Eleven years after it launched, Messenger will end its mission on 30 April by crashing down, more than 4,100 orbits and four years after it reached its perch above and around the planet.
Among the mission’s biggest discoveries was that both of Mercury’s polar regions contain water ice despite the planet’s close proximity to the sun and daytime temperatures that can reach 800F (430F) elsewhere.
Data from Messenger also revealed that “most of these deposits don’t consist of water ice directly at the surface, but rather water-ice covered by a dark layer”, said Sean Solomon, Nasa’s principal investigator for the mission. That dark layer, roughly 30cm thick, is “much colder than the average mercury material”.
Solomon added that some hypotheses hold that “this dark material is in fact organic, carbonaceous material delivered to Mercury” from the outer solar system – meaning that the “the building blocks for organic chemistry and life” may be resting on the planet closest to the sun.
“We don’t see anything in the geological features that indicate running water as we see on Mars,” said the planetary science director, James Green. “It’s not likely on a regular basis that there’s liquid water on Mercury.”
The mysterious dark material may have come to the planet by way of debris from space crashing on to Mercury’s surface, in parallel to the idea that a comet or meteorites may have brought basic organic material to Earth.
That is not to say life has ever existed or ever will exist on a planet with conditions as inhospitable as those on Mercury, Solomon and his colleagues assured reporters. Sunlight is so powerful on Mercury that sodium in the atmosphere glows, and radiation so strong that the glowing sodium atoms give the planet an orange, comet-like tail.
Temperatures also drop to -290F (-180C) , and volcanoes and hollows dot the landscape.
Another of Nasa’s surprising discoveries on Mercury was that the planet is as “volatile rich” with elements and minerals as Mars or Earth, which directly contradicts what scientists had predicted. Nasa in part analyzed such materials by studying deposits exposed by volcanic eruptions.
The discovery of the surprisingly diverse makeup of Mercury means that scientists will have to overhaul their ideas of how the planet formed, and by extension how the solar system became the way it is.
Messenger has sent data about the volcanoes of Mercury and the chemistry of its lava, which may help solve the mysteries of the planet’s origin and “phenomenally dense” core.
Another mystery of the planet is its “offset magnetic field”. Unlike Earth, which has a magnetic field located at the center of the planet, Mercury’s field is strangely off to the planet’s side and more than 20% closer to the north pole, creating an asymmetrical field.
Nasa will continue collecting data from Messenger until the probe succumbs to Mercury’s gravity. The probe survived using solar power and reached its target after an odyssey that forced it to “borrow gravity” from other planets. Nasa “solar sailed” the spacecraft into Mercury’s range.
Green and Solomon said they hope for an eventual lander mission to Mercury, to analyze the dark material and electrical field patterns of the planet.
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