Mars is the planet that never grew up
25 May 2011
Mars, the magnificent Red Planet that has inspired a thousand science fiction fantasies, is just a baby - it has never grown up. Scientists in the US have analysed isotopes in meteorites that resemble Martian geology and have discovered that the planet stopped growing while its solar system siblings carried on developing. This would account for the fact that Mars is 10 times smaller than Earth.
Planets are thought to have evolved from dust particles that collided and stuck together, forming objects called planetesimals. These in turn coalesced to create 'embryos', which further condensed with one another to form planets.
Isotope levels in meteorites show that Mars never grew larger than a planetary embryo |
Each of these processes takes millions of years. But researchers have now shown that Mars stopped developing only 2-4 million years after the formation of the solar system. Earth, on the other hand, carried on growing normally until it reached 'adulthood' about 50 million years later.
Nicolas Dauphas of the University of Chicago and Ali Pourmand, of the University of Miami, looked at isotope ratios in meteorites known to have a similar composition to the Martian mantle - the layer surrounding the core. They measured the decay of hafnium-182 into tungsten-182 using inductively coupled mass spectrometry.
When planetismals collide to form an embryo, the metal content of the incoming mass migrates to the core of the resulting body. So when an embryo is formed, it will have almost no metal within its mantle. If there is tungsten-182 in the mantle millions of years later, this will have come from the decay of hafnium, and so the age of the sample can be calculated. 'It is a very accurate tracer of what happens in the first few tens of millions of years,' Dauphas tells Chemistry World.
The analysis showed that, after the formation of the solar system, Mars stopped growing 2-4 million years later. Earth,...