To date 33 missions to Mars have been attempted. The first 4 were launched from the Soviet Union between 1960 and 1962 and either did not reach Mars or failed to send back any information. The American Mariner missions started in 1964 and in that year Mariner 4 photographed Mars, showing its surface to contain numerous impact craters. More photographs were taken by Mariners 6 and 7 during their flybys of Mars. Mariner 9 in 1971 was the first spacecraft to enter martian orbit and, once a major dust storm had subsided, its cameras revealed for the first time the true diversity of the martian surface with canyons, ancient flood plains, dune fields and shield volcanoes. The first topographic and geological maps were constructed with the Mariner 9 data. Between 1971 and 1973 the Soviet Union landed two modules, Mars 3 and 6, at the rim of the Hellas basin and within the ancient highlands but little information was returned.
Viking 1 Panorama. The lansdcape is a mixture of poorly sorted rocks (basalt clasts) thought to have been deposited rapidly from a catastrophic flood. Small drifts (less than 2 m) are also visible. The robotic boom and spacecraft can be seen in the foreground. Image from National Space Science Data Center.
The next major step forward in martian exploration was the Viking missions which returned data from 1976 to 1980. This mission was in two identical orbiter-lander pairs - Viking 1 and Viking 2. The satellites went into orbit around Mars and starting returning images prior to the landings (see location of landing sites here.) This allowed the final selection of landing sites to be made after high resolution images had been taken by the orbiters and so hazardous areas were avoided. More recent missions e.g. Pathfinder in 1997 have also used Viking imagery to choose landing sites. The mosaic maps of Mars (and its moons) which are now familiar are based on the Viking Orbiters' 52603 images. The Viking 1 lander reached the martian surface in Chryse Planitia at 22.4oN, 48.0oW in flood plains beyond Maja Vallis in July 1976 and Viking 2 landed in September within the Utopia Planitia basin 48.0oN, 225.5oW 170 km from the 90 km diameter impact crater Mie. The rocks at the Viking 2 site have a small size range and are thought to be ejecta from Mie. The landers carried a range of equipment including panoramic cameras, X-ray Fluorescence for major element analyses of rock and soil, environmental sensors (ie wind speed, temperature, atmospheric pressure), and a gas chromatograph mass spectrometer (gcms). The mass spectrometer was used to measure the atmospheric composition, predominantly CO2 with 2.7% N2, 1.6% 40Ar, with traces of noble gases, H2O and O2, O3. It is this data that was used to demonstrate that some of the SNC meteorites contain trapped martian atmosphere. The most publicized experiments were those concerned with the search for traces of life. There were 3 main biology experiments using the gcms: Pyrolitic Release (PR) for measuring the uptake of CO2 and synthesis of organic compounds; Gas Exchange (GEX) looking for the release of both oxidized and reduced gases; Labelled Release (LR) looking for the decomposition of added organic nutrients. These 3 experiments were designed to test for the effects of photosynthesis (e.g. incorporation of CO2) and other chemical tracers of biological activity. Results from the GEX experiment did show the release of O2 when water was added to soil, and this raised the possibility of photosynthetic processes. However, it was subsequently clearly demonstrated that this could be explained by the presence of strongly oxidizing substances in the top most part of the soil. Such oxidising substances would be formed through uv irradiation of the surface of minerals. No organic molecules were detected by the gcms and none of the 3 biology experiments showed signs of life. More experiments using different approaches to this question might yet produce positive results. For instance, the Beagle 2 lander will sample material underneath rocks and up to 2 m beneath the surface where the effects of uv-induced decomposition of any organic molecules could be minimized.
Pathfinder Panorma, Sagan Memorial Station. Image from National Space Science Data Center.
Pathfinder landed in the Ares Vallis outwash channel (19.1oN, 33.2oW) in the Chryse Basin in July 1997. Like the Viking 1 site, there was a mixture of rocks (about 16% of the surface), soil, cemented hardgrounds and drift. Both sites are of Hesperian age. The varied size distribution and shape of the rocks on the ground surface of Ares Vallis are consistent with a catastrophic flood origin. Unlike Viking, Pathfinder contained a mobile robot on wheels called Sojourner. This was guided around the various rocks seen at the landing site and an X-ray Spectrometer on board Sojourner was used to measure the compositions of rocks and soil, take photographs and spectral measurements. The red-coloured coatings on the rocks were believed to contain ferric oxides, such as Fe-Ti-spinels, and ferric hydroxides. Over the 83 sols (martian days) mission eight rocks, six soil-like materials and one indurated soil/hardground were analysed. The soil and hardground were enriched in SO3 and Cl reflecting the presence of salts such as halite and sulphates. The H2O and carbonate contents of martian soil are not accurately known for either the Viking or Pathfinder analyses but can be constrained within the range 0.2-2.0 wt% H2O and < 2 wt% for CO3. No carbonate minerals have been identified. The major element compositions of martian soil are very similar between the Viking 1,2 and Pathfinder analyses as a result of homogenisation of the surface mobile deposits by aeolian activity. No clays have been firmly identified. The rocks analysed by Pathfinder at Ares Vallis have slightly more fractionated compositions than the Viking samples, having higher SiO2, TiO2 and Al2O3 contents. They have been compared to terrestrial andesite compositions whereas the Viking results are more similar to olivine-rich basalts. However, all the analyses have low Al and high Fe contents compared to terrestrial igneous rocks (but similar to the martian meteorites) suggesting Al-poor and Fe-rich mantle source regions. Primitive magmas from which rocks with SiO2-rich Pathfinder rock compositions could have formed through crystal-melt fractionation may be represented by the shergottite martian meteorites. See the Bibliography, Links, Glosssary for more information about the Pathfinder and Viking data.
