Abstract
Exploring Mercury
PhD student Mark Bentley explains how and why he got involved Mark Bentley is studying for a PhD in planetary science. He is helping to design and build instruments for a forthcoming ESA mission to explore the surface of Mercury. Mark Bentley
Space has excited and inspired me for as long as I can remember; my earliest memory of this is being allowed to stay up 'really late' to watch the Space Shuttle Columbia land in 1981, at the age of five. Science in general has always interested me. Although I probably didn't recognize it as such at the time, my fascination with collecting all sorts of equipment (or as my parents called it, 'junk') and finding out what made them tick was an early demonstration of this.
At school it seemed natural to take science subjects (Physics, Chemistry and Maths A-levels) and then to consider University though physics was not my first thought. I was all set for the respectable career of computer science, not realizing that my space interests could lead anywhere, until I flicked through the first prospectus I received. By luck it was from Leicester University, and while computer science was offered it also had something called 'Physics with Space Science and Technology'. The rest, as they say, is history...
After graduating I spent the following two years working for a UK company developing satellite simulators. But then I started thinking about doing a PhD attracted by the flexibility of directing my own research.
I knew that I wanted something that involved space science and the element of discovery, but also something that looked at the engineering and technology of a space mission. The timing was fortuitous shortly after I committed myself to a PhD, the European Space Agency announced the selection of BepiColombo, a mission to Mercury, as one of its 'Cornerstone' (large scale) missions. Here was a mission big on science (no spacecraft has ever orbited Mercury, let alone landed on it) and technology as well!
So that takes me to where I am now in my first year at the Planetary and Space Sciences Research Institute of the Open University in Milton Keynes. If everything goes according to plan, three years later I will be Dr Bentley and know a whole lot more about Mercury! So what am I now? A physicist at heart, but I guess 'planetary scientist' is more accurate...
The great thing about studying the planets is that the field can be stretched to encompass just about any aspect of science you care to choose from biology, through engineering, to physics and more. Planetary science fits well with the modern 'trend' for multidisciplinary research as well as being on the leading edge of modern science, and one of the most international areas of study.
In studying our solar system we aim to learn more about the processes that formed the planets and ultimately life itself. For the foreseeable future the nine major bodies and their associated moons are our only glimpse back in time to the early life of our corner of the Universe.
Over the past few decades, a relatively short period of time, we have expanded our understanding of the planets by orders of magnitude. Instruments like the Hubble Space Telescope have enabled more and more detailed images of both the near and far, whilst robotic space probes have extended scientists' senses to the far corners of the solar system.
The two least studied planets lie at the two extreme ends of our system. Pluto sits at the outer edges of the solar system, a small icy ball that astronomers even argue about calling a planet. Mercury, messenger of the Gods, is a relative inferno, closer to the Sun than any other body.
Mercury is not an easy target for spacecraft. Tucked deep in the Sun's gravitational well, any mission must lose about 60% of its orbital energy in order to match Mercury's orbit. The only spacecraft to visit Mercury to date was Mariner 10, a NASA mission flown in the mid-70s. It had far too much energy to enter orbit and could just make several quick passes, leaving an incomplete image of only half of the planet. This, and observations made from Earth, provide almost all of our knowledge of Mercury. Earth observations, however, are hampered by the planet's proximity to the Sun, making observations possible only at dawn and dusk. A mosaic of images of Mercury from the NASA Mariner 10 spacecraft. ©NASA
In the mid-80s improved radar equipment allowed high resolution mapping of surface features from the Earth. Amongst the results were two tantalising mysteries: a large dome feature, similar in some ways to shield volcanoes seen on Mars, observed on the unimaged side of the planet and complex scattering of returned radar from distinct areas around the poles, suggesting that water ice may exist in craters there.
Both NASA and the European Space Agency (ESA) are now planning missions to Mercury. The US team are using a newly discovered trajectory that will allow them to reach Mercury using traditional chemical propulsion, incorporating various planetary flybys so-called 'gravity assist' manoeuvres. The European team, on the other hand, has proposed a much more complex mission. In order to get to Mercury, ESA have adopted a novel technology knows as 'solar electric propulsion' (SEP). The basic principle is that electrical energy is produced using solar cells, and this is used to accelerate ions of gas, producing a continuous, if low thrust. The upshot is that the mission is much less constrained by the alignment of the planets and other trajectory concerns and can complete the journey in only two and a half years.
