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updated 9/28/2012 4:05:42 PM ET 2012-09-28T20:05:42

A NASA rover's discovery of an ancient streambed on Mars is exciting, but it’s far from the first solid evidence that the Red Planet was once a warmer and wetter place.

On Thursday, scientists announced that the Curiosity rover had found rocky outcrops containing large and rounded stones cemented in a conglomerate matrix. The discovery suggests that water had flowed fast and relatively deep — perhaps hip-deep, in fact — through the area billions of years ago.

"This is the first time we're actually seeing water-transported gravel on Mars," Curiosity co-investigator William Dietrich of the University of California, Berkeley, said in a statement.

But Curiosity 's find didn't exactly surprise mission scientists. They chose to set the $2.5 billion robot down in the Red Planet's huge Gale Crater, after all, because Mars-orbiting spacecraft have spotted signs there of long-ago water activity — from channels and alluvial fans to minerals that form in the presence of liquid water.

And these more recent observations build on evidence for a wet ancient Mars that goes back four decades and has been accumulating ever since. [ The Search for Water on Mars (Photos) ]

Eyes in the sky
Perhaps the first compelling signs that the Red Planet's surface — a frigid and dry place today — once harbored liquid water came from NASA's Mariner 9 spacecraft.

Mariner 9 launched toward Mars in May 1971 and later that year became the first probe ever to orbit another planet. Mariner 9's images showed canyons — including the enormous Valles Marineris, which is named after the spacecraft — and what appeared to be riverbeds.

A succession of other NASA orbiters — from the twin Vikings in the mid-1970s to Mars Odyssey and the Mars Reconnaissance Orbiter (MRO), which remain active today — have spotted many more landforms that speak of long-ago erosion by liquid water.

MRO has observed streaks in the Red Planet's Newton Crater that shift over the course of a few months, suggesting that water might even be flowing seasonally on Mars today.

The evidence is also mineralogic as well as topographic. Instruments aboard MRO and other craft have detected water-associated minerals such as clays and sulfates in numerous locations across the Red Planet.

NASA / JPL-Caltech / Univ. of Arizona
This image combining orbital imagery with 3-D modeling shows flows that appear in spring and summer on a slope inside Mars' Newton crater.

Ground-truthing
Over the past decade, rovers have ground-truthed some of the observations made from orbit, strengthening the case for a wet ancient Mars.

For example, NASA's Opportunity rover found multiple deposits of odd, iron-rich spherules after landing on the Red Planet in January 2004. These so-called "blueberries" are concretions created by the action of mineral-rich water inside rocks, scientists say.

Opportunity's twin, Spirit, discovered strong evidence of an ancient hydrothermal system near its landing site back in 2007. And last December, researchers announced that Opportunity had found a thin vein of gypsum while poking along the rim of Mars' Endeavour Crater.

"There was a fracture in the rock, water flowed through it, gypsum was precipitated from the water. End of story," Steve Squyres of Cornell University, Opportunity's principal investigator, told reporters at the time. "There's no ambiguity about this, and this is what makes it so cool."

Searching for habitable environments
Here on Earth, life thrives pretty much anywhere liquid water is found, which explains the intense interest in searching for signs of the stuff on Mars.

NASA / JPL-Caltech / Cornell / ASU
This color view of a mineral vein called "Homestake" comes from the panoramic camera (Pancam) on NASA's Mars Exploration Rover Opportunity.

Indeed, the past decade or so of NASA's activities at the Red Planet have been geared toward "following the water." Curiosity's mission marks a transition to the next phase in the hunt for past or present Mars life: searching for habitable environments.

Curiosity is about 50 days into a two-year mission to determine if the Gale area can, or ever could, support microbial life. This is a long and involved process that requires more than the confirmation of an ancient streambed, researchers said.

"The question about habitability goes just beyond the simple observation of water on Mars to re-creating the environments in greater detail, with an understanding of the chemistry that was going on at that time, to ask if this is the kind of place that micro-organisms could've lived," Curiosity chief scientist John Grotzinger of Caltech in Pasadena told reporters Thursday.

"That's still to be determined, and that's the research the team is working on," he added.

Curiosity's mission may also shed light on when and why Mars dried out long ago. Scientists plan to drive the 1-ton robot partway up Mount Sharp, which rises 3.4 miles (5.5 kilometers) into the Red Planet sky from Gale's center.