Probably one of the most successful planetary missions ever is that of the Mars Global Surveyor MGS (1997). This was launched from Earth at the same time as Pathfinder (in a different spacecraft) which has provided highly detailed topographic, gravity, magnetic, thermal emission spectra, infrared thermal mapping and photographic data. After going into an initially elliptical orbit, MGS entered a nearly circular 2 hour polar orbit and it is from this that mapping of the martian surface has taken place. From an orbital height of about 380 km the Mars Orbiter Camera can take images of up to 1.4 m per pixel and resolve objects of about twice that size at its highest resolution. This has produced much new information about geologically recent gullies on Mars and the metre-scale layered structure of the martian uppermost crust (see section on martian geology) and some of the images have been used in this website. The Mars Orbiter Laser Altimeter MOLA (see Bibliography, Links, Glosssary has now produced a highly detailed topographic map (up to 256 pixels per degree in places). For instance, the basins underlying the northern plains - Polar, Chryse, Utopia, Isidis have been characterized together with Lunar Mare type wrinkle ridges. The topographic data has also been together with gravity data to construct models of the thickness of martian crust which varies from around 3 km under Isidis to > 50 km under the Tharsis volcanic province. See the rotating Mars globe for an idea of what MOLA has produced. The thermal emission spectra have been used to identify two surface type endmembers a SiO2-poor basalt and an andesitic basalt. These end members may be the spectral expressions of the basalts and andesites described at the Viking and Pathfinder sites. The andesitic spectral signature is most widespread in the northern plains and the SiO2-poor basalt covers much of the southern highlands.
Mars Odyssey reached Mars around Oct. 2001 and achieved a circular mapping orbit by January 2002. This orbiter is providing detailed information on the identity and distribution of surface minerals and H2O. The 3 main instruments are THEMIS (Thermal Emission Imaging System, similar but with greater spectral resolution than the MGS experiment), for determining the distribution of minerals; a Gamma Ray Spectrometer to detect H (and therefore by proxy H2O ice or H-bearing minerals); thirdly there is a radiation monitor.
Variation in Hydrogen Concentration across Mars
Epithermal neutrons provide a measure of hydrogen in the martian surface. There is a high hydrogen content in surface soils south of 60oS and around the north polar cap (cold, blue colours). There may be a separate explanation for regions near the equator that contain large H abundances. There it may be in the form of chemically and/or physically bound water e.g. in clays because water ice is unstable near the equator
Recently Boynton et al. (Science, 2002) have published some results from the Gamma Spectrometer experiment. This spectrometer relies on the natural (cosmic ray induced, via the the ejection of neutrons) flux of gamma rays from the top metre of the martian surface. By correlating the radiation flux from the Pathfinder landing site they have obtained a calibration for H and so H2O contents. By using this calibration across Mars (see figure above) they have suggested that there are substantial concentrations of water ice around mid to south polar latitudes and across some northerly latitudes. Such ice would probably be derived from atmospheric water vapour. It is the localised melting of such ice deposits that may be responsible for the gullies in the southern part of the ancient highlands which have been discovered by the Mars Global Surveyor narrow angle images.
An opposition of Mars and Earth occurred in 2003 and both NASA and ESA utilised it. NASA has landed two Mars Environmental Rovers (MER) at two different localities on Mars.
In January 2004 the Mars Environmental Rover Spirit landed safely in Gusev Crater (175oE, 13oS) a large, ancient impact crater. Its twin Opportunity arrived on the 24th January 2004 at Terra Meridiani (353oE, 2oS) an area thought to contain iron-oxide deposited from water at some stage in its history. This landing site is particularly exciting because it shows the first clear outcrop of underlying sedimentary rocks seen in any of the landing missions (see section on martian named features)
Meridiani Planum rock outcrop within small impact crater at the Opportunity landing site, Jan. 2004. Image from JPL/NASA.
