Dr Lutz Richter with a model of the Mars drill PLUTO
Interview with Dr Lutz Richter from the Institute for Space Simulation at the German Aerospace Center, Cologne-Porz, Principal Investigator for the Mars drill 'PLUTO'.
Dr Richter, DLR is contributing an important experiment to the mission in the form of the borer system PLUTO, as well as the HRSC on the orbiter. How would you sum up what this mission is about?
Dr Lutz Richer:
Mars Express will undertake a broad study of the planet Mars with a scope only comparable to the American Viking missions of the 1970s. The Mars Express orbiter will orbit the planet for four years, on an elliptical path between 250 and 11 000 kilometres above the surface. From this orbiter, we will study the characteristics of the surface, the Martian atmosphere and the deep subterranean structure. Finally, the lander Beagle 2 has the task of taking chemical and mineralogical measurements on surface material and searching for indications of earlier primitive life in samples from beneath the surface.
German scientists have made some key contributions to the instruments onboard Mars Express: the HRSC stereo camera comes from DLR and the geophysical radio probe of the planet using the orbiter’s communication system will be managed by the University of Cologne. There is also a good deal of German involvement in other instruments on the orbiter, such as the OMEGA infrared spectrometer (which will map surface minerals, particularly in search of the past effects of water), the PFS spectrometer (for studying the surface and the atmosphere) and the SPICAM instrument (for studying the atmosphere). The MARSIS radar system, developed and scientifically guided by Italy and the US, will send out its signals up to a depth of five kilometres, allowing us to make inferences about the stratification of the planet's crust and the presence of a water table.
Indeed, the question of the history of water on Mars is a central one, as is the question of what forms water might take if present on the planet today and how it is distributed, for example as ice embedded in the ground, exposed on the surface (at the poles) or as water vapour in the atmosphere. The key thing is to find out how often, where and for how long water was able to exist in liquid state on the surface, because this has a bearing on whether life could have formed on Mars as it did on Earth
PLUTO is mounted on the Beagle 2 lander. Where does the name Beagle 2 come from and what other experiments are installed on the lander?
Dr Lutz Richter:
In the mid-19th century, Charles Darwin made several research voyages on a ship called the 'Beagle', and studied plant and animal species in various parts of the world. His findings gave rise to his theory of the evolution of life on Earth. Because one of the objectives of the Mars Express lander is to search for biological traces on Mars, our British colleagues chose the name ‘Beagle 2’. They presented the lander proposal to ESA and, after a successful competitive selection process, were awarded financing for Beagle 2.
The approach to be followed by the lander in its search for life originated with Prof. Colin Pillinger of the Open University in Britain, who together with his team studied a number of the ‘Mars meteorites’ and, like other groups of researchers, identified traces of organic material. It is unclear whether this is connected with any biological processes or whether these traces could have originated from Earth and reached the inside of the rock before being found and examined.
In order to rule out the possibility of terrestrial contamination, or a distortion of the findings through influences from Earth, Pillinger proposed that the Beagle 2 lander be sent to Mars with a miniaturised instrument that could undertake measurements similar to those that Pillinger would otherwise undertake in his own laboratory. The method being used is known as stepped combustion. With this method, the sample is gradually heated up to higher temperatures in the presence of oxygen.
A mass spectrometer detects the escaping gases, searching for carbon from organic molecules which would burn at around 300º Celsius, unlike the carbon from carbonate rocks which is only released at 600º - 800º. We can also measure the distribution of the stable carbon isotopes (12 and 13) within the organic carbon, because on Earth, this is an unmistakable indicator of life processes. So all we are doing is applying a very simple criterion to identify chemical traces of earlier life processes on Mars.
With the Viking missions, many more assumptions were made about the nature of the expected microbial life-forms, and these missions failed to provide conclusive results. What is significant about Beagle 2 is that it will take the very first soil samples from depths of up to 1.5m, thus avoiding the problem identified after Viking: that samples taken near the surface cannot retain organic material for long because of UV radiation from the Sun and presumed oxidising substances (e.g. oxygen radicals) that are formed photochemically. DLR’s ‘mole’ borer system PLUTO is essential to the sampling process in that it gives access to the deep samples. Other instruments on the lander, mounted on a gripper arm, will analyse the mineralogy and chemistry of the rocks around the lander. The main thing we are searching for here is signs of water having affected the terrain around the landing site at some point in the past. Other, lightweight sensors will measure the weather conditions in the ground-level atmosphere.
