Space

Rare meteorite may have formed the basis for the planet Mercury

Rare meteorite may have formed...
Image of the planet Mercury captured by MESSENGER in 2008
Image of the planet Mercury captured by MESSENGER in 2008
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Image of the planet Mercury captured by MESSENGER in 2008
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Image of the planet Mercury captured by MESSENGER in 2008
Artist's impression of the MESSENGER spacecraft in orbit around the planet Mercury
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Artist's impression of the MESSENGER spacecraft in orbit around the planet Mercury

A new study led by researchers from MIT may haveidentified the starting material from which the planet Mercury wascreated. The closest planet to the Sun is thought to have initiallyformed from enstatite chondrite – an incredibly rare material onEarth, only found in 2 percent of meteorites known to have struck ourplanet.

Earth and the other planets in our solar systemare believed to have formed through a process known as accretion. Some 4.6 billion years ago, the protostar that was our Sun wassurrounded by a disk of matter composed of dust and gas. Over time,these particles clumped together, steadily gaining mass until theyformed meteoroids, which collided and merged with similar bodies,causing them to grow ever larger until they became planets.

This dramatic process rendered the early planetsballs of molten metal, which took millions of years to cool andharden. A further 4.5 billion years of dynamic activity on Earth hasworked to erase much of the surface evidence for our planet's violentformation history. However, Mercury's extreme volcanic past coupledwith a relative lack of subsequent activity has resulted in surfacescars that remain to this day.

Older geological regions on the surface of thescorched planet are identifiable by a higher count of asteroidimpacts, while a smoother surface indicates a younger region. This issignificant, as a younger sample would have a different chemicalcomposition compared to its older counterpart, as the materials wouldhave been altered by processes occurring in the core of the planetprior to being deposited on the surface through volcanic eruption.

In order to determine Mercury's rate of cooling,as well as the starting material from which the planet was formed,the researchers turned to observations made by NASA's MESSENGER spacecraft during its time orbiting the planet between 2011 - 2015.

During this time, the probe's X-ray spectrometercollected detailed readings of lava deposits in 5,800 separatelocations, which allowed planetary scientists to determine theirindividual compositions. The team first correlated the composition ofthe 5,800 data samples with the age of the terrain based on craterdistribution.

Artist's impression of the MESSENGER spacecraft in orbit around the planet Mercury
Artist's impression of the MESSENGER spacecraft in orbit around the planet Mercury

Samples estimated to be around 3.7 billion yearsof age were found to have a markedly different chemical makeup whencompared to 4.2 billion year-old samples. To gain a deeperunderstanding of the cooling process, the researchers decided torecreate two examples of Mercury's lava deposits in a laboratorysetting.

The team created synthetic rocks made up fromchemical building blocks derived from the mineral ratios detected inthe older and younger lava deposits. Having melted therocks in a furnace to mimic the state of the materials at the pointof eruption, the researchers applied additional heat and pressure inan attempt to simulate a reverse of the cooling process experiencedby Mercury in the few billion years following its formation.

The team were watching for the formation ofminiature crystals within the faux samples, which would denote thepoint at which the planet's rocky core began to melt. According tothe laboratory recreation, the older deposits would have melted at adepth of 360 km (224 miles) below the surface of Mercury, at atemperature of around 1,650 ºC (3,002 ºF). Meanwhile the youngersamples detected by MESSENGER may have melted at a depth of 160 km(99 miles) having reached temperatures of 1,410 ºC (2,570 ºF).

The results suggest that around 4.2 billion yearsago, Mercury experienced a dramatic period of cooling, which saw theplanet's temperature drop by 240 ºCoverthe course of only 500 million years. An analysis of the crystalsfound in the samples also point toward enstatite chondrite as the substance that comprised Mercury's early core. This materialis so rare on Earth, that only a little more than 200 enstatitechondrite meteorites have been discovered to date.

Unfortunately,evidence that the rare meteorite constituted the startingmaterial for Mercury is far from conclusive. In order to prove thetheory, scientists would need to get their hands on a number of lavadeposit samples from the planet's surface.

Source: MIT

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