lundi 27 juin 2016
NASA Rover Findings Point to a More Earth-like Martian Past
NASA - Mars Science Laboratory (MSL) patch.
June 27, 2016
Chemicals found in Martian rocks by NASA's Curiosity Mars rover suggest the Red Planet once had more oxygen in its atmosphere than it does now.
Researchers found high levels of manganese oxides by using a laser-firing instrument on the rover. This hint of more oxygen in Mars' early atmosphere adds to other Curiosity findings -- such as evidence about ancient lakes -- revealing how Earth-like our neighboring planet once was.
This research also adds important context to other clues about atmospheric oxygen in Mars' past. The manganese oxides were found in mineral veins within a geological setting the Curiosity mission has placed in a timeline of ancient environmental conditions. From that context, the higher oxygen level can be linked to a time when groundwater was present in the rover's Gale Crater study area.
Image above: This scene shows NASA's Curiosity Mars rover at a location called "Windjana," where the rover found rocks containing manganese-oxide minerals, which require abundant water and strongly oxidizing conditions to form. Image Credits: NASA/JPL-Caltech/MSSS.
"The only ways on Earth that we know how to make these manganese materials involve atmospheric oxygen or microbes," said Nina Lanza, a planetary scientist at Los Alamos National Laboratory in New Mexico. "Now we're seeing manganese oxides on Mars, and we're wondering how the heck these could have formed?"
Microbes seem far-fetched at this point, but the other alternative -- that the Martian atmosphere contained more oxygen in the past than it does now -- seems possible, Lanza said. "These high manganese materials can't form without lots of liquid water and strongly oxidizing conditions. Here on Earth, we had lots of water but no widespread deposits of manganese oxides until after the oxygen levels in our atmosphere rose."
Lanza is the lead author of a new report about the Martian manganese oxides in the American Geophysical Union's Geophysical Research Letters. She uses Curiosity's Chemistry and Camera (ChemCam) instrument, which fires laser pulses from atop the rover's mast and observes the spectrum of resulting flashes of plasma to assess targets' chemical makeup.
In Earth's geological record, the appearance of high concentrations of manganese oxide minerals is an important marker of a major shift in our atmosphere's composition, from relatively low oxygen abundances to the oxygen-rich atmosphere we see today. The presence of the same types of materials on Mars suggests that oxygen levels rose there, too, before declining to their present values. If that's the case, how was that oxygen-rich environment formed?
"One potential way that oxygen could have gotten into the Martian atmosphere is from the breakdown of water when Mars was losing its magnetic field," said Lanza. "It's thought that at this time in Mars' history, water was much more abundant." Yet without a protective magnetic field to shield the surface, ionizing radiation started splitting water molecules into hydrogen and oxygen. Because of Mars' relatively low gravity, the planet wasn't able to hold onto the very light hydrogen atoms, but the heavier oxygen atoms remained behind. Much of this oxygen went into rocks, leading to the rusty red dust that covers the surface today. While Mars' famous red iron oxides require only a mildly oxidizing environment to form, manganese oxides require a strongly oxidizing environment, more so than previously known for Mars.
Lanza added, "It's hard to confirm whether this scenario for Martian atmospheric oxygen actually occurred. But it's important to note that this idea represents a departure in our understanding for how planetary atmospheres might become oxygenated." Abundant atmospheric oxygen has been treated as a so-called biosignature, or a sign of extant life, but this process does not require life.
Curiosity has been investigating sites in Gale Crater since 2012. The high-manganese materials it found are in mineral-filled cracks in sandstones in the "Kimberley" region of the crater. But that's not the only place on Mars where high manganese abundances have been found. NASA's Opportunity rover, exploring Mars since 2004, also recently discovered high manganese deposits thousands of miles from Curiosity. This supports the idea that the conditions needed to form these materials were present well beyond Gale Crater.
