100,000 zaps and a Sievert

CuriosityNASA’s Mars Science Laboratory (MSL) rover Curiosity chalked-up another milestone on October 30th, 2013, when the laser which comprises part of the Chemistry and Camera instrument (ChemCam)  system mounted at the top of the rover’s mast, was fired for the 100,000 time.

The shot was one of a series of 300 fired at a total of 10 locations on a rock called “Ithaca”, and was taken at a range of 4.04 metres (just over 13 feet) from  the target. The laser is used to vaporise tiny amounts of an object (the target area being around the size of a pin head), producing a spark of plasma (ionised gas). This spark is observed via a telescope which also forms a part of the ChemCam system, and the spectrum of light from the spark is analysed to identify chemicaal and mineral elements within it.

The rock “Ithaca” shown here, with a rougher lower texture and smoother texture on top, appears to be a piece of the local sedimentary bedrock protruding from the surrounding soil in Gale Crater. This image, taken by Curiosity’s Mast Camera (Mastcam), was captured on Sol 439 (October 30th, 2013). The rectangle indicates the area where ChemCam used its laser and remote micro-imager to inspect “Ithaca”. That inspection included the 100,000th laser shot fired by ChemCam on Mars

Each pulse from the laser delivers more than a million watts of power for about five one-billionths of a second. The technique used by ChemCam, called laser-induced breakdown spectroscopy, has been used to assess composition of targets in other extreme environments, such as inside nuclear reactors and on the sea floor. Experimental applications have included also environmental monitoring and cancer detection. MSL is the first mission to use the technique on another planet.

A graph showing a spectrum recorded by ChemCam which averages data from multiple laser firings at the same point on the rock “Ithaca” on Sol 439, and which included to 100,000th laser shot. The spectrum is typical of Martian volcanic (basalt) material. Although Ithaca is a sedimentary rock, the particles in the sediments that became the rock originated in igneous source rocks. The elements identified from the spectrum include a standard major-element suite of silicon, magnesium, aluminium, calcium, sodium, potassium, oxygen and titanium. Chromium and manganese, though not labelled, were also present (click to enlarge)

Virtually every shot taken by the laser yields a spectrum of data which is returned to Earth. Most targets get zapped at several points with 30 laser pulses at each point. An international team of scientists and students is mining information from ChemCam to document the diversity of materials on the surface of Gale Crater and the geological processes that formed them.  The range of materials recorded so far includes dust, wind-blown soil, water-lain sediments derived from the crater rim, veins of sulphates and igneous rocks that may be ejecta from other parts of Mars.

Since reaching the 100,000 total in late October, the laser has been fired a further 2,000 times. ChemCam also includes a micro-imager camera, which has taken over 1600 images during the time the rover has been operating on Mars.

The ChemCam mast element on Curiosity
The ChemCam mast element on Curiosity


Four Billion Years

In a little more than a year on the Red Planet, Curiosity has determined the age of a Martian rock, found evidence the planet could have sustained microbial life, taken the first readings of radiation on the surface, and shown how natural erosion could reveal the building blocks of life. MSL team members presented these results and more from Curiosity in six papers published on December 9th, 2013 by Science Express and presented them in press briefings and talks at the Fall Meeting of the American Geophysical Union in San Francisco.

As a  part of its operations, Curiosity has carried out a number of sample drillings into rocks on Mars. The second rock, dubbed “Cumberland” from which the rover obtained cuttings for analysis is now the first rock ever to be dated while sitting on another planet. Analysis of the mineral ingredients in the cuttings obtained from “Cumberland” estimates the age of the rock to be about 3.86 to 4.56 billion years; this matches estimates as to the overall age of Gale Crater itself, obtained through other means, which suggests that the techniques being used to analyse the samples gathered by the rover are reliable.

“The age is not surprising, but what is surprising is that this method worked using measurements performed on Mars,” Kenneth Farley, from the California Institute of Technology and a co-author of one of the new papers, said. “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.”

Before they could measure rocks directly on Mars, scientists estimated their ages by counting and comparing the numbers of impact craters on various areas of the planet. The crater densities are correlated with ages based on comparisons with crater densities on the moon, which were tied to absolute dates after the Apollo lunar missions returned rocks to Earth.

The Cumberland sample analysis was a fundamental and unprecedented measurement which had been considered unlikely even as recently as the rover’s arrival on Mars in August 2012. To obtain it, Farley and his colleagues adapted a 60-year-old radiometric method for dating Earth rocks that measures the decay of an isotope of potassium as it slowly changes into argon, an inert gas. Argon escapes when a rock is melted. This dating method measures the amount of argon that accumulates when the rock hardens again.

The researchers also assessed how long Cumberland has been within about an arm’s reach of the Martian surface, where cosmic rays striking the atoms of the rock produce build-ups of gasses Curiosity can measure. The analysis of different gases present in the rock yielded exposure ages in the range of 60 million to 100 million years. This suggests shielding layers above the rock were stripped away relatively recently. Combined with clues of wind erosion Curiosity has observed, the exposure-age discovery points to a pattern of windblown sand eroding relatively thick layers of rock, which form a retreating vertical face, or scarp.

“The exposure rate is surprisingly fast,” Farley said. “The place where you’ll find the rocks with the youngest exposure age will be right next to the downwind scarps.”

