The Mars rover Curiosity has found something — something noteworthy, in a pinch of Martian sand. But what is it?
NASA/JPL-Caltech, via Malin Space Science Systems, via EPA
The Curiosity rover in a self-portrait stitched together from 55 images taken by a camera at the end of one of its arms. Data from the rover’s first soil sample is now being analyzed by scientists.
The scientists working on the mission who know are not saying. Outside of that team, lots of people are guessing.
The intrigue started last week when John P. Grotzinger, the Mars mission’s project scientist, told National Public Radio: “This data is going to be one for the history books. It’s looking really good.”
And then he declined to say anything more.
Fossils? Living microbial Martians? Maybe the carbon-based molecules known as organics, which are the building blocks of life? That so much excitement could be set off by a passing hint reflects the enduring fascination of both scientists and nonscientists with Mars.
“It could be all kinds of things,” said Peter H. Smith, a planetary scientist at the University of Arizona who was the principal investigator for NASA’s earlier Phoenix Mars mission but is not involved with Curiosity. “If it’s historic, I think it’s organics. That would be historic in my book.”
Dr. Grotzinger and other Curiosity scientists will announce their latest findings on Monday in San Francisco at a meeting of the American Geophysical Union.
Do not expect pictures of Martians, though.
Guy Webster, a spokesman for NASA’s Jet Propulsion Laboratory in Pasadena, Calif., which operates Curiosity, said the findings would be “interesting” rather than “earthshaking.”
Mr. Webster noted that “a really big announcement,” if one should occur, would most likely be made at NASA headquarters in Washington and not at an academic conference.
Whatever is revealed will be linked to the work of Curiosity’s sophisticated chemistry laboratory instrument, Sample Analysis at Mars — SAM, for short. The rover’s robotic arm dropped the first bit of sand and dust into the instrument on Nov. 9, and the scientists have been analyzing and contemplating ever since.
One of the main goals of SAM is to identify organic molecules, but it would be a big surprise for organics to show up in a first look at a sand sample selected more as a test exercise than with the expectation of a breakthrough discovery.
Curiosity will be headed toward layers of clays, which could be rich in organics and are believed to have formed during a warm and wet era early in the planet’s history. But Curiosity has months to drive before arriving at those locations.
And the Curiosity scientists have learned through experience that it pays to double-check their results before trumpeting them. An initial test of the Martian atmosphere by the same instrument showed the presence of methane, which would have been a major discovery, possibly indicating the presence of methane-generating microbes living on Mars today. But when the scientists ran the experiment again, the signs of methane disappeared, leading them to conclude that the methane found in the first test had come from air that the spacecraft had carried to Mars from its launching spot in Florida.
Mr. Webster, who was present during the interview with NPR, said Dr. Grotzinger had been talking more generally about the quality of data coming back from Curiosity and was not suggesting that the data contained a breakthrough surprise. “I don’t think he had in mind, ‘Here’s some particular chemical that’s been found,’ ” Mr. Webster said. “That’s not my impression of the conversation.”
On Twitter, Curiosity chimed in: “What did I discover on Mars? That rumors spread fast online. My team considers this whole mission ‘one for the history books.’ ” (The public information staff at the Jet Propulsion Laboratory writes the posts for the rover.)
Now, three different lines of evidence back the water-ice interpretation. Infrared laser pulses fired at the planet by MESSENGER's Mercury Laser Altimeter have revealed bright regions inside nine darkened craters near the planet's north pole2. These bright regions, thought to be water ice, line up perfectly with ultra-cold spots that, according to a thermal model of the planet that takes into account Mercury's topography, should never be warmer than –170 °C3.But “radar does not uniquely identify water ice,” says David Lawrence, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. Sulphur, for example, could have produced a similar radar signature.
Stores of ice confirmed on Sun-scorched Mercury
MESSENGER finds evidence of pure water ice near planet's north pole.
Water ice is abundant in Mercury's dark polar craters.
NASA/JOHNS HOPKINS UNI APPLIED PHYS LAB/CARNEGIE INST OF WASHINGTON
Talk about a land of fire and ice. The surface of Mercury is hot enough in some places to melt lead, but it is a winter wonderland at its poles — with perhaps a trillion tonnes of water ice trapped inside craters — enough to fill 20 billion Olympic skating rinks.
The ice — whose long-suspected presence1 has now been confirmed by NASA's orbiting MESSENGER probe — seems to be much purer than ice inside similar craters on Earth's Moon, suggesting that the closest planet to the Sun could be a better trap for icy materials delivered by comets and asteroids. Three papers detailing the findings are published today in Science2, 3, 4.
Despite Mercury’s blistering 400 °C temperatures, the floors of many of its polar craters are in permanent shadow, because the planet's rotational axis is perpendicular to its orbital plane, so its poles never tip towards the star. Indeed, radar pinged to the planet from Earth in the past 20 years has revealed bright regions1 near the poles consistent with metres-thick slabs of pure water ice.
Now, three different lines of evidence back the water-ice interpretation. Infrared laser pulses fired at the planet by MESSENGER's Mercury Laser Altimeter have revealed bright regions inside nine darkened craters near the planet's north pole2. These bright regions, thought to be water ice, line up perfectly with ultra-cold spots that, according to a thermal model of the planet that takes into account Mercury's topography, should never be warmer than –170 °C3.But “radar does not uniquely identify water ice,” says David Lawrence, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. Sulphur, for example, could have produced a similar radar signature.
A third team, using MESSENGER's Neutron Spectrometer, has spotted the telltale signature of hydrogen — which they think is locked up in water ice — in those same regions4. "Not only is water the best explanation, we do not see any other explanation that can tie all the data together," says Lawrence, lead author of the spectrometer study.
So where did the water come from? The bright icy spots identified by MESSENGER's laser are surrounded by darker terrain which receives a bit more sunlight and heat. The neutron measurements suggest that this darker area is a layer of material about 10 centimetres thick that lies on top of more ice, insulating it.
Dark materials
This darker material around the bright spots may be made up of complex hydrocarbons expelled from comet or asteroid impacts, says David Paige, a planetary scientist at the University of California, Los Angeles, and first author of the thermal-model paper3.
Paige and his colleagues suggest that when these icy bodies slam into Mercury, their components migrate over time — by repeatedly vaporizing and precipitating — to the cooler poles, where they get stuck in the frigid polar craters.
But even there, sunlight will sometimes hit parts of the craters' interiors, vaporizing the water ice and leaving behind ‘lag deposits’ of hydrocarbons that gradually become thicker and darker as they are chemically altered by sunlight.
Small impacts should have buried the surface if the ice were a billion years old, and the MESSENGER researchers believe it might be much younger than that, perhaps 50 million years old.
"The ice deposits we are looking at are not ancient," says Paige.
No comments:
Post a Comment