OlympusMons94 t1_jdjwm8c wrote

The gradual freezing out of the outer core to grow the inner core indirectly sustains the magnetic field generated within the outer core. (And even then "only" for the past ~0.5-1.5 billion out of 4.55 billion years of Earth's history, because the inner core didn't exist before then.)

However, the inner core itself doesn't really do anything practical or of non-academic interest. Aside from very slowly getting bigger, it just spins along (very, very slightly more or less rapidly) with the rest of Earth. The motions that power the dynamo are strictly within the molten portion of the core. Essentially, the combined motion due to Earth's rotation and core convection organize into rotating columns within the outer core, which sustain Earrh's intrinsic magnetic field. (See dynamo theory.)

The geodynamo is NOT because of the inner core rotating relative to the outer core, which it barely does. Because the inner core is a solid within a relativley low viscosity liquid, it can rotate at a slightly different rate. If anything, the electromagnetic forces associated with the dynamo act on the inner core in a way roughly analogous to an induction motor, exerting torque on the inner core and spinning it relative to the outer core. (The inner core never spins at a rate very different from the rest of Earth--inertia, conservation of angular momentum and energy, etc.) Even in this sense, which is not nearly as dramatic or impactful as clickbait would have one believe, the inner core is a passive component.



OlympusMons94 t1_jddydm8 wrote

However, the solid inner core does have a lower percentage of light elements than the molten outer core surrounding it. The light elements preferentially, although not entirely, stay in the molten core. As the inner core grows from the molten core freezing out, the concentration of light elements in the remaining melt gradually increases. The rising of light elements through the remaining liquid core is the main source of energy (and crucially, entropy) and driver of convection that have sustained Earth's dynamo since the inner core first formed some time in the past ~0.5-1.5 billion years (latent heat of freezing is a minor contribution). Operating a dynamo through this mechanism requires that the molten core have cooled enough to start freezing, and overall be compositionally well-mixed, without significant layering (stratification) by density, i.e. light element concentration.

The need to explain the dynamo also relates to the question of how much radiogenic (and thus heat producing) elements, particularly potassium, are actually in the core. The traditional idea, generally suppoeted by geochemistry and minerla physics, is that this amount is negligible. However, with evidence from the rock record of a dynamo for the past 3.5-4.2+ billion years, this leaves a long gap where it is more difficult to explain what drove the geodynamo.

Prior to the formation of the inner core, the compositional convection due to freezing would not have existed to power the dynamo. Therefore, a different mechanism must have powered the early geodynamo. The primordial heat left over from Earth's formation should not, by itself, be enough to sustain thermal convection for billions of years until inner core nucleation. For geophysicists, the long-lived geodynamo is much easier to explain with a thermally driven dynamo supported by the heated generated by radioactive isotopes such as potassium-40. There are, of course, other proposed explanations, such as the the precipitation of light elements near the core-mantle boundar, that is the top of the then-entirely molten core (Mittal et al., 2020; Wilson et al., 2022).

Returning to a more direct possible answer to part of u/VillageNo4 's question, the inner core might be in a 'superionic' state such that the iron metal behave like a solid, while the light elements that did get incorporated into it behave like a liquid (Wang et al., 2021 and He et al., 2022). (See also https://www.sciencenews.org/article/earth-inner-core-superionic-matter-weird-solid-liquid.) The high temperatures and pressures deep in planetary interiors can produce materials that are very exotic compared to what we see in everyday life. (c.f. Jupiter's liquid metallic hydrogen mantle and possible 'solid' core, which if it exists would not have a well-defined surface.).


OlympusMons94 t1_jc3ndu7 wrote

On Earth, volcanic or man-made gases can lead to acid rain, with a bit of strong acids such as sulfuric acid (H2SO4) dissolved in water. On Venus, there is very little water. What little is left has mostly combined with SO3 to form H2SO4. The clouds and rain are made of very concentrated, even anhydrous (no water as a solvent), sulfuric acid. This rain never reaches the surface. Because of the heat, it evaporates before reaching ~30 km altitude. (A phenomenon called virga, which is common with H2O on Earth when the surface air is very dry.)


