Life on Mars?Recent discoveries on Mars have re-kindled the long-running debate between scientists about the existence of Martian life.
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Key textIn August 1996, scientists from the National Aeronautics and Space Administration (NASA) announced they had detected possible signs of life on Mars; the announcement caused heated discussions within the scientific community. A series of more recent discoveries has caused scientists to once again debate the possibility of life on Mars.Where was NASA's 'evidence' found? The early evidence didn't arrive in the form of a little green man or even a little green bug. It was a rock. Not much bigger than a potato and given the dry name of ALH84001, this rock was discovered in Antarctica in 1984. It stands out as a rock among rocks because it is a meteorite and it came from Mars (Box 1: Mars: Earth's cool cousin). We know where it comes from because it has a chemical 'fingerprint' that matches samples taken from Mars. Life on Mars...PAH! NASA scientists noticed several intriguing things about ALH84001 which they thought indicated that life may once have existed on Mars. The first of these were tiny blobs, the size of fullstops. Called carbonate rosettes, they were similar to the rosettes produced by bacteria in ponds on Earth as they metabolise minerals. In addition, chemical compounds known as polycyclic aromatic hydrocarbons (PAHs) were found in and around the carbonate rosettes. ALH84001 contains an unusual mixture of certain lightweight PAHs. The NASA scientists concluded that these might have been produced as once-living organisms decomposed. Another piece of evidence centred on the discovery of tiny crystals of magnetite and iron sulphide embedded in places where the carbonate rosettes had dissolved. The scientists noted that some bacteria on Earth also manufacture similar crystals. The fourth and final piece of evidence was perhaps the most controversial. Using an electron microscope, the NASA scientists found elongated and egg-shaped structures within the carbonate rosettes which they interpreted to be tiny fossils of Martian microbes. If not life on Mars, then what? Mars, which once may have had a climate similar to Earth's, has long been the subject of earthly speculation about life on other planets. Nevertheless, most scientists not directly involved in the study of ALH84001 were sceptical when the results were announced amidst worldwide excitement. The scepticism has increased as further tests on ALH84001 have been carried out. There are several plausible explanations for the phenomena described above that don't require the presence of life. Natural geological processes alone could have caused all the observed 'evidence'. For example, carbonate globules similar to the meteorite's rosettes have formed through chemical reactions in volcanic rocks in Norway. Another possibility is contamination. Some other meteorites have been shown to be contaminated by chemicals and organisms from Earth. The NASA scientists argued that most of the carbonates contained isotopes associated with Mars and that PAHs were more concentrated inside the rock than on its surface. Others have concluded that the meteorite picked up at least some of the PAHs (which are common everywhere on Earth) as it sat in the ice for 13,000 years. In any event, these compounds occur on dust particles in space where they almost certainly formed non-biologically. Keep on looking The debate about life on Mars continued when methane was detected in the Martian atmosphere in 2003. On Earth, over 90 per cent of our methane is produced by living things. Could living things also be responsible for Martian methane? Or was it produced by reactions between rocks and underground water?
One way to settle the ongoing debate about life on Mars is to go back there. In January 2004 two NASA 'rovers' – robotic geologists – landed on Mars. They found clues suggesting past water activity, essential for life as we know it, on the planet. Then in 2008 NASA's Phoenix Mars Lander confirmed what scientists had long suspected – that water exists on Mars. Phoenix also confirmed that Martian soil contains magnesium and potassium and has a slightly alkaline pH. Some people went so far as to say that we could be growing alkali-loving plants like turnips and asparagus in it. However, the presence of very high levels of salts and perchlorate has raised more questions about Mars' habitability. Australian scientists have a range of expertise that can help in the search for evidence of life on Mars. Professor Malcolm Walter, Director of the Australian Centre for Astrobiology at the University of NSW, has worked closely with NASA in their astrobiology program and has previously advised them on the best landing sites for probes searching for evidence of life (or former life) on Mars. 'Hot springs are a great place to live if you're a bacterium,' says Professor Walter. 'So what we should be doing on Mars is focusing on former hot springs.' (Box 2: Cyanobacteria: the simple things of life). In fact life on Earth is thought to have originally evolved in similar warm, saline conditions. The thought of finding bacteria in Martian hot springs may not immediately excite the imaginations of Hollywood movie producers. But the discovery of life elsewhere in the universe – in any form – could have a profound effect on the way we perceive ourselves. Are we alone, or does the universe teem with life? (Box 3: ET, can you speak up?). Related Nova topics:
