A new study led by researchers from MIT may have identified the starting material from which the planet Mercury was created. The closest planet to the Sun is thought to have initially formed from enstatite chondrite – an incredibly rare material on Earth, only found in 2 percent of meteorites known to have struck our planet.
Earth and the other planets in our solar system are believed to have formed through a process known as accretion. Some 4.6 billion years ago, the protostar that was our Sun was surrounded by a disk of matter composed of dust and gas. Over time, these particles clumped together, steadily gaining mass until they formed meteoroids, which collided and merged with similar bodies, causing them to grow ever larger until they became planets.
This dramatic process rendered the early planets balls of molten metal, which took millions of years to cool and harden. A further 4.5 billion years of dynamic activity on Earth has worked to erase much of the surface evidence for our planet's violent formation history. However, Mercury's extreme volcanic past coupled with a relative lack of subsequent activity has resulted in surface scars that remain to this day.
Older geological regions on the surface of the scorched planet are identifiable by a higher count of asteroid impacts, while a smoother surface indicates a younger region. This is significant, as a younger sample would have a different chemical composition compared to its older counterpart, as the materials would have been altered by processes occurring in the core of the planet prior 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 spectrometer collected detailed readings of lava deposits in 5,800 separate locations, which allowed planetary scientists to determine their individual compositions. The team first correlated the composition of the 5,800 data samples with the age of the terrain based on crater distribution.
Samples estimated to be around 3.7 billion years of age were found to have a markedly different chemical makeup when compared to 4.2 billion year-old samples. To gain a deeper understanding of the cooling process, the researchers decided to recreate two examples of Mercury's lava deposits in a laboratory setting.
The team created synthetic rocks made up from chemical building blocks derived from the mineral ratios detected in the older and younger lava deposits. Having melted the rocks in a furnace to mimic the state of the materials at the point of eruption, the researchers applied additional heat and pressure in an attempt to simulate a reverse of the cooling process experienced by Mercury in the few billion years following its formation.
The team were watching for the formation of miniature crystals within the faux samples, which would denote the point at which the planet's rocky core began to melt. According to the laboratory recreation, the older deposits would have melted at a depth of 360 km (224 miles) below the surface of Mercury, at a temperature of around 1,650 ºC (3,002 ºF). Meanwhile the younger samples 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 years ago, Mercury experienced a dramatic period of cooling, which saw the planet's temperature drop by 240 ºC over the course of only 500 million years. An analysis of the crystals found in the samples also point toward enstatite chondrite as the substance that comprised Mercury's early core. This material is so rare on Earth, that only a little more than 200 enstatite chondrite meteorites have been discovered to date.
Unfortunately, evidence that the rare meteorite constituted the starting material for Mercury is far from conclusive. In order to prove the theory, scientists would need to get their hands on a number of lava deposit samples from the planet's surface.