The Last Dispatch of a Doomed Spacecraft

Exactly a week ago today, the MESSENGER orbiter crashed into Mercury's surface, becoming the first spacecraft ever to make contact with the solar system's innermost planet.

The mission was a huge scientific success even in its final moments, which it spent diligently collecting the first close-up observations of Mercury. This last payload of information may take years to fully process, but today, a study published in Science Express will detail one of the first major discoveries—the age of Mercury's magnetic field.

As it turns out, the field is somewhere between 3.7 to 3.9 billion years old, only about a few hundred million years younger than the planet itself. This is important to know because Mercury is the only other terrestrial planet in the solar system besides Earth with a magnetic field, making it a valuable second dataset for studying the origins, evolution, and behavior of planetary magnetism.

The data could also shed light on other open questions in science, such as the origins of life on Earth, which would not have been possible without our planet's protective magnetic field. On top of that, it affects our ambition to extend life to other worlds in our solar system—Mars's lack of a magnetosphere, for example, is one of the major obstacles to colonizing it.

"The science from these recent observations is really interesting and what we've learned about the magnetic field is just the first part of it," said planetary scientist Catherine Johnson, the lead author of the study, in a statement.

Indeed, when I asked Johnson if the age of the field was in line with what she might have expected, she replied that her team had no expectations about its age at all (for reference, the Earth's magnetic field is about 3.5 billion years old). That really demonstrates that MESSENGER was plowing through totally uncharted territory during its descent, providing scientists with an unprecedented opportunity to get a handle on Mercury's most elusive mysteries.

Using data from the orbiter's magnetometer, Johnson's team was able to reconstruct the magnetic field's ancient past. "We measured small magnetic fields that result from magnetized rocks," she told me.

"The signals are weak, only observed at low altitudes, and are observed in repeated flyovers of the same region so we know they are from the crust, not the core, or from above the planet," Johnson continued. "The age information comes from the spatial association of the magnetic fields with regions whose surface age is at least 3.7 to 3.9 billion years old."

"Age information [itself] comes from the number of impact craters per unit area—older surfaces are more heavily cratered," she added.

This is shaping up to be just the first of many revelations about Mercury based on MESSENGER's last moments in its skies, and Johnson said she still has plenty more data to sift through regarding Mercury's unusual magnetic field which is 150 times weaker than the Earth's field.

Meanwhile, other teams associated with the orbiter are looking for carbon on the surface, or more comprehensive evidence of water ice patches, so there will be no shortage of research to parse over the coming months. MESSENGER itself may have retired to life as a brand new Mercurian crater, but it looks as if its impressive scientific legacy here on Earth is just getting revved up.