More ongoing results have revealed the presence of K-bearing sulphates in the layered sedimentary rock and nodules of haematite (iron oxide). Chemical analyses of the sulphate were made with the Rover's X-ray spectrometer. This clearly shows that liquid water or brine was present at the site and that it evaporated to leave behind salt minerals. This in turn is consistent with the warmer and wetter conditions that planetary geologists have long thought have occurred at least occasionally in Mars's history and especially during the earliest (Noachian) times. This work is consistent with research on meteorites, including that done at the Open University and Natural History Museum which has shown that the nakhlite meteorites contain an assemblage of evaporite minerals deposited by evaporating brines.
Meridiani Planum rock outcrop close-up view showing bedding and circular rock drill marks which are a few cm diameter. Feb. 2004. Image from JPL/NASA.
Meridiani Planum rock outcrop close-up view showing rounded haematite mineral nodule approx. 1 cm diameter in layered sedimentary rock. Feb. 2004. Image from JPL/NASA.
Mars Express has successfully gone into orbit around Mars and has started to return images and data. In the next few years it is planned to produce a high resolution stereo map of the martian surface. One of the highlights of the mission so far is the confirmation of water-ice in the southern polar regions. The issue of glaciation has also risen to the front of Mars research. The Beagle 2 space probe was launched on June 2nd and landed on Mars around 3 am, December 25th 2003. However no contact has been made with it and it is presumed to have been unsuccessful. Perhaps some of the unique and innovative designs can be reused for components of other European missions that are being planned such as BeagleNet. BeagleNet is a proposal to land 2 or more small landers from orbit with seismic instruments and other instruments similar to Beagle 2. The Mars Express Orbiter is currently in orbit returning data: stereo photos with the HRSC instrument, radar, thermal emission spectrometry.
The Beagle 2 space probe weighed 60 kg. In contrast, Viking 1 and 2 spacecraft weighed 3500 kg and some of the other early missions weighed up to 6200 kg. At the Natural History Museum I led the site selection process with a team from various institutions. Beagle 2 is named after H.M.S. Beagle, which is the ship Charles Darwin sailed upon between 1831 to 1836, during which voyage he made many of the observations that led him to write Origin of Species. The Lander (~30 kg) was designed to search for extinct life via isotopically fractionated organic material on and below (< 2 m) the surface using a mass spectrometer and stepped combustion-based system. It would also study the inorganic chemistry and mineralogy of the landing site with its suite of other instruments. An aim was to search for extant life by looking for traces of methane within the atmosphere. The Beagle 2 instrument suite (~9 kg) consists of: a stereo camera pair with 24 filters including 11 narrow band mineralogy filters (440-1000 nm) distributed between the two cameras (these will also produce a 48o diagonal field of view); a 4 colour microscope with a resolution of ~ 8 mm for examination of rock and soil; an X-ray-Fluorescence Spectrometer for elemental composition (similar to that used on Pathfinder); a Gamma-ray Mssbauer Spectrometer for determining iron oxidation and chemistry; a suite of environmental sensors measuring temperature, pressure, dust opacity, UV radiation; the Gas Analysis Package which could measure atmospheric gas composition as well as searching for organics via the stepped combustion method.
Beagle 2 planned to land or landed unsuccessfully in the Isidis basin (see figure below). One of the later planning ellipses is at 11.6oN, 90.75oE (269.25oW). one of the later planning ellipses is 115 x 50 km with a 74.9o azimuth (clockwise from north) and covers 4500 km2. Isidis is a 1600 km diameter ancient (Noachian) impact basin which has subsequently been filled in by lava flows and sediments on top. It also shows signs of Hesperian to Amazonian phreatic volcanic activity in the presence of some volcanic cones. The geological formations within the landing ellipse are the Amazonian Smooth Plains and Hesperian Ridged Plains. The origin of these units is not clear but probably involves sedimentary processes. The pattern of thermal inertia in the basin - with higher values concentrated around the southern margin - suggests that rocks and mud were brought over the basin floor from the adjacent Noachian Highlands. The volcanic cones indicate that there has been a long history of magma-ice interaction in the basin.
Location of Beagle 2 landing ellipses used for initial planning (length 495 km) and final (no. 4) 115 x 50 km ellipse used for placing the target co-ordinates within Isidis Planitia. Diagram from 1/64th MOLA DEM. 1,2,3 Beagle landing ellipses for flight path angles of 15o, 18o, 20o. The 1500 km diameter 'Main Ring' and the 1100 km Inner Ring [Frey et al., LPSC, 2000] are shown. Aps Amazonian Smooth Plains Deposits, Apk Amazonian Knobby Plains Unit, Hr Hesperian Ridged Plains, Hvr Ridged Plains member of Vastitas Borealis Formation, Hs Syrtis Major Formation, Np undivided Noachian Plateau sequences [Greeley and Guest, 1987]. Dotted lines are the boundaries between these units. G indicates two of the ghost craters, partially filled by Amazonian sediment, in the south of the basin. Inset shows the RGB MOLA 1/64th DEM of the Isidis basin with elevation scale. Topographic data from MOLA Science Experiment Team.