BepiColombo, ESA's Mercury mission, will actually consist of three spacecraft! The planetary orbiter will stay close to Mercury and perform remote sensing and mapping of the surface environment. The magnetospheric orbiter, now going to be built by the Institute for Space and Astronautical Science (ISAS) in Japan, will fly in a highly eccentric orbit that takes it from within a few hundred kilometres of the surface to a distance of several planetary radii. This means it will fly in and out of the magnetosphere, the magnetic 'bubble' formed by interaction of the planetary magnetic field with the solar wind. The third and final element is termed the 'MSE' the Mercury Surface Element, or in plain terms a lander, and this is where my research comes in.
There is only so much that remote observation can tell us about a planet. The only true way of verifying what we are seeing is to literally go and 'dig the dirt'. The lander on BepiColombo is designed to do just that, using inflated airbags to cushion its descent to the surface. This 'soft landing' will take place in the polar regions of Mercury, where the surface temperature is moderate—between -50 and +70 °C at the sub-solar point at Mercury's closest approach to the Sun the temperature can reach over 400 °C!
It is the potential for making these surface measurements that forms my PhD research. There are a whole series of fundamental questions that scientists would like to answer about Mercury. For example: why is the planet much denser than the other 'terrestrial' bodies? And how has such a small planet got a magnetic field? The answers to these questions need data from several complementary sources. The first step is to identify the science goals, then look at what measurements could be made to resolve or constrain these questions, and finally consider the physics of obtaining this data.
My project focuses on the surface and sub-surface material on the planet. The surface of Mercury, like the Moon, has been shaped by the impacts upon it and this is still very much in evidence from images of the planet. Craters of many different sizes are evident over most of the surface. These impacts also break up rocks on the surface and produce a finer distribution of particles, known as regolith. The stratigraphy of this material can therefore tell us something about the change in impact environment over time. A conceptual design of the BepiColombo Mercury Surface Element (lander) ©ESA. Conceptual image of the BepiColombo spacecraft at Mercury ©ESA.
As well as being interesting in its own right, the regolith also interacts with almost all other aspects of the Mercurian environment. By analysing the regolith we will be able to find out about Mercury's thin atmosphere and also (because the magnetosphere affects the amount of solar wind hitting the planet's surface) changes in the magnetosphere. Planets like the Earth and Jupiter rely on an electrically conductive ionosphere to close the current systems generated by the magnetosphere. Some researchers believe that on Mercury these currents could flow through, or very close to, the surface itself!
Designing and building instruments to work in an environment like the surface of Mercury is one of the major challenges I face. Not only must they be capable of surviving extremes of temperature and vibration they must also be small enough to fit into a total lander payload mass of just 7 kg and complete their investigations within the one week expected lifetime of the MSE.
In order to take measurements in more than one place, the lander must be equipped with some limited form of mobility. A 'micro-rover' will be carried and deployed after landing, a miniature tracked vehicle that will carry instruments (probably an alpha x-ray spectrometer) to specific target rocks and areas around the lander. To keep things simple the rover will be physically and electronically connected to the lander by a flexible tether.
The lander will also carry a 'mole', a slender cylinder (currently being developed for the Beagle-2 Mars lander) with an internal hammering mechanism. Once pushed into the top layer of soil the mole will be able to drive itself down, pushing aside or breaking small rocks, to a depth of several metres, taking measurements as it goes.
Over the past few months we have been studying some of the instruments which could be carried by the mole. Concentrating on just one of these it is easy to see how quickly you run into problems!
If the MSE lands near the poles, one of the most fascinating activities would be to look for evidence of water ice. In recent years researchers looking at life on the Earth have shown that if water is present, even in the most inhospitable of environments, life often finds a way to survive. The possibility of water on any planet is therefore an exciting prospect!
One possible way to look for ice either at or near the surface is to extract a sample using the mole as it penetrates the regolith, heat it at a constant rate and record the amount of energy used to maintain that rate. This technique, differential scanning calorimetry, can observe phase changes in materials and so help to identify them.
The technical challenges of performing even this simplistic analysis task are quite daunting. We have to design and build a sample acquisition mechanism that can withstand launch and landing and work at extreme temperatures, heat a sample down a borehole and reject excess heat and the electronics must fit into a 2 cm diameter by 50 cm long mole.
So although BepiColombo will not launch until 2009 and will not arrive at Mercury until 2012, there's more than enough work to keep me busy until then!