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Mercury was magnetic four billion years ago

Magnetic field Scientists have found evidence of a four billion year old ancient magnetic field on Mercury, according to new data from NASA's Messenger mission.

The discovery reported in the journal Science, means Mercury has the oldest confirmed magnetic field of any terrestrial world in the inner solar system.

The authors made their discovery by examining data from a series of low altitude orbital passes undertaken by Messenger at heights often less than 50 kilometres above Mercury's crater-strewn surface.

"We've measured some very weak fields in rocks that were magnetised in a very ancient field on Mercury, which tells us that Mercury had a magnetic field not just today, but also almost four billion years go," says the study's lead author Dr Catherine Johnson of the University of British Columbia in Vancouver.

"Until this observation all we really knew about the magnetic field's longevity was that it's been around since the first time we were able to observe it, which was with the Mariner 10 mission.

"So this tells us that there was a field about four billion years go, and the simplest interpretation of that would be that Mercury has actually had a magnetic field during that whole time."

NASA's Mariner 10 spacecraft first detected a global magnetic field around Mercury during its flybys of the planet in the mid 1970s.

The Messenger spacecraft, which in 2011 became the first probe to orbit Mercury, confirmed the earlier observations and undertook more detailed studies until its mission was completed last month.

Mercury's global magnetic field provided scientists with evidence that the 2,440 kilometre wide planet still retains a partially molten iron core which acts as a geodynamo to produce the magnetic field.

Detecting remnant magnetic signatures

Finding remnant magnetisation in Mercury's crust was made more difficult because 95 to 99 per cent of the magnetic signatures being detected by Messenger are created by other sources.

These include magnetic fields produced by Messenger itself, the global magnetic field generated by Mercury's geodynamo, and the interaction between Mercury's magnetic field and the Sun's solar wind, which creates electical currents that also produce magnetic fields above the planet.

"So it's been really important to understand all the magnetic fields measured by Messenger and be able to remove that from our observations in order to see that tiny little signature that's left over in the remnant rocks," says Johnson.

The remnant magnetic signatures were detected in two regions, the Suisei Planitia north of Shakespeare basin, and just south of the lobate scarp tectonic feature called Carnegie Rupes.

Johnson and colleagues also detected a third weaker magnetic signature over a region near 170° East.

The finding that Mercury's core dynamo field was present early in the planet's history provides important insight into the evolution of the planet's core and the dynamo action within it.

Dating a rock

Johnson and colleagues determined the dates of the magnetised rocks on Mercury by first looking at the number of craters in a given area.

An area that's more heavily cratered than other areas has been exposed to impactors longer than the other areas, and is therefore older.

The authors were able to calibrate the number of craters per area to an actual age and date, thanks to earlier studies of the Moon.

Scientists had perviously counted the number of craters in a given area on the Moon, and also chemically dated rock samples from the same area that were brought back by the Apollo astronauts, allowing the crater density and rock age to be calibrated.

Older than Earth

"In terms of the inner planets of our solar system, this is the longest lasting geodynamo generated magnetic field that's been observed so far, including the Earth," says Johnson.

"On Earth we have observations of magnetization of rocks back to about three and a half billion years, while our observations of Mercury are of magnetic signatures in rocks that are 3.7 to 3.9 billion years old and perhaps even older.

"The problem on Earth is that it's hard to find really old rocks beyond about 3.5 billion years as they've been reheated, squeezed and squashed, so it's hard to figure out their magnetisation dates."

Launched in 2004, Messenger had been in space for over a decade.

During the four years it orbited Mercury, Messenger provided scientists with vast amounts of data about the tiny planet closest to the Sun.

Related: Goodbye Messenger! Five things we discovered about Mercury