Curiosity’s primary mission is to find evidence for an ancient environment that could have supported life, a goal it achieved some 10 months ago after its first sample drilling operation into a rock dubbed “John Klein” in the “Yellowknife Bay” area of Mars. Analysis of the samples gathered there, coupled with other measurements obtained within “Yellowknife Bay” revealed the region would once have been an environment favourable for microbial life. The clay-rich lakebed region offers the key chemical elements for life, plus water not too acidic or salty, and an energy source. The energy source is a type used by many rock-eating microbes on Earth: a mix of sulphur- and iron-containing minerals that are ready acceptors of electrons, and others that are ready electron donors, like the two poles of a battery.

Analysis of the composition and layering of rocks in the “Yellowknife Bay” area have led to new estimates of when habitable conditions existed in the area and for how long they may have persisted. It is thought that Mars had enough fresh water to generate clay minerals and to perhaps support microbial life more than four billion years ago. However, the planet underwent drying that left any remaining liquid water acidic and briny, and somewhat less habitable for microbes.

How the lake within Gale Crater may once have appeared, billions of years ago. Water flowed down from the crater rim (arrows) and pooled in the linear depression between the crater rim and the flank of “Mount Sharp”. This view combines image data from the Context Camera on NASA’s Mars Reconnaissance Orbiter and colour information from Viking Orbiter imagery captured in the 1970s. The scale bar in the lower right corner shows a scale of 25 kilometres (15 miles). North is towards the top of the image

Overall, study of the chemistry and mineralogy of the “Yellowknife Bay” samples suggest the habitable environment there existed during a part of Martian history called the Hesperian Era, when parts of the planet were already becoming drier and more acidic, and roughly equates to the time of the oldest evidence for life on Earth. Analysis suggests that the conditions persisted for millions to tens of millions of years. During that time rivers and lakes probably appeared and disappeared, and that even when  the surface was dry, the subsurface was probably wet, as evidenced by mineral veins deposited in rock fractures by the passage of underground water.

The mission’s Principal Scientist, John Grotzinger, said of the findings, “This habitable environment existed later than many people thought there would be one. This has global implications. It’s from a time when there were deltas, alluvial fans and other signs of surface water at many places on Mars, but those were considered too young, or too short-lived, to have formed clay minerals. The thinking was, if they had clay minerals, those must have washed in from older deposits. Now, we know the clay minerals could be produced later, and that gives us many locations that may have had habitable environments, too.”

Humans on Mars

Another aspect of the Mars Science Laboratory mission is investigate the radiation environments of interstellar space and on the surface of Mars in order to gain a greater understanding of the risks human explorers will face when both en route to / from Mars and when operating on the planet’s surface.

Analysis of data gathered during the interplanetary cruise phase of the mission was released in June 2013, revealing that Curiosity, cocooned inside its aeroshell received a daily dosage of around 1.8 milliSieverts a day.

The papers published on December 9th, 2013, included data on the radiation environment the rover encountered during its first ten months on Mars, both from galactic cosmic rays (GCRs) which originate outside of our solar system, and from solar energetic particles (SEPs) associated with solar flares and coronal mass ejections from the sun. This data shows that the rover was exposed to a daily average of some 0.67 milliSieverts. This means that the likely round-trip exposure for a human embarking on a mission to Mars will be around 1 Sievert (Sv).

A 1 Sv exposure accumulated over time is associated with a five percent increase in risk for developing fatal cancer. NASA has established a three percent increased risk of fatal cancer as an acceptable career limit for its astronauts currently operating in low-Earth orbit, within the planet’s protective magnetosphere.

Further to this, Curiosity’s mission is currently taking place when the solar cycle is at a low ebb. There have been no significant solar storms affecting Mars, and more than 95% of the total measured by the rover is from cosmic rays. Therefore, when the solar cycle is taken into account, the cumulative radiation exposure an astronaut may encounter during a mission to Mars could be somewhat higher than the 1 SV indicated by the analysis of Curiosity’s exposure.

In gathering this kind of data, Curiosity is providing engineers with vital information which will help them build better models to anticipate the radiation environment human explorers will face, which in turn should help in developing new technologies to help shield astronauts in deep space, particularly from the ravages of solar radiation.

Measurements with the MSL Radiation Assessment Detector (RAD) on Curiosity during the flight to Mars and now on the surface of Mars enable an estimate of the likely radiation exposure humans would face in a Mars mission which would require 180 days to reach and return from Mars, with a 500-day stay in between. The total exposure would be around 1 SV, with roughly equal contributions from the three stages of the expedition. Average doses for other activities / events is shown for comparison

The data also helps in understanding the likely habitability of the Martian surface, and what it may have meant – or may mean – for Martian microbes. If any organic chemicals that are potential signs of life did exist within rocks at about 5 centimetres (2 inches), below the planet’s surface, they would be depleted up to 1,000-fold in about 650 million years by radiation at the exposure rate measured in Curiosity’s first 10 months. However, “Cumberland” has been exposed to cosmic rays’ effects for only about 60-100 million years. With such a young exposure age, enough organic material could still be present in “Cumberland” to be detectable. Even if Mars has never supported life, the planet receives organic molecules delivered by meteorites, which should leave a detectable trace.

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All images and video courtesy of NASA / JPL

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