The Planets

This model of Mariner 10 shows the spacecraft in flight. In a highly complex mission, the craft used the gravitational pull of a planet to direct it and large solar panels acted as sails whenever scientists needed to correct Mariner’s course.

© NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Mercury’s elliptical path around the Sun shifts slightly with each orbit, such that its closest point to the Sun moves forward with each pass. This discovery could not be verified with Newtonian physics and it took Albert Einstein’s theory of relativity to finally explain it.

At first sight many things about Mercury simply don’t make sense. During its 88-day orbit around the Sun it travels in a lopsided, elliptical orbit, which means it can be as far away from the Sun as 70 million kilometres but occasionally as close as 46 million kilometres. This is by far the most irregular of orbits of all the planets, but it is not the end of Mercury’s oddity. Temperatures at midday can rise to 430 degrees Celsius on the surface, but at night, because it’s a small planet and it has no atmosphere, temperatures fall to minus 170 degrees, giving it the greatest temperature swing of any known body in the Solar System. Its rotation is also unusual, gravitationally locked to the Sun in what is known as a 3:2 spin orbital resonance. This means the planet spins precisely three times on its axis for every two orbits, which in turn means that its day is twice as long as its year. In effect, you could be travelling over its surface at walking pace and keep the Sun at the same point in the sky as you strolled through eternal twilight.

As planetary scientist Nancy Chabot explains, ‘A day on Mercury is not like a day on Earth. It has a very unusual orbit … It has to go around the Sun twice to have one complete solar day on the planet, where the Sun goes from directly overhead to directly overhead and this actually takes 176 Earth days.’ Because of the planet’s orbit, there are places on the Mercurian surface where a hypothetical observer would be able to see the (two and a half times larger in the sky) Sun appear to rise and set twice during one Mercurian day. It rises, then arcs across the sky, stops, moves back towards the rising horizon, stops again, and finally restarts its journey towards the setting horizon.

© HarperCollins

At the end of a long, circuitous route, Messenger finally enters Mercury’s orbit on 18 March 2011, the first spacecraft to do so.

‘With the beginning today of the primary science phase of the mission, we will be making nearly continuous observations that will allow us to gain the first global perspective on the innermost planet.’

Sean Solomon, Messenger mission

Most of Mercury’s anomalies can be explained by the orbital mechanics of its journey around the Sun, except, that is, for the odd elliptical orbit that takes it on such an oval-shaped, elongated course. This irregularity has puzzled astronomers for centuries and hints at an ancient planet that was very different from the Mercury we see today.

5 … 4 … 3 Main engines start 2 … 1 … and zero and lift off of Messenger on NASA mission to Mercury … a planetary enigma in our inner solar system

To truly begin to understand Mercury’s history we had to wait nearly 40 years before we could return to her. On 18 March 2011, NASA’s Messenger spacecraft became the first to enter Mercury’s orbit, and over the next four years it succeeded in not only photographing 100 per cent of the planet’s surface, but also collecting extensive data on its geology.

But before any of this could happen, Messenger had to take perhaps the most circuitous route in the history of our exploration of the Solar System. Just passing close to Mercury to take a few snaps, as Mariner 10 did, is hard enough, but actually entering into its orbit was thought to be either too difficult to achieve or too costly to execute. As cosmochemist on the Messenger mission Larry Nittler explained, ‘There are two major challenges to getting a spacecraft into orbit around Mercury: gravity and money. When you go from Earth to Mercury, you’re falling into the gravitational well of the Sun, which makes you accelerate faster and faster as you get closer. And, if you were to go straight from Earth to Mercury, this means that you would basically just zip right by the planet, or you would need to bring an incredible amount of fuel to put the brakes on, more than you could actually afford.’

© HarperCollins

Messenger’s six-year, seven-month, 16-day journey to Mercury took it on a complex route involving several gravity assist manoeuvres before it entered the planet’s orbit.

A number of missions never made it further than pencil and paper, while others floundered and failed at the proposal stage. It was only when Chen-wan Yen, a NASA engineer from the Jet Propulsion Laboratory (JPL), provided a trajectory that could not only get a craft into orbit but could do it at an estimated bargain-bucket cost of 280 million dollars, that the Messenger mission could really begin to take flight.

Taking off from Cape Canaveral on 3 August 2004, Messenger began a six-year, seven-month, 16-day journey to Mercury that would take it on a 7.9-billion-kilometre trajectory before it entered into orbit around the smallest of all the planets. To arrive at Mercury with the right speed and on the right course would require a complex route that would entail a number of gravity-assist manoeuvres around the Earth, Venus and Mercury itself to reduce the speed of the craft relative to Mercury. So, combined with the brief firing of its large rocket engine to finally insert it into orbit, this mission profile allowed Messenger to complete its voyage without the need to carry the vast reserves of fuel required to slow its passage through the firing of rockets. This design made the craft lighter and cheaper, but ultimately much slower. Almost seven years was a long time to wait for the team patiently charting its progress across the stars. Larry Nittler described Messenger’s course as ‘sneaking up on [Mercury] by taking a seven-year journey, flying around the Sun many times, doing multiple flybys around Mercury and Venus, and each time transferring some of [the] craft’s speed and energy to the planet, so it could slow down, so that when we finally got to Mercury after seven years, we were able to fire our engine just a little bit, to slow down [even more] and get captured by the weak gravitational field of the planet’.

