Earth and Mars: shared past, shared future?
Dr Ralf Jaumann, Co-Investigator of the High Resolution Stereo Camera (HRSC) on Mars Express
Interview with Dr Ralf Jaumann, Commercial Director of the Institute of Planetary Research at the German Aerospace Center (DLR), Berlin-Adlershof.
Question: Dr Jaumann, there are similarities and differences between the geology of Earth and the geology of Mars. Why is Mars so interesting from a scientific point of view?
Dr Jaumann: Of all the planets in the Solar System, Mars is without a doubt the one that is most similar to Earth – not only in terms of its outer appearance but also a number of fundamental geophysical parameters, such as internal structure and the rough composition of the planet. We’re also familiar with Martian surface structures such as volcanoes, canyons, river deltas, polar caps and deserts from their presence here on Earth. Although the Martian atmosphere is considerably thinner than that of Earth, the interaction between the atmosphere and the surface of Mars means that a number of processes that take place on Mars are similar to those on our own planet.
What Mars doesn’t have - at least today – is water, the ‘driving force’ behind a large number of erosive processes that continually reshape the surface of our planet. On Mars, erosive processes happen much more slowly than on Earth. For example, all over Mars you find craters that are billions of years old, whereas on Earth, all traces of asteroid impacts disappear within a matter of a few million years. So what we see on Mars today is to some extent a snapshot of early Martian history, a ‘window’ into the geological past of our Solar System. Most of the surface of Mars, particularly the ‘old highlands’ in the southern hemisphere, was formed more than two billion years ago. This can be seen from small-scale forms as the result of current wind erosion.
But we also know from photographs of Mars that water must certainly have played a very important role on this planet at one time. This immediately raises the question of whether it could have supported life, and whether this life was only episodic or short-lived before disappearing again. Or whether there are still ‘niches’ where life is actually present, protected from cosmic radiation. The question of water and life on another celestial body is also a geological one, because water leaves behind ‘geological’ traces in the form of valleys, layered deposits, deltas and other flow forms.
Our neighbour Venus, which is closer to the Sun, may well be not dissimilar to Earth as a whole body, being almost as large as our ‘blue planet’. But the surface of Venus, and in particular its impenetrable atmosphere, have fewer ‘Earth-like’ structures than Mars and we can be reasonably sure that water has been of little significance on the surface of Venus, because at the temperatures experienced so close to the Sun, it is never stable and would evaporate immediately. This makes Mars a more interesting focus for planetary geology.
Question: Like Earth, Mars has seasons, determined by the planet’s orbit around the Sun. Can the associated processes on Mars be compared with those on Earth?
Dr Jaumann: Absolutely! The most striking example is the seasonal expansion and melting of the polar caps. Because the rotational axis of Mars is more ‘tilted’ on its orbital path, at an angle of more than 25 degrees, seasonal fluctuations are even more extreme than on Earth. The highest and lowest temperatures vary enormously: on the warmest days the temperature at the equator may reach 20 degrees Celsius, while during the coldest polar nights the temperature can sink to minus 140 degrees Celsius. These differences in solar energy, to some extent absorbed by the atmosphere, try to balance themselves out, which gives rise to what we call ‘weather’ here on Earth.
Mars has weather too. There are fierce dust storms, for example, that rage over huge distances and leave their traces on the surface. Nowadays, wind and ‘weather’ are the most active processes shaping the surface of Mars. But Martian weather today lacks water: there is no rain or snow, and no rivers or glaciers form - two very important erosive processes here on Earth. Today, Mars is a dry desert planet.
But in the four and a half billion years of Martian history, there must have been periods during which drainage systems were fed with water or valleys were eroded by glaciers. It has even been surmised that a vast ocean once existed in the strikingly flat, low-lying northern hemisphere of the planet, an ocean that could have been fed by rivers in the now dry valleys of the southern highlands. However, Mars as a planetary body is too small to be able to retain a dense atmosphere, because its gravity is two thirds less than that of Earth: the volatile gases in an atmosphere of this type are lost to space.