OlympusMons94 t1_jc3jq6v wrote

CO2 near Venus' surface is a supercritical fluid, which is neither gas nor liquid, but has properties of both. At present, the CO2 is more gas-like, but in the past Venus' surface pressure may have been even higher, possibly enough to support a more liquid-like supercritical CO2 (Bolmatov et al., 2014).

There is also supercritical CO2 within Earth's crust. Supercritical and even actual liquid CO2 are released by hydrothermal vents on the sea floor.


OlympusMons94 t1_jasfhg2 wrote

Sphaghettification is not linked to the event horizon, or necessarily black holes or general relativity at all. For a supermassive black hole, spaghettification would not occur until well within the event horizon. For a stellar black hole, spaghettification would occur outside the event horizon.

Even just strictly following Newtonian mechanics, the tidal forces from being sufficiently close to a sufficiently high and compact mass would stretch you out and rip you apart. Of course, Newtonian gravity is not a very good approximation in situations where that would apply to objects as small as the human body, let alone molecules. (But for large objects, see Roche limit and the "stretch marks" on Mars' moon Phobos caused by tides.)


OlympusMons94 t1_ja08cqp wrote

Like animals, plants are aerobic organisms, and must also consume the sugar they make via aerobic respiration (C6H12O6 + 6O2 -> 6CO2 + 6H2O). So from just making and "burning" food, the mass of water is conserved by plants, less any sugar they store for later use.

(Backing up a bit, in photosynthesis, the oxygen atoms that go into the sugar come from the CO2, while the oxygen from the water (that makes 89% of H2O's mass) is released as oxygen into the surrounding air.)

On average the mass of water in and outside of a plant that isn't growing is in dynamic equilibrium, except for the changes in water temporarily stored in or moving through the plant (e.g., in sap). But for a plant that is growing, including just storing food, its overall mass increases. Most of this mass is carbon and oxygen from CO2, but also some is hydrogen from "destroyed"/"lost" water. (The rest of the water molecule is released as oxygen into the surrounding air.)

The bulk of a plant is composed of carbon, oxygen, and a bit of hydrogen. Some of this is sugar (food) that is temporarily stored for later use. Most of this is cellulose and hemicellulose, which are polymers (long, chemically bound chains) of sugar molecules, which comprise the structure of the plant. (Cellulose has the chemical formula (C6H5O10)n, where n is some big number of the C6H5O10 units. Note that C6H5O10 is a simple sugar, minus 2 H's and an O, or H2O. The combination of simple sugars to make cellulose actually releases water, so that somewhat reduces the net water consumed by a growing plant. But I digress.)

Switching gears entirely, there are many other non-biological factors that affect the amount of water on or above Earth's surface through goelogic time. It's not at all a trivial matter of whether the amount of water is increasing or decreasing through time, or at over a given time peirod. Volcanoes release water from the interior. Chemical weathering of rocks puts some of the water into the chemical structure of minerals. Subduction returns some of the water and "water"-containing minerals to the interior. Some water vapor is broken down into H and OH by ultraviolet sunlight, and some of those (especially the H) escape into space. (Comets and asteroids also deliver a bit of water and hydrated minerals, but beyond the very early Earth, this is negligible.)


OlympusMons94 t1_j9nbq1w wrote

Which shenanigans?

Following US export laws?

Wanting to be paid for services rendered?

Not servicing enemy-occupied territories?

Not being able to instantly keep up with rapid advances and the fog of war to add service to recently-deoccupied territories?

Or just Musk's naive and ignorant tweets about Crimea and referendums that have no more bearing on Starlink or anything else in the real world than him challenging Putin to a duel?


OlympusMons94 t1_j96r48u wrote

Atmospheric composition doesn't matter. Density does some for small objects, but any rock big enough to make the large craters visible in images like this won't be stopped by an atmosphere.

