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Now, I'm not questioning claims that any planet in the Solar System is atmospherically or externally inhospitable, but claims made that other planets have geology that is extremely hostile to colonization (or even, in the case of Venus, further exploration). Some of these claims are:

  1. Mars's core is frozen.
  2. Mars has no recent volcanic activity.
  3. There are no large deposits of carbonates or other carbonaceous minerals/materials on Mars.
  4. The total quantity of water on Mars is less than 1% that on Earth.
  5. A perchlorate layer covers the entire Martian surface.
  6. The soil of the Moon, Mars, and Venus would need to be massively modified to support plant life.
  7. Venus was volcanically resurfaced in its entirety.
  8. There are never any breaks in Venus's cloud layer, no matter how small or infrequent.

Judgements like this seem very premature given our lack of hard data, especially that of direct measurements of rock strata. For instance, about the first claim, only one functional siesmometer has been sent to the surface, which has only been there for a year and lacks any impact tests, and the only instrument sent to measure internal heat has so far malfunctioned. The 2nd and 7th claims are almost exclusively based on crater counts, which can give tremendously different age values depending on the expected impact rate. The 3rd is based exclusively on remote sensing methods unable to identify underlying strata, the 5th is based on a handful of landings, and the 6th has been called into question by research old and new. For the 8th, no modern space mission has even carried the instrumentation required to confirm this (that is, a high-resolution visible-light camera). All of these seem only possible to confirm or deny with much more extensive exploration, especially on and under the ground.

So, then, what am I missing, if anything? Why is it acceptable to make judgements like these off the information we have? I mean, imagine if a textbook of Earth's geology was writen solely on the observations of a handful of satellites as well as 9 pseudo-randomly-dispersed geologists allowed only a couple tens of kilograms of shallowly-penetrating tools and analysis devices (some defective) each, of which 5 of them are buried up to their knees in rock.

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    $\begingroup$ I think there's a good question in here somewhere, but it's a little hard to get to the bottom of this because you make claims about claims without citing and quoting any specific examples. It's hard to answer when it's not at all clear what it is you've been reading. Can you link to or cite a few specific examples of these claims so that others can read them and see exactly what it is that's been claimed that you take exception to? Thanks! $\endgroup$ – uhoh Dec 21 '19 at 6:30
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    $\begingroup$ In a comment or by editing the question? (BTW, a problem about these claims is that they are often not found mostly in peer-reviewed work, but in science journalism and in the words of "soul-crushing engineer" types. And there's the fact they are based on very little hard evidence.) $\endgroup$ – Grant Hartlage Dec 21 '19 at 7:06
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    $\begingroup$ Oh it's always better to modify your original post. Comments are actually considered temporary and might be deleted without notice in the future. I think many astronomers are regularly irked by the way things are oversimplified in the popular press, and may not want to be reminded. It's better if you can ask about the astronomical or planetary-science aspects directly if possible. $\endgroup$ – uhoh Dec 21 '19 at 7:17
  • $\begingroup$ The scientific community isn't of one mind, but many individuals. That said, yes it can happen that assumptions or judgments are given too much credit, but that's not the scientific method. The scientific method is evidence based and peer reviewed, not assumed. Each of your examples could be the subject of whether it's the pseudo-science of assumption or the hard-science of evidence. Doing all 8 in one answer gets long. There's also the problem, as @GrantHartlage points out, of tracking down the source of the claim, and not blaming the scientist for a journalist's summary. $\endgroup$ – userLTK Dec 21 '19 at 11:24
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It is not simple to address all these points since they are simple expressions for complex processes.

1.) Mars's core is frozen

Translating frozen as solid. It is still unclear if Mars' core is solid or liquid, see @userLTK's answer: it is curently seen as being viscous. A liquid core could help maintaining a magnetic field. Mars has a very weak magnetic field that does not shield it like the earth's magnetic field does. This is adverse to the development of complex organic molecules and the basics of simple life. Maybe InSight brings us more insight into this one.

2.) Mars has no recent volcanic activity.

That appears to be incorrect. Mars has no plate tectonics that offer cycles (e.g.carbon). One can speculate that tectonis are helpful for a biologic evolution and complex life, offering renewal of needed elements, thermostats, being helpful in case of planetwide overfreezing, ... but this is debated and speculative.

There is possible polar volcanism and recent lava flows in the calderas of the Tharsis Montes (Olympus Mons and neighbours).