Question: Looking back at previous missions to the Red Planet, what do we already know? And what is there that’s still worth finding out?
Dr Jaumann: Since the first reconnaissance missions to Mars in the 1970s, our knowledge of Mars has increased enormously. The two American Viking missions in 1976 achieved some groundbreaking results. Firstly, the photographs taken by the two orbiters enabled us to map the surface of the planet with high-quality image resolution, and secondly the two Viking landers sent back from Martian soil what is still the most important discovery: Mars is dry, and no traces of life have yet been found.
Nonetheless, these results triggered even more intensive exploration of our neighbouring planet, particularly the spectacular, detailed images of the gigantic volcanoes and the polar caps with their striking structure, but most of all the dried-up ‘river systems’ and canyons. The hypotheses could now be tried out for real: if there were water on another planet in the inner solar system, Mars would be the first place to look for it. And that’s exactly what they did: since the 1990s, advances in microelectronics have allowed us to tackle the key questions with much better and miniaturised instruments.
But there was a serious setback with the failure of the Russian mission Mars 96, particularly from the point of view of European Mars research. However, this was followed in 1997 by the successful landing of the Mars Pathfinder, when the small rover ‘Sojourner’ explored the area around a landing site for the first time. Sure enough, Sojourner did not find any water, even though the chosen landing site was a primeval river delta. The Americans suffered a setback of their own with the loss of the Mars Polar Lander in 1999, but also celebrated two major successes in Mars research with the two orbiters Mars Global Surveyor and Mars Odyssey.
Question: We obviously already possess a great deal of knowledge about Mars thanks to these missions. Where should future research now be focused?
Dr Jaumann: What we need now is even better pictures of the surface of Mars: in higher resolution, covering the entire planet and, most importantly, linked to topographical information, to form a realistic three-dimensional picture of the planet’s surface. This is the only way in which we can quantitatively define, diagnose and compare geological phenomena such as volcanoes, lava flows, river valleys and craters – and not only that but understand the processes which created them. And finally, with the selective use of specific colour channels in photographs taken from orbit, we can draw conclusions about the composition of a surface, in other words its mineralogy. All of this is handled on Mars Express by the High Resolution Stereo Camera (HRSC), which provides simultaneous, full-coverage colour imaging of a structured surface with high resolution, in three dimensions. The pictures taken by Mars Express will take Mars research another huge leap forwards. And let me just add that the HRSC was developed here at DLR in Berlin-Adlershof!
Question: Dr Jaumann, if you had to define the concept of ‘life’, would the same assumptions apply on Mars as on Earth? Couldn’t life on other planets develop in totally different forms?
Dr Jaumann: That’s a difficult question to answer, since Earth is all we know about and even then we don't know enough about how life developed from the first hydrocarbon molecules that had the ability to multiply and their own metabolism. We don’t know whether life originally developed on Earth, whether it only developed on Earth, or whether it was brought to Earth by a meteorite from a nearby celestial body, possibly even Mars. This may sound rather far-fetched and is in fact quite improbable, but ‘biologically’ speaking it is a possibility. The time it would take for a chunk of rock, hurled out into space from Mars, to get into an orbit that crossed the path of Earth’s orbit is not so long that a microbe inside such a meteorite could not survive the journey in the not-so-extreme cold of the inner Solar System.
In fact, it’s impossible to come up with a generally accepted definition of ‘life’. It is generally agreed that life on Earth is characterised by three features: the ability to reproduce, a metabolic process for the conversion of energy, and the ability to ‘evolve’ or develop new characteristics. The conditions required for this would also seem to be undisputed: you need to have water, energy and organic molecules, in other words hydrocarbons. But already we’re entering an area of contention. Does it have to be the carbon atom, with its ability to make four chemical bonds and thus highly varied ones? Or, in another part of the universe, could this work with the silicon atom, which has a similar structure and also has a valency of 4? Astronomy is going through a phase of enormous advances. In just a decade we have discovered over a hundred planets that orbit around stars, but research has not yet advanced far enough to answer the question of whether an organic or hydrocarbon chemistry exists on these distant worlds that could give rise to life that is capable of reproducing.
