Dr. Jurgen Rahe
Code SL
NASA Headquarters
Washington, DC 20546

Dear Jurgen:

The Mars Science Working Group met at NASA Ames on March 27th and 28th. I was very pleased that you were able to join us for part of the meeting. I am writing to you to summarize our key findings and recommendations.

The primary purpose of this meeting was to consider the scientific issues associated with a Mars sample return mission. In order to involve a broad segment of the Mars science community in this effort, our meeting was preceded by a two and a half-day workshop, hosted by Geoff Briggs. We greatly appreciate the support that NASA provided for this activity. Geoff will be producing a detailed workshop report the coming weeks. In the paragraphs below, I will summarize what will be the major conclusion of that report.

First, we feel that the long-term focus for a program of sample returns from Mars should be to determine whether life has ever existed on Mars. This includes not just the search for life itself, but also assessment of past climatological and geological conditions on the planet, and whether they were suitable for the development of life. We believe that this focus addresses one of the most profound questions in planetary science. We also note that answering it will require careful investigation of many scientifically important characteristics of Mars.

Mars is a very complex planet, and we cannot hope to do this job with a single sample return mission. A carefully-planned campaign of sample returns will be required. We note that the resources of the Mars Surveyor program are modest, and that the first sample return mission will probably have to be very simple --- perhaps little more than a technology demonstration mission --- in order to be affordable. This simplicity may apply in many areas: landing accuracy, mobility, and sample selection, acquisition, handling, and preservation. A simple approach will inevitably limit the scientific value of the first samples, and it is essential that both NASA and the science community recognize this fact. Subsequent sample returns can become more complex and ambitious as we learn more about Mars and as technological capabilities improve.

A point that we discussed at length was how the missions that precede the first sample return can contribute to the scientific value of the samples. Because samples can be collected most effectively from an environment that is known and understood a priori, there are clear scientific benefits to getting the samples from a site that has already been explored on a previous mission. There may also be technical benefits to such an approach. We therefore suggest that suitability for subsequent sample return should be a factor in selection of sites for landers sent to Mars prior to the actual first sample return mission.

We also considered in more general terms how much information about Mars is needed to allow for intelligent sample return site selection. We concluded that the currently-planned set of NASA Mars missions will provide the data needed to allow selection of a scientifically adequate first sample.

The most serious gap in our knowledge of Mars that will be left by the currently-planned mission set and that is related to sample site selection has to do with surface mineralogy. After Mars Global Surveyor and Mars '96, we will have mineralogical data in the 5-50 micrometer region at 3 km/pixel, and in the 1-5 micrometer region at tens of km/pixel. Improving our ability to recognize mineralogy using visible through mid-IR data at much higher spatial resolution than this would probably allow for significantly improved sample site selection. We suggest that NASA consider augmenting the present mission set with such a capability when the opportunity to do so arises.

Some capabilities will be essential for any mission. Two of the most obvious ones are surface mobility and compositional sensing ability. Even at the simplest of landing sites, considerable geologic diversity is expected due to processes like cratering that mix materials from a range of depths. The ability to detect compositional differences among prospective samples is required to recognize diversity. In fact, for some biology-related sampling objectives, like finding carbonates, good compositional measurements may be needed to locate a useful sample at all. The ability to move around on the surface is required to take advantage of diversity, and will become increasingly important at complex and interesting sites. Because the scientific quality of returned samples will depend on the mobility and compositional sensing capability of the sample return mission vehicle(s), high priority should be placed on developing these capabilities, and on working them into the sample return campaign as soon as possible.

Accurate landing is another capability that is important for science and that should be developed soon. For the first sample return mission the target material must cover an area larger than the landing error ellipse. A small landing ellipse would therefore make a large number of prospective materials available for investigation.

Most thinking to date about a Mars sample return has focussed on rocks and soils. However, we point out that it is also scientifically important to return a sample of the martian atmosphere. The details of atmospheric composition can hold many important clues to the evolution of the planet's volatiles and climate. Fortunately, it may be technically simple for some atmospheric gas to be captured along with the surface samples. We hope that this objective can be accomplished early in the sample return campaign. There is no need to isolate the atmospheric gas from the other samples on the first sample return mission, though it would be desirable to do so on some subsequent mission so as to avoid the possible effects of rock or soil devolatilization.

There was a great deal of discussion of the types of sample sites that should be given highest scientific priority. We identified three particularly attractive types of sample site, all of which should be visited at some point in a comprehensive sample return campaign. They are, in no particular order:

The workshop report will describe the scientific objectives associated with each of these in detail. We have looked at these three site types from the standpoint of the three goals of the Mars exploration program: Life, Climate, and Resources. For each of these three goals, the priorities among the site types are as follows:

Life: Ancient lakebed sediments have highest priority, with ancient highland crust second, and volcanic plains third.