3.) There are no large deposits of carbonates or other carbonaceous minerals/materials on Mars.

There are carbonates, but not organic. That means that in the past organisms have not built up sequences of sediments like on earth. And it is true that remote sensing will not suffice for a qualified final judgement. But there is nothing more to do atm.

4.) The total quantity of water on Mars is less than 1% that on Earth.

Unclear, but probably not totally nonsensical. Though there is water, or rather brines. Water isn't exactly rare in the solar system. Mars probably had a water ocean early, but may have lost it qickly to space. Otoh it can be debated if amounts of water have been sequestered in the Martian mantle. This water could sum up to several meters worth of water coverage.

5.) A perchlorate layer covers the entire Martian surface.

Exaggerated, but perchlorates were found in samples of Marsian regolith and under UV radiation result in a bacteriocidal environment. No potatoes for Marc ...

6.) The soil of the Moon, Mars, and Venus would need to be massively modified to support plant life.

Well, that is indeed a belittlement. It is far beyond our capbilities to mobilize the necessary masses and energies. One could imagine greenhouses; and test runs have been/are being made in Antarctica. But who wants greenhouse stuff every day ;-). There is this study concluding that even if we could mobilize all the CO2 in all available reservoirs including the upper crust of Mars, that would only rise the atmospheric pressure by a few percent. Not enough to significally warm the atmosphere or walk around without a pressure suit (oxygene supply allways needed anyway).

7.) Venus was volcanically resurfaced in its entirety.

I have no idea, but so are large parts of Earth, constantly actually. But on the Venusian surface there are ~450°C and 95bar atmospheric pressure. The atmosphere is almost pure CO2. Excluding life until further notice is a valid guess.

8.) There are never any breaks in Venus's cloud layer, no matter how small or infrequent.

Ok.

We could address these points one by one and search for proper publications of data from spacecraft, remote sensing and rovers and apply the principle of uniformity (that's what NASA does, but don't use it as a religion ...) and discuss further. But this has all been done multiple times. All in all, for Venus the judgement "is dead" is pretty natural, given the conditions. Mars may have seen microbial life in its beginning, let's say its first few hundred million years (speculative !), but today this is unlikely, though not impossible. More data needed, as you say.

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I've read your question a couple of times and I think I see the gist of what you're going for, but if I'm way off, please let me know.

Pulling 2 sentences:

. . . but claims made that other planets have geology that is extremely hostile to colonization (or even, in the case of Venus, further exploration).

. . . .

Judgements like this seem very premature given our lack of hard data, especially that of direct measurements of rock strata.

It sounds like your asking why scientists are pesimistic, especially in regards to colonization of other planets and the 2nd part, why do scientists jump to conclusions like "Mars has a solid core" - and I remember when that was the going hypothesis.

My answer is that pessimism is useful, especially with space travel, colonization or billion dollar missions. Nobody wants another hubble that takes blurry images, or worse, astronauts dying.

Both the dreamers, who want to expand into space and do everything we can, and the pessimists who tell us why this or that won't work are useful parts of the design and planning of missions. Space travel is crazy expensive and when people are involved, often very dangerous and it's good to have people to think of reasons why a mission might fail. I realize I'm off your subject a little bit, but pessimism has a place in any kind of big-budget planning.

Onto the 2nd and I think, more interesting part, why do scientists say things like Mars has a solid core, when in 2003 this was found to be incorrect.

First, not all scientists were saying it. Probably most scientists were in the "we don't know" camp. But "We don't know" doesn't make as good an article, so the scientist or team of scientists who says "Mars core froze solid, that's why it lost it's magnetic field", is more likely to get quoted than the larger number of scientists who say "We don't know why Mars lost it's magnetic field". - and I'm old enough to remember the "Mars has a solid core" articles - it was what was presented to the public, or at least, based on the articles I read. My mind is telling me that I read that more recently than 2003, but my mind is unreliable about such things. Google tells me that Mars' core is liquid.

Now . . . liquid is a bit of a lie as well, it's more like the consistency of pitch, but it's liquid enough to flow a little, or, not frozen solid and rigid (rigid objects that large still bend due to tidal forces), but rigid doesn't flow though it might rotate, and I'm getting a little sidetracked. Point is, these measurements are subtle, but the going theory is that Mars has a liquid core.

What you're missing is the scientific method and there's lots of articles about this, easily searched. I think a lot of them make it a little more complicated than they need to, so I'm going to summarize.

