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Water on Mars has a long history

The European Space Agency's Mars Express spacecraft found this patch of water ice in an unnamed crater on Mars in 2005. Photograph: ESA/DLR/FU Berlin (G. Neukum)

Curiosity's discovery of a dry river bed adds to a 40-year-old body of evidence for once-flowing water on Mars

It was November 1971. The scene at Nasa was a tense one, similar to that of Curiosity's landing back in August. Ground controllers held their collective breath as Mariner 9 approached Mars. If everything went to plan, the spacecraft would make history by being the first manmade object to go into orbit around Earth's neighbouring planet.

Mariner 9 was an octagonal spacecraft just less than a metre and a half across, with four cross-like solar panels sticking out another two metres from its body. Its mission was to map Mars. At the appointed time, the retrorockets fired and Mars' gravity took hold of the spacecraft.

Soon afterwards, data began to flow back to Earth from the cameras. Images built up on the monitor screens line by line but the scientists stared in mounting disappointment. Nothing was visible. The whole planet was engulfed in a dust storm. All the exquisite geology that the spacecraft was meant to see lay buried under a blanket of suspended particles.

All the personnel could do was wait for the dust to settle, and pray that the spacecraft would wait too. Even today spacecraft can be fickle things. Back then, they were positively cranky. They could fail at any time, for any of a thousand different unanticipated reasons. Mariner 8, for example, had been destroyed during its launch.

Time was of the essence as the dust storm raged. Days turned to weeks until the Martian skies finally began to clear in January. As it did, one titanic mountainous peak after another broke through the murk. These were the extinct volcanoes of the Tharsis region.

As the dust dropped out of the atmosphere, so the geology of Mars revealed itself: canyons and valleys, craters and plateaux. TheMariner rift valley was named after this spacecraft. And then there were the meandering channels. These were the real prize. Unmistakable to geologists, they were dried-up rivers. Water had once flowed on Mars.

It is now 40 years since that first observational proof was beamed back to Earth. Every Mars mission since has corroborated the finding and yet new evidence for rivers or lakes still excites us more than anything else. The only news that could trump it would be the discovery of life on Mars.

Friday's announcement that Nasa's Curiosity rover was driving across adry river bed allows us to imagine a time in the past when there was another blue planet in the solar system.

The finding itself is not a surprise. From orbit, the landing region displayed an 'alluvial fan', which is similar to a river delta, where river-borne sediments are deposited. Instead, the excitement is because Curiosity can deliver the ground truth. According to Nasa, the sizes and shapes of stones offer clues to the speed and distance of a long-ago stream's flow. This is information that is impossible to estimate with any accuracy from orbital pictures.

So Curiosity continues the grand tradition begun by Mariner 9 of detailing the evidence for Mars' past water. As its mission continues it may also see something else that Mariner 9 did: a dust storm.

Martian dust storms are most likely to take place when the planet approaches the Sun. This will happen next in January 2013. At that time, the planet will receive almost 50% more energy from the Sun than when it is at its furthest point. Even now, the warmth and light are building.

This extra heating melts the carbon dioxide ice from the polar caps to create a thicker, higher pressure atmosphere. As the gas tears around the planet, it can lift dust into the atmosphere and blanket the globe.

Early results from Curiosity's 'weather station' instruments suggest that the pressure in the Martian atmosphere could already be rising faster than expected. The trouble is that no one understands the Martian atmosphere well enough to know whether this makes a storm more or less likely.

If one does blow up, Curiosity is guaranteed a good view. Unlike Mariner 9, however, Nasa's latest Martian explorer will not be looking down from orbit, it will be right in the thick of it.

In 1972, Mariner 9 returned the first images of dry rivers on Mars. This is Nirgal Vallis in the Red Planet's southern hemisphere. Photograph: NASA/JPL-Caltech.



Did a Massive Extraterrestrial Body Impact Earth 12,000 Years Ago?

