Le robot américain Curiosity, sur Mars depuis sept semaines, a découvert des graviers et cailloux provenant du lit d'un ancien ruisseau, confortant les hypothèses d'un passé humide de la planète rouge, ont annoncé jeudi des scientifiques de la mission.

D'autres indices de la présence passée d'eau sur Mars avaient déjà été observés mais jamais de tels sédiments laissés par l'écoulement d'eau, ont-ils précisé.

Des orbiteurs ont depuis longtemps pris des images de canaux à la surface de Mars dont on supposait qu'ils ont été creusés par l'écoulement d'eau dans le passé.

Mais "c'est la première fois que nous voyons des graviers transportés par de l'eau sur Mars", a dit William Dietrich de l'Université de Californie, un des scientifiques de la mission.

"Ceci est une transition entre des hypothèses quant à la taille des matériaux transportés par un écoulement d'eau et une observation directe de ces derniers", a-t-il ajouté.

Les images transmises par Curiosity montrent des graviers, des cailloux et du sable cimentés dans une couche de roches conglomérées de 10 à 15 cm d'épaisseur datant probablement de "plusieurs milliards d'années", a par ailleurs précisé, lors d'une conférence de presse, William Dietrich.

Les ruisseaux pourraient avoir existé pendant "des milliers voire des millions d'années", a-t-il ajouté.

La taille de ces cailloux --qui varie de celle d'un grain de sable à celle d'une balle de golfe-- et leur forme donnent une idée de la vitesse et de la distance de l'écoulement de ce ruisseau.

"A partir de la taille de ces cailloux (dont certains se sont détachés de la roche, ndlr), on peut en déduire que l'eau s'écoulait à environ 0,91 mètre par seconde" et avec une profondeur d'un mètre environ, soit la distance de "la cheville à la hanche", a précisé William Dietrich.

"La forme de ces graviers révèle qu'ils ont été transportés et leur taille confirme qu'ils n'ont pas été transportés par le vent mais par le flot de l'eau", a souligné Rebecca Williams du Planetary Science Institute à Tucson (Arizona), membre de l'équipe scientifique de Curiosity.

La forme arrondie de certains de ces cailloux indique qu'ils ont été transportés sur de longues distances depuis le haut du bassin où un "canal" appelé "Peace Vallis" rejoint l'écoulement alluvial.

L'abondance de canaux dans ce bassin laisse penser que ces écoulements d'eau étaient continus ou répétés au cours d'une longue période, et non pas occasionnels ou même durant seulement quelques années, selon ces scientifiques.

Ces derniers pensent que Curiosity se trouve au milieu d'un réseau d'anciens ruisseaux et rivières.

Ces chercheurs pourraient utiliser les instruments de Curiosity pour déterminer la composition chimique de cette couche de roche conglomérée qui pourrait révéler davantage de caractéristiques de l'environnement humide dans lequel ces sédiments se sont formés.

Cette découverte a été faite sur un site situé au nord du cratère Gale sur l'équateur martien et au pied du mont Sharp, une montagne de 5.000 mètres d'altitude se trouvant à l'intérieur, destination finale de Curiosity.

Curiosity avait étudié son premier morceau de roche au début de la semaine, surtout pour tester son bras robotique, et a repris sa route pour se rendre à Glenelg, zone géologiquement intéressante toute proche située à l'intersection de trois types de terrains.

C'est là que la Nasa espère trouver des roches intéressantes à analyser et effectuer les premiers forages dans le sol martien.

La destination finale de Curiosity est le mont Sharp distant de huit kilomètres, un trajet qui prendra au moins trois mois à raison de cent mètres par jour, selon la Nasa.

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Des photos envoyées par Curiosity, compilées par nos confrères du HuffPost américain
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  • 'Still Life with Rover'

    This full-resolution self-portrait shows the deck of NASA's Curiosity rover from the rover's Navigation camera. The back of the rover can be seen at the top left of the image, and two of the rover's right side wheels can be seen on the left. The undulating rim of Gale Crater forms the lighter color strip in the background. Bits of gravel, about 0.4 inches (1 centimeter) in size, are visible on the deck of the rover. This mosaic is made of 20 images, each of 1,024 by 1,024 pixels, taken late at night on Aug. 7 PDT (early morning Aug. 8 EDT). It uses an average of the Navcam positions to synthesize the point of view of a single camera, with a field of view of 120 degrees. Seams between the images have been minimized as much as possible. The wide field of view introduces some distortion at the edges of the mosaic. (NASA)

  • This image shows the landing site of NASA's Curiosity rover and destinations scientists want to investigate. Curiosity landed inside Gale Crater on Mars on Aug. 5 PDT (Aug. 6 EDT) at the green dot, within the Yellowknife quadrangle. The team has chosen for it to move toward the region marked by a blue dot that is nicknamed Glenelg. That area marks the intersection of three kinds of terrain. The science team thought the name Glenelg was appropriate because, if Curiosity traveled there, it would visit it twice -- both coming and going -- and the word Glenelg is a palindrome. Then, the rover will aim to drive to the blue spot marked "Base of Mt. Sharp", which is a natural break in the dunes that will allow Curiosity to begin scaling the lower reaches of Mount Sharp. At the base of Mt. Sharp are layered buttes and mesas that scientists hope will reveal the area's geological history. These annotations have been made on top of an image acquired by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter.

