Question for the experts: what are the tests and analysis that we cannot do via a rover-based lab and hence need to send the samples to Earth? Asking out of curiosity (pun not intended!).
Simple answer from a space nerd: Rovers are small, light, and have very little energy. So what we can't do is anything that requires a large or heavy device or anything that requires significant energy. They break/fail. They take forever to make happen.
Want to burn 20kg of soil to see if there's any trace of specific elements? You'll need a furnace. Those are both heavy and high-energy. You could do a smaller scale test on a rover of 2g perhaps, but what if the traces are very, very small? Plus, whatever analysis tools you would use now have to also be extremely small and light, which means less capabilities.
Also, rovers have limited size: we can only do a small number of tests per rover because the rover's utility belt of tools is only so large. If your experiment is really easy to do but it's not a high enough priority, it won't happen.
Lastly, rovers die. You might spend 5 years building a tool, 2 years sending it on the next rover to Mars, only to have it crash or die before it's time to run your experiment.
But if we sent 300kg of soil back from Mars, we could do every test we can think of, carefully, with the best tools humanity can make and as much energy as we need.
Edit: +1 to what Something1234 said too! You can't clean out apparatuses easily after an experiment!
These are all reasons why sending humans is so important. They will explore more of Mars in a week than rivers have in 50 years. They will arrive with a testing lab and do all sorts of tests immediately.
If Starship meets its performance targets, it’s first missions will bring hundreds of scientists to a Mars, with thousands of tons of equipment and supplies. In-orbit and in-situ refueling are game changing technologies that will literally reduce the cost of deep space manned missions by a thousandfold.
So instead of sending a rover with 4x or 10x energy, the answer is a mission that requires enough energy to launch the samples all the way back to Earth? This makes me smile.
I do agree though. We can expend 1000x the energy of the return trip once the samples are back on earth. And we also have the luxury of unlimited time to invent new tests and new tech to apply to whatever is left of the samples.
No, rover is easier; return system has to fly all of its mass back to Earth, rover never leaves Mars.
In which case you might be asking what the advantage of a return system is in the first place, the answer being for much the same reason that we have "stages" of rockets. The return system gets a fresh exponential curve to work with. If you tried to ship both at once it would be a nightmare. Plus there are all the advantages of shipping things back, regardless of what the rocket equation says. Science via a rover is great but it just can't match numerous full laboratories with humans in them.
The part that disappoints me a bit is that I still don't see us launching fuel into space for space refueling very often. Rocket equation bites no matter what you do, but launching a Falcon fuel of nothing but fuel (to the extent possible) shrinks the solar system a lot. It's still pretty big after that, but it's a different place, even with conventional chemical rockets. I hope we'll see it soon. I'm not sure it'll be quite as simple as "SpaceX will have a Mars Starship there before the mission can get there", but there is a real prospect of making it so space missions takes weeks instead of years if we can get space refueling figured out. A lot of other things come into reach as well, like satellite reclamation.
Not an expert, but the test chambers on a rover can't be cleaned easily or without a massive amount of complexity. They are also fairly limited and very limited use. While a sample collected and sent back can be split and used in many ways.
Better question: What will we learn that we cannot learn from all the Martian rocks we already have on earth.
There have been plenty or Martian meteors found in Antarctica, and no doubt many more if we redoubled our efforts to find them. Many once claimed to have found evidence of life in these rocks already. Some of us are even old enough to remember President Clinton's speech on the subject. (That isn't a deepfake video. The US president really did talk about the discovery of life on mars.)
> Better question: What will we learn that we cannot learn from all the Martian rocks we already have on earth.
These particular samples will be from the present-day surface, with known geographic (aerographic?) origin, not affected by potentially millions of years of interplanetary travel, and not contaminated by sitting around on Earth for who knows how long.
That would be true if we were looking for life currently alive on mars, but this is a search for fossil life, life that once existed but died off perhaps a billion years ago. Fossils would survive a million-year trip just fine.
There has only been one serious attempt to detect current life, the labeled release experiment on the Viking lander. It came back positive. If we are seriously interested in detecting current life we don't need a sample return. Any rover can dump some rock into a nutrient bath and put the results under a microscope. If anything is growing/moving, life is detected. Viking didn't have the bandwidth for such things. Lets do that a couple times first.
> That would be true if we were looking for life currently alive on mars
I mean, why not?
> Fossils would survive a million-year trip just fine.
Sure, but getting to pick where we take the rocks from helps us maximize the possibility of fossils in the sample. The rovers have found rocks with evidence of water - being able to pick those instead of a random chunk (which might've been blasted from deep bedrock) found in Antarctica has its appeal.
> There has only been one serious attempt to detect current life, the labeled release experiment on the Viking lander. It came back positive.
This seems like a good argument for sample return.
There is a big, big difference between, "we found a rock on earth that is probably from Mars that was blasted here some time in the past," and, "we went to a specific area of Mars and got a rock and brought it back." Not least of which is the intrinsic value and aspirational nature of us actually fetching things from other planets ourselves, with intent and purpose, rather than luckily stumbling upon some.
Excellent question. Obviously there are lots of things they couldn't afford to get into a lander. Question is what could they have put into a lander, for much less than $billions, that they still haven't?
I am no expert, but I can imagine there are many kinds of laboratory equipment that is way too heavy to pit on a rover, perhaps mass spectrometers, etc.
Biological wxperiments is a big one. There is kind of outstanding question of perclorates in martian soil and its toxicity to earth organisms, plants, etc.
The mars rover does have a spectrometer. It's called ChemCam [1], it uses a laser to vaporize a small amount of matter and them captures the spectrographs of the resulting plasma with a camera. That doesn't take away from your point, but it is a cool piece of equipment.
Can't we just shoot a orbital space laser at mars then from mars orbit? If we're already sending a very heavy craft that way... should be able to do spectral analysis from space.