Climate: Ancient highland crust and ancient lakebed sediments have high and approximately equal priority, with volcanic plains third.

Resources: All three are of high priority.

While the scientific priorities appear clear, the technical challenges associated with each type of site are not yet well known. A choice among them for the first sample return mission will have to wait until the capabilities of that mission are better understood.

A number of other issues dealing with sample masses, sample selection procedures, sample preservation, and sample curation were addressed at the workshop. These will be discussed at greater length in Geoff's report.

While our primary focus was on early sample return missions, we also gave some preliminary thought to science that could be done much later in a sample return campaign. Types of sites that would be attractive later in the campaign include:

We can also identify some technological capabilities that should be added to the program in this timeframe. These include the ability to rove over large regions (many tens of kilometers or more), the ability to sample deep in the regolith (depths of meters), and the ability to drill short distances (centimeters) into large, strong rocks and extract samples from their interiors.

While our emphasis here has been on sample return, we stress that some key Mars science objectives in the Life-Climate-Resources triad require other sorts of missions. Important examples are an array of seismometers to investigate internal structure (this will be done to some extent by InterMarsnet), a large array of meteorological stations to investigate atmospheric circulation, and an aeronomy orbiter to investigate the upper martian atmosphere and its interaction with the solar wind.

In addition to sample return, we also dealt with two topics related to international cooperation at Mars: InterMarsnet and Mars Together.

Heinrich Wa¨nke brought us up to date on the status of the InterMarsnet selection, and Jeff Plescia explained to us NASA's position: that NASA will support InterMarsnet if and only if it is selected by ESA this spring for a 2003 launch. We support this position. We also reaffirm the strong scientific endorsement that we gave InterMarsnet at our last meeting. Moreover, we note that InterMarsnet could be a very effective scientific precursor to a sample return mission by characterizing three prospective sample sites in detail. The three model landing sites discussed in the InterMarsnet Phase A Report are from intermediate-age volcanic plains, ancient cratered highlands, and an ancient lakebed. These are the same three types of sites we have endorsed for sample return, and we feel that InterMarsnet could go a long way toward performing scientific validation of such sites.

Roger Bourke, Pete Ulrich, and Sasha Zakharov spoke to us about Mars Together. We were impressed by both the scientific return and the risk of this venture. There was a good deal of discussion of the "launch vehicle dividend" that this mission might provide. This dividend was defined to us as the amount of money that NASA will save by using a Russian launch vehicle in 2001, minus the amount NASA will spend as a consequence of doing business with Russia. The magnitude of this dividend is not known now, nor even whether it is positive or negative. We were pleased to hear from Roger that a very high priority will be placed on estimating soon what this dividend might be. We also note that there are other potential benefits like increased launch mass. Until we know what the financial and technical benefits of Mars Together are, we must restrict our comments on Mars Together to purely scientific issues.

We note that Mars Together, if successful, can provide an enormous scientific dividend. Along with delivering one of NASA's planned 2001 missions to Mars, it would also allow a substantial Russian landed element (nominally a rover) to be placed on the surface. Because of the science that this Russian vehicle might generate, we endorse Mars Together even if the financial dividend proves to be zero. We assume, of course, that data from all Mars Together spacecraft would be shared by both the US and Russia.

We are concerned with the apparent risks associated with Mars Together. These risks seem to be more programmatic than technical, and at the moment they are difficult to assess. At a minimum, we feel that it would be unwise to let participation in Mars Together jeopardize the science on the US-launched 2001 mission. This could happen, for example, if the launch vehicle dividend were negative and large. Imprudent risk, both technical and programmatic, to the Russian-launched mission must also be avoided, and we are prepared to provide advice on the science trades in this area as the risks become better understood.

Our hope, of course, is that the financial dividend will be positive. If it is, we can think of a number of ways it could be used to enhance the overall science of Mars Together. These include the following:

The scientific benefits of these uses are quite variable, and we are prepared to prioritize among them on scientific grounds once an estimate of the size of the financial dividend is available.

That summarizes the major findings and recommendations from our meeting. As always, please contact me with any questions or comments that you might have. Our next meeting will be in Washington, DC on August 1st and 2nd. I hope that you'll be able to join us for that one as well.


Best wishes,

Steve Squyres
Chairman, MarsSWG


cc: W. Huntress
P. Ulrich
H. Brinton
J. Plescia
J. Boyce
C. Pilcher
G. Cunningham
A. Spear
J. McNamee
N. Haynes
D. Shirley
R. Bourke
S. Miller
J. Campbell
L. Lowry
C. Weisbin
A. Chicarro
G. Scoon
A. Zakharov
MarsSWG members

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