Question: Under what conditions would it be possible for life to develop in the Solar System?
Dr Jaumann: We have to bear in mind the fact that, when the first life-forms began to develop in our Solar System, the planets were ‘hostile to life’ as we would understand it now. Their atmospheres contained much more carbon dioxide than they do today, and no free oxygen. In the first few hundred million years, it was very much hotter on the surface of these planets and each planet was subjected to bombardments of meteorites and asteroids many thousands of times larger than today. One of the consequences of this may have been that, three and a half to four and a half billion years ago, when the planets were consolidating, the first life-forms developed in isolation either on Earth or on another body but were then completely annihilated by a severe asteroid impact, because an event with such high energy levels would ‘sterilise’ the whole of the Earth’s crust. This is an entirely realistic assumption. But does this mean that life developed anew each time following the same pattern or did it start off with a different structure depending on circumstance? This is a question that gives rise to a great deal of controversy amongst scientists.
Question: Could life have developed on Mars under these conditions?
Dr Jaumann: All these areas of discussion, and also the uncertainty that still surrounds all the available theories, don’t just apply to Earth but are intended to be universally applicable. Science is currently putting them to the test for our entire solar system. And that’s where we come full circle. We have no reason to suppose that life couldn’t have developed on Mars if the necessary conditions for triggering reproduction and metabolism were present: water, hydrocarbons and the necessary amount of energy. This last factor is the direct result of a planet’s distance from the Sun: it defines what we call the ‘habitable zone’, a zone in which water is stable on the surface of a planet and where the sun’s radiation provides ‘moderate’ conditions that are hospitable to life. Today Mars is beyond this zone, with Earth occupying the best position in the middle and Venus being too close to the Sun.
But with Mars, we’re confronted with a borderline case. If you just ‘nudge’ a few parameters, for example if in models you assume other values for axial tilt, solar radiation or the composition of the early atmosphere, you get a Mars with a small-scale greenhouse effect and therefore water in a stable condition as H2O on the surface.
In addition to other scientific tasks, the current missions to Mars all have the same objective: to find direct or indirect evidence of the existence of water on Mars - whether in the past or even today - both from Martian orbit and on the ground, using remote sensing equipment and the miniature chemistry lab on board Beagle 2 and the two American exploration rovers. And finally, to search for hydrocarbon compounds of organic origin. Only one thing is certain: life cannot have played a very important role on Mars, because otherwise we would have discovered traces of it a long time ago, such as sedimentary rocks with characteristic chemical signals that could be detected from orbit. In addition to this, the Martian environment today is very hostile to life by Earth standards. Without a protective magnetic field like Earth, Mars is exposed to cosmic radiation that is highly destructive to cells because of its high UV energy levels.
Question: What makes the HRSC so valuable to Mars research? How does this camera work and what kind of advances we can achieve with it?
Dr Jaumann: As a matter of fact, if the Mars Express mission is successful and can be prolonged for one Martian year (or two Earth years), Mars will be better mapped than Earth in terms of topographical data by the high-resolution stereo camera. Earth has of course been measured with high precision in densely populated parts of the continents. But there are many areas, particularly at the poles, which are not particularly well charted topographically. Mars may be smaller than Earth, but its surface is approximately the same size as the total land mass of Earth.
You can imagine what a huge, valuable collection of image data we will have at the end of the mission for planetary research, images that are much better quality than those provided by the mapping project of the Viking probes. The goal of the camera team is to map half of the planet in stereo with a resolution of 10 - 20 metres per pixel and with 20 - 40 metres vertical accuracy. 75% of the surface will be mapped with at least 40 metres per pixel and the whole of Mars with a resolution of at least a hundred metres. Using the HSRC telescope tube, it should also be possible to chart 2% – 3% of the surface with a resolution of 2 – 5 metres. That may not sound like much, but it’s the equivalent of the total surface area of the first 15 member states of the EU!
