Volume: 04, Issue: 07 11/30/2005 
An infrared OMEGA image of Candor Chasma, showing bright and brown deposits (red markers) that are rich in the mineral kieserite, a hydrated magnesium sulphate. Image courtesy ESA/OMEGA/HRSC.
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MARSIS 'radargram' images showing echoes obtained from an approximately 250 km diameter circular structure in the subsurface of Mars, interpreted to be a buried impact basin. Image courtesy ASI/NASA/ESA/Univ. of Rome/JPL.
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This topographic map, based on Mars Orbiter Laser Altimeter data, shows the MARS Express groundtracks and the arc structures detected by MARSIS that are interpreted to be part of a buried impact basin about 250 km in diameter. Image courtesy ASI/NASA/ESA/
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This MARSIS radargram image (top) shows data from the subsurface of Mars in the layered deposits that surround the north pole. Image courtesy ASI/NASA/ESA/Univ. of Rome/JPL/MOLA Science Team.
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Other Articles in This Issue:
Mars Rovers Celebrate Anniversary, Explore Possibility of Life
Venus Express Observes Earth, Moon
Mars Reconnaissance Orbiter Passes Halfway Mark
Chat about Spaceflight with a NASA Astronaut

ESA Spacecraft Makes Intriguing New Discoveries

The European Space Agency’s (ESA) Mars Express spacecraft has made a trio of new discoveries: buried craters, underground ice, and evidence of large aquifers.

For the first time in the history of planetary exploration, the MARSIS - Mars Advanced Radar for Subsurface and Ionospheric Sounding – instrument aboard ESA's Mars Express has provided direct information about the deep subsurface of Mars.

First, MARSIS found an almost circular structure, about 250 km in diameter, shallowly buried under the surface of the northern lowlands of the Chryse Planitia region in the mid-latitudes on Mars. Scientists have interpreted it as a buried basin of impact origin, possibly containing a thick layer of water-ice-rich material.

"The detection of a large buried impact basin suggests that MARSIS data can be used to unveil a population of hidden impact craters in the northern lowlands and elsewhere on the planet," said Jeffrey Plaut, MARSIS Co-Principal Investigator. "This may force us to reconsider our chronology of the formation and evolution of the surface."

To draw this picture of the subsurface, the MARSIS team studied the echoes of the radio waves emitted by the radar, which passed through the surface and then bounced back in the distinctive way that told the 'story' about the layers penetrated. These echo structures form a distinctive collection that include parabolic arcs and an additional planar reflecting feature parallel to the ground, 160 km long. The parabolic arcs correspond to ring structures that could be interpreted as the rims of one or more buried impact basins. Other echoes show what may be rim-wall 'slump blocks' or 'peak-ring' features.

The planar reflection is consistent with a flat interface that separates the floor of the basin, situated at a depth of about 1.5 to 2.5 km, from a layer of overlying different material. In their analysis of this reflection, scientists do not exclude the intriguing possibility of a low-density, water-ice-rich material at least partially filling the basin.

MARSIS also probed the layered deposits that surround the north pole of Mars in an area between 10º and 40º East longitude. The interior layers and the base of these deposits are poorly exposed. Two strong and distinct echoes coming from the area correspond to a surface reflection and subsurface interface between two different materials. By analysis of the two echoes, the scientists were able to draw the likely scenario of a nearly pure, cold water-ice layer thicker than 1 km, overlying a deeper layer of basaltic regolith, apparently ruling out the hypothesis of a melt zone at the base of the northern layered deposits.

In addition to the MARSIS discoveries, the OMEGA instrument onboard Mars Express has found implications of substantial quantities of liquid water in the early history of Mars. OMEGA (Observatoire pour la Mineralogy, l'Eau, les Glaces et l'Activité) is the craft’s visible and infrared mapping spectrometer.

The data collected by OMEGA reveal the presence of specific surface minerals that imply the long-term presence of large amounts of liquid water on the planet. These hydrated minerals provide a clear mineralogical record of water-related processes on Mars.

OMEGA detected the presence of two different classes of hydrated minerals - phyllosilicates and hydrated sulphates - over isolated but large areas on the Martian surface. Both minerals are the result of a chemical alteration of rocks; however, their formation processes are very different and point to periods of different environmental conditions in the history of the planet.

Phyllosilicates have a thinly layered structure and are products of igneous minerals sustaining a long-term contact with water. (An example is clay.) Phyllosilicates were detected by OMEGA mainly in the Arabia Terra, Terra Meridiani, Syrtis Major, Nili Fossae and Mawrth Vallis regions, in the form of dark deposits or eroded outcrops.

Hydrated sulphates, on the other hand, are formed as deposits from salted water, and most sulphates need an acid water environment to form. They were spotted in layered deposits in Valles Marineris, extended exposed deposits in Terra Meridiani, and within dark dunes in the northern polar cap.

The detection and mapping of these two different kinds of hydrated minerals point to two major climatic episodes in the history of Mars: an early, moist environment in which phyllosilicates formed, followed by a more acid environment in which the sulphates formed. These two episodes were separated by a Mars global climatic change.

For images and more information, visit the ESA website at http://www.esa.int.

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