ESA Science & Technology - Publication Archive

Photographs of the martian surface are undeniably beautiful, but uncovering the truth behind the sinuous river beds and suggestive gullies is not easy. Spacecraft and rover missions have turned up plenty of evidence that water once flowed on Mars, but today the planet's surface is bone dry. There is water ice at the poles and hints of a frozen sea near the equator - but any large reservoirs of water are likely to be kilometres down where conditions are warmer. So the hunt for liquid water, which might provide a habitat for martian life, has gone underground.

The 150 km wide Holden crater lies in an area characterized by high density of valley networks implying conditions conducive to forming of water-related environments. We undertook geological mapping and a stratigraphic survey in order to probe the evolution of water-related landforms and their paleoenvironmental implications. Our investigations lead us to propose that the Holden area was subjected to a "wet" lacustrine phase of Hesperian age and an "icy" phase during the Amazonian. Deltaic, coastal, and lacustrine environments occurred during the ??wet?? phase, some displaying a cyclic depositional pattern presumably related to autogenic processes. Water was delivered to the basin by the Uzboi Vallis and by surface runoff channels from a series of drainage basins along the crater walls. Fan delta geometries and coastal onlap enabled estimation of major water levels. Two levels of major stand of the water have been recognized, possibly reflecting allogenic controls. Geologic units related to this "wet" lacustrine phase were subsequently eroded by glacial abrasion and plucking and were disconformably overlain by glacial deposits of Amazonian age, defining an "icy" phase. These features are consistent with a warm-based glacier entering the Holden crater through the wide Uzboi Vallis to form a proglacial lake in the central part of the crater. Changes in sedimentary units reflect changes of depositional environments probably connected with climatic variation.

Images from the Mars Express HRSC (High-Resolution Stereo Camera) of debris aprons at the base of massifs in eastern Hellas reveal numerous concentrically ridged lobate and pitted features and related evidence of extremely ice-rich glacier-like viscous flow and sublimation. Together with new evidence for recent ice-rich rock glaciers at the base of the Olympus Mons scarp superposed on larger Late Amazonian debris-covered piedmont glaciers, we interpret these deposits as evidence for geologically recent and recurring glacial activity in tropical and mid-latitude regions of Mars during periods of increased spin-axis obliquity when polar ice was mobilized and redeposited in microenvironments at lower latitudes. The data indicate that abundant residual ice probably remains in these deposits and that these records of geologically recent climate changes are accessible to future automated and human surface exploration.

The majority of volcanic products on Mars are thought to be mafic and effusive. Explosive eruptions of basic to ultrabasic chemistry are expected to be common, but evidence for them is rare and mostly confined to very old surface features. Here we present new image and topographic data from the High Resolution Stereo Camera that reveal previously unknown traces of an explosive eruption at 30° N and 149° E on the northwestern flank of the shield volcano Hecates Tholus. The eruption created a large, 10-km-diameter caldera ~350 million years ago. We interpret these observations to mean that large-scale explosive volcanism on Mars was not confined to the planet's early evolution. We also show that glacial deposits partly fill the caldera and an adjacent depression. Their age, derived from crater counts, is about 5 to 24 million years. Climate models predict that near-surface ice is not stable at mid-latitudes today, assuming a thermo-dynamic steady state. Therefore, the discovery of very young glacial features at Hecates Tholus suggests recent climate changes. We show that the absolute ages of these very recent glacial deposits correspond very well to a period of increased obliquity of the planet's rotational axis.

It is thought that the Cerberus Fossae fissures on Mars were the source of both lava and water floods two to ten million years ago. Evidence for the resulting lava plains has been identified in eastern Elysium, but seas and lakes from these fissures and previous water flooding events were presumed to have evaporated and sublimed away. Here we present High Resolution Stereo Camera images from the European Space Agency Mars Express spacecraft that indicate that such lakes may still exist. We infer that the evidence is consistent with a frozen body of water, with surface pack-ice, around 5° north latitude and 150° east longitude in southern Elysium. The frozen lake measures about 800 x 900 km in lateral extent and may be up to 45 metres deep - similar in size and depth to the North Sea. From crater counts, we determined its age to be 5 ± 2 million years old. If our interpretation is confirmed, this is a place that might preserve evidence of primitive life, if it has ever developed on Mars.

Discoveries made with the High Resolution Stereo Camera on the Mars Express orbiter show that, as recently as a few million years ago, the surface of Mars was being shaped by flowing water, lava and ice.