There are similarities between Mars and Earth that make the possibility of life there even more plausible. Although much smaller than Earth, Mars has a similar density and a similar mineral composition. The day length on Mars is only just over 24 hours, and the angle that the axis of rotation forms with the orbital plane is currently nearly the same on both planets, causing both to have seasons. The hitch is that Mars is cold. The average temperature at the surface of the planet is about -63°C at the equator and as low as -110°C at the poles. Although living things can survive on Earth under extreme conditions, life is unlikely to thrive at these temperatures. Nevertheless, the existence of life or former life cannot be ruled out. There may be 'hotspots' under the surface where heat produced by the planet's core creates temperatures warm enough for microbial life. In addition, Mars may once have been much warmer than it is today. Scientists have detected evidence that water and carbon dioxide have escaped the planet into space. If these gases were once present in the planet's atmosphere in large enough quantities, they could have created a 'greenhouse' effect similar to the one which keeps Earth warm. Indeed, it is possible that about 4 billion years ago the conditions on both Earth and Mars were suitable for life. Some big questions thus remain unanswered. Was the beginning of life on Earth an extraordinary fluke, or is the process so easy that it also occurred on our nearest planetary neighbour at about the same time? Could life have hitched a ride on a meteorite between the two planets, as some scientists suggest? If life did start on Mars, what happened to it? Related sites
They are still with us. Indeed, blue-green algal blooms are becoming increasingly evident in Australia's rivers and lakes as human activities create conditions in which they thrive. Although commonly referred to as blue-green algae, cyanobacteria are not actually algae. Cyanobacteria, and bacteria in general, are prokaryotic life-forms. This simply means that their cells do not have distinct nuclei – their genetic material mixes in with the rest of the cell. This characteristic is distinctive of bacteria and archaea; all other life-forms on Earth consist of eukaryotic cells in which the genetic material is contained inside a membrane. Bacteria (and archaea) are hardy creatures. They can survive in hot, cold, salty, acidic and alkaline environments in which eukaryotes would perish. Despite this, they have a bad image: after all, bacteria cause many diseases in humans, some of them fatal. Yet, without them we may not be here at all. Most scientists believe that microbes were the earliest life-forms, simple creatures that fed on carbon compounds that were accumulating in Earth's early oceans. In the harsh conditions that were present then, no other organism could have survived. Slowly, other microbes evolved that could use the sun's energy to manufacture their own food. Cyanobacteria then went a step further: they started to extract hydrogen from water during photosynthesis, releasing oxygen as a by-product. Over time, enough oxygen accumulated in the Earth's atmosphere to allow the evolution of oxygen-breathing organisms. But we may owe bacteria more than the air we breathe. It is possible that eukaryotic cells, of which humans are made, evolved from bacteria about 2 billion years ago. The theory is that larger prokaryotic cells started 'swallowing' smaller ones. Eventually, the small cells became the membrane-enclosed nuclei of the larger cells, and eukaryotic cells came into existence. Regardless of how it happened, the evolution of eukaryotic cells was a significant milestone in the history of life on Earth. As conditions became more favourable, ever more complex organisms began to evolve. Over three billion years later, we have reached a point in our own evolution where we can peer down a microscope at perhaps a thousand of these tiny life-forms drifting in a drop of water. Are we looking at our ancestors? Related site
OK, there may be microbes on Mars, stunning enough if proved true. Yet perhaps the bigger question is whether any intelligent life exists elsewhere in the universe. The prospect both scares and excites people. Scientists and radio-astronomers have started the search for extraterrestrial intelligence (SETI) in a systematic manner. Several international organisations, including the SETI Institute and the SETI League, are using radio telescopes to 'listen' for signals that might have been produced by intelligent life. Such signals are most likely to be in the form of electromagnetic radiation. This travels at the speed of light and is generally unlikely to be scattered or absorbed as it flashes across space. The universe is fairly quiet in the microwave region of the electromagnetic spectrum, so most SETI enthusiasts tune their receivers to look in this band. In 1995 the SETI Institute started Project Phoenix at the Parkes radio telescope in NSW. This initial phase of Project Phoenix searched for extraterrestrial signals from 202 sun-like stars up to 155 light years away. Project Phoenix used radio telescopes to scour 800 nearby stars for signs of life. The Parkes telescope detected some cosmic noises, but none that could be attributed to aliens. Nevertheless, it has continued to play an important role in more recent SETI projects. Now anyone can become involved in the search for extraterrestrial intelligence through their personal computer. SETI@home is a project run through the University of California that makes use of the general public's computers to increase the amount of radio telescope data that can be analysed. As the search continues, it is worth thinking about what would happen if a call did come in from outer space. Will our global society be shattered by it, or more closely bonded? Will the reaction be one of fear, or hope? Should we try to make contact with the extraterrestrials? If we did, there's one crucial question that we cannot answer: what on Earth will they think of us? Related sites
August 2009, pages 18-21
January/February 2002, pages 38-40 Down under Mars research a step closer (by Jennifer Laing) Describes an attempt to find Mars-like sites in the Australian outback, where scientists could test equipment and technology.
