Curiosity rover delivers key pieces of Martian puzzle with recent findings
A team of scientists working on the Mars Science Laboratory (MSL) rover Curiosity mission, have announced their latest findings of their studies of Yellowknife Bay, the area that the Curiosity rover is currently investigating on Mars. These include the first-ever age measurements of rock formations on the surface of another world. The new findings were presented during the first day of the 2013 Fall meeting of the American Geophysical Union or ‘AGU’, on December 9 in San Francisco, California.
Even before the rover had arrived on Mars on August 5, 2012, the planetary science community knew that following a succesful landing, the mission would provide crucial data concerning the habitability of the Red Planet. The robotic explorer didn’t fail to deliver. After Entry, Descent and Landing (EDL), Curiosity set out to begin its 2-year primary mission of characterising possible past habitable environments around its landing site, inside Gale crater. During its first months on Mars, Curiosity had already achieved this goal, consistently revolutionizing our view of the Red Planet along the way. The rover kept delivering data by discovering evidence of past flowing streams of water, making the first-ever radiation measurements on the Martian surface, drilling on rocks and zapping them with the laser on its Chemistry & Camera (or ‘ChemCam’) instrument, to measure their chemical composition. Now, Curiosity completed another historic first, by making the first-ever age measurement of a rock, in-situ on the surface of another planet.
Age Determination process
The age measurement results come from a study led by Kenneth Farley, a participating scientist on the MSL mission. The measurements were actually two-fold. The first, concerned the formation age of minerals inside a rock called “Cumberland” and the second concerned the amount of time the rock outcrops on Yellowknife Bay have been exposed to the surface. Cumberland was drilled by Curiosity in during May 2013. The rover then took samples of the resulting powdered rock and inserted them into its Sample Analysis at Mars, or SAM instrument, to investigate the rock’s chemical and isotopic composition. The results showed that the samples were somewhat rich in potassium-40, an isotope of potassium that is known to decay into argon-40, with a half-life of 1.2 billion years. Since argon is a noble gas, it doesn’t chemically interact with other elements. So, any argon present in the samples, could only be the result of potassium decay. Knowing the rate of potassium to argon decay and the amount of argon in the samples, Falray’s team used potassium-argon radiometric dating, a method that is commonly applied on Earth, but had not been used in space before. Radiometric dating came up with an age of approximately 4.2 million years, give or take 400 million.
“The age is not surprising, but what is surprising is that this method worked using measurements performed on Mars,” said Farley. “When you’re confirming a new methodology, you don’t want the first result to be something unexpected. Our understanding of the antiquity of the Martian surface seems to be right.” Scientists were pleased to find that the estimate to come out of the radiometric dating method, was also in complete agreement with the estimate given by the crater-counting method previously used to calculate the overall age of the rocks. “This is important, because first of all, it validates the crater-counting models that were used to estimate these ages. This is an absolutely critical part of establishing Mars’ history – to have an age determination of crater-counting, and the radiometric method actually confirms for the first time, that these models are valid,” he added during his presentation at the recent AGU meeting.
The other important finding unveiled at the meeting, was the surface exposure age of the wider Yellowknife Bay area, where both of Curiosity’s drilling targets, the ‘Cumberland’ and ‘John Klein’ rocks lay. By carefully studying the geomorphology of Yellowknife Bay, scientists came to the conclusion that the landscape is constantly reshaped by wind erosion. In essence, Mars’ surface is sand-blasted by wind, constantly exposing previously underground layers of rock to the surface. Mars also doesn’t have a global magnetic field like Earth, so it is constantly bathed in lethal galactic cosmic rays, and ultraviolet radiation coming from the Sun. All this radiation penetrates to a depth of 3 meters below the surface, producing isotopes of various noble gases after colliding with the molecules on the ground and rocks. By measuring the amount of these isotopes inside Cumberland, and by knowing the half-life of their radioactive decay, scientists can estimate how long the rocks at Yellowknife Bay have been at, or within 3 meters from the surface. The results was somewhat surprising to scientists.
“The age that we obtained with all three of the different isotopes that we’ve used, is about 80 million years,” noted Farley. Considering the age of 4.2 billion years of the Cumberland rock itself, it turns out that it was only exposed at the surface in very recent geologic times.
Importance of surface exposure age for habitability
These age determination measurements are very important to the search for organic molecules on Mars as well.
While Curiosity examined the powdered rock samples from Cumberland with its SAM instrument, it detected an abundance of chlorine oxides, like hydrogen chloride (HCl) and perchlorate (ClO4− ). The presence of such oxi-chlorinated compounds on the Cumberland samples was to be expected, since the soil on the surface of Mars had been found to be rich in perchlorate, by NASA’s Phoenix mission in 2008. But the scientists detected strong signs of carbon and nitrogen as well, in the form of chemical compounds like carbon dioxide (CO2) and nitric oxide (NO) respectively.
