Mercury – magnetic field

Thanks to NASA orbiter flybys made in the months close to the crash site, traces of magnetization in the crust of the planet were found. This was based on a core of molten iron that in ancient times, that would act as a dynamo. The study was published in Science.
The investigation of MESSENGER orbiter of NASA was supposed to last just one year. Instead, the probe has stick up until last week – the crash took place in the evening of 30 April – in orbit for four years, around the most hellish place between the planets of the Solar System. Orbits that in recent months have become increasingly bold increasingly close: just a few tens of kilometers, to the point that in May, the magnetometer board, to identify changes due to localized crustal rocks. And to reconstruct the history of its magnetic field – produced by the moving molten iron in the heart of the planet – the date of origin being between 3.7 and 3.9 billion years ago.


Credit: NASA, Johns Hopkins University Applied Physics Laboratory, Carnegie Institution of Washington

Mercury and Earth are the only two bodies of the inner solar system to have a magnetic field (Mars seems to have had in the past, but now has disappeared). Of course, that of our planet is much more intense (about a hundred times), but since 1974 – since the time of the flyby probe Mariner 10 – that we know of the existence of that of Mercury, although some of its peculiarities have emerged only recently, thanks to MESSENGER.
It could not be otherwise, because in the meantime no other mission has had the courage to face what is considered to be the “Eiger North Face” – the “wall of death” – of planetary exploration. Yes, because to approach Mercury is very risky, and at least two reasons. First, because there is to be burned: the heat, over there, is so intense that MESSENGER, while protected by a heat shield ad hoc, has had to make to the last large elliptical orbits, so getting away periodically in search of ‘ a bit ‘of refreshment for the on-board instrumentation. Then for the tremendous force of gravity exerted by the sun: to prevent the orbiter they were fatally sucked, NASA engineers have studied the path approaching the millimeter, employing about seven years since its launch (2004) to that of ‘actual entry in orbit (2011).
Prudence that was wisely put aside in the last months of the mission, when there was nothing to lose. With the life of the probe now marked by the inevitable running out of fuel, the minimum height of the orbit – maintained in previous years always above the 200 km – has been decreasing up to risk sliding dizzying steps: just 15 km from the ground, little more than the cruising altitude of a scheduled here on Earth.


Credit: NASA, University of British Columbia

And it was a risk that has largely paid off, not only for the resolution of the images. If in fact until the first months of 2014 the magnetometer board could not detect any significant magnetic signal from the crust of the planet, here is that with the fall of the flyby appeared the first variation (the “blue lines” picture opposite) , gradually more intense As the probe approached to the ground, and modulated by a dominant component corresponding to about 320 km away. Sign likely, scientists think of the presence of a layer of magnetized rocks at between 7 and 45 km depth. Not only the geography and the extension of the regions of maximum intensity of the magnetic signal suggests, says the study just published in Science Express, the magnetization of the rocks is very ancient, dating back to an era precisely between 3.7 and 3.9 billion of years ago.
“The knowledge that the latest observations have allowed us to do is really interesting,” says planetologist who led the study, Catherine Johnson, of the University of British Columbia (Canada), “and what we found on the magnetic field is not only the beginning. Without these observations we could never know how it evolved the Mercury’s magnetic field over time. “

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