What is the Earth’s Mantle Made Of?

The Earth's layers, showing the Inner and Outer Core, the Mantle, and Crust. Credit: discovermagazine.com

Like all the other terrestrial planets, (Mercury, Venus, and Mars) the Earth is made up of many layers. This is the result of it undergoing planetary differentiation, where denser materials sink to the center to form the core while lighter materials form around the outside. Whereas the core is composed primarily of iron and nickel, Earth’s upper layer are composed of silicate rock and minerals.

This region is known as the mantle, and accounts for the vast majority of the Earth’s volume. Movement, or convection, in this layer is also responsible for all of Earth’s volcanic and seismic activity. Information about structure and composition of the mantle is either the result of geophysical investigation or from direct analysis of rocks derived from the mantle, or exposed mantle on the ocean floor.

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Could A Planet Be as Big as a Star?

How big do planets get? Can they get star sized?

Everybody wants the biggest stuff.

Soft drink sizes, SUV’s, baseball caps, hot dogs and truck nuts.

Astronomers mostly measure stars in terms of mass and use the Sun as a yard stick. This star is 3 solar masses, that star is 10 solar masses, and so on.

We’re pandering to those of you who want the most massive stuff as opposed to the most volumetric stuff. So if you want the biggest truck, but don’t care if it’s got the most truck atoms in one place, this might not be for you.

How massive can planets get, and where can I order a custom one more massive than a star?

It all depends on what your planet is made of. There are two flavors of planets, gas and rock.

Gas planets, like Saturn and Jupiter are pretty much made of the same stuff as our Sun.

Jupiter’s pretty big, but it’s actually only about 1/1000th the mass of our star. If you made it more massive. by crashing about 80 Jupiters together, you’d get the same amount of mass as the smallest possible red dwarf star.

And all that mass would compress and heat up the core and it would ignite as a star.

Artist's View of Extrasolar Planet HD 189733b
Artist’s View of Extrasolar Planet HD 189733b

Extrasolar planet astronomers have turned up some pretty massive gas planets. The most massive so far contains 28.7 times the mass of Jupiter.

That’s so massive it’s more like a brown dwarf.

But if you had a planet entirely made of rock, like the Earth. It would need to be much, much larger before its core would ignite in fusion.

It would need to be dozens of times the mass of our Sun.

Stars with 8-11 stellar masses can fuse silicon. So a rocky planet would need millions of times the mass of the Earth before it would have that kind of pressure and temperature.

So you could get a situation where you have more mass than the Sun in a rock flavored world, and it wouldn’t ignite as a star. It would get pretty warm though.

No star can burn iron. In fact, when stars develop iron in their core, that’s when they shut down suddenly and you get a supernova.

Feel free to collect all the iron in the Universe together and lump it into a ridiculously huge pile and no matter how long you stare at for, it’ll never boil or turn into a star.

It might turn into a black hole, though.

Artist's impression of Kepler-10c (foreground planet)
Artist’s impression of Kepler-10c (foreground planet)

The largest rocky planet ever discovered is Kepler 10c, with 17 times the mass of Earth.

Massive, but nowhere near the smallest star.

There’s new research that says that heavier elements blasted out of supernovae might collect within huge star forming nebulae, like gold in the eddies of a river. This metal could collect into actual stars. Perhaps 1 in 10,000 stars might be made of heavier elements, and not hydrogen and helium.

Metal stars.

So, it’s theoretically possible. There might be corners of the Universe where enough metal has collected together that you could end up with a star that’s made up of planety stuff. And that’s pretty amazing.

What do you think? If we found one of these giant metal stars, what should we call it?

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A New Look at Apollo Samples Supports Ancient Impact Theory

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New investigations of lunar samples collected during the Apollo missions have revealed origins from beyond the Earth-Moon system, supporting a hypothesis of ancient cataclysmic bombardment for both worlds.

Samples of Apollo 16 breccia that contain chondritic material (JSC)

Using scanning electron microscopes, researchers at the Lunar-Planetary Institute and Johnson Space Center have re-examined breccia regolith samples returned from the Moon, chemically mapping the lunar rocks to discern more compositional detail than ever before.

What they discovered was that many of the rocks contain bits of material that is chondritic in origin — that is, it came from asteroids, and not from elsewhere on the Moon or Earth.

Chondrites are meteorites that originate from the oldest asteroids, formed during the development of the Solar System. They are composed of the initial material that made up the stellar disk, compressed into spherical chondrules. Chondrites are some of the rarest types of meteorites found on Earth today but it’s thought that at one time they rained down onto our planet… as well as our moon.

The Lunar Cataclysm Hypothesis suggests that there was a period of extremely active bombardment of the Moon’s surface by meteorite impacts around 3.9 billion years ago. Because very few large impact events — based on melt rock samples — seem to have taken place more than 3.85 billion years ago, scientists suspect such an event heated the Moon’s surface enough prior to that period to eradicate any older impact features — a literal resurfacing of the young Moon.