Scientists at first thought that the presence of carbon and nitrogen, was a result of its delivery from Earth inside the SAM instrument. Yet, these two compounds were detected in far larger quantities than those believed to have been carried from Earth, making scientists confident that most of the carbon and nitrogen that was detected, was of Martian origin. That doesn’t necessarily mean that something was alive inside the Cumberland samples, but it shows that the rock outcrops of Yellowknife Bay could easily preserve organic compounds.
This conclusion was reinforced by independent studies, made by David Vaniman of the Planetary Science Institute in Tucson, Ariz. And Scott McLennan of Stony Brook University in Stony Brook, N.Y. by examining the clay minerals found inside the Yellowknife Bay rocks. The breakthrough discovery of clay minerals had come in 2012, from Curiosity’s first rock drill on Mars, on a neighboring rock called ‘John Klein’. Clay minerals are known to be formed in the presence of water, and their abundance at Yellowknife Bay hinted at a past wet and habitable environment, conducive to life. The important discovery from the new studies, is that those minerals were formed in-situ inside the rocks on Yellowknife Bay, after the latter had been deposited there from the rim of Gale Crater, carried by flowing water. This in-situ formation means that habitable environments were present in broader areas on Mars and persisting for much more time than previously thought, maybe more than millions to tens of millions of years.
Even if life has never flourished on Mars, clay minerals are very good agents for the preservation of organic compounds inside rocks, deposited either by meteorites, or by other geological processes. But the surface exposure of rocks, would be a fatal threat for any organics, which would be destroyed by galactic cosmic rays and solar radiation, over a period of hundreds of millions of years. Yet, the discovery that the Cumberland rock has been exposed to the surface for approximately only 80 million years, makes scientists feel confident that enough organics could still be present, for Curiosity to detect. This surface age exposure measurement, gives a baseline to scientists for determining the best places to look for organic compounds, from calculating the surface exposure of rocks.
Radiation exposure measurements and importance on future human missions
One of the instruments onboard Curiosity, is RAD, short for Radiation Assessment Detector. The instrument was turned on even before Curiosity’s arrival on Mars, measuring the levels of cosmic and solar radiation in interplanetary space, and after Curiosity’s landing on the surface of Mars.
One of the findings that the scientists unveiled at the AGU meeting, were the radiation measurements on the surface of Mars made by RAD, during the first 300 Sols after Curiosity’s landing. What the results showed, was that the Martian surface was dominated by the presence of galactic radiation only, with just one solar particle event being recorded. In addition, the recorded levels of galactic radiation on the surface varied significantly, showcasing daily and seasonal variations. Even though the Sun is currently at its Solar maximum, this maximum is amongst the weakest of the past 60 years, thus having a minimal effect on the radiation environment on Mars that RAD has recorded.
More important to the prospect of future human exploration of Mars, Curiosity’s results helped scientists to calculate the total radiation exposure an astronaut would get from a turnaround trip there. It turns out that the total radiation exposure an astronaut would get during a 6-month transit to Mars, would be in the range of 650-700 mSv. This happens to be the same radiation exposure from a 500-day stay on the surface of Mars. That’s not surprising, considering that the Martian surface is better shielded from radiation than interplanetary space. These levels are still a bit higher than NASA’s currently accepted limits. It is calculated that 1000 mSv of radiation would increase a person’s risk of getting cancer, by 5 percent. The space agency’s current limit for astronauts on the International Space Station, is 3 percent. For comparison, the average radiation dose for a 6-month stay on the ISS, is 75-90 mSv.
“Our measurements provide crucial information for human missions to Mars,” says Don Hassler of Southwest Research Institute in Boulder, Colo. and principal investigator for Curiosity’s RAD instrument. “We’re continuing to monitor the radiation environment and seeing the effects of major solar storms on the surface at different times in the solar cycle, will give additional important data. Our measurements also tie into Curiosity‘s investigations about habitability. The radiation sources that are concerns for human health also affect microbial survival as well as preservation of organic chemicals.”
The fascinating discoveries that were presented in the recent AGU meeting, not only helped to expand our knowledge and understanding of the Martian environment, but proved to be a turning point in Curiosity’s mission as well.
As John Grotzinger, project scientist for the Curiosity rover, noted:
“Really what we’re doing in the mission, is turning the corner from a mission which is dedicated to the search for habitable environments, to a mission that is now dedicated to a search for that subset of habitable environments which also preserves organic carbon. And that’s the step we need to take, as we explore for evidence of life on Mars.”
Yet, all the results which have come from Curiosity’s first 16 months on Mars, are only a prelude to its main science mission: to reach the base of the 5.5-km high Mount Sharp, which is located within Gale Crater. Once there, Curiosity will start investigating the mountain’s different geologic layers, something that would give scientists a significant insight to the Red Planet’s different geologic epochs, and how Mars’ changing climate affected its habitability and the possible presence of life.
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