Why is the density of Mercury 40% larger than the density of Mars?

[Bjarne]

Why is the density of Mercury 40% larger than the density of Mars?

 

[granpa]

because its mostly iron?

 

[mgb_phys]

Why is still up in the air!

It could be that it was hit by something early in it's life that knocked off most of it's out crust. A similair thing happened to Earth forming the moon, the Earth's density is even high than mercury's and the moon is composed of Earth's early crust.

It could also be that radiation pressure form the sun stripped off the lighter outer layers as the planet was being formed.

 

[granpa]

because its smaller? perhaps all the planets started out that way

 

[Bjarne]

All planets are made of the same dust, so the density "should" be the same.
( I do not believe that something have knocked off the surface of Mercury )

Off course also the inner heat and pressure de play a role for the density, do someone have a idea off how much?

 

[granpa]

evidently quite a bit. Earth has a density of 5.5 yet is made mostly of rocks which on the surface have a density of just slightly over 3. the density of iron is 7.87 and makes up about (3400/6330)^3=0.155 of the earth. this gives an expected density for the Earth of 3.755 (so pressure is probably a major factor)

mercury is also dense. 5.43
mars is twice as massive yet much less dense. 3.94
so the density of mercury probably isn't due to pressure.

http://en.wikipedia.org/wiki/Structure_of_the_Earth#Core

 

[granpa]

All planets are made of the same dust, so the density "should" be the same.
( I do not believe that something have knocked off the surface of Mercury )

Off course also the inner heat and pressure de play a role for the density, do someone have a idea off how much?

rocks that collide pulverize one another while metal objects that collide may stick together. so i would expect that the first sizable objects to form in the early solar system would be metal. only later would rock dust gravitate onto them. even then it might be more prone to being blasted back off into space.

 

[Bjarne]

so i would expect that the first sizable objects to form in the early solar system would be metal

But metal is created under pressure. That mean less mass = less metal.
BUT this is not the way it seems to work at Mercury?

We can off course guess what causes the different density.
But guessing is to my opinion the last option - Can we get this any closer?

Secondly!
The inner heat of Venus and Mercury is much higher, because it has always been “trapped”.

If these two planets were orbiting in Mars (cold) orbit, - these two planets would shrink, and the density of these 2 hot planet would have been even greater,.

Hence “the mystery” would also be greater, - because Mercury would have an even larger density than now, also the density of Venus would increase and probably be greater than the density of the Earth. This would happen even though Venus and Mercury both are smaller than the earth, and even though both planets also have smaller pressure.

To make a long history short:
Both Venus and Mercury have in reality larger density than the Earth.
It should have been the opposite, because the Earth has the greatest pressure.

How much will inner heat and pressure really affect the density of the planets?

 

[granpa]

metal is indeed produced under pressure but in stars not planets.

heat does cause things to expand slightly but not that much.

 

[Bjarne]

Iron is so fare I understand produced inside a planet
But not metal heavyer than that
right ?

 

[granpa]

Iron is so fare I understand produced inside a planet
But not metal heavyer than that
right ?

um...inside stars.

 

[Borek]

http://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry57.html

 

[LURCH]

I have always assumed that, when the early solar system was forming from a cloud of gas and dust particles, the heavier bits were more toward the center. This would be especially true after the sun ignited, blowing the lighter elements away.

Is this contrary to the current mainstreanm model of Solar System formation?

 

[Enthalpy]

Also my opinion, that lighter elements were blown away.
Also, denser elements tend to have a higher melting and boiling point, so a nearer Sun evaporates lighter ones.

Less mainstream: if at some epoch elements behave like a gas, then the Sun's (works with planets as well) gravity makes a wonderful elements and isotopes separator, with 30km/s speed instead of 300m/s in uranium enrichment. I like this one for explaining isotopic compositions of planets.

"Little" drawback: when, where, how should elements be gases?

 

[Chaos' lil bro Order]

Why is still up in the air!

A similair thing happened to Earth forming the moon, the Earth's density is even high than mercury's and the moon is composed of Earth's early crust.
.

You really shouldn't present what is 1 of 4 leading theories on the moon's formation as the verbatim fact.

 

[Algr]

But not metal heavyer than that

There are many elements heavier then iron, but stars produce LOTS of iron, and less of other things.