NASA Weighs Use of Rover to Image Potential Mars Water Sites
Ever since it was announced that there may be evidence of liquid water on present-day Mars, NASA scientists have wondered how best to further investigate these long, seasonally changing dark streaks in the hope of finding evidence of life -- past or present -- on the Red Planet.
"It's not as simple as driving a rover to a potential site and taking a scoop of soil," said Jim Green, NASA's director of planetary science. "Not only are these on steep slopes, we need to ensure that planetary protection concerns are met. In other words, how can we search for evidence of life without contaminating the sites with bugs from Earth?"
Pending approval of a mission extension, NASA's Curiosity Mars rover will continue to climb to progressively higher and younger strata on Mount Sharp, investigating how long the ancient, water-rich environments found so far persisted as Mars dried out. Reaching those destinations would bring the rover closer to locations where dark streaks are present on some slopes. On the way, the route would allow the one-ton rover to capture images of the potential water sites from miles away and see if any are the seasonally changing type.
Image above: This May 11, 2016, self-portrait of NASA's Curiosity Mars rover shows the vehicle at the "Okoruso" drilling site on lower Mount Sharp's "Naukluft Plateau." The scene is a mosaic of multiple images taken with the arm-mounted Mars Hands Lens Imager (MAHLI). Image Credits: NASA/JPL-Caltech/MSSS.
The features of interest have been observed by NASA's High-Resolution Imaging Science Experiment (HiRISE) camera on the Mars Reconnaissance Orbiter (MRO). They appear as dark lines that appear to ebb and flow over time. Planetary scientists think these gullies or recurring slope lineae (RSLs) may appear seasonally as a form of briny water at or near the surface of the Red Planet under warmer conditions.
There are two RSL candidates that may be within Curiosity's reach, on the side of the 3.1-mile-high (5-kilometer-high) Mount Sharp. The rover's Remote Micro-Imager (part of ChemCam) would be the main instrument for imaging the possible sites. The goal would be to study the regions over time to see if there are any changes and to rule out other causes for the changes, such as dry avalanches.
How close could the rover safely get to an RSL? "That's exactly the question that needs to be addressed early in the process," said Catharine Conley, NASA's planetary protection officer. "Kilometers away -- it's unlikely that it would be an issue. In terms of coming much closer, we need to understand well in advance the potential for Earth organisms to come off the rover, and that will tell us how far away the rover should stay."
Conley notes that while the Martian environment is considered harsh for many organisms, that's not necessarily the case for all of them -- particularly microbes that might be hiding within the nooks and crannies of a robotic explorer.
The darkish streaks are considered "special regions" on Mars, where extra precautions must be taken to prevent contamination because of the suspected presence of liquid water, considered a prerequisite for life.
The Mars Science Laboratory (MSL) spacecraft launched from Cape Canaveral, Florida, on Nov. 26, 2011, arriving on the Red Planet on Aug. 6. 2012. NASA's most ambitious Mars mission to date, its goal was to study the Martian environment and determine if Mars is, or was, suitable for life. A decision on the rover's potential extended mission is expected in the next several months.
NASA's Jet Propulsion Laboratory, a division of Caltech in Pasadena, California, built the rover and manages the Curiosity mission for NASA's Science Mission Directorate, Washington.
Los Alamos National Laboratory leads the U.S. and French team that jointly developed and operates ChemCam. NASA's Jet Propulsion Laboratory, a division of Caltech in Pasadena, California, built the rover and manages the Curiosity mission for NASA's Science Mission Directorate, Washington.
NASA Confirms Evidence That Liquid Water Flows on Today’s Mars
Mars Science Laboratory (MSL) or "Curiosity" rover: http://mars.nasa.gov/msl/
ChemCam instrument on Curiosity rover: http://www.msl-chemcam.com/
Images (mentioned), Text, Credits: NASA/JPL/Guy Webster/Los Alamos National Laboratory/Laura Mullane.
Publié par Orbiter.ch à 17:57