The Moon provides negligible shielding. It covers only a tiny portion of the sky. Hold out your little finger at arm's length. The Moon is half as wide (that's wide, not long) in the sky as that little finger. Imagine how good a shield that tip of your little finger would be. Well, the Moon is smaller and it's not at arm's length. It's almost 400,000 km away. Ther eis a lot of room in between.

Earth is also a much bigger target with much stronger gravity compared to the Moon.

Jupiter is about as likely to send objects toward Earth as divert them away.

Weathering, erosion, and covering with water and sediment (as well as vegetation) because of our thick atmosphere and water are important.

Besides that, Earth has a lot of volcanism to resurface face cratered areas. That is also why the dark lunar maria we can see on the Moon are so lightly cratered compared to the lighter surrounding highlands. The maria are giant plains of frozen lava. (Much of the maria surfaces are still really old, though. A relate dlld point is that there were a lot more eimpacts very early in the solar system's history.)

Lastly, Earth also has active plate tectonics, which deforms craters on land, completely subducts craters on the ocean floor within a couple hundred million years, and is related to Earth's volcanic activity.

Because of geologic activity, Venus, Europa, Enceladus, Io, and Pluto all have surfaces with few large or obvious craters. Their surfaces have all been resurfaced by lava or ice within the past few hundred million years.



OlympusMons94 t1_j6ng2lg wrote

Liquids (and solids) are much less compressible than gases, but they are still compressible. Constant volume (incompressibikity) is just (sometimes) a useful simplifying assumption. (In other contexts like sound/seismic wave speed, it would be, well, complicating to say the least, given that would result in an infinite wave speed.)

If you have a tall enough, a column of metal, or even rock, it will deform under the pressure from its own weight. A penny is just very small and light, and deformation is negligible. Like a solid, a liquid will also not deform without without some force being applied, but the type of deformation is different.


OlympusMons94 t1_j6arh8y wrote

I fail to see what crewed launch/landing of Starship from/on Earth with crew has to do with anything I said. Or for the most part, Falcon Heavy going to the Moon either. Once the upper stage, be it Falcon's or ICPS, performs the TLI burn in LEO parking orbit, its job is done. They don't need to do anything at or near the Moon.(Perhaps you mean Dragon launched by FH, but I'm not suggesting that either.)

What I am saying is that you can't land people on the Moon without a moon lander, which is a spacecraft capable of supporting humans in deep space. SLS and Orion being ready first or not doesn't change that. Between the generic Moon lander requirements, and the requirements imposed on the HLS (by waiting in NRHO for Orion then going back and forth from there to the surface), the HLS must be a very substantial spacecraft.

If the HLS Starship is capable of supporting humans in NRHO and to and from the Moon, it is just as capable of supporting humans in space between LEO and NRHO. The delta-v required to go from LEO to NRHO and back to LEO is much less than required by the actual lander. So a spacecraft identical to the HLS could serve as the ferry between LEO and NRHO. We already have capsules capable of taking crew to and from LEO, and docking with spacecraft (be it the ISS or the HLS copy). Therefore, by the time the HLS is ready and SLS/Orion have a use, SLS/Orion could be replaced by a copy of the HLS and currently existing vehicles.


OlympusMons94 t1_j6akuun wrote

Once again, Starship delays are irrelevant. It isn't and wasn't ever needed as a launch vehicle for a Moon program, nor was SLS. SLS/Orion, or whatever launch vehicle and capsule are used for Artemis, can't do anything useful until they have a working lander, however long that takes. But once Starship HLS is ready, you might as well make the most of it and replace SLS/Orion. (No other proposed HLS is nearly as far along as Starship, or even under contract to NASA yet. But the same could apply to most any hypothetical HLS, given all of the work that is left to it because of SLS/Orion's shortcomings.)