Cosmos
New Scientist
13 August 2008, pages 6-7 Phoenix uncovers history of liquid water on Mars (by Rachel Courtland) Discusses the discovery of water and perchlorate by the Phoenix Mars Lander.
4 August 2008 Phoenix Mars lander 'tastes' first sample of water ice (by Devin Powell) Confirms the presence of water on Mars.
16 July 2008 Mineral maps show widespread water on early Mars (by Maggie McKee) Describes the amount and location of water on early Mars.
26 June 2008 The most extreme life-forms in the universe (by Anna Davison) Examines organisms that survive in extreme conditions on Earth and their relevance to extraterrestrial life.
26 June 2008 Martian soil could grow turnips, Phoenix finds (by Rachel Courtland) Describes analysis of soil on Mars by the Phoenix lander.
29 May 2008 Was Mars too salty for life? (by Stephen Battersby) Proposes that the salt levels discovered on Mars may be too high to support life.
23 May 2008, pages 35-39 Fire and ice: What really happened to water on Mars (by Stuart Clark) Reviews possible explanations for what has happened to previous surface water on Mars and its implications for life.
20 March 2008 Chloride salts on Mars may have preserved past life (by Maggie McKee) Discusses whether chloride deposits on Mars may harbour life.
25 July 2007, pages 42-45 Mars: What flows beneath (by Ivan Semeniuk) Provides an overview of the objectives of the Phoenix Mission.
12 July 2003, pages 24-39 This special issue discussing life beyond Earth contains the following articles:
Scientific American 18 December 2007 Martian meteorite harbors life's building blocks (by JR Minkel) Proposes that organic molecules inside space rock were probably the result of chemical reactions
May 2007, pages 24-33 The mystery of methane on Mars and Titan (by Sushil K. Atreya) Proposes reasons for the presence of methane on Mars and Titan.
August 2001, pages 36-41 The ice of life (by David F. Blake and Peter Jenniskens) Describes the connection between interstellar ice and organic compounds.
December 1997, pages 36-41 The case for relic life on Mars (by Everett K. Gibson Jr, David S. McKay, Kathie Thomas-Keprta and Christopher S. Romanek) An early article that reviews the evidence from meteorite ALH84001 suggesting previous life existed on Mars.
Water and life on Mars? (Denver Museum of Nature and Science, USA)
Looks at the connection between water and the existence of life, and discusses ways to look for evidence of water on Mars.
After 10 years, few believe life on Mars (USA Today, USA) Evaluates the evidence of life from the meteorite ALH84001
The NASA University of NSW Pilbara Education Project (Australia)
A multimedia resource that investigates early life on Earth with a focus on the Pilbara region.
Australian Broadcasting Corporation
How Mars works (How Stuff Works, USA)
Covers the major geologic features of Mars, its climate, how it was formed and the possibility of the existence of life.
On the question of the Mars meteorite (Lunar and Planetary Institute, USA)
An older article that answers basic questions about the meteorite from Mars (ALH84001) and reviews the evidence for Martian bacteria in the meteorite.
asteroid. A small Solar System body that orbits the sun. Most asteroids are thought to be the result of debris left over from the formation of the solar system and occur in the region between Mars and Jupiter. Collisions among the bodies in the asteroid belt and adjacent planets displace fragments and place them on a collision course with Earth. The asteroid belt is thought to be the source of most meteorites. electromagnetic radiation. Energy that travels through space in the form of waves. The highest frequencies are gamma-rays; the lowest frequencies are radio waves (microwaves are a type of radio wave). All electromagnetic radiation travels at light speed – 300 000 kilometres per second in a vacuum. Shorter wavelength radiation (eg, ultraviolet) carries more energy and is likely to be more harmful to living tissue. electron microscope. An instrument that uses electrons, instead of light, to produce a magnified image of an object. The magnification that can be achieved is about one thousand times that of a light microscope. eukaryotic. Describes those cells that have their genetic material (chromosomes) contained within a nucleus. meteorite. A fragment of an asteroid or a planet that has been broken off by a collision and eventually falls on the Earth. It consists of solid matter which survives the descent and lands on the Earth's surface. polycyclic aromatic hydrocarbon (PAH). An organic compound containing only hydrogen and carbon. The atoms are organised into a number of stable, unsaturated ring structures, like benzene. The main sources of PAHs on Earth are vehicle exhaust and smoke from burning fossil fuels. prokaryotic. Describes those cells, or organisms, that do not have their genetic material enclosed within a nucleus. Bacteria and archaea are prokaryotic.
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This topic is sponsored by the Kanagawa Prefectural Museum of Natural History.
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