  1. Develop a working hypothesis (example, Mars - small planet, it's core froze solid)
  2. Develop a test for the hypothesis, which ideally provide evidence in favor of, or in disagreement with the hypothesis. It's worth noting that a GOOD hypothesis, often refers to whether it's testable, not whether the claim is correct.
    1. Publish your results - let others review them, repeat the test, reach a general consensus or disagree.

The heart of the scientific method is the testability part and the part where other scientists check the work. Not to sound like a Borg, but to the scientific method, belief is irrelevant, assumption is irrelevant, only the results and those who are trusted to review the results matter. The results should dictate the answer.

So, when a scientist or a team of scientist said at some point "Mars has a solid core, that's why it lost it's magnetic field" - that was a claim, which is little more than a logically constructed argument/educated guess. It wasn't an evidence backed study. Scientist know the difference between a claim and an evidence backed study because of their training and they are completely different things, kind of like how a lawyer recognizes the difference between an emotional or ethical argument and a legal argument. But people who read science articles may not see the difference, and so when a claim is proved false (like the Mars has a solid core claim) - oh my, they were wrong.

The funny thing about science is that it's OK to be wrong. They're not picking stocks or playing chess where making the right move is what matters. Science is about discovery and it's the testable hypothesis that moves science forward. Being wrong is OK, in fact, scientists have won the Nobel prize for being wrong.

The discovery of dark energy, basically everybody was wrong on that prediction, so much so that the data had to be double and triple checked before it was believed. The discovery of the cosmic background radiation - they thought it was interference from bird droppings at first and spent time cleaning up their radar dishes before they went back to look for whatever it was they were looking for. It was what they found that they weren't looking for that was the big discovery.

Bell's theorum is a fun one. Bell believed he'd found a way to disprove spooky action at a distance and when his experiment was finally put together, it wound up proving what he believed he would disprove (and proof isn't quite the right word, it's more a math word than quantum physics, but I think you get the gist).

Point is, it's the testable hypothesis that moves science forward and sometimes wins the Nobel prize. Not the idea and not even being right, but what can be demonstrated, that is, proved right or proved wrong.

Claims, or logically constructed arguments have a place in science. This happened because of that, and then "oh look, we were wrong". To a scientist, this is part of the scientific process and anyone who reads an article, or who goes back to the original research can usually work out the difference between a claim and an evidence backed study. Not everyone who writes articles makes that difference clear and many people who read the articles don't even know there is a difference. Hence, it's not always a good idea for scientists to talk to the press. Things can be misinterpreted.

I happen to think that a handful of your examples are true, but to the scientific method, it doesn't matter what I believe, it matters what can be proved - like that Tom Cruise movie. Scientists are actually pretty good sports most of the time about being wrong, because when a claim is proved wrong it opens the door to new discovery.

This isn't because science is sloppy. It's because the scientific method permits inquiry and logical prediction, such as "We think this because of that" and if what they think is later proved incorrect, that teaches the scientist something. In fact to the scientist, it's kinda cool when things don't turn out as expected. The public, however, can be less forgiving.

That's my take, anyway, hope that helps.

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    $\begingroup$ A lot of text, but well spoken :-) These answers take time. I edited my answer to incorporate current view of the Martian core. Thanks. $\endgroup$ – user31179 Dec 24 '19 at 9:46
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    $\begingroup$ @ebv while I might be making a bad assumption, nobody really knows why Earth has an internally generated magnetic field and Mars doesn't, but one idea is that Earth's outer core is very slowly cooling and becoming part of the solid inner core. That phase transition increases density and releases heat (like how water to ice releases heat, ice to water takes in heat), this may generate the circulation of the outer core that creates the magnetic field. (or it might be something else - perhaps the Moon and tidal forces play a role). And, I know I write too long answers. I'm working on that. $\endgroup$ – userLTK Dec 24 '19 at 20:12
  • $\begingroup$ You could post a question "What causes earth's magnetic field" ... ;-) $\endgroup$ – user31179 Dec 24 '19 at 20:20
  • $\begingroup$ @ebv I could, and let me back up a little, Earth's magnetic field is believed to be caused by convection currents, driven by both heat and Coriolis effect, but why Earth which rotates about the same period as Mars, has a relatively strong field and Mars has almost none, that question is a little trickier. I'm pretty comfortable with the we don't know yet and more research is needed answer to this question and I've read up on it a fair bit, for a layman at least. $\endgroup$ – userLTK Dec 25 '19 at 18:55

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