But since you insist: Orion has been in development since 2006, and SLS since 2011. SLS was based on the earlier Ares and Shuttle. Engines are arguably the most difficult part of a launch vehicle. The core stage engines and boosters for SLS were developed in the 1970s. The upper stage is a repurposed Delta III/IV upper stage using an improved version of an engine first developed in the 1950s. After all of that, SLS still flew nearly 6 years after, and cost twice as much as, what was originally planned.

Starship is a brand new and revolutionary vehicle and should be expected to take longer than SLS to develop. The earliest mention of anything like ITS, BFR, or Starship by SpaceX goes back only to 2012, and even mention of hydrogen fueled "Raptor" engines only goes back to 2009 (since 2012, the fuel has been methane). A Starship design similar its to current form (e.g., switching from carbon fiber to stainless steel) didn't even start until late 2018.

Despite all of that, Starship should make its orbital flight well within a year of SLS. The HLS Starship will of course be extra/different, but that was not contracted until 2021, and even then was delayed (at least on the NASA side) by Blue Origin. Orion has yet to fly with a full life support or any docking systems. SLS will still be using its "interim" upper stage through Artemis III.

I'm not sure what the fixation with radiation hardening is. SpaceX and others have all the access they need to NASA'a data and work on radiaiton. (Furthermore, resillience to radiation is also simpler to brute force with the mass and redundancy afforded by Starship.) SLS itself (and Dragon and Starliner) are not operated beyond LEO, so that is no more an issue than for any other rocket (or LEO capsule). Only Orion (or, as I am proposing a second HLS) and the HLS need to be designed for the deep space environment. Yet again, until the HLS is ready, no one from NASA is landing on the Moon. So even supposing it takes 20 years to get a radiation-hardened HLS, that won't change anything. Whenever the HLS design is ready, we might as well use it for all of Artemis' beyond LEO flight.

The funding models for SLS and Starship are also very different. NASA funding for the HLS is milestone based, and only paid after completing previously agreed upon milestones. SLS is funded in advance through Congress (often getting more than the administration requests). All of Boeing et al's costs are paid for plus a bonus, i.e. cost plus. (In theory, their poor performance should nix the plus part, but that didn't happen.)

Edit: typos


OlympusMons94 t1_j69xl7y wrote

SLS and Orion could be replaced by a second HLS Starship and a couple of Dragons or Starliners for ferrying astronauts to and from LEO. All are under contract to NASA right now, and unlike even Orion, Dragon (and hopefully soon Starliner) have actually caried astronauts to dock with another spacecraft. Any argument to the effect of "Starship/HLS hasn't been demonstrated yet" is as irrelevant as SLS/Orion are without a lander.

If we are talking in hind sight, there is no maybe about it. 15-20 years ago, distributed lift with the Atlas V and Delta IV (with Ariane 5 cooperation and an on-ramp for the future Falcon 9/Heavy) could have been used with distributed lift. No new launch vehicle needed to have been developed, let alone SLS or Ares.


OlympusMons94 t1_j2bne1r wrote

No. An induced magnetosphere (like Venus, Mars, Europa, Titan, comets, etc. have) doesn't require or have anything to with the core. It just requires the presence of some kind of atmosphere, in which the magnetic field is to be induced.

An intrinsic magnetosphere (like the Sun, Ganymede, Earth, and the other five planets have) is by definition generated in the interior of a planet, and for rocky/terrestrial planets lile Earth and Mercury this would tend to be in the metallic core (as opposed to the rocky mantle). But gas giants and ice giants generate their intrinsic magnetic fields above their core. For example, Jupiter's and Saturn's magnetic fields are generated in their liquid metallic hydrogen mantles.

An "active core" isn't really a scientific term, and can have different meanings in popular discourse. The usual, better meaning is that there is an active dynamo in the core, generating an intrinsic planetary magnetic field. But the absence of an intrinsic magnetic field and the core therefore not being "active" in this way does not imply the core is solid (let alone not rotating; all cores rotate along with the rest of the planet). There needs to be additional forcing to generate a dynamo. (For example in the case of Earth's core, the freezing out of the inner core causes the outer core to convect. Planetary rotation twists this vertical convective motion into spirals and this combined motion drives the dynamo.)