There’s also evidence that there was a common source for the impactors, based on composition of the chondrites. What event took place in the Solar System that sent so much material hurtling our way? Was there a massive collision between asteroids? Did a slew of comets come streaking into the inner solar system? Were we paid a brief, gravitationally-disruptive visit by some other rogue interstellar object? Whatever it was that occurred, it changed the face of our Moon forever.

Curiously enough, it was at just about that time that we find the first fossil evidence of life on Earth. If there’s indeed a correlation, then whatever happened to wipe out the Moon’s oldest craters may also have cleared the slate for life here — either by removing any initial biological development that may have occurred or by delivering organic materials necessary for life in large amounts… or perhaps a combination of both.

Timeline for the Lunar Cataclysm Hypothesis (LPI)

The new findings from the Apollo samples provide unambiguous evidence that a large-scale impact event was taking place during this period  on the Moon — and most likely on Earth too. Since the Moon lacks atmospheric weathering or water erosion processes it serves as a sort of “time capsule”, recording the evidence of cosmic events that take place around the Earth-Moon neighborhood. While evidence for any such impacts would have long been erased from Earth’s surface, on the Moon it’s just a matter of locating it.

In fact, due to the difference in surface area, Earth may have received up to ten times more impacts than the Moon during such a cosmic cataclysm. With over 1,700 craters over 20 km identified on the Moon dating to a period around 3.9 billion years ago, Earth should have  17,000 craters over 20 km… with some ranging over 1,000 km! Of course, that’s if the craters could had survived 3.9 billion years of erosion and tectonic activity, which they didn’t. Still, it would have been a major event for our planet and anything that may have managed to start eking out an existence on it. We might never know if life had gained a foothold on Earth prior to such a cataclysmic bombardment, but thanks to the Moon (and the Apollo missions!) we do have some evidence of the events that took place.

Sample of lunar impact melt breccia, showing exterior and chondrule-filled interior. (Click for sample report.) Source: JSC

The LPI-JSC team’s paper was submitted to the journal Science and accepted for publication on May 2. See the abstract here, and read more on the Lunar Science Institute’s website here.

And if you want to browse through the Apollo lunar samples you can do so in depth on the JSC Lunar Sample Compendum site.

Were Martian Rocks Weathered by Water?

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There are many ways rocks can be textured. Wind erosion, water erosion, the escape of volcanic gases during their formation (in the case of igneous rocks)… all these forces can create the pitted textures found on many rocks on Earth… and perhaps even on Mars. And according to a report published by a group of planetary geologists led by James Head of Rhode Island’s Brown University, another method may also be at play on Mars: melting snow.

Here on Earth in the hyper-arid dry valleys of Antarctica, water from melting snow erodes the surfaces of dark boulders, creating pitted textures similar to what has been found at many locations on Mars.

In order for that process to be truly analogous, though, a few conditions would have to be met on the red planet. First, the atmospheric pressure must be high enough to allow water to remain – if only temporarily – in a liquid state. Water that instantly boils away won’t have enough time to chemically attack the rock. Second, the rock itself must be at least warm enough to not freeze the water (again, must be liquid.) And third, there must actually be water, snow or frost present.

While one or more of these factors may be currently present in locations on Mars, they have not yet been found to exist all together in the same place. But that’s just what’s been found now… in Mars’ geologic past these may all have very well existed either in isolated locations or perhaps even planet-wide.

The paper’s abstract states:

For example, increases in atmospheric water vapor content (due, for example, to the loss of the south perennial polar CO2 cap) could favor the deposition of snow, which if collected on rocks heated to above the melting temperature during favorable conditions (e.g., perihelion), could cause melting and the type of locally enhanced chemical weathering that can cause pits.

In other words, if the dry ice at Mars’ south pole had melted at one point, freed-up water vapor could have fallen on rocks elsewhere as snow. If Mars were at a point in its orbit closest to the Sun and therefore experiencing warmer temperatures the snow could have then melted – especially upon darker rock surfaces.

Still, it’s possible – or even probable – that the weathering did not occur at a consistent rate across the entire surface of the rocks. Some sides may have weathered faster or slower than others, depending on how they were exposed to the elements. But if there’s one thing Mars has had a surplus of, it’s time. Even if the processes outlined in the report are indeed the cause of Mars’ pitted rocks, they have likely been in play over many hundreds of millions – even billions – of years.

Read the team’s report on the Journal of Geophysical Research here.

Thanks to Stu Atkinson for his color work on the images from Opportunity. Check out his blog The Road to Endeavour for updates on the rover’s progress.

How Are Rocks Formed?

As a terrestrial planet, Earth is divided into layers based on their chemical and rheological properties. And whereas its interior region – the inner and outer core – are mostly made up of iron and nickel, the mantle and crust are largely composed of silicate rock. The crust and upper mantle are collectively known as the lithosphere, from which the tectonic plates are composed.

It in the lithosphere that rocks are formed and reformed. And depending on the type of rock, the process through which they are created varies. In all, there are three types of rocks: igneous, sedimentary, and metamorphic. Each type of rock has a different origin. Therefore, the question, “How are rocks formed?” begs three distinct answers.

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