Feel free to take your mass produced Fiat hatchback (or whatever you drive) on a trick through Antarctica. Or the Amazon. Or the Sahara. Or up Mount Everest...
Not a valid analogy.
The environments on Mars are very much varied. The only way to make something suitable for all of them at once is to over engineer it and have most of that engineering useless for the environment it is in. Every component present designed to be used in an environment the probe is not in reduces science payload.
MERs were designed for a typical surface of Mars - sandy, rolling plains. They obviously can't go up/down steep slopes, cliffs, or navigate too rocky a terrain (neither can MSL, for that matter), so chaos terrains, fossae, mensae, etc. would be difficult (also for other reasons - shadowed areas can't be explored by solar-powered probes, for instance). But that still leaves huge areas of Mars for exploration, and this kind of probe can easily do this kind of job.
So, it's not like sending a Fiat from European roads to Brazilian rainforest to Antarctica to the Himalayas, it's far from it. It would be like sending a desert-adapted vehicles from Sahara to some other desert environment. It's not like the MERs can only operate in the general areas where they landed, that's so not true.
You will note that you are talking about orbital probes, or basically probes all doing the exact same thing in near the exact same environment. Orbital probes are just far simpler things as they have to do a far simpler thing.
It's not far simpler, in many ways it's pretty difficult. The point was, you can create a set of basic designs (inner solsys probe, outer solsys probe, Mars rover, outer solsys moon lander, etc. etc.) and then adapt them for particular mission needs for a fraction of the cost of developing an entirely new probe, especially if this was available to other space agencies.
So says you. I bet any engineer working on the science package of these things disagrees with you.
I bet he doesn't. The two identical MERs landed on different places of the planet, and due to the nature of their landing systems, the engineers were not sure where
exactly they'd land. That's why they can deal with variety of navigable terrains, which happen to cover most of Mars. That's a fact, deal with it.
UNLESS, as Czerth said, you are looking for breath instead of depth. I am sure there is a very good reason scientists pick the second over the first. Maybe one day we will be at the point were the cost of delivery allows us to conduct redundant tests for various reasons, but right now if you are going to go through the cost of getting there you might as well get the biggest bang you can.
MERs are basically a (rather awkward) robotic alternative to a human geologist. If you want to understand a place from a geological standpoint, you need far more than one short field trip. This is what the MER mission(s) essentially amounted to. Far more exploration is needed to get a picture of Martian geological history - we need to find out if Mars ever had large oceans or not, when the volcanoes ceased to be active, when did the atmosphere begin eroding, etc. etc. etc. More MER-based vehicles would be a cost-efficient way of doing such a large scale geological survey of Mars in the absence of a human landing (which would be preferable for so many other reasons).
(I am not arguing against MSL here; I am arguing against the general wastefulness of robotic exploration programmes which develop great probes, only to abandon them and spend fortunes developing different ones. I think if a design is successful, it should be utilized to its maximum potential, until it clearly cannot cope with the new mission requirements.)
I bolded the part that makes your mass produced idea undesirable. It is extremely varied, which is why expecting a one size fits all vehicle to tackle it is dubious at best.
I didn't say the probes couldn't go there, because they could. When I said "extremely varied" I meant their origin and history as well as appearance. For example, we need to know how much of Mars' surface was underwater, and for how long. The first MERs found rocks that can only form in the presence of water. But are there others? At different places around Mars? We don't know - we can guess based on some very clever orbital scans, but having more rovers cover more areas of Mars would be much more preferable.
For instance, we could choose a number of locations which we think are representative of a particular type of terrain we find on Mars, and send a probe to each. Then compare the results. I suspect our knowledge of the planet's past would grow exponentially then.
The same with MSL - if it finds something very interesting, we will still be in the dark concerning the scope of what we've found. Is it just a localized phenomenon? An anomaly? Or¨is this kind of thing ubiquitous on Mars? Or just on one hemisphere? Well, send more probes to find out! The basic design is available, building Mk.2 versions will be (relatively) cheap and fast, and sending these every 2-4 years will keep the planetary exploration division of NASA busy for decades while producing results the agency needs. The accumulated experience will also be useful for future missions of different kinds.
). Good luck to the mission - hopefully it will find something really interesting.
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