The large-area coverage at a resolution of 10-20 metres per pixel in colour and three dimensions with the High Resolution Stereo Camera Experiment on the European Space Agency Mars Express Mission has made it possible to study the time-stratigraphic relationships of volcanic and glacial structures in unprecedented detail and give insight into the geological evolution of Mars. Here we show that calderas on five major volcanoes on Mars have undergone repeated activation and resurfacing during the last 20 per cent of martian history, with phases of activity as young as two million years, suggesting that the volcanoes are potentially still active today. Glacial deposits at the base of the Olympus Mons escarpment show evidence for repeated phases of activity asrecently as about four million years ago.Morphological evidence is found that snowand ice deposition on the Olympus construct at elevations of more than 7,000 metres led to episodes of glacial activity at this height. Even now, water ice protected by an insulating layer of dust may be present at high altitudes on Olympus Mons.

Data from the Observatoire pour la Mineralogie, l'Eau, les Glaces, et l'Activite (OMEGA) on the Mars Express spacecraft identify the distinct mafic, rock-forming minerals olivine, low-calcium pyroxene (LCP), and high-calcium pyroxene (HCP) on the surface of Mars. Olivine- and HCP-rich regions are found in deposits that span the age range of geologic units. However, LCP-rich regions are found only in the ancient Noachian-aged units, which suggests that melts for these deposits were derived from a mantle depleted in aluminum and calcium. Extended dark regions in the northern plains exhibit no evidence of strong mafic absorptions or absorptions due to hydrated materials.

The OMEGA/Mars Express hyperspectral imager identified hydrated sulfates on light-toned layered terrains on Mars. Outcrops in Valles Marineris, Margaritifer Sinus, and Terra Meridiani show evidence for kieserite, gypsum, and polyhydrated sulfates. This identification has its basis in vibrational absorptions between 1.3 and 2.5 micrometers. These minerals constitute direct records of the past aqueous activity on Mars.

The Observatoire pour la Mineralogie, l'Eau, les Glaces, et l'Activite (OMEGA) imaging spectrometer observed the northern circumpolar regions of Mars at a resolution of a few kilometers. An extended region at 240°E, 85°N, with an area of 60 kilometers by 200 kilometers, exhibits absorptions at wavelengths of 1.45, 1.75, 1.94, 2.22, 2.26 and 2.48 micrometers. These signatures can be unambiguously attributed to calcium-rich sulfates, most likely gypsum. This region corresponds to the dark longitudinal dunes of Olympia Planitia. These observations reveal that water alteration played a major role in the formation of the constituting minerals of northern circumpolar terrains.

The Observatoire pour la Mineralogie, l'Eau, les Glaces, et l'Activite (OMEGA) visible-infrared imaging spectrometer extensively observed regions of Mars with latitudes above 70° N in late 2004 (heliocentric longitude from L_s 93° to L_s 127°). The extent of water ice at the surface and the size of ice grains were monitored as a function of time. Bright, small-grained frost, which initially covered a large fraction of the polar cap, waned in favor of large-grained ice. In outlying regions, dominated by largegrained ice, the albedo increased over the period. Evaluating the dust content was model dependent. However, contamination of ice by dust was low.

The Observatoire pour la Mineralogie, l'Eau, les Glaces, et l'Activite (OMEGA) investigation, on board the European Space Agency Mars Express mission, is mapping the surface composition of Mars at a 0.3- to 5-kilometer resolution by means of visible/near-infrared hyperspectral reflectance imagery. The data acquired during the first 9 months of the mission already reveal a diverse and complex surface mineralogy, offering key insights into the evolution of Mars. OMEGA has identified and mapped mafic iron-bearing silicates of both the northern and southern crust, localized concentrations of hydrated phyllosilicates and sulfates but no carbonates, and ices and frosts with a water-ice composition of the north polar perennial cap, as for the south cap, covered by a thin carbon dioxide-ice veneer.

The Mars Express Observatoire pour la Mineralogie, l'Eau, les Glaces, et l'Activite (OMEGA) hyperspectral image data covering eastern Terra Meridiani indicate the ubiquitous presence of molecular water in etched terrain materials that disconformably overlie heavily cratered terrains and underlie the hematite-bearing plains explored by the Opportunity rover. Identification of crystalline water in kieserite (MgSO₄IH₂O) is linked to materials exposed in a valley and plateau to the north of hematite-bearing plains. The mineralogical similarities between the etched terrain deposits examined with OMEGA data and the layered rocks examined by Opportunity imply that the ancient aqueous environments inferred from analyses of the rover data extend over regional scales.

