Nick Shuttleworth Tutor: Prof. McEwen
THE SEARCH FOR LIFE ON MARS
This article discusses the way in which the search for life on Mars is being carried out. Information about findings from meteorites such as ALH 84001 is given and discussed. The history of missions to study Mars is given with more detail on Mars Express and Beagle 2. Finally future missions are considered. This article reaches the conclusions that PAHs alone are of little use as biomarkers in meteorites and that a great deal has been learnt about Mars by the many craft sent to study it.
It is clear that there are several things which are important if life is to exist: a liquid medium such as water, a source of energy and a number of elements. Although it is too cold at present, it appears from photographs and data produced by many missions, that at some point liquid water did exist on the surface of Mars. Energy for life does not exist on Mars as it is provided for Earth, as the distance to the Sun in too great for the same kinds of photosynthesis to occur, but geochemical energy could serve as a good substitute. When these facts are taken with the Pathfinder findings that the rocks on Mars contain magnesium, iron, aluminium and phosphate, which are all good candidates for life support, it is easy to see that life could have existed at some point.1 This text concerns findings and facts relating firstly to meteorites and secondly to Mars missions, past present and future.
Several meteorites have been found on Earth which it has been proved originated on Mars. ALH 84001 was discovered in the Antarctic in 1984 and was found to contain polycyclic aromatic hydrocarbons (PAHs). The distribution of PAHs observed was interpreted as being indigenous to the meteorite, possibly derived from an ancient Martian biota. However, analysis showed that the PAHs observed were most likely due to a mixture of extraterrestrial PAHs and PAHs in the ice meltwater. As a result it was found that PAHs are not useful biomarkers in the search for extinct or extant life on Mars.2 Other studies of ALH 84001, using high-resolution scanning and transmission electron microscopy, found carbonate globules which are similar to some terrestrial bacterially induced carbonate precipitates. Although inorganic formation is possible the only scenario which explains the presence of the PAHs is that which suggests that the PAHs and globules could be fossil remains of past Martian biota.3
THE HISTORY OF MARS MISSIONS
The first missions to Mars were not intended as a search for life, they were simply exploratory. Marsnik 1 and 2 (also known as Korabl 4 and 5 or Mars 1960A and B) were intended to perform a flyby of the planet and return surface images to Earth however both missions suffered launch failures, failing to obtain Earth parking orbit and re-entering.4
Sputnik 22, launched in 1962, was designed to image the surface and send back data on cosmic radiation, micrometeoroid impacts and Mars' magnetic field, radiation environment, atmospheric structure and possible organic compounds. Communications were lost when the spacecraft was 106,760,000 km from Earth. This mission can be considered as the first to show an interest in the existence of life on mars as it was intended to look for organic compounds.5
During the period 1964 to 1971 a further 8 missions took place. Mars 3 in 1971 was the first mission to put a lander on the surface, however 20 seconds after landing the instruments stopped working, thought to be due to a dust storm. The images and data returned by the orbiter revealed mountains as high as 22 km, atomic hydrogen and oxygen in the upper atmosphere, surface temperatures ranging from -110 C to +13 C, surface pressures of 5.5 to 6 mb, water vapour concentrations 5000 times less than in Earth's atmosphere, the base of the ionosphere starting at 80 to 110 km altitude, and grains from dust storms as high as 7 km in the atmosphere.6
Perhaps the most important missions so far, Viking 1 and Viking 2 launched in 1975. The primary mission objectives were to obtain high resolution images of the Martian surface, characterise the structure and composition of the atmosphere and surface, and search for evidence of life. The last of these objectives was to be achieved by taking surface samples and analyzing them for signs of life however these biological experiments found no evidence of life.7
The two most important missions in terms of finding conditions conducive to life ever having existed on Mars were Mars Global Surveyor and Mars Odyssey. Global Surveyor carried a wide array of equipment including the Mars Orbiter Camera (MOC), the Mars Orbiter Laser Altimeter (MOLA), a Magnetic Fields Investigation (MAG) and the most important, in terms of finding evidence of life, the Thermal Emission Spectrometer (TES). Data produced by TES gives strong evidence of water at the Martian south pole. This data was built upon by Odyssey in 2001 using the Gamma-Ray Spectrometer (GRS) which was designed to map the elemental composition of the surface and determine the abundance of hydrogen in the shallow subsurface.8 Measurements showed significant amounts of hydrogen which was most likely due to water ice in the upper few feet of the Martian surface.9
At this time there are two missions studying Mars, the Mars Express orbiter carrying the Beagle 2 lander and the twin NASA rovers Spirit and Opportunity. Mars Express is an ESA mission whose primary objectives are to obtain global high resolution photo-geology, mineralogical mapping and mapping of the atmospheric composition, study the subsurface structure, the global atmospheric circulation, and the interaction between the atmosphere and the subsurface, and the atmosphere and the interplanetary medium.10 To accomplish these aims Mars Express carries a wide array of experiments as its payload; the Energetic Neutral Atoms Analyser (ASPERA) will study the way in which the solar wind interacts with the Martian atmosphere, shedding light on the processes by which water vapour and other gases may have left the atmosphere in the past. The High Resolution Stereo Colour Imager (HRSC) will be used to produce a map of the distribution of rocks and minerals on the surface of Mars. The Radio Science Equipment (MaRS) will by used to measure variations in gravity, pressure and temperature over the surface. The Subsurface Sounding Radar/Altimeter (MARSIS) will map the distribution of water and ice in the upper portions of the Martian crust. It will analyse reflected radio waves in the upper 2-3 kilometres to determine the subsurface structure. The Infrared Mineralogical Mapping Spectrometer (OMEGA) will analyse sunlight reflected from the surface to determine the mineral content of the Martian surface as well as the molecular composition of the atmosphere. The Planetary Fourier Spectrometer (PFS) will measure the global distribution of water vapour in the atmosphere. The Ultraviolet and Infrared Atmospheric Spectrometer (SPICAM) will measure the composition of the atmosphere over much smaller volumes than the PFS experiment. It will also use stellar occultation to measure the vertical profiles of carbon dioxide, temperature, ozone, aerosols and clouds.11 Of the above PFS and MARSIS are the most important in terms of finding evidence of life. The discovery of the presence of water over a long time period (tens of millions of years) and in large enough quantities would greatly increase the chances that life has existed at some point on Mars.
