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Mercury

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NASA’s Messenger probe to Mercury has shown off a stunning new color map of the planet.

The map comprises thousands of images acquired by the spacecraft during its first year in orbit.

This is not how we would see Mercury, which would look like a dull, brownish-grey globe to our eyes.

Rather, the map represents an exaggerated view of Mercury that is intended to highlight variations in the composition of its rock.

“Messenger’s camera has filters that go from the blue to the near-infrared of the spectrum, and we are able to use computer processing to enhance the very subtle but real colour differences that are present on Mercury’s surface,” explained Dr. David Blewett from the Johns Hopkins University Applied Physics Lab.

“The areas that you see that are orange – those are volcanic plains. There are some areas that are deep blue that are richer in an opaque mineral which is somewhat mysterious – we don’t really know what that is yet.

“And then you see beautiful light-blue streaks across Mercury’s surface. Those are crater rays formed in impacts when fresh, ground-up rock is strewn across the surface of the planet,” said David Blewett.

Dr. David Blewett displayed the map here in Boston at the annual meeting of the American Association for the Advancement of Science (AAAS).

He was giving a sneak preview of the data that is about to be deposited in NASA’s planetary archive.

This will include a black-and-white, or monochrome, map of the entire surface of Mercury at a resolution of 200 m per pixel (the color map has a resolution of 1 km per pixel and is just short of 100% coverage).

NASA's Messenger probe to Mercury has shown off a stunning new color map of the planet

NASA’s Messenger probe to Mercury has shown off a stunning new color map of the planet

The mission so far has been a triumph, which ought to make the current request to NASA management for an operational extension a very easy case to make.

Messenger’s observations have thrown up many surprises and challenged a lot of assumptions.

The probe has revealed Mercury’s rich volcanic history. It has confirmed the existence of great lava plains, but also uncovered evidence for explosive volcanism.

We know now, too, that the planet has ice in shadowed craters.

“It’s got polar ice caps. Who’d have thought that?” said Dr. David Blewett.

In addition, the probe’s instruments have detected relatively high abundances of sulphur and potassium in surface materials.

These are volatile elements that should not really be present on such a scale on a planet that orbits so close to the Sun with its searing heat.

But these elements may help explain many puzzles, like the nature of those opaque terrains. These could get their dark hue from the presence of sulphides.

The compounds could also lie behind the intriguing “hollows” that pockmark great swathes of Mercury’s surface.

Shallow with irregular shapes, the depressions often have bright halos and bright interiors. When scientists look around the Solar System for similar phenomena, the best comparison would appear to be the depressions that form in the carbon dioxide ice at the poles of Mars.

Those features are thought to arise when the CO2 ice sublimates away – that is, when it transforms directly from a solid state to a gaseous state.

“Well, Mercury’s surface isn’t made of ice – it’s scorching hot next to the Sun. But it seems that there is some sort of sublimation-like loss in the solid, silicate rocks that is causing these hollows to initiate and enlarge.

“It may be that a combination of high temperatures and what’s called severe space weathering destroys sulphide minerals in the rocks, causing them to crumble and open up a depression.”

Messenger is in great shape should NASA management agree to a mission extension. The probe is thought to have enough fuel to operate until 2015.

And by then, new spacecraft will be on their way to Mercury. Under a joint venture known as BepiColombo, Europe and Japan are sending two satellites that should arrive at the innermost world in 2022.

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NASA’s Messenger probe has found further tantalizing evidence for the existence of water ice at Mercury’s poles.

Though surface temperatures can soar above 400 C, some craters at Mercury’s poles are permanently in shadow, turning them into so-called cold traps.

Previous work has revealed patches near Mercury’s poles that strongly reflect radar – a characteristic of ice.

Now, the Messenger probe has shown that these “radar-bright” patches line up precisely with the shadowed craters.

Messenger is only the second spacecraft – after Mariner 10 in the 1970’s – to have visited the innermost planet. Until Messenger arrived, large swathes of Mercury’s surface had never been mapped.

The bright patches were detected by ground-based radio telescopes in the 1990s, but as co-author Dr. Nancy Chabot explained: “We’ve never had the imagery available before to see the surface where these radar-bright features are located.”

The researchers superimposed observations of radar bright patches by the Arecibo Observatory on the latest photos of Mercury’s poles taken by the MDIS imaging instrument aboard Messenger.

“MDIS images show that all the radar-bright features near Mercury’s south pole are located in areas of permanent shadow,” said Dr. Nancy Chabot, from Johns Hopkins University Applied Physics Laboratory (JHUAPL).

“Near Mercury’s north pole such deposits are also seen only in shadowed regions, results consistent with the water-ice hypothesis.”

NASA’s Messenger probe has found further tantalizing evidence for the existence of water ice at Mercury's poles

NASA’s Messenger probe has found further tantalizing evidence for the existence of water ice at Mercury's poles

However, Dr. Nancy Chabot cautions, this does not constitute proof, and for many craters, icy deposits would need to be covered by a thin layer (10-20 cm) of insulating debris in order to remain stable.

Maria Zuber, from the Massachusetts Institute of Technology (MIT), who is a co-investigator on the Messenger mission, said: “The most interesting interpretation of [the radar observations] is that they were due to water ice.

“Sulphur had been proposed, there had also been some suggestion it was roughness – though there was no reason craters at the poles should be rougher than those at low latitudes.”

“The new data from Messenger… is strengthening the evidence that there is some sort of volatile there, and water-ice seems quite likely.”

Maria Zuber said information from several instruments on Messenger was currently being analyzed in order to answer the ice conundrum: “I think this is a question that we can come to a definitive answer on, as opposed to <<we think it may be this>>,” the MIT researcher explained.

On Wednesday, scientists from the Messenger mission published findings that Mercury had been geologically active for a long period in its history.

Data from the probe shows that impact craters on the planet’s surface were distorted by some geological process after they formed.

The findings, reported in Science magazine, challenge long-held views about the closest world to the Sun.

Scientists also presented a new model of Mercury’s internal structure, which suggests the planet’s huge inner core is encased in a shell of iron sulphide – a situation not seen on any other planet.

Messenger was launched in 2004, and entered orbit around its target in March last year. NASA recently announced that its mission would be extended until 2013.

 

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