Often, "active core" is instead or additionally taken to indicate or be synonymous with active volcanism or tectonics. But these are driven by processes in the mantle and crust, and not directly related to the core, let alone the magnetic field. So this idea of an "active core" is "not even wrong".


OlympusMons94 t1_j29lrit wrote

Tectonic and volcanic activity are caused by processes in the crust and mantle. Though the mantle very slowly flows and deforms like tar or putty, it is overwhelmingly solid (albeit very hot) rock. Magma exists only in certain regions of the crust and mantle, and even then mostly as a partial melt in a solid matrix, like water in a sponge, or slush.

The core does provide some heat from below, which helps power that activity, in addition to the heat in the mantle leftover form Earth's formation, and the heat in the mantle and crust generated by radioactive decay. But the core is not directly involved, and it being solid or liquid (it is both, actually, with a solid inner core and molten outer core) wouldn't necessarily preclude or permit volcanic or tectonic activity.


OlympusMons94 t1_j29jrnd wrote

It doesn't. That's just an outdated, incorrect idea.

Fast moving charged particles from the solar wind colliding with the upper atmosphere can gradually strip away some of the atmosphere through a process called sputtering. Magnetic fields shield from and redirect charged particles, so they can reduce this type of atmospheric loss (but planetary magnetic fields also contribute to atmospheric loss in other ways).

The motion (from convection and rotation) of the electrically conducting molten iron in Earth's outer core generates a magnetic field around the planet. Because this magnetic field is generated within the planet, it is described as an intrinsic magnetic field. The idea was that this is required to prevent the solar wind from stripping away the atmosphere.

However, Venus has a very thick atmosphere, and being closer to the Sun is subjected to a stronger solar wind than Earth. Yet, Venus lacks an intrinsic magnetic field (likely because its core, while molten, is not convecting). Because it lacks an intrinsic magnetic field, the upper atmosphere is exposed to the solar wind and its magnetic field, which induces a weak magnetic field in Venus' upper atmosphere. This induced magnetic field in turn protects the atmosphere from sputtering escape more or less like the intrinsic magnetic field would. The induced magnetosphere is not unique to Venus. Any atmosphere, be it Venus', Mars', or a comet's, exposed to the solar wind will develop an induced magnetic field. As such, atmospheric loss from sputtering is relatively insignificant for not only Earth with its intrinsic magnetic field, but for Venus and Mars as well.

What matters more for the ability to retain an atmosphere is ultimately the balance of a planet's gravity against the motions of gas particles caused by uncharged solar radiation, that is light, which is not shielded by magnetic fields. If the energy from sunlight causes the gas particles to reach escape velocity, they are lost to space. This is thermal escape, and encompasses a number of different processes.

Of particular relevance to Mars, ultraviolet light from the Sun breaks apart CO2 and water vapor molecules, producing ions which move faster than Mars' relatively low escape velocity. Venus and Earth have much higher gravity, so have been more able to hold onto their CO2/oxygen and nitrogen atmospheres. (Although at present, Mars isn't losing its atmosphere much faster than Earth or Venus are. It must have lost atmosphere emuch more rapidly in the past, particularly because the younger Sun would have emitted more UV radiation.)

As it is, though, Venus has lost almost all of its surface/atmospheric water because of solar UV and hydrogen escape. The runaway greenhouse effect it experienced evaporated/boiled any oceans, putting the H2O in the atmosphere where it could be broken up into hydrogen and OH/oxygen. Because hydrogen is so light, it is much more easily lost from the atmosphere to thermal escape than heavier gases like nitrogen, oxygen, or CO2.