Was the landing site of Beagle 2 chosen at random or was it picked out specially for particular criteria?
Dr Lutz Richter:
The landing site in Isidis Planitia was specially chosen with technical requirements in mind. For one thing it is relatively low-lying, which means the landing parachute has more air in which to slow down the probe, and for another thing it is near the equator, which means the temperatures will be warmer than in the higher northern latitudes when the lander touches down. Nevertheless, there is evidence of earlier effects of water in this area because the former impact crater Isidis, which is about three and a half to four billion years old, was obviously filled in later by sediment, and water may have played a part in this process. The surface structures visible today are also similar to tuff cones on Earth, which are created by volcanic activity in the presence of water in the ground.
When Beagle 2 lands on Mars on 25 December 2003, how will you know it has arrived safely?
Dr. Lutz Richter: Beagle 2 cannot transmit directly to Earth, because the lander is too small for that and doesn’t have enough energy at its disposal. Two-way communication will be achieved via the Mars orbiter, which has a standardised UHF radio relay system. This system will be able to communicate with the current American landers (as of January) and with Beagle 2. On the scheduled arrival date of Mars Express, the European orbiter won’t yet be available because after the capture manoeuvre it still has to perform several other manoeuvres before the beginning of January in order to enter the planned final orbit around Mars. But the American Mars Odyssey orbiter, which has been circling Mars since October 2001, will be able to receive signals and data from Beagle 2 and transmit commands to the European lander right from the outset, each time it passes over the landing site (every 12 hours). Odyssey's first flyover will occur at 06:30 CET on 25 December, about two hours after the landing. If this brief contact (lasting about 10 minutes) is successful, we will receive the first data from Beagle shortly afterwards, including two black and white wide-angle images of the lander’s surroundings on the ground. From the time when the lander separates on 19 December until it lands, it will not be possible to receive any data.
If the first attempt at radio contact with the lander is unsuccessful, this doesn’t necessarily mean that the landing went wrong. It’s more likely to be down to unfavourable orientation on the planet’s surface or the settings on the communication system (orbiter and lander). In case this happens, on the evening of 25 December Beagle will send out its carrier frequency (a signal with no data) that can be picked up by the radiotelescope at Jodrell Bank in Britain. This will confirm that the lander has reached the ground safely.
Question: PLUTO is often described in the media as a high-tech mole. What is so special about it and most importantly, how will you know what exactly PLUTO has found?
Dr Lutz Richter: PLUTO works on the ‘mole’ principle, which means it’s not a rotating borer. This wouldn’t be an option, because a bore system that could be used in space to drill at depths of 1.5 metres would be too heavy and too large for Beagle 2. This particular ‘mole’ is a 28cm-long metal cylinder with a diameter of 2cm, connected to a cable containing an electrically powered impact mechanism. This mechanism generates an impact from inside against the tip every four seconds. With every impact, the device moves a few millimetres deeper into the soil material, which is pushed to the sides by the impact effect. The rebound from each impact is less than the force of the original impact and is absorbed by the borer’s shell rubbing against the surrounding soil material. This allows the device to drive itself in without the body of the borer having to be braced from above.
The PLUTO mole mainly functions as a sampling tool in that it opens its tip at a specified depth and takes in a few cubic millimetres of soil each time. After each drilling run, the connection cable winds up to draw the mole back out of the ground, aided by reverse impacts if necessary. Because the PLUTO system is mounted on the gripper arm of the lander, the arm can then move PLUTO such that the tip of the ‘mole’ can pass the sample over to the instrument inside the lander that burns the material and searches for organic traces. The way in which the mole penetrates the ground will also provide us with our first information about the solidity of the Martian soil beneath the surface and data about any stratification present. The PLUTO mole will also measure the soil temperature, which will allow us to draw inferences about the physical properties of the Martian soil. We’re also already developing the next PLUTO, which will be designed to take a wide range of measuring instruments into the ground with it to carry out analyses in situ without having to bring the sample up to the surface.
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