We report a detection of methane in the martian atmosphere by the Planetary Fourier Spectrometer onboard the Mars Express spacecraft. The global average methane mixing ratio is found to be 10 +/- 5 parts per billion by volume (ppbv). However, the mixing ratio varies between 0 and 30 ppbv over the planet. The source of methane could be either biogenic or nonbiogenic, including past or present subsurface microorganisms, hydrothermal activity, or cometary impacts.

The Analyzer of Space Plasma and Energetic Atoms (ASPERA) on board the Mars Express spacecraft found that solar wind plasma and accelerated ionospheric ions may be observed all the way down to the Mars Express pericenter of 270 kilometers above the dayside planetary surface. This is very deep in the ionosphere, implying direct exposure of the martian topside atmosphere to solar wind plasma forcing. The low-altitude penetration of solar wind plasma and the energization of ionospheric plasma may be due to solar wind irregularities or perturbations, to magnetic anomalies at Mars, or both.

Using the Fourier Transform Spectrometer at the Canada France Hawaii Telescope, we observed a spectrum of Mars at the P-branch of the strongest CH₄ band at 3.3 Œm with resolving power of 180,000 for the apodized spectrum. Summing up the spectral intervals at the expected positions of the 15 strongest Doppler-shifted martian lines, we detected the absorption by martian methane at a 3.7 sigma level which is exactly centered in the summed spectrum. The observed CH₄ mixing ratio is 10+/-3 ppb. Total photochemical loss of CH₄ in the martian atmosphere is equal to 2.2x10 cms, the CH₄ lifetime is 340 years and methane should be uniformly mixed in the atmosphere. Heterogeneous loss of atmospheric methane is probably negligible, while the sink of CH₄ during its diffusion through the regolith may be significant. There are no processes of CH₄ formation in the atmosphere, so the photochemical loss must therefore be balanced by abiogenic and biogenic sources. - Remainder of abstract truncated -

Editor: Andrew Wilson
Scientific Coordination: Agustin Chicarro

This ESA Special Publication focuses on the Mars Express scientific instrumentation and its state about a year after launch in order to include some initial scientific discoveries. In spite of the Beagle 2 failure, the lander's payload is also thoroughly described here because it is of the highest scientific value. Furthermore, the orbiter instruments are looking specifically for possible evidence of past or present life. No other mission to Mars since NASA's Viking missions in the 1970s has made exobiology so central to its scientific goals.

In this paper we define and describe morphological features that have colloquially been termed "spiders" and map their distribution in the south polar region of Mars. We show that these features go through a distinct seasonal evolution, exhibiting dark plumes and associated fan-shaped deposits during the local defrosting of the seasonal cap. We have documented the seasonal evolution of the cryptic region and have found that spiders only occur within this terrain. These observations are consistent with a geyser-like model for spider formation. Association with the transparent (cryptic) portion of the seasonal cap is consistent with basal sublimation and the resulting venting of CO₂ gas. Also consistent with such venting is the observation of dark fan-shaped deposits apparently emanating from spider centers. Spiders are additionally confined to the polar layered deposits presumably due to the poorly consolidated and easily eroded nature of their upper surface.

Mars Express, planned to be the first 'flexible mission' in the revised ESA Long-Term Scientific Programme, is based on a fast implementation scenario and will be launched towards Mars in June 2003 by a Soyuz/Fregat launcher. The mission is cost-capped at 150 MECU and will be submitted for approval by ESA's Science Programme Committee in November 1998. Its payload has already been selected and European industry will submit bids for design and development phases (Phases-B/C and D) at the beginning of September 1998.

The studies that were carried out in 2001 around the possibility of re-using the Mars Express platform have proven the effectiveness of a fast approach between ESA and the scientific community. It was only in March 2001 that ESA issued a 'Call for Ideas' to react to the identified possibility of a low-cost mission based on the re-use of the platform developed for Mars Express, which will be launched in 2003. A strict schedule was imposed, in order to benefit from these special circumstances, aiming at a launch date for the new mission in 2005, which is the next 'window' for missions to Mars. In response to the Call for Ideas, the scientific community presented a wealth of interesting and challenging proposals.The studies that were carried out in 2001 around the possibility of re-using the Mars Express platform have proven the effectiveness of a fast approach between ESA and the scientific community. It was only in March 2001 that ESA issued a 'Call for Ideas' to react to the identified possibility of a low-cost mission based on the re-use of the platform developed for Mars Express, which will be launched in 2003. A strict schedule was imposed, in order to benefit from these special circumstances, aiming at a launch date for the new mission in 2005, which is the next 'window' for missions to Mars. In response to the Call for Ideas, the scientific community presented a wealth of interesting and challenging proposals.