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As glad as I am that Perseverance's landing was successful, I'm a bit confused as to why there appears to be so little science equipment aboard the rover (by volume) and why the mission seems to be focused on returning samples in the future instead of analyzing them in-situ today. For example, here is a picture of the science equipment on the rover which shows how only a tiny fraction of the rover actually consists of science equipment. Sure, the rover needs volume for power, mobility, communications, computation, etc. but wouldn't the volume dedicated to the sample caching system have been better utilized for sample analysis?

More specifically, any samples the rover takes aren't going to be back at Earth until 2031, and that's not only optimistic but also assuming everything goes right (landing on Mars is still like 50/50). Right now the Mars Sample Return mission architecture is looking quite complex and has not only a rover, lander, rocket, and satellite involved, but also a heaping of major international cooperation which, while good, surely can't help keep the project on time. To a pessimist, the high-level view smacks of purposeful government-induced-sunk-cost policymaking a-la, "Well, we already spent over $2B getting the rover to collect samples, so now not running a sample return mission would mean that money just went down the drain!" and thus lock in Congressional/NASA funding for this project for at least another decade. Furthermore, even if it's ultimately successful, I wouldn't be surprised if the whole project slips a launch window or two because of the complexity and the samples only end up back on Earth in the mid 2030's. By then, a faster sample-return mission (or like a fully decked out science SpaceX Starship) could've taken place.

This leads me to the question:

How was it decided that Perseverance's Sample Caching return is preferable to on-site scientific analysis?

Of course, there are several possible reasons I've thought up but none of them have convinced me so far (and my reasoning might be completely wrong, so please correct me!):

  • "To learn more about Mars, we need the samples in the big labs we have on Earth" -- While this is certainly true, I'm not convinced that we've reached a hard limit of what's possible to do on site.
  • "Local analysis was Curiosity's mission" -- Yes, and curiosity was built over 10 years ago. I imagine we would be able to build more compact and better local analysis equipment today.
  • "Sample return is cheaper than packing expensive science equipment into the rover" -- The rover is already expensive enough that cost is basically no object.

EDIT: Specifically, are there any primary-source documents or write-ups from NASA which detail their decision-making process in the conception of the Perseverance Mission?

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  • $\begingroup$ "The rover is already expensive enough", Perseverance is relatively cheap and less costly than Curiosity. "cost is basically no object", I'm sure everyone involved wished that were true. It comes down to money, mass, power, and volume. You can throw money at the mass problem to a point, but NASA has a budget and can use that money for other things. RTGs produce limited power. Payload fairings only get so big. $\endgroup$ – Schwern Feb 22 at 18:56
  • $\begingroup$ @Schwern You're right, perhaps I should've been clearer: when I say "cost" I'm talking about pure dollars. $2.2 billion is a lot of money and even if it is cheaper than Curiosity, I can't really imagine a meeting where someone said, "Well, let's do the sample caching instead of including the onboard lab module because the lab costs 100 million more". Basically, I don't think that when the mission planners and engineers sat down to decide what goes on the rover, things were ruled out because they cost too much money (please correct me if I'm wrong about this though). $\endgroup$ – Dragongeek Feb 22 at 19:45
  • $\begingroup$ Yes, absolutely they do have to worry about money. NASA has a budget. Mission planners have to "sell" their mission and cost is a big factor; they have to get the most science for their buck. More mass means more money at an exponential rate. So yes, they had a mass budget (and power and volume budgets) and had to decide which instruments they could cram in. I'll write up some more about the selection process in an answer. $\endgroup$ – Schwern Feb 22 at 20:13
  • $\begingroup$ This doesn't seem like a small amount of science equipment to me. You may be underestimating the complexity of a rover with no science. $\endgroup$ – user253751 Feb 23 at 11:22
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I'll just address the last point:

"Sample return is cheaper than packing expensive science equipment into the rover" -- The rover is already expensive enough that cost is basically no object.

Well, it is simply not the case that cost is no object. They didn't decide to put a rover with a mass of a tonne on Mars because, well, that seemed like a good mass: they worked really hard and decided that, they could, if everything went OK, plausibly get such a thing to Mars. If they wanted to put a rover with a mass of 2 tonnes on Mars they'd probably need a new launch system (I don't know this, but I suspect). That quite likely means the mission doesn't happen.