They're keen to explore Mount Sharp's base, which harbors clays and sulfates, orbital observations have shown. About 2,300 feet (700 meters) up, however, these deposits peter out.

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If Curiosity climbs high enough to cross this threshold, it could help scientists piece together a history of wet Mars, dry Mars and the transition between the two, researchers have said.

Follow Space.com senior writer Mike Wall on Twitter @michaeldwallor  Space.com @Spacedotcom. We're also on Facebookand  Google+.

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Photos: Curiosity's space odyssey to Mars

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  1. Caring for Curiosity

    NASA's Curiosity rover is as big as a compact car and weighs a ton ... and it's on Mars. Here's where the journey began. A white-room team works on the six-wheeled spacecraft on Aug. 13, 2011, at the Payload Hazardous Servicing Facility at NASA's Kennedy Space Center in Florida. (NASA via Getty Images) Back to slideshow navigation
  2. Liftoff!

    An Atlas 5 rocket rises from Cape Canaveral Air Force Station in Florida on Nov. 26, 2011, with NASA's Mars Science Laboratory spacecraft sealed inside its payload fairing. That spacecraft, in turn, enclosed and protected the Curiosity rover. (NASA via Getty Images) Back to slideshow navigation
  3. Mohawk Guy at work

    Activity lead Bobak Ferdowsi works inside the Spaceflight Operations Facility for NASA's Curiosity rover at NASA's Jet Propulsion Laboratory in Pasadena, Calif., on Aug. 5, 2012 - the rover's landing day. Ferdowsi, who adopts a fresh hairdo for each space mission, became an Internet sensation thanks to his stars-and-stripes Mohawk and his youthful manner. (Brian van der Brug / Pool via Getty Images) Back to slideshow navigation
  4. Curious about Curiosity

    Jasper Goldberg and Andreas Bastian, both 22, watch live NASA coverage of Curiosity's descent to Mars on the giant video screen in New York's Times Square. (Andrew Burton / Reuters) Back to slideshow navigation
  5. Watching and waiting

    Steve Collins waits for word at the Jet Propulsion Laboratory's mission control room during the "seven minutes of terror" as Curiosity approaches the surface of Mars on Aug. 5. Collins was working at JPL in 1993 when NASA's Mars Observer probe was lost just before its scheduled arrival at the Red Planet. (Fred Prouser / Reuters) Back to slideshow navigation
  6. Touchdown!

    The Mars Science Laboratory team in the Mission Support Area at NASA's Jet Propulsion Laboratory reacts after learning that the Curiosity rover has landed safely on Mars. The happy news came at 10:31 p.m. PT Aug. 5 (1:31 a.m. ET Aug. 6). (Bill Ingalls / NASA via Reuters) Back to slideshow navigation
  7. From Mars to Times Square

    Spectators in New York's Times Square cheer the announcement that NASA's Curiosity rover successfully landed on Mars. (Peter Foley / EPA) Back to slideshow navigation
  8. Great catch!

    As it flew high above, NASA's Mars Reconnaissance Orbiter captured this picture of the Curiosity rover and its parachute descending to the Martian surface on Aug. 5. The inset image has been processed to bring out additional detail in the view of the rover and the chute. (NASA / JPL-Caltech via AP) Back to slideshow navigation
  9. Flying saucer

    A color image shows the Mars Science Laboratory's heat shield, as seen by a camera on the Curiosity rover during the spacecraft's descent on Aug. 5. The picture was obtained by the Mars Descent Imager instrument, also known as MARDI, and shows the 15-foot (4.5-meter) diameter heat shield when it was flying away 50 feet (16 meters) below the spacecraft. This image shows the inside surface of the heat shield, with its protective multilayered insulation. (NASA / JPL-Caltech / MSSS) Back to slideshow navigation
  10. The mountain ahead

    One of the first views from NASA's Curiosity rover, which landed on Mars on Aug. 5, shows the rover's shadow in the foreground and a 3-mile-high mountain known as Aeolis Mons or Mount Sharp in the distance. That mountain is the rover's eventual destination. The picture was taken through a "fisheye" wide-angle lens by one of the rover's hazard avoidance cameras. (NASA / JPL-Caltech via AFP - Getty) Back to slideshow navigation
  11. Miles and miles on Mars

