‘We couldn’t quite believe it, in fact we thought the data was wrong … we spent over two months looking at and double-checking the information but it was correct, Messenger had found a high level of volatiles such as sulphur, sodium and potassium on the surface.’
Nancy Chabot, planetary scientist, Messenger mission
Right up until the end of its mission in 2015, Messenger continued to uncover many of Mercury’s secrets, including a few very particular surprises. Using a combination of photography, spectroscopy and laser topography, Messenger revealed tantalising evidence that even this close to the Sun, water ice can exist on the surface of a planet. Even though the Sun blasts much of Mercury’s surface, the tilt of its rotational axis is almost zero, so there are craters and features around the planet’s poles that never see direct sunlight. Combined with the lack of atmosphere, these regions are forever exposed to the freezing temperatures of space, and it’s in this environment that Messenger was able to record the clear signature of water ice. Here, in the eternal night of a polar crater, it’s cold enough for ice to survive for millions of years, just metres away from the savage ferocity of the Sun’s light.
However, Messenger’s most startling discovery was still to come. The mission objectives had been developed to explore the deep history of Mercury and provide data to test against our theories of the formation and early life of the planet. Messenger was equipped with a collection of spectrometers designed to analyse the composition of Mercury at different depths. The Messenger team had worked on a detailed set of predictions outlining the chemistry of the planet, but as the spacecraft began to sniff at the Mercurian surface it soon became clear that our assumptions had not been quite right.
As the gamma-ray and X-ray spectrometers analysed the elements on Mercury’s surface they began to measure the unexpected characteristic signature of a number of elements such as phosphorus, potassium and sulphur at much higher levels than they were expecting. Up to this point, the working hypothesis had been that during the formation of Mercury (and all the rocky planets), as the rock condensed and combined to form the planet, the heavier elements like iron would sink towards the centre, forming the bulk of the core, while the lighter elements, such as phosphorus and sulphur, would remain near the surface. These more volatile elements would then be expected to be stripped away from the surface, particularly on a planet like Mercury, which is so close to the Sun. And yet the Messenger data confirmed high levels of potassium, and sulphur was detected at ten times the abundance of the element on Earth or the Moon. Both are volatile elements, easily vaporised, and when this close to the Sun, they simply should not have survived the planet’s birth.
On top of that, the Messenger data confirmed what we had long suspected about the structure of Mercury, that it is the densest of all the planets, with a massive iron core making up 75 per cent of the planet’s radius compared to just over 50 per cent here on Earth. The core creates a strange lopsided magnetic field, indicating that the internal dynamics of the planet are different to anything we have seen before.
All of this adds up to making Mercury something of a mystery, as nothing quite accords. The eccentric orbit, the abundance of volatile elements on the surface and the oversized iron core all point to the planet having a history far more complex than was first imagined, and the best explanation we have to make sense of the Messenger data is that Mercury was not born in its current sun-scorched position. It has long been supposedly known that the orbits of the planets are eternal, stable loops that sustain the structure of the Solar System in an endless rhythm, but everything we are learning now suggests that this is far from the whole story.
MEASURING MERCURY
Messenger was equipped with seven scientific instruments to collect data, including the Mercury Dual Imaging System (MDIS), Gamma-Ray and Neutron Spectrometer (GRNS), X-Ray Spectrometer (XRS), Magnetometer (MAG), Mercury Laser Altimeter (MLA), Mercury Atmospheric and Surface Composition Spectrometer (MASCS) and Energetic Particle and Plasma Spectrometer (EPPS). All these instruments communicated with the spacecraft through Data Processing Units (DPUs) and had to be mounted on the spacecraft with a view of Mercury but without interference from the Sun. They were designed to withstand the extreme temperatures the craft would encounter.
© NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Topography
Messenger’s MLA equipment was able to measure the difference in elevation across the northern hemisphere of Mercury, revealing it to be 10 kilometres between the lowest and highest regions.
Temperature
Messenger recorded expected information about the temperature of the planet, that the craters which were sunlit reached high temperatures, reflected in the red colouration of these images.
© NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Geology
In this enhanced colour mosaic, the smooth volcanic plains of the Caloris basin are coloured yellow, with the craters picked out in blue.
© NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
© WALTER PACHOLKA, ASTROPICS / SCIENCE PHOTO LIBRARY
As predictable as the sunrise, Mercury keeps its place in our solar system, visible in the glow of dawn over Haleakala National Park, Hawaii.
A SECRET HISTORY
Mercury, like all four of the rocky siblings, was formed of molten rock. A few million years later, as the young planet began to cool, its crust solidified and its journey around the Sun transformed from being part of a swirling cloud into a clearly defined passage, an orbit. The path the infant Mercury travelled, however, was most probably far removed from the course it now holds. The young Mercury was born not as the closest planet to the Sun but at a much greater distance, far beyond the orbit of Venus, beyond Earth, perhaps even beyond Mars. This was a planet that came into being in the mildest region of the Solar System. It was far enough away from the Sun to allow volatile elements like sulphur, potassium and phosphorus to be folded into its first rocks without being vaporised away by the heat of the Sun, but maybe near enough for its surface to be warmed, perhaps even just the right amount for liquid water to settle on its surface. This may well have been a planet big enough to hold an atmosphere, a watery world upon which all the ingredients of life could well have existed. Mercury, it seems, really did have its own moment in the sun, but these hopeful beginnings were not to last.
Today it’s hard to imagine the planets in any orbit other than our night sky. They feel eternal, permanent, and so it’s natural to think of the Solar System as a piece of celestial clockwork, a mechanism running with perpetual and unchanging precision, marking out the passage of time. In time frames that we can comprehend – days, weeks, months and years – the motion and trajectory of the planets is just that: clockwork. We use these markers to plot out the 24 hours of a day, 365 days of a year, and the lunar cycle is, of course, intimately linked to our months. Beyond that, Newton’s laws of universal gravitation first described in 1687 allow us to this day to plot out the trajectories of all the heavenly bodies far into the future and back into the distant past. This predictability of motion is what allows us to plot great astronomical events, such as eclipses and transits, far into the future. It’s why, for example, we can predict that on 14 September 2099 the Sun, Moon and Earth will be in precise alignment to create the final total solar eclipse of the twenty-first century across North America.
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