up on top of each other.
From Mars to Earth
The team examined six Martian meteorites, all from a type called Nakhlites. Despite falling to Earth at different times and in different locations, these rocks all come from the same place on Mars.
Around 11 million years ago, an asteroid slammed into the surface of Mars, gouging a crater and launching rocks into space. Eventually some of these rocks reached Earth. The team used radiometric dating methods to examine the ages of the six meteorites.
Dr Caroline Smith, Principal Curator of Meteorites at the Museum, was part of the study.
She says, 'This study is the first to be able to accurately and precisely date different Nakhlite meteorites, including samples from the Museum collection.
'Previously scientists thought they came from one thick lava flow or from magma that solidified within the Martian crust, and there were many complicated hypotheses developed to explain their formation.'
The results showed that although their chemical composition and texture is similar, the meteorites were formed at different times from different layers of lava flows. The oldest layer formed around 1.42 billion years ago, the others over the next 93 million years.
Dr Smith continues, 'The results from our study are exciting as they prove these meteorites actually come from at least four different eruptions of a Martian volcano over a time period of several tens of millions of years. This is much longer than we see for similar volcanoes on Earth.'
The researchers have identified a likely candidate for the crater on Mars where the asteroid impact happened.
By analysing this crater's dimensions and calculating how many layers of rock the asteroid uncovered, the team suggest that the volcanoes were growing at the rate of just over half a metre every million years.
A much more active past
As Dr Benjamin Cohen from the University of Glasgow, who led the research, points out, this rate of growth seen 1.4 billion years ago is much too slow to account for giants like Olympus Mons.
He says, 'When we are talking about a volcano that could be upwards of 10 kilometres tall, a crater that excavated only a few tens of metres deep only represents a very small portion of its history.
'For the Martian volcanoes to have grown so large, Mars must have been far more volcanically active in the distant past.'
The data matches that of remote-sensing crater-counting studies of Martian volcanoes, which indicate that volcanism occurred at much lower rates in the recent past, compared to early in the planet's history.
Dr Cohen adds, 'Studying Martian meteorites - including specimens from the Natural History Museum - allows scientists to understand more about the formation and evolution of the largest volcanoes in the solar system.'