    This image, released on Aug. 9, shows part of the deck of NASA's Curiosity rover as seen by one of the rover's navigation cameras. The rover's pointy low-gain antenna and its paddle-shaped high-gain antenna are among the pieces of hardware visible in the foreground. The rim of Gale Crater can be seen at the horizon. (NASA / JPL-Caltech via AFP - Getty) Back to slideshow navigation
  12. Curiosity's crime scene

    The four main pieces of hardware that arrived on Mars with NASA's Curiosity rover are pinpointed in this image from NASA's Mars Reconnaissance Orbiter, taken 24 hours after landing. The heat shield was the first piece to hit the ground, followed by the back shell attached to the parachute. The rover itself was lowered to the ground on cables by its rocket-powered sky crane. The cables were cut, and then the sky crane flew away to its own crash landing. (NASA / JPL-Caltech via Getty Images) Back to slideshow navigation
  13. What a blast!

    This is a portion of the first 360-degree black-and-white panoramic view acquired by the navigation cameras aboard NASA's Curiosity rover. Two disturbed areas are visible in the foreground, where the rocket thrusters on Curiosity's sky crane blasted away the surface gravel to reveal bedrock below. The high country of Gale Crater's rim can be seen in the distance. (NASA / JPL-Caltech via AFP - Getty) Back to slideshow navigation
  14. First color picture

    An image from the Curiosity rover's Mars Hand Lens Imager, or MAHLI, provides the first color view of the north wall and rim of Gale Crater. The picture was taken by the MAHLI camera at the end of Curiosity's stowed robotic arm. The view appears fuzzy because of the dust that has settled on the camera's removable cover. (NASA / JPL-Caltech / MSSS via AP) Back to slideshow navigation
  15. Mars in living color

    A color image from NASA's Curiosity Rover shows the pebble-covered surface of Mars. This is a portion of the first color 360-degree panorama from NASA's Curiosity rover, made up of thumbnails, which are small copies of higher-resolution images. The mission's destination, a mountain at the center of Gale Crater called Aeolis Mons or Mount Sharp, can be seen in the distance rising up toward the left. Blast marks from the rover's descent stage are in the foreground. (NASA / JPL-Caltech / MSSS / Getty Images) Back to slideshow navigation
  16. Pew-pew

    This composite image, with magnified insets, shows the results of the first laser test by the ChemCam instrument aboard NASA's Curiosity rover on Mars. The composite incorporates a Navcam image taken prior to the test, with insets taken by the camera in ChemCam. The circular insert highlights the rock before the laser test. The square inset is further magnified and processed to show the effect of the laser blasts on Aug. 19. (LANL / MSSS / JPL-Caltech / NASA via Reuters) Back to slideshow navigation
  17. Looking ahead

    The Mars Curiosity rover's robotic arm takes aim at Mount Sharp in a mosaic that combines navigation-camera imagery from Sols 2, 12 and 14 (Aug. 8, 18 and 20). Mount Sharp, also known as Aeolis Mons, is a 3-mile-high mountain within Gale Crater that will be the rover's ultimate destination. (Marco Di Lorenzo / Ken Kremer / JPL-Caltech / NASA) Back to slideshow navigation
  18. Future mapped out

    NASA's Curiosity rover landed inside Gale Crater at the green dot, within the Yellowknife quadrangle, on Aug. 5. The team has decided to send it first to the region marked by a blue dot, that is nicknamed Glenelg. That area marks the intersection of three kinds of terrain. Then the rover will aim for the blue spot marked "Base of Mt. Sharp," where a natural break in Martian sand dunes will provide an opening for Curiosity to begin scaling the lower reaches of Mount Sharp. (Univ. of Ariz. / JPL-Caltech / NASA via Reuters) Back to slideshow navigation
  19. First steps

    Curiosity's navigation camera system looks back at the wheel tracks from the rover's first test drive on Aug. 22. The $2.5 billion rover made its first moves a little more than two weeks after its arrival on Mars. (JPL-Caltech / NASA via AFP - Getty Images) Back to slideshow navigation
  20. Mount Sharp