So the sort of science equipment we can put on Mars needs to:

  • be small enough to fit in the rover;
  • be light enough to be within the very stringent mass budget;
  • be able to withstand the journey to Mars, including being strapped to the top of a rocket and shaken vigorously and then spending 7 months in a very nasty radiation environment and the remainder of its life in a fairly nasty radiation environment;
  • use no more power than is available on the rover (the MMRTG provides 110 watts at launch, which must run everything on the rover).
  • be able to work unattended with no possibility of repair, ever;
  • not require any consumables not easily found on Mars, which probably means 'nothing we can't take with us other than power';
  • not interfere with any other science experiments.

These are quite stringent requirements. A lot of the things that might be interesting fail them dismally.

For example, let's say we're interested in organic compounds, which we probably are. Well a very good way to look at information about organic compounds is NMR: this is particularly interesting because NMR is non-destructive. NMR machines are big: small ones are merely large, pictures of large ones often include people walking around the top of the machine. I don't know how heavy they are, but probably many many tonnes: most of an NMR machine is huge magnets which are not famously light things.

Oh, and these magnets are cryogenic: NMR machines need supplies of liquid helium to keep their magnets superconducting.

If I have a sample on Earth I want to do NMR on I just book some time in my local NMR facility. If I have a sample on Mars I want to do NMR on, I build a spacecraft to bring the sample from Mars to Earth and then book some time in my local NMR facility.

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I don't think that when the mission planners and engineers sat down to decide what goes on the rover, things were ruled out because they cost too much money

They absolutely do.

Let's clear up some misconceptions.

  1. NASA has a budget.
  2. We can examine samples on Earth far better than any robot could.
  3. After some spectacular failures, the US has gotten very good at landing on Mars.

Vision and Voyages for Planetary Science in the Decade 2013-2022

Specifically, are there any primary-source documents or write-ups from NASA which detail their decision-making process in the conception of the Perseverance Mission?

Every 10 years the National Research Council (now National Academies of Sciences, Engineering, and Medicine) asks planetary scientists what they would like NASA to research and then recommends missions for the next decade. Vision and Voyages for Planetary Science in the Decade 2013-2022 decided a Mars sample return was the highest priority.

That document will provide the details you're looking for. Though it is just a recommendation it carries great weight with mission planners. The whole document appears to be available online for free.

Why Sample Return?

Scott Hubbard, NASA’s first Mars program director, sums it up in Mars 2020: Its Origins, Science and Technology.

Bringing samples back to Earth is critical for three reasons that have stood the test of time: utilizing instruments that cannot be shrunk to spacecraft size; engaging hundreds of scientists across dozens of laboratories; and most importantly, being able to follow the pathways of discovery as new experiments are conducted. As capable as Curiosity is, the instrument suite is fixed.

When designing a probe the mission planners have to make an educated guess about what instruments will be most useful. If they discover something they didn't expect, they must wait until the next mission to send fresh instruments. A sample return allows anyone to propose any experiment using any instrument.

Vision and Voyages Chapter 6 - Mars: Evolution of an Earth-Like World has a section on the Importance of Mars Sample Return. Here's some excerpts.

The analysis of carefully selected and well-documented samples from a well-characterized site will provide the highest science return on investment for understanding Mars in the context of solar system evolution and for addressing the question of whether Mars has ever been an abode of life.

Two approaches to the study of martian materials exist—that using in situ measurements and that employing returned samples. The return of samples allows for the analysis of elemental, mineralogic, petrologic, isotopic, and textural information using state-of-the-art instrumentation in multiple laboratories. In addition, it allows for the application of different analytical approaches using technologies that advance over a decade or more and, most importantly, the opportunity to conduct follow-up experiments that are essential in order to validate and corroborate the results. On an in situ mission, only an extremely limited set of experiments can be performed because of the difficulty of miniaturizing state-of-the-art analytical tools within the limited payload capacity of a lander or rover. In addition, these discrete experiments must be selected years in advance of the mission’s launch. Finally, calibrating and validating the results of sophisticated experiments can be challenging in a laboratory and will be significantly more difficult when done remotely.

They go on to give examples of inconclusive results from in-situ analysis and more detail about the return.

