Has the iron core of Mars really solidified?
In the Nova episode "Origins: Earth is Born", Neil DeGrasse Tyson states, "But Mars is just a fraction the size of the Earth, so it cooled more rapidly. And as it cooled, its molten iron core hardened. As a result, Mars stopped generating its magnetic shield." (February 28, 2004)
This comment from Tyson is consistent with what I have long believed. But I wonder.
The melting point of iron (Fe) is around 1538 °C.
The the geothermal gradient for Mars is estimated to be around 6 °C per km.
If the estimate of 6 °C per km is the ballpark, then I would expect that iron is molten as a mere 256 km of depth. At the core of Mars (over 3000 km in depth) I would expect the temperatures to be far higher than 1538 °C.
It seems to me that it is impossible for iron to be in solid form at Mars' core.
@zephyr This is a topic that is not trivially researchable by a layperson: http://adsabs.harvard.edu/abs/2015EGUGA..1715240D
Mars core may or may not be solid, I'm of the opinion that it's not solid.
See recent NASA press release here
and a much earlier study here
It has been known since 2003 that at least part of Mars’ interior is
molten, based on how easily the Sun’s gravity distorts the planet’s
shape, but no one knew whether it is completely liquid, or whether
there is a solid inner core like Earth’s.
and Nasa's 2003 announcement here
on Neil deGrasse Tyson, link here, but I'm not sure it's from him, so . . I might delete later.
I've heard (but should provide a source) it said that Mars core has solidified. I remember NdGT saying that, but if he says it on a show, it's really the show saying, not him. Anyway, I'll try to get that point worked out.
But, these seemingly conflicting statements might still have some relation (I think). A basic explanation is that the difference between liquid and solid under the pressures you get inside a planet are somewhat inexact. The other thing is, to generate a magnetic field, you need more than just a liquid metal around a solid metal, but you need dynamic spiraling. See here and here.
Venus rotates very slowly, this rotation means that Venus has essentially zero Coriolis effect, which might help it generate a permanent magnetic field. Earth's relatively rapid rotation and Coriolis effect, and perhaps it's very gradually slowing rotation due to the Moon, help Earth maintain it's magnetic field. A very slow variation in the rate of rotation of the solid core and outer solid mantle could generate enough spin to maintain the magnetic field.
The point of all this, planetary magnetic fields are complicated but they likely require the right type of permanent flow within the liquid part of the core, probably around a solid iron core. Earth maintains that perhaps in part due to higher core temperatures which enable more easy flowing as well as the release of heat through condensation as the outer core solidifies onto the inner core and perhaps also, due to the Moon. Mars at one time in it's past probably had a magnetic field similar to or perhaps even stronger than Earth, evidenced by some powerful crustal fields that remain. See here.
I don't know why he uses the word "solidified" when other research says liquid. That might be a good question to ask him, but, Neil deGrass Tyson may be right that the cooling of the smaller Mars may have caused it to lose it's magnetic field, but based on article's I've read, he's incorrect to imply it solidified, but the liquid core may have become solid enough that the dynamics that could have once caused a magnetic field could have stopped, at least, I would speculate that this is possible but I don't think anyone knows for 100% certain. The history of planetary magnetic fields as well as the precise makeup of planetary cores and what turns on or off a magnetic field inside a planet isn't well understood.
and Zepher's answer is also important that the temperature of a liquid iron core at pressures inside of a planet are very different than the melting temperature at 1 ATM. Earth's inner core is hotter than it's outer core but the greater pressure keeps it's inner core solid while it's outer core is liquid.
Re *I assume this is a direct quote, see here.* I would assume the exact opposite. That blog appears to be written by Greg Bentall rather than Tyson, Einstein, Drake, or Sagan (also tagged in that blog post). Bentall appears to be mixing and matching what's he's read, possibly with a layperson's misunderstanding, and possibly some now out-dated stuff. Note well: given other stuff I've seen from Tyson, I truly doubt he's a proponent of the rare earth hypothesis. (Compare with the bulk of that blog post.)