Question: How does the High Resolution Stereo Camera work?
Dr Jaumann: The HRSC is quite an unusual camera, one which has never flown on a planetary mission before. It works in a similar way to a flat-bed scanner, the kind many people use with their PCs. Each of the nine sensors consists of 5184 light-sensitive semiconductor elements, each measuring 7 microns, arranged in a line. These are arranged in parallel in the focal plane of the lens and are moved across the area to be photographed by the forwards movement of the spaceship, like a broom being pushed in front of you. Each activated sensor maps one strip of terrain per scanning sequence, with a width of 5184 pixels and a length that varies according to choice but may extend to several tens of thousands of pixels. The length of the charted area is only limited by the memory and transmission capabilities of the probe.
The recorded image signals then proceed following a precisely defined plan. The spacecraft transmits the compressed data to Earth, where it is received either by the ESA antenna in Australia or NASA's Deep Space Network in California, Spain or Australia. From there, the data is sent to the ESA’s control centre in Darmstadt (Germany), where it is checked for completeness before being passed on to the DLR’s Institute for Planetary Research at Berlin-Adlershof, where it is processed by the HRSC Experiment Team. Finally, the director of the camera experiment, Professor Neukum from the Freie Universität Berlin, gives his research team the go-ahead to begin scientific analysis of the image data. The scientists’ findings are then published as quickly as possible and presented to other specialists in the field for discussion. However, the most important findings and the most spectacular images are released to the general public immediately.
Question: You will need a few years yet to analyse all the scientific findings of the Mars Express mission. But what’s next? What missions will the DLR be involved in over the coming years?
Dr Jaumann: Mars Express is in every respect a major mission, and if all goes well, a hugely important mission given the many first-class experiments being done. It will add enormously to our fundamental understanding of the planet Mars. However, the DLR Institute for Planetary Research is also involved in other missions which are extremely important to planetary research.
In mid-2004, the US/European spaceship Cassini-Huygens will reach Saturn and spend several years there carrying out a wide-ranging set of experiments to discover more about the gaseous planet, its rings and especially its many-shaped moons from orbit. There are still many unanswered questions in the exploration of the outer solar system too, some of which are associated with the possibility of the existence of water and the development of life, as is the case with Mars. It is surmised that on Saturn’s largest moon, Titan, enveloped by cloud, there could be methane oceans and ice continents. This is where the ESA probe Huygens will be landing. The DLR’s Institute for Planetary Research will be primarily concerned with the analysis of spectrometer data from Cassini about the moon’s solid surfaces.
February 2004 will see the launch of the comet mission Rosetta, having been slightly delayed. However, it won’t be making its rendezvous with the comet Churyumov-Gerasimenko until the year 2014.
The DLR Institute for Planetary Research also plans to look at other important questions within the inner solar system. For example, we are extremely interested in the geology of the innermost planet Mercury. Towards the end of this decade, a European mission called Bepi Colombo is scheduled to perform some very important experiments on this planet, a mission in which we will be involved.
In 2010 the Dawn mission will be heading for the third largest body in the asteroid belt, the small planet Vesta. This mission will be launched by the Americans in 2006 and the camera experiment is being developed by the DLR Institute in collaboration with the Max Planck Institute for Solar System Research..
But Mars will of course continue to be a focus of interest, because despite the huge amount of research being done, one thing is certain: even with Mars Express, Beagle 2 and the American rovers Spirit and Opportunity we cannot find answers to all our questions. NASA plans to bring the first rock sample from Mars back to Earth within the next 20 years.
Europe will certainly also be making further contributions to the exploration of the red planet; as well as various plans for future unmanned missions to Mars, a project to explore Earth's sister planet is also in the pipeline in the form of Venus Express. And we shouldn’t forget that our nearest neighbour in space, the Moon, still hides a few clues to help our understanding of the primitive history of Earth and the inner Solar System.
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