August 24, 2012
Returning samples to Earth for analysis is one of planetary sciences’ holiest of grails. Although many different, complex measurements on returned samples are possible, one of the most important ones – from the standpoint of geologic study – is to determine the age of a rock. Ages are determined by obtaining precision measurements of the amounts of different isotopes of certain elements (some of which are radioactive and decay at known rates). By comparing the ratio of these radioactive elements to their daughter products, the amount of time that has elapsed since the rock formed can be calculated and thus, the age of that rock can be inferred. If we know from which regional unit the rock comes, we can infer the ages of major events in planetary history. This is one of the principal reasons why planetary scientists crave samples from other worlds.
We’ve determined the ages of most of the more than 380 kg of rock and soil samples returned from the Moon during Apollo. Using that information and the geological mapping of the Moon from photographs, we were able to deduce the time sequence of major lunar and Earth-Moon system events. Broad-scale, regional relations determined by remote mapping allowed us to identify the relative timing and significance of major units, while the returned rock samples allowed us to assign absolute ages to those same units. The method proved so effective in reconstructing lunar history, that sample return became an idée fixe of the planetary science community, who strongly desired applying this approach to another planet. Because questions surrounding its potential as a reservoir of life and because its nature permits the landing, retrieval and return (barely) of samples, Mars, with its complex, well mapped surface geology, was the object of most immediate interest.
When the planetary community wrote its recent “decadal survey” (a report outlining the highest priority robotic missions to undertake in the coming ten years), sample return came in as the highest priority for Mars (so high, that in effect, the decadal study told NASA to do a Mars sample return or do nothing). Once Mars sample return was studied in detail, cost became an issue. NASA robotic missions are classified according to the cost category they fall under. The most expensive missions are “Flagship” missions, whose costs exceed $2 billion (the current MSL “Curiosity” rover Flagship mission cost about $2.6 billion). A Mars sample return would require not one but three separate Flagship-class missions: one to rove and collect the samples, another to launch the samples into orbit around Mars, and finally a mission to collect those samples from Mars orbit and return them to the Earth. Using a variety of scenarios, the effort would cost over $10 billion, with a possible price tag exceeding $20 billion. This staggering cost quickly shelved Mars sample return while planetary scientists scrambled for something to fill in a possible multi-decadal gap with no mission.
The question became, “Can a different and cheaper approach begin to address some of the key issues for which sample return is thought to be essential?” Although many kinds of measurements can be done on returned samples, radiometric dating is one of the most critical and one thought to be possible only in laboratories on the Earth. By using the absolute age of a single unit to bracket the timing of a host of different units mapped from remote sensing data, a single rock from a surface outcrop of a clearly defined unit of regional significance might enable us to calibrate the geologic time scale of Mars. So the question before us is, “Is it possible to measure the absolute age of a rock remotely?”
Several groups around the country have been investigating the possibility of creating a small, portable laboratory for radiometric dating. These instruments could be miniaturized and flown aboard a future robotic rover. Rocks could be selected for analysis as the vehicle roams across the planet. If such a rover were sent to areas of known geological context (e.g., a large, regional lava flow), rocks dated by the rover would define an absolute age for the flow. A large lava flow would have numerous impact craters on it (the more densely a surface is cratered, the older it is). For Mars, we now have to estimate (i.e., guess) how old its units are by comparing crater densities with those for lava flows on the Moon (from which the Apollo astronauts returned samples). Although this approach is better than nothing, Mars has had its own cratering history and direct comparison to lunar history may not be valid. A few solid absolute ages for lavas of widely varying age on Mars could “tie down” the cratering curve, such that we would not only date the flows we visit, but we could with precision, confidently estimate the ages of many other geological units not visited.
Indicative of a healthy and engaged science community, not all are convinced that ages obtained from an automated lab would be as useful as the high precision results that would be obtained from state-of-the-art terrestrial laboratories. But a collection of imprecise ages from a variety of different units on Mars is better than no dates from any unit at all. Given the astronomical costs and high technical risk of robotic sample return from Mars, the idea that we might be able to measure ages remotely looks increasingly attractive and practical. This technique could also be applied to other planetary objects. A properly equipped rover could make numerous measurements of the ages of craters and lava flows over a wide area on the Moon, where such information could be tied into the existing high-quality (but incomplete) lunar time scale. Remote age dating would also be useful on planets from which launch of a sample return vehicle is nearly impossible, such as Venus (with a dense atmosphere and a very high surface gravity).
As sample return missions escalate in cost and difficulty, we should investigate how much can be learned about a planet’s history short of sample return. A properly equipped robotic rover could blaze a new “Lewis and Clark Trail,” traversing large distances and making precision measurements along the way – returning information of inestimable value for a relatively low price.
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