OlympusMons94 t1_iyjajjv wrote

U-238 can undergo spontaneous fission, which results in the emission of neutrons. If a U-238 nucleus absorbs a neutron, it becomes U-239, which quickly decays to Np-239, and then Pu-239.

But there are other isotopes of Plutonium besides Pu-239. This includes Pu-238, which can also occur naturally in small amounts as a result of double beta decay of U-238.

Yet another naturally occurring isotope of Plutonium is Pu-244, which has a relatively long half-life of 80.6 million years. This is produced by the r-process, and some trace amounts of this have been found in the ocean floor.

ETA: https://en.wikipedia.org/wiki/Plutonium#Occurrence


OlympusMons94 t1_ixxukz1 wrote

G and mass M can be measured with less precision individually, but the product G*M = mu, the standard gravitational parameter, can be measured with higher precision, and is more useful anyway, since most applications require or can use GM, not (necessarily) G and/or M separately.

But unless the distance from the mass is great enough to treat it as a single point mass, then well below even 6 digits of precision, the actual force of gravity at a given location cannot be treated as simply GM/r^2 . So a more precise value of G, M, or even GM, will not, by themselves, give a more accurate answer. Planets and other "spherical" celestial bodies are not perfectly spherical, and have topography, and an internal mass distribution that is not radially symmetric. Depending on the location and reference frame, you also have to consider centrifugal acceleration.

Because of the equatorial bulge and centrifugal acceleration, the effective force felt on Earth's surface is on average 1% lower at Earth's equator than at the poles. The non-uniform gravity field of Earth, and even more so the Moon, must be considered for spacecraft and artificial satellites orbiting these bodies.


OlympusMons94 t1_isyssqt wrote

Asteroids are not a concern. Real asteroids are not like the movies or the classic game. They are spread far apart. Even in the main belt between Mars and Jupiter they average ~1,000,000 km apart. There are a lot of them, so by happenstance you could pass relatively close to one, as New Horizons flew by an asteroid at a distance of over 100,000 km on its way to Jupiter and Pluto. (Or you could expend a potentially significant amount of fuel to deliberately fly by one at a closer but still-safe distance, as Galileo flew within 2,400 km of 243 Ida on its way to Jupiter.)

Micrometeroids are somewhat more of a concern, but mainly in low orbit around a massive object like Earth or Neptune, much less in deep space. (Neptune has faint dusty orbital rings.) For that, something like a Whipple shield would greatly reduce the risk and damage from micrometeoroids and other debris up to ~1 cm in size. (Earth has both natural and artificial debris orbiting it, so shielding is especially important for the ISS, which is spending decades in low Earth orbit.)


OlympusMons94 t1_is2lu3h wrote

If you just went to a place where the free fall acceleration was 9.75 m/s^(2), you wouldn't notice any difference unless you measured it. The average American would weigh about 1 lb or 0.5 kg-weight less than they usually do. Just due to eating and other bodily processes, your weight varies by more than that over the course of a day.

There is nothing special about the *exact* value of 9.8 m/s^(2) (or the standard gravitational acceleration of 9.80665 m/s^(2)) other than that it is a standard chosen for convenient reasons. It is just the approximate average free fall acceleration on Earth's surface. The actual value varies over the surface of Earth because it is not a perfect sphere. Not only are there an equatorial bulge and flattened poles, but there is topography like mountains. The acceleration due to gravity decreases with the square of distance from the center of mass, so it is higher at the poles and lower at the equator, and lowest (on the surface) at mountaintops near the equator. Centrifugal acceleration from Earth's rotation also acts to reduce the effective or net acceleration. Centrifugal acceleration is zero at the poles and highest at the equator. Altogether, the average acceleration the equator is about 1% lower than at the poles. The lowest acceleration on Earth's surface is about 9.76 m/s^(2), at the summit of Mt. Chimborazzo in Ecuador.

Now, if something happened to the physical constants of the universe or even just the properties of Earth so that the average acceleration became 9.75 m/s2, that could be significant--quite possibly an "everybody dies" scenario.