Mission Selection and Budgets

NASA has a limited budget decided by the US Congress. Missions must compete to provide the most science for the buck. Vision and Voyages made it their first selection criteria. From their Executive Summary...

To assemble this program, the committee used four criteria for selecting and prioritizing missions. The first and most important was science return per dollar. Science return was judged with respect to the key science questions identified by the planetary science community; costs were estimated via a careful and conservative procedure that is described in detail in the body of this report...

Mars Sample return is their top priority.

The highest-priority flagship mission for the decade 2013-2022 is the Mars Astrobiology Explorer-Cacher (MAX-C), which will begin a three-mission NASA-ESA Mars Sample Return campaign extending into the decade beyond 2022.

A major accomplishment of the program recommended by the Committee on the Planetary Science Decadal Survey will be taking the first critical steps toward returning carefully selected samples from the surface of Mars.

But cost was a major concern.

At an estimated cost of \$3.5 billion as currently designed, however, MAX-C would take up a disproportionate share of NASA’s planetary budget. This high cost results in large part from the goal to deliver two large and capable rovers—a NASA sample-caching rover and the ESA’s ExoMars rover—using a single entry, descent, and landing (EDL) system derived from the Mars Science Laboratory (MSL) EDL system. Accommodation of two such large rovers would require major redesign of the MSL EDL system, with substantial associated cost growth.

They recommended it be cut down to \$2.5 billion to have a chance of obtaining the money from Congress.

The committee recommends that NASA fly MAX-C in the decade 2013-2022, but only if the mission can be conducted for a cost to NASA of no more than approximately \$2.5 billion FY2015... It is likely that a significant reduction in mission scope will be needed to keep the cost of MAX-C below \$2.5 billion.

It turns out even that was optimistic. MAX-C was cancelled due to 2011 budget cuts putting a cap of \$1 billion on any flagship project far below what the NRC recommended.

Out of the ashes came Mars 2020, originally announced at the end of 2012 with a budget of \$1.5 billion. It would cut costs by building on Curiosity, sometimes literally by using spare parts. The eventual cost is \$2.8 billion over 10 years with \$2.2 billion of that for the rover.

Instrument Selection

In December 2013 NASA put out an Announcement of Opportunity for Mars 2020 "to solicit proposals for Mars 2020 surface-science investigations and exploration technology investigation". I'll leave that to be read for details.

NASA received 58 proposals (that's a lot) and selected 7 at an estimated cost of $130 million to develop and build.

NASA had to select instruments which would provide the most science for their goals using the limited money, mass, power, and volume available. Mass is the greatest challenge of spaceflight. Delivering 1000 kg of rover requires 531,000 kg of rocket and fuel and 900,000 kg of trust. Because of the Tyranny of the Rocket Equation each kilogram of payload mass can add dozens or hundreds of kilograms of rocket, fuel, and landing equipment which all cost money.

More instruments mean more payload volume. Space is vast, but space inside a payload fairing is limited to roughly 5m in diameter and about 15m in height (the Atlas V 541 used to launch Perseverance has a very tall payload faring, but includes the 13m upper stage). There's only so much which can be crammed in, and some simply cannot fit at all.

Finally, more instruments means more power and power is very limited on Mars. Perseverance uses a very reliable Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) providing 110 Watts, barely enough to run a desktop PC, supplemented with batteries, and it only gets weaker. Spacecraft have flown with more powerful RTGs, Cassini-Huygens carried 3 RTGs delivering over 800 watts at launch, but that adds cost and mass.

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One of the reasons for the sample caching that was not mentioned yet in any of the other answers, is that we want to preserve pristine samples from before humans went to Mars.

It is looking at least plausible that we will send life (more precisely humans) to Mars in one of the next 5 transfer windows. It is highly likely that the number of Mars missions will increase in the next transfer windows. Therefore, the chances of contaminating Mars will go up.

Having pristine, guaranteed uncontaminated samples in hermetically sealed containers allows us to study the influence of human presence on Mars.

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It is important to not underestimate the value and power of "the big labs we have on Earth". They are still doing experiments on the examples returned from the Apollo missions.

And besides, we still have a grand total of 0 Mars samples here on Earth1. We could benefit a whole lot from at least an example or two.

1except of course for Martian meteorites. Hat tip to @Wyck for an estimated 292 catalogued instances

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