Mars Express scientists find a different Mars underneath
Wed Dec 13, 2006 at 21:02 UTC
With results that the principal investigator of the Mars Express MARSIS radar, Giovanni Picardi, from the University of Rome 'La Sapienza', describes as unprecedented, Mars is showing scientists that it has an older, craggier face buried beneath its surface.
The results were obtained by MARSIS, the pioneering sounding radar on board ESA's Mars Express orbiter, and provide important new clues about the still mysterious geological history of Mars.
Observations by MARSIS, the first sub-surface sounding radar used to explore a planet, strongly suggest that ancient impact craters lie buried beneath the smooth, low plains of the northern hemisphere of Mars. The technique uses echoes of radio waves that have penetrated below the surface.
MARSIS found evidence that these buried impact craters - from about 130 to 470 kilometres in diameter - are present under much of the northern lowlands. The findings appear in the 14 December issue of the journal Nature.
With MARSIS "it's almost like having X-ray vision," said Thomas R. Watters of the National Air and Space Museum's Center for Earth and Planetary Studies, Washington, and lead author of the results. "Besides finding previously unknown impact basins, we've also confirmed that some subtle, roughly circular, topographic depressions in the lowlands are related to impact features."
Studies of how Mars evolved aid understanding of early Earth. Some signs of the forces at work a few thousand million years ago are harder to detect on Earth because many of them have been obliterated by tectonic activity and erosion.
Image Credit: ESA/ASI/NASA/Univ. of Rome/JPL/Smithsonian
High resolution image (3 MB)
This image shows a ground-range projection of the 'radargram' obtained by the MARSIS sounding radar on board ESA's Mars Express on 6 July 2005 (orbit 1892), when the spacecraft was flying over the Martian lowland plains of Chryse Planitia.
Rim walls and interior ring structures of impact basins produce parabolic-shaded echoes. The inset shows that parabolic-shaped echoes in the radargram project to circular arcs, indicating the presence of a buried impact basin.
Image Credit: ESA/ASI/NASA/Univ. of Rome/JPL/Smithsonian
High resolution image (2.5 MB)
This image shows the locations of the buried basins detected by the MARSIS sounding radar on board ESA's Mars Express in July 2005. Locations and inferred diameters from MARSIS echoes are shown in black on a MOLA colour-coded shaded relief.
The locations and diameters of quasi-circular topographic depressions with diameters larger than 200 kilometres are plotted in white. White polygons show the area covered by MARSIS orbits. Where there is left/right ambiguity in the location of a MARSIS basin that coincides with a quasi-circular topographic depression, the plotted location reflects the minimum offset between the centres of the MARSIS basin and the quasi-circular topographic depressions themselves.
Image Credit: ESA/ASI/NASA/Univ. of Rome/JPL/Smithsonian
High resolution image (1.3 MB)
(Top) This radargram obtained thanks to data form the MARSIS sounding radar on board ESA's Mars Express on 4 July 2005 (orbit 1886), showing parabolic-shaped echoes from the rim walls of a buried impact basin. (Bottom) The parabolic echoes project to circular arcs on the surface and indicate the location of a 210 km-diameter impact basin in Amazonis Planitia buried by young lava flows. The dashed white circles are approximate fits to the arcs (echoes labelled in the radargrams). The echoes are interpreted to be from the top and bottom rim wall of the basin.
RIGHT: (Top) This MARSIS radargram was obtained thanks to data gathered on 7 July 2005 (orbit 1897) and shows a parabolic-shaped echo from the rim wall of a buried impact basin. (Bottom) The surface projection of the parabolic echo indicates a 140 km-diameter basin in Amazonis Planitia buried by young lava flows. The dashed white circles are approximate fits to the arcs (echoes labelled in the radargrams). The echoes are interpreted to be from the far rim wall of the basin.
The best fit to the echo indicates that the orbit track is offset from the basin centre, thus the left/right ambiguity in the data does not allow a unique determination of the basin centre. The two bottom images show the subsurface echoes projected on MOLA colour-coded shaded relief.
Image Credit: ESA/ASI/NASA/Univ. of Rome/JPL/Smithsonian
High resolution image (1.2 MB)
Radargrams and ground-range projections of MARSIS data obtained in July 2005 when Mars Express was flying over the Chryse Planitia region of Mars. The two bottom images show the subsurface echoes projected on MOLA colour-coded shaded relief. Echoes are plotted in time-delay versus position along the orbit track. The peak surface return is corrected to agree with the MOLA topography.
The top left radargram was obtained thanks to MARSIS data on 9 July (orbit 1903), while the top right one was obtained thanks to data gathered on 6 July (orbit 1892). Parabolic echoes that project as arcs on the surface are interpreted to be from the near and far rim walls (relative to the orbit track) of buried impact basins.
The dashed white circles are approximate fits to the arcs (echoes labelled in the radargrams). Echoes are interpreted to be from the far rim wall of an approximately 220-km-diameter basin (corresponding echoes in a. and b. labelled as 'Far wall'), superimposed on a larger, 310-km-diameter basin that may have echoes from the top and bottom of the near rim wall (corresponding echoes in a. and b. labelled as 'Near wall').
The new findings bring planetary scientists closer to understanding one of the most enduring mysteries about the geologic evolution and history of Mars. In contrast to Earth, Mars shows a striking difference between its northern and southern hemispheres. Almost the entire southern hemisphere has rough, heavily cratered highlands, while most of the northern hemisphere is smoother and lower in elevation.
Since the impacts that cause craters can happen anywhere on a planet, the areas with fewer craters are generally interpreted as younger surfaces where geological processes have erased the impact scars. The surface of Mars' northern plains is young and smooth, covered by vast amounts of volcanic lava and sediment. However, the new MARSIS data indicate that the underlying crust is extremely old.
"The number of buried impact craters larger than 200 kilometres in diameter that we have found with MARSIS," said Jeffrey Plaut, MARSIS co-Principal Investigator, from the Jet Propulsion Laboratory, California, "tells us that the underlying crust in the northern lowlands must be very ancient, dating to the Early Noachian epoch (lasting from the planet's birth to about 4 thousand million years ago)." The Early Noachian was an era in which impact cratering was very intense across the Solar System.
The results suggest that the northern lowlands crust is as old as the as the oldest exposed southern highlands, also dated in the Noachian epoch, and that the dichotomy between northern and southern hemispheres probably formed very early in the history of Mars.
"These results are truly interesting and unprecedented," added Giovanni Picardi, MARSIS Principal Investigator, from the University of Rome 'La Sapienza'. "MARSIS can contribute to understand the geology of Mars through the analysis of the surface and sub-surface morphology. In addition, with a detailed analysis of the instrument's data, we can also obtain valuable indications about the composition of the materials."
European Space Agency News Release