© HarperCollins

The highly elliptical path taken by Messenger to finally enter Mercury’s orbit at 00.45 UTC on 18 March 2011.

© NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Messenger’s mission was a deeper exploration of the cratered landscape and geology of Mercury. One major discovery from its imaging work was evidence of water ice in its polar craters.

© NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Messenger took images of Mercury’s south pole on several orbits, allowing scientists to monitor the region through changing illumination.

Appropriately, when Messenger finally entered into Mercury’s orbit at 00.45 UTC on 18 March 2011, the path it settled into was highly elliptical. This orbit took it on a 12-hourly cycle from 200 kilometres above the planet’s surface to 10,000 kilometres away from it. It may seem like an odd orbit for a craft with the singular aim of getting as close to Mercury as possible, but this was an essential part of the design of the mission, vital to protect Messenger from the fierce heat radiated by the scorching hot surface of Mercury. The sunlight reflected from the surface is so powerful it would have literally melted the solder holding the spacecraft together if it wasn’t given time to cool down between its closest approaches to the planet.

Protected by an enormous ceramic solar shield and its eccentric orbit, Messenger could begin its work. For two years the spacecraft mapped pretty much every bit of the surface of Mercury, and the images beamed back to Earth revealed a planet that’s been in the firing line for billions of years. Too small to hold on to an atmosphere that might protect it from meteorites, and lacking any processes to recycle old terrain, Mercury’s ancient surface is the most cratered place in the Solar System.

© NASA/JPL

This computer photomosaic of Mercury’s southern hemisphere was created from images taken by Mariner 10 on its flyby of Mercury, giving scientists a tantalising glimpse of this elusive planet.

Cosmochemist Larry Nittler explains the reason behind Messenger’s elliptical orbit

‘The way we addressed the problem of heat from the planet was to be in an extremely elliptical orbit, where we flew in very close over the North Pole, and took observations close to there, but then flew very far over the South Pole, like 10,000 kilometres. And so a couple of times a day we’d zoom in over the North Pole, get our data close, but the instruments would heat up, so then we’d fly and get different data farther out from the planet while we cooled, and in this way – heat up, cool down – we kept everything below the danger temperatures where instruments could be damaged.’

MAPPING MERCURY

The Mariner 10 mission had enabled scientists to see about half of the planet, so the first full view of the terrain of Mercury came from the flybys of Messenger. As planetary scientist Nancy Chabot explains, ‘Before Messenger, we had only seen 45 per cent of the planet and we saw some stuff during the flybys before we went into orbit, but after orbiting the planet we have now mapped 100 per cent of the planet and seen nearly everywhere. There are some permanently shadowed regions which are still mysterious … but after mapping the full planet, we have a good idea of what the surface looks like and craters are absolutely a dominant land form. This planet has been sitting there for billions of years and been hit over and over, and it hasn’t had a lot of processes to destroy those craters.’

‘Scars are just another kind of memory.’

M.L. Stedman

Amongst the thousands upon thousands of craters on Mercury, the largest by far is Caloris Planitia, a lowland basin 1,525 kilometres in diameter that is thought to have formed in the early years of the Solar System, around 3.9 billion years ago. It was first spotted as Mariner 10 sped past in 1974, but due to the trajectory and timing of the craft only half of it was lit, so the full character of this crater remained a mystery for another 30 years until Messenger could photograph it in all of its glory. Taking one of its very first photos, Messenger revealed Caloris to be bigger than had been previously estimated, encircled by a range of mountains rising 2 kilometres from the Mercurian surface, whose peaks create a 1,000-kilometre boundary around the lava plains within.

© NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

This colour mosaic of Mercury’s Caloris basin was created using images taken by Messenger in 2014.

© NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Messenger photographed Mercury’s geology in great detail, capturing this crater within the vast Caloris basin.

On the other side of the mountains, the vast amount of material that was lifted from the planet’s surface at the moment of impact formed a series of concentric rings around the basin, stretching over 1,000 kilometres from its edge. The collision that created Caloris hit Mercury with such force that it also had more global consequences. Messenger photographed in great detail an area named (in the not particularly scientific vernacular) ‘the weird terrain’, a region at the planet’s diametrically opposite point, the antipode, to Caloris. This area of strange geological formations distinct from the rest of the surrounding terrain was likely created by the seismic shockwave of the Caloris impact reverberating through the whole of the planet.

© NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

In this 3D view of Mercury’s north polar region, the areas marked in yellow show evidence of water ice.

© NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

The Mercury Atmosphere and Surface Composition Spectrometer (MASCS) instrument and the Mercury Dual Imaging System (MDIS) aboard Messenger enabled scientists to create these images, which use colours to map out the mineral, chemical and physical makeup of Mercury.

© NASA/Goddard Space Flight Center Science Visualization Studio/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Radio tracking data sent by Messenger has enabled scientists to create maps of the gravity field of Mercury. In this image, Mercury’s gravity anomalies are depicted in colours: red indicates mass concentrations around the Caloris basin (centre) and the Sobkou region (right).

‘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.