Nasa spacecraft that found water on Mercury prepares to crash into planet

Mercury

Messenger’s 11-year voyage of discovery to end on 30 April
Mission has forced rethink about origin of planet closest to sun

This artist’s rendering provided by Nasa shows the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (Messenger) spacecraft orbiting around Mercury.
This artist’s rendering provided by Nasa shows the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (Messenger) spacecraft orbiting around Mercury. Photograph: AP

Alan Yuhas

Mercury’s ‘dynamic and complex world’ revealed by Nasa’s Messenger
First time planet has high-resolution maps after Nasa spacecraft orbited planet over course of a year

Alok Jha

Mercury is the smallest of the solar system’s eight planets and, for decades, also its most neglected by humans. While Mars, Venus, Jupiter and Saturn have been probed and photographed in exquisite detail during the space age, the closest planet to the Sun has had to make do with a few flybys from the Mariner 10 spacecraft in the early 1970s.

Now Mercury has its own high-resolution maps, down to the scale of kilometres, made from thousands of images taken by Nasa’s Messenger spacecraft as it orbited the planet over the course of a year.

“Messenger has revealed Mercury to be a fascinating, dynamic and complex world,” said David Blewett at the Applied Physics Laboratory at Johns Hopkins University and a scientist for the Messenger mission. Part of the reason, he said, that it had taken more than 30 years to revisit the planet since the Mariner 10 flybys was that most people thought Mercury was probably like the moon and largely inert and boring.
Global Map Of Mercury From Messenger.
Global Map Of Mercury From Messenger. Photograph: Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/Nasa

“We know now that it is an oddball planet,” said Blewett, speaking ahead of a briefing on Friday at the annual meeting of the American Association for the Advancement of Science in Boston. “It’s the smallest of the eight planets but has the highest density. The interior structure is different than the other planets. The geologic surface is different to the moon and Mars. The surface composition is enigmatic because … it consists of rock types that we don’t have much experience with. It has a global, Earth-like magnetic field, Venus and Mars do not.”

The new global colour map is an enhanced image that shows the different compositions of rocks on the surface of Mercury. The more orange areas are volcanic plains while the make-up of the rocks in the deep blue areas is unknown. Though Messenger was able to detect the abundance of individual elements on Mercury’s surface – including iron, titanium, sulphur and potassium – without rock samples to study, scientists cannot determine the exact compounds or minerals in which those elements are arranged.
Enhanced-color close-up of hollows on Mercury
Enhanced-color close-up of hollows on Mercury Photograph: Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/NASA

Mercury orbits the sun in an eccentric orbit that gets as close as 46m kilometres and never goes further than 70m kilometres from our star (earth, for comparison, stays about 150m kilometres away). This means the surface of the planet can reach more than 400C.

Nasa’s Messenger spacecraft made its first flyby of the planet in 2008 and began to orbit in early 2011, since when it has been mapping and measuring the surface of the planet. In November 2012, Nasa announced there was water ice and tar-like organic molecules at Mercury’s poles. Because the planet’s axis of rotation around the sun barely tilts from the vertical, the deepest parts of craters at the polar regions never get sunlight and the temperatures there are about -200C. The presence of ice and organic compounds lends weight to the idea that the planet was pummeled by icy comets during the early years of the solar system.

The biggest surprise for researchers is that, on the surface, the abundance of relatively volatile elements such as potassium and sulphur is so high. Most of the models for the formation of Mercury predict that these elements should have evaporated away during the planet’s formation.
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“The big problem in making Mercury is how do you get a planet that has such a huge iron core and such a small proportion of rock? One of the ideas was that maybe there was a giant impact that stripped off much of the rocky outer layers and left you with unusually high proportion of iron in the core,” said Blewett. But Messenger’s data, he said, meant planetary scientists would have to come up with new ideas.

One alternative candidate is that the chemical conditions in the inner part of the nebula where Mercury was forming were different than scientists had expected, and this probably allowed iron to condense first, while the rockier parts of the planet were swept away. “That model seems to predict the correct sulphur and potassium abundance for the surface and that’s probably the leading contender right now,” said Blewett.

Mercury Nasa Space Maps

Preparing for the Messenger probe’s final days before it smashes into Mercury, Nasa presented its accomplishments on Thursday, including the discovery of water on the planet closest to the sun.

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.
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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.