Beagle 2 is the lander associated with Mars Express. It was built specifically to answer the question of the possibility of past life on Mars. Among many other tasks it will look at the presence of water, existence of carbonate minerals, occurrence of organic residues, complexity and structure of organic material, isotopic fractionation between organic and inorganic phases and seek trace atmospheric species indicative of extant life. The Gas Analysis Package (GAP) contains a mass spectrometer which will be used to analyse the amount of carbon dioxide released when soil and rock samples are heated. This process will confirm the presence of carbon in a sample as any carbon compound present will burn to give carbon dioxide. GAP will also be able to detect methane in the Martian atmosphere. Due to the oxidising conditions on the surface of Mars methane is quickly destroyed, so any detection of its presence would lead to the assumption that it must come from a continuous supply such as is produced on Earth by many biological processes. The Planetary Undersurface Tool (Pluto) will be deployed by Beagle 2 to collect samples from beneath the surface in areas which appear to have been inundated by large volumes of water. Pluto will also use a grinding disk to remove the layer of dust on rock surfaces, due to weathering, so that samples can be taken. As well as the above experiments concerned with the search for evidence of life, Beagle 2 will also carry many other experiments including 3 cameras, spectrometers to find the nature of iron on the surface and quantify major elements and environmental sensors to measure surface UV flux, air temperature, air pressure, wind velocity and dust momentum and direction.12
Spirit and Opportunity are twin NASA rovers which will explore the Martian surface during 2004. Spirit is currently on the surface with Opportunity in transit. The two rovers are the largest and most mobile to date and each have a wide ranging scientific payload including a Panoramic Camera (PanCam), a Microscopic Imager (MI), a Miniature Thermal Emission Spectrometer (Mini-TES), a Mossbauer Spectrometer (MB) an Alpha Particle X-ray Spectrometer (APXS), a Rock Abrasion Tool (RAT), Magnet Arrays and Engineering Cameras (Hazcoms and Navcoms).13 Spirit and Opportunity are not vehicles designed to find evidence of life but the data which they return will likely go further in indicating whether life was ever possible on Mars.
Mars Reconnaissance Orbiter (MRO) will launch in August 2005 and is expected to reach Mars in March 2006. It will orbit Mars for one Martian year (687 days) studying the surface using a visible stereo imaging camera (HiRISE) and a visible/near-infrared spectrometer (CRISM). MRO will also be equipped with a shallow subsurface sounding radar (SHARAD) which will search for underground water.
The French Space Agency (CNES) is looking at the feasibility of launching NetLander in 2007. NetLander will consist of four landers which will be used to make simultaneous seismological, atmospheric, magnetic, ionospheric and geodetic measurements. The most important objective of these measurements will be to search for ground reservoirs of water and ice.
Mars Science Laboratory is a long duration rover planned for a 2009 launch. Its primary objectives will be to assess the biological potential of at least one target area, characterize the local geology and geochemistry, investigate planetary processes relevant to habitability, including the role of water, and to characterize the broad spectrum of surface radiation.
A great deal has been learnt about the possibility of extinct and extant life on Mars through various media. Studies of meteorites have shown compounds which are most likely formed organically although whether they are of extraterrestrial origin is not certain. Photographs from Mars orbiters have shown geological features which appear to have been formed due to erosion by a liquid although no liquid water can currently exist on the surface. Data from many missions, specifically Global Surveyor and Odyssey, have revealed what are generally accepted to be vast frozen lakes under the Martian poles. This evidence of water in conjunction with elemental findings suggests that it is possible that life existed on Mars but is yet to be found.
“So life could have originated on Mars. That doesn’t mean that it did, or that it’s there now. But it’s reason enough to look.” 14
The Search For Life In The Universe - Pacchioli
2 Polycyclic aromatic hydrocarbons (PAHs) in Antarctic Martian meteorites, carbonaceous chondrites, and polar ice - Becker et al.
3 Search for past life on Mars: possible relic biogenic activity in martian meteorite ALH84001 McKay et al.
NSSDC Master Catalog
NSSDC Master Catalog
NSSDC Master Catalog
NSSDC Master Catalog
NSSDC Master Catalog
NSSDC Master Catalog
ESA Mars Express: Science and Technology
Beagle 2 : the British led exploration of Mars
NSSDC Master Catalog
14 Bruce Jakosky
, Professor of Geological Sciences, University of Colorado
International Strategy For The Exploration Of Mars - International Mars Exploration Working Group
Mars And The Development Of Life - Hansson
An ESA Study For The Search For Life On Mars - Westall et al.
On The Search For Extant Life On Mars - Klein
The Search For Life On Mars: The Role Of Rovers - Stoker
Search For Life On Mars: Evaluation Of Techniques - Schwartz et al.
Human Exploration Of Mars: The Reference Mission Of The NASA Mars Exploration Study Team - Hoffman and Kaplan