New research results are consistent with a controversial theory that an extraterrestrial body – such as a comet – impacted the Earth approximately 12,900 years ago, possibly contributing to the significant climatic and ecological changes that date to that time period. The paper in the Proceedings of the National Academy of Sciences (PNAS) includes significant findings about the nature of so-called "microspherules" that were found at a number of prehistoric sites, based on research done at North Carolina State.

While the findings are interesting in themselves, the paper is only the latest in a heated scholarly debate focused on whether such an "impact event" took place. The debate dates back to a 2007 paper, in which researchers reported finding evidence at multiple sites of a significant impact event. The evidence cited in that paper included a large increase in the abundance of magnetic microspherules at the study sites. The microspherules are metallic spheres in the range of 10 to 50 micrometers. For comparison, human hair is 50 to 100 micrometers in diameter.

Specifically, the 2007 team found hundreds to thousands of these microspherules in each kilogram of dirt they sampled at the Younger Dryas Boundary (YDB) layer from several sites. The YDB marks the period when the Earth's climate reverted to conditions similar to the ice age and populations of prehistoric animals, such as mammoths, appear to have dropped off precipitously. It also marks the period when theClovis culture in North America seems to have experienced a significant population decline or some significant cultural modification.

Samples were also taken from layers above and below the YDB. Microspherules were found in much greater numbers in the dirt samples taken from the YDB, as compared to the samples from the other layers. These microspherules have a variety of natural and artificial sources, including impact events, volcanoes and industrial pollution. Most types of microspherules are easily distinguished from one another.

However, in 2009, another team of researchers published a paper calling the 2007 findings into question. The researchers had examined two of the sites cited in the 2007 paper – the Blackwater Draw site in New Mexico and the Topper site in South Carolina, as well as 5 others – and reported that its researchers were unable to find increased numbers of the relevant microspherules in the YDB at all but one site – and even that site was questionable.

Now the new PNAS paper finds that the 2009 study relied on flawed protocols. Perhaps more importantly, the researchers behind the new study have re-examined the Blackwater Draw and Topper sites – as well as a third site in Maryland common to the 2009 study– and were able to find microspherules in amounts consistent with the 2007 hypothesis at each site.

"Our study replicates only a small subset of the research reported in 2007 and within those narrow limits, our results are consistent with theirs. Much research remains to be done to prove or disprove the hypothesis," says Dr. Malcolm LeCompte of Elizabeth City State University, who is lead author of the new PNAS paper.

LeCompte brought some of these microspherules to the Analytical Instrumentation Facility (AIF) at NC State, which provides both analytical instrumentation and expert staff to help researchers analyze and characterize materials and material structures at the micro and nanoscale.

"They wanted to know what's in these spherules and where they came from," says Charles Mooney, the scanning electron microscope (SEM) lab manager at AIF. "We analyzed the microspherules with an SEM, which allowed us to obtain high-resolution images of the microspherules. We also collected x-rays generated by electron beam-sample interactions to tell us what elements were in each sample," Mooney explains. "This told us that the microspherules were largely made up of iron, aluminum, silicon, and occasionally titanium, with one spherule containing significant amounts of rare earths, such as cerium."

Dr. Dale Batchelor, director of operations at AIF, also sliced open some of the microspherules using an analytical instrument composed of a both a focused ion beam (FIB) and an SEM to examine their interior structure and composition. Interestingly, some of the microspherules were partially hollow, but exhibited internal crystal structures when cross sectioned with the FIB.

"To our knowledge this is the first instance of the FIB technique being used to cross section YBD microspherules – in effect exploratory surgery on the microscale," Batchelor says. "The FIB is the scalpel and the SEM is the eye."

Most of the microspherules were made up of elements in proportions similar to the composition of the Earth's crust and not, as some had proposed, meteorite material. In addition, the surface characteristics of the microspherules indicate that they were heated to a molten temperature and then cooled rapidly. "This is consistent with the theory of an impact event, but falls short of proof positive," says LeCompte.

More information: doi: 10.1073/pnas.1208603109 Journal reference: Proceedings of the National Academy of Sciences.