  • his image shows a closer view of the landing site of NASA's Curiosity rover and a destination nearby known as Glenelg. Curiosity landed inside Gale Crater on Mars on Aug. 5 PDT (Aug. 6 EDT) at the blue dot. It is planning on driving to an area marked with a red dot that is nicknamed Glenelg. That area marks the intersection of three kinds of terrain. Starting clockwise from the top of this image, scientists are interested in this brighter terrain because it may represent a kind of bedrock suitable for eventual drilling by Curiosity. The next terrain shows the marks of many small craters and intrigues scientists because it might represent an older or harder surface. The third, which is the kind of terrain Curiosity landed in, is interesting because scientists can try to determine if the same kind of rock texture at Goulburn, an area where blasts from the descent stage rocket engines scoured away some of the surface, also occurs at Glenelg. The science team thought the name Glenelg was appropriate because, if Curiosity traveled there, it would visit the area twice -- both coming and going -- and the word Glenelg is a palindrome. After Glenelg, the rover will aim to drive to the base of Mount Sharp. These annotations have been made on top of an image acquired by the High Resolution Imaging Science Experiment on NASA's Mars Reconnaissance Orbiter. (NASA)

  • Scientists have now named the four marks near NASA's Curiosity rover where blasts from the descent stage rocket engines blew away some of the Martian surface material. Scientists have named the scour marks, clockwise from the most north: Burnside, Goulburn, Hepburn and Sleepy Dragon. These names were chosen by the science team from a list of rock formations in northern Canada because they all have something to do with heat, for example "burn" or "dragon." This cropped image is part of a larger panorama from Curiosity's Mast Camera (see pia16051).

  • Goulburn Scour Mark

    This cropped image from NASA's Curiosity rover shows one set of marks on the surface of Mars where blasts from the descent-stage rocket engines blew away some of the surface material. This particular scour mark is near the rear left wheel of the rover and is the left-most scour mark on the left side of this larger panorama from Curiosity's Mast Camera (see PIA16051). This scour mark is named Goulburn after a 2-billion year-old sequence of rocks in northern Canada.

  • This image (cut out from a mosaic) shows the view from the landing site of NASA's Curiosity rover toward the lower reaches of Mount Sharp, where Curiosity is likely to begin its ascent through hundreds of feet (meters) of layered deposits. The lower several hundred feet (meters) show evidence of bearing hydrated minerals, based on orbiter observations. The terrain Curiosity will explore is marked by hills, buttes, mesas and canyons on the scale of one-to-three story buildings, very much like the Four Corners region of the western United States. A scale bar indicates a distance of 1.2 miles (2 kilometers). Curiosity's 34-millimeter Mast Camera acquired this high-resolution image on Aug. 8, 2012 PDT (Aug. 9 EDT). This image shows the colors modified as if the scene were transported to Earth and illuminated by terrestrial sunlight. This processing, called "white balancing," is useful to scientists for recognizing and distinguishing rocks by color in more familiar lighting. (NASA)

  • Readying ChemCam

    This image shows the calibration target for the Chemistry and Camera (ChemCam) instrument on NASA's Curiosity rover. The calibration target is one square and a group of nine circles that look dark in the black-and-white image. The calibration target set can be seen in the middle left in this image, to the right of the rover's power source. The materials used in these circles are the types of materials scientists anticipated they might encounter on Mars. The square is a titanium alloy with a painted edge. The ChemCam instrument will be firing a series of powerful, but invisible, laser pulses at a target rock or soil. It is located on the rover's mast, near the Navigation camera that took this image. A telescopic camera known as the remote micro-imager will show the context of the spots hit with the laser. This image was taken by the right-side Navigation camera on Aug. 16, 2012. (NASA)

  • Curiosity's First Rock Star

    This mosaic image shows the first target NASA's Curiosity rover aims to zap with its Chemistry and Camera (ChemCam) instrument. ChemCam will be firing a laser at this rock, provisionally named N165, and analyzing the glowing, ionized gas, called plasma, that the laser excites. The instrument will analyze that spark with a telescope and identify the chemical elements in the target. The rock is just off to the right of the rover. This image is part of a set of images obtained by Curiosity's Mast Camera on Aug. 8 PDT (Aug. 9 EDT). See PIA16051 for the larger mosaic. (NASA)

  • Head of Mast on Mars Rover Curiosity

    This view of the head of the remote sensing mast on the Mars Science Laboratory mission's rover, Curiosity, shows seven of the 17 cameras on the rover. Two pairs of Navigation cameras (Navcams), among the rover's 12 engineering cameras, are the small circular apertures on either side of the head. On the top are the optics of the Chemistry and Camera (ChemCam) investigation, which includes a laser and a telescopic camera. The Mast Camera (MastCam) instrument includes a 100-millimeter-focal-length camera called MastCam-100 or M-100, and a 34-millimeter-focal-length camera called the MastCam-34 or M-34. The two cameras of the MastCam are both scientific and natural color imaging systems. The M-100 looks through a 1.2-inch (3-centimeter) baffle aperture, and the M-34 looks through a 2.1-inch (5.3-centimete) baffle aperture. (NASA)

  • Curiosity's First Rock Star, Up-Close

    This close-up image shows the first target NASA's Curiosity rover aims to zap with its Chemistry and Camera (ChemCam) instrument. ChemCam will be firing a laser at this rock, provisionally named N165, and analyzing the glowing, ionized gas, called plasma, that the laser excites. The instrument will analyze that spark with a telescope and identify the chemical elements in the target. The rock is just off to the right of the rover. This image is part of a set of images obtained by Curiosity's Mast Camera on Aug. 8 PDT (Aug. 9 EDT). See pia16051 for the larger mosaic.


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  • La descente de Curiosity

  • Le parachute de Curiosity photographié par l'orbiteur MRO

    futura-sciences.com <a href="http://www.futura-sciences.com/fr/news/t/astronautique/d/en-image-le-parachute-de-curiosity-photographiac-par-lorbiteur-mro_40525/" target="_hplink">raconte la séance photo</a>...

  • Le Mont Sharp

  • Le Mont Sharp

  • Paysage