A Cheap Date

The MoonRise mission concept, in its most recent iteration, in cooperation with the Canadian Space Agency. The mission should fulfill a need to obtain a baseline sample of the 4 billion year-old South Pole-Aitken basin, only a small part of which spills over onto the Moon's nearside in line of sight with flight directors on Earth [NASA/NLSI].

Paul D. Spudis
The Once and Future Moon
Smithsonian Air & Space

  

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.

Originally published at his Smithsonian Air & Space blog The Once and Future Moon, Dr. Spudis is a senior staff scientist at the Lunar and Planetary Institute. The opinions expressed are those of the author and are better informed than average.

Related Posts:
How the Mars community shot itself in the foot (March 10, 2012)
LROC: The Soviet lunar sampling missions (March 16, 2012)
The Last Sampler: Failure, then Success (March 17, 2012)
LROC: Retracing the steps of Apollo 15 / Constellation ROI (April 17, 2010)
The Rosetta Stone of the Solar System (November 19, 2009)
The Keepers of the Moon (July 8, 2008)

How the Mars community shot itself in the foot

Mars Sample Return (MSR) as envisioned in 2006 [NASA].

Paul D. Spudis
The Once and Future Moon
Smithsonian Air & Space
 

The recent release of the administration’s FY 2013 budget gave some scientists a bit of a shock.  Planetary science (considered a “jewel in the crown” of the space agency) has been identified for cutting, over 20% during the next five years.  A particularly painful cut comes to the agency’s robotic Mars exploration program.  Planned missions in cooperation with the Europeans and future missions designed to lead up to the return of a surface sample from Mars were eliminated from the budget.  In effect, the successful program of Mars missions created after the embarrassing failure of the Mars Polar Lander over a decade ago is being scrapped.

The administration digested the National Research Council (NRC) Decadal Survey in planetary science (released last spring) before writing their new budget.  The study process for this report involves getting the relevant scientific communities to determine and lay out their priorities.  The assumption is that the scientific community can best determine the most relevant goals and questions in planetary science and therefore design mission concepts to address them.  Through a variety of working groups and forums, the desires of the community are made known and a report is written around them.  Typically, planetary scientists organize their working groups around objects of study, such as the inner (rocky) planets, small bodies (asteroids and comets), and giant planets.  For the latest Decadal Survey, the Mars community had its own separate group. Mars is, of course, a rocky, inner planet, and for decades has held sway in the planning process, both for robotic and human missions.

NASA’s highest scientific priority for Mars exploration is to determine if it has now, or has ever had life.  The chosen mission concept to address this question is to return samples of the surface of Mars to the Earth.  This is a very difficult task.  Mars is a big planet with a deep gravity well.  At its closest, it is several tens of millions of miles from the Earth, leaving robotic machines controlled from the Earth with long time delays (up to tens of minutes).  Safely landing on Mars is hard enough – taking off again and navigating back to Earth with samples safely in hand, is at least an order of magnitude more difficult.

Yet the new Decadal Survey made Mars sample return its only priority in the area of Mars science – the report offered no alternative missions for consideration.  Moreover, the sample return mission concept presented by the Decadal Survey required not one, but three separate “Flagship” missions (i.e., those having total costs exceeding $1 billion).  In a complex scenario, the mission concept called for a Mars lander to deliver a rover, explore and collect samples and then store them on the surface.  A second mission years later would rendezvous with the stored samples on the surface of Mars, transfer them to an ascent vehicle, and place the samples in orbit around the red planet.  The third and final mission would rendezvous with this orbital vehicle, dock with it and return the samples to the Earth.  From initial landing to sample return would take over a decade and cost many billions of dollars.  Moreover, in this series of three sequential and very complex missions, one single-point failure could spell the end of the entire effort.

When the Office of Management and Budget (OMB) saw this plan and its price tag, they thought it was too much money for too complicated a mission.  Unfortunately, the Mars subgroup left no “back-up” options in the Decadal Survey – it was do the sample return trio or do nothing.  Hence, the new budget proposes nothing.  Of course, a big part of the reason that this mission trio was a non-starter was to preserve funding for the James Webb Space Telescope (JWST), which at its current estimated $8 billion cost (and counting), effectively makes most other space science endeavors non-starters.

Cry “Havoc!” and let slip the dogs of war!  The planetary science community was stunned.  The Planetary Society organized a letter writing campaign, demanding that Congress intervene and save the “Mars program.”  Scientists complained that their highest priority as expressed in the Decadal Survey had been discarded without any real thought and debate (much as the Vision for Space Exploration had been thrown away two years ago).  In partial response, the agency is setting up an ad hoc group to study some less expensive, interim Mars missions (something that the Decadal Survey should have done).  Presently, all of planetary science is in danger of severe cutbacks.  And the final bill for JWST has yet to be delivered.

The MoonRise mission concept, in its most recent iteration planned in cooperation with the Canadian Space Agency, fulfills the need to obtain a baseline sample of the 4 billion year-old South Pole-Aitken basin, only a small part of which spills over onto the Moon's nearside, in line of sight with flight directors on Earth [NASA/NLSI].

What can be learned from this these events and applied to the exploration of the Moon?  Like the Mars community, the lunar science community has made sample return the centerpiece of their mission wish list.  A South Pole-Aitken (SPA) basin sample return has been proposed as a New Frontiers mission and studied in detail twice over the last nine years – and passed over for selection twice.  Yet the new Decadal Survey once again makes this mission its top priority in lunar science.  Moreover, for this mission to be scientifically successful in its goal of dating the impact that created the SPA basin (the biggest and oldest impact crater on the Moon) it must not only complete the sample return, it must collect samples whose context can be reconstructed and fully understood.  As discussed here previously, given the difficulty of such reconstruction for the Apollo samples (which were carefully documented and collected by trained field observers), an unambiguous outcome for this robotic mission is exceedingly unlikely.

Certainly, returning a sample from the Moon is less difficult than doing it from Mars, so the two tasks are not directly comparable.  Yet, there are a number of missions to both the Moon and Mars that could be done for less money and would significantly advance our understanding of their histories and processes.  For example, an entirely new field of scientific study is the generation, movement and fate of water on the Moon, a problem rich in both scientific and exploration potential.  This new field could be investigated profitably by a series of properly instrumented, small robotic missions.

These issues and questions were known at the time that the Decadal Survey was conducted, so there is little excuse for ignoring them, except for the community’s fixation on sample return missions.  In part, this obsession exists because it provides a large part of the research community with something to do.  NASA money has built many expensive laboratories to analyze extraterrestrial materials and new lunar and planetary samples are needed to keep them operating.  But the full potential of remote, in situ analysis – coupled with careful and clever geological planning – has not been given enough thought by the scientific community.

Will the lunar science community also shoot itself in the foot?  If so, it will simply be finishing a job started by this administration two years ago with the cancellation of the VSE.  Fans of human spaceflight please take note:  the process of undertaking these “Decadal Surveys” has been widely praised and advocated as a model for determining the goals and objectives of the human space program.  Considering the consequences of this latest effort in planetary science, one might want to re-think that scenario.

Originally published March 8, 2012 at his Smithsonian Air & Space blogThe Once and Future Moon, Dr. Spudis is a senior staff scientist at the Lunar and Planetary Institute. The opinions expressed are those of the author and are better informed than average.