Saturday is Sexy… Sunday is Movies Night
By Stuart Gary
Later this week, NASA’s Messenger mission to Mercury will go out with a bang ending its 10-year journey. Along the way the spacecraft has made some startling discoveries, reports ABC Star Stuff’s Stuart Gary
The golf-cart-sized probe will crash onto Mercury’s primitive crater-strewn surface when it runs out of fuel — probably around April 30.
Launched back in 2004, NASA’s Mercury Surface, Space Environment, GEochemistry, and Ranging probe is the first spacecraft to go into orbit around Mercury.
“Messenger’s answered lots of questions about the planet nearest the Sun, but it’s also opened up a Pandora’s box,” says Monash University‘s Emeritus Professor Andrew Prentice, who has been closely studying the mission’s results.
It took the 485-kilogram probe six-and-a-half years to reach the solar system’s smallest planet on 18 March 2011.
Until then, we knew little about the 4880-kilometre-wide planet.
The only previous visitor to the planet was NASA’s Mariner 10 spacecraft which made three close fly-bys in the mid 1970s.
The discovery of water ice so close to the searing heat of the Sun was a huge surprise.
Despite an average surface temperature of 427°C, the planet could have over a trillion tonnes of frozen water on the permanently shadowed floors of impact craters at its poles.
“The discovery of water ice was unexpected, although there had been earlier hints in radar observations” says Prentice.
“There would have been no water when Mercury formed, that would have arrived later together with other volatile materials in comet and asteroid bombardments.”
Prior to Messenger, we had hints that Mercury’s surface contained volatile chemicals such as potassium, sodium, sulphur and chlorine.
In theory these chemicals shouldn’t be there because the planet formed so close to the Sun.
“If Mercury formed as an iron-rich planet, it shouldn’t have had many volatile elements,” says Prentice.
As Messenger mapped Mercury’s surface it discovered these chemicals are located around asteroid and comet impact sites.
“Messenger shows us that a lot has happened in the last 4.5 billion years to change Mercury’s surface,” says Prentice.
“When you look carefully at these volatiles, especially potassium, we found that the ratios of potassium to thorium were identical to those found in Martian meteorites.
“So Mercury probably did form as an iron-rich planet, which was later bombarded by asteroids and comets containing volatiles.”
Wrinkles on the planet’s surface, known as scarps, show that Mercury has shrunk by over seven kilometres in radius over the past four-and-a-half billion years.
“Messenger completed the job of mapping the surface of Mercury, and it did so in amazing detail,” says Prentice.
NASA’s Mariner 10 spacecraft was only able to image one side of the planet.
“The measurements made back then failed to find any evidence of shrinkage,” says Prentice.
Messenger helped resolve the riddle of why Mercury has such a large iron core that occupies almost 50 per cent of its volume — over twice as much as Earth’s core.
“It was considered a bit of an oddity because it’s so extremely dense compared to all the other planets and we thought it may have formed in a different way,” says Prentice.
“Some theories suggested Mercury originally formed as a far bigger planet, which lost most of its mantle in a giant collision.
But it appears to have formed the same way as the other terrestrial planets Venus, Earth and Mars.
“We now realise that Mercury formed in a region so hot, that only iron condenses out of the protoplanetary nebula at this location.”
“Just as the gas giants all formed beyond the snow line between Mars and Jupiter, where it’s cold enough for water and ices to condense, Mercury formed at an iron line.”
Messenger confirmed Mariner 10’s observations that Mercury has a magnetic field.
This is surprising because scientists believed that Mercury’s iron core was completely solid because the planet is so small it should have cooled and solidified all the way though.
The Earth’s magnetic field is produced by the movement of the liquid iron outer core around the solid iron inner core — a geo-dynamo.
“This means that like Earth, Mercury must have a large liquid iron reservoir to generate a magnetic field,” says Prentice.
But how part of the core could remain liquid is still a mystery.
Prentice suggests that Mercury has five times more thorium and uranium relative to the amounts seen on Earth.
“The radioactive decay of all this uranium and thorium makes the crust a blisteringly hot ceramic mantle which also insulates the iron core, preventing it from solidifying all the way through and allowing a geo-dynamo to continue producing a magnetic field,” says Prentice.
When its fuel finally runs out, Messenger will crash into the planet at over 14,000 kilometres per hour leaving a crater 16 kilometres wide. Unfortunately, it will crash into the side of the planet facing away from Earth so we will not see it in real time.
But it will leave a lasting legacy that will be picked up by future missions.
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