    Mount Sharp is seen in the distance in an image taken Aug. 23 by the 34-millimeter Mast Camera on Curiosity. The gravelly area around Curiosity's landing site is visible in the foreground. Farther away, about a third of the way up from the bottom of the image, the terrain falls off into a depression (a swale). Beyond the swale, in the middle of the image, is the boulder-strewn, red-brown rim of a moderately-sized impact crater. Father off in the distance, there are dark dunes and then the layered rock at the base of Mount Sharp. Some haze obscures the view, but the top ridge, depicted in this image, is 10 miles (16.2 kilometers) away. Scientists enhanced the color to show the Martian scene under the lighting conditions we have on Earth. (NASA via AFP - Getty Images) Back to slideshow navigation
  21. Hip-hop on Mars

    Will.i.am of the Black Eyed Peas sings at NASA's Jet Propulsion Laboratory on Aug. 28. Will.i.am's "Reach for the Stars" officially became the first song broadcast from Mars, thanks to a signal beamed from Curiosity. (Nick Ut / AP) Back to slideshow navigation
  22. Mmm, Marsberries!

    Small spherical objects fill the field in this Martian mosaic combining four images from the Microscopic Imager on NASA's Opportunity rover. The Sept. 6 view covers an area about 2.4 inches (6 centimeters) across, at an outcrop called Kirkwood in the Cape York segment of the western rim of Mars' Endeavour Crater. Shortly after its landing in 2004, Opportunity spotted similar spherules that were nicknamed "blueberries," but these berries are not as rich in iron, posing a scientific puzzle. (NASA/JPL-Caltech/Cornell Univ./ USGS/Modesto Junior College via EPA) Back to slideshow navigation
  23. 'Do I look fat?' Curiosity checks its belly

    A mosaic of photos taken on Sept. 9 by the Mars Hand Lens Imager on NASA's Curiosity rover shows the underside of the rover and its six wheels, with Martian terrain stretching back to the horizon. The four circular features on the front edge of the rover are the lenses for the left and right sets of Curiosity's hazard avoidance cameras, or Hazcams. Because of the different perspectives used for different images, some of the borders of the photos don't line up precisely. (NASA via AFP - Getty Images) Back to slideshow navigation
  24. A Martian rock called Jake

    NASA's Curiosity rover stopped about 8 feet (2.5 meters) in front of this Red Planet rock on Sept. 19, the mission's 43rd Martian day, or sol. The pyramid-shaped chunk was the first rock that the Curiosity rover touched for science's sake. It was named "Jake Matijevic" in honor of a top engineer who worked on every one of NASA's rover missions — but passed away just days after Curiosity's landing. Jake the rock, which measures about 10 inches (25 centimeters) tall, provided a good reference point for the rover's sophisticated instruments. (NASA via Getty Images) Back to slideshow navigation
  25. Rover's footprint

    NASA's Curiosity rover cut a wheel scuff mark into a wind-formed ripple at the "Rocknest" site on Mars to give researchers a better opportunity to examine the particle-size distribution of the material forming the ripple. The rover's right navigation camera took this image of the scuff mark on the mission's 57th Martian day, or sol (Oct. 3), the same sol that a wheel created the mark. For scale, the width of the wheel track is about 16 inches (40 centimeters). (Handout / Reuters) Back to slideshow navigation
  26. Rover's self-portrait

    The Curiosity rover used the Mars Hand Lens Imager (MAHLI) to capture the set of thumbnail images stitched together to create this full-color self-portrait in this Oct. 31, 2012 image from NASA. (NASA via Reuters) Back to slideshow navigation
  27. Life on Mars?

    The first sample of powdered rock extracted by the drill of Curiosity is seen on Feb. 20, 2013. Powder drilled out of a rock on Mars contains the best evidence yet that the Red Planet could have supported living microbes billions of years ago, the team behind NASA's Curiosity rover said March 12, 2013.

    Curiosity rover sees life-friendly conditions in ancient Mars rock. (NASA/JPL/Caltech/MSSS via EPA) Back to slideshow navigation
  28. The big picture

    This picture isn't from the Curiosity rover - it's a 2003 image from the Hubble Space Telescope, showing the full disk of Mars. The big picture hints at how much we'll be learning about the Red Planet during Curiosity's two-year, $2.5 billion mission. And that's just the beginning: Scientists hope the nuclear-powered rover will last years or even decades longer. (NASA via AP) Back to slideshow navigation
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  1. Image: NASA's Jet Propulsion Lab Holds Viewing Of Mars Curiosity Rover Landing
    NASA via Getty Images
    Above: Slideshow (28) Curiosity's space odyssey to Mars
  2. Image:
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    Slideshow (12) Month in Space: January 2014

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