I recall someone theorized that Mercury was once a jovian planer that had all it's atmosphere blown off from being too close to the sun.

All the planets are quite different from each other... maybe it was just random chance how each one turned out.

 

[Widdekind]

Also my opinion, that lighter elements were blown away.
Also, denser elements tend to have a higher melting and boiling point, so a nearer Sun evaporates lighter ones.

Less mainstream: if at some epoch elements behave like a gas, then the Sun's (works with planets as well) gravity makes a wonderful elements and isotopes separator, with 30km/s speed instead of 300m/s in uranium enrichment. I like this one for explaining isotopic compositions of planets.

"Little" drawback: when, where, how should elements be gases?

The Sun, and all the planets, formed from a collapsing clump of gas & dust, called a "Nebula". At those low densities (~1000 per m³ ?) and temperatures (~10 K ?), all elements are gases.


I believe it is Consensus that planetoids forming closer to stars lose more of their volatiles, leaving behind an increasingly "over dense" core, like Mercury. Please consider the Galilean Moons of Jupiter:

the characteristics of these worlds [= Jovian moons] are consistent with a decreasing average density with increasing distance from Jupiter, implying that the relative amount of water-ice crust increases w.r.t. the rock core... Jupiter must have been hotter in the past than it is today, [so] Io would have been close enough to have had most of its volatiles evaporate away. Moving progressively farther out, Europa would have been able to hold onto some water, Ganymede even more, and Callisto (being the coldest of the Galilean moons at the tie of its formation) would have retained the largest percentage of volatiles.

The consequences of this evolution can be seen in each of the Galilean moons. Consider them in sequence beginning with the one farthest from Jupiter. Callisto apparently cooled and solidified rapidly after material accreted out of the local Subnebula around Jupiter. As a result, its surface continued to collect dust as the nebula thinned, blanketing the moon w/ dark material. Having solidified in the early stages of the formation of the solar-system, Callisto was also subject to frequent impacts of the still-abundant objects that traveled among the newly formed planets and moons. Evidence of the nebular dust accretion and the impacts remains today.

Ganymede, solidifying somewhat more recently, has a newer surface than Callisto's*...

Thus, since temperature increases towards the center of collapsing (sub)Nebulas, planetoids condense out of the gaseous phase from the outside in, from colder to hotter. Thus, Io (closest to Jupiter) is the densest Jovian moon, while Mercury (closest to Sun) is the densest planet. Just like Jupiter's Io, the Sun's Mercury condensed (millions of years ?) after Mars, the Moon, Earth & Venus. By that time, all the "lighter elements were blown away", precisely as you say. This left Mercury "under-massed" (less material left) and "over-dense" (it was mostly iron, nickel, and dense rocks).

* Carroll & Ostlie. Introduction to Modern Astrophysics, pp. 837-38.​

 

[Widdekind]

Here is another, relevant thread:

https://www.physicsforums.com/showthread.php?t=238190&highlight=formation+zone"​

Several posters report that the Sun did, indeed, blow out most of the gas & dust in the Inner System, leaving behind only the proto-planetary cores, from which formed the rocky planets.

 

[Widdekind]

The Solar System formed from the outside inwards:

Radiometric dating indicates that the asteroids formed early (four million years after the sun did), followed by the formation of Mars (10 million years after), then Earth (50 million years after*)​

* http://www.sciam.com/article.cfm?id=the-genesis-of-planets&page=6​

 

[Widdekind]

In their inner disks, Stars vaporize even heavy metals:

"Dust” in this context simply means microscopic bits of water ice, iron and other solid substances... Surrounding each star is a rotating disk of leftover material, the wherewithal for making planets. Newly formed disks contain mostly hydrogen and helium gas. In their hot and dense inner regions, dust grains are vaporized; in the cool and tenuous outer parts, the dust particles survive and grow as vapor condenses onto them.

Astronomers have discovered many young stars that are surrounded by such disks. Stars between one million and three million years old have gas-rich disks, whereas those older than 10 million years have meager, gas-poor disks, the gas having been blown away by the newborn star or by bright neighboring stars. This span of time delineates the epoch of planet formation. The mass of heavy elements in these disks is roughly comparable to the mass of heavy elements in the planets of the solar system, providing a strong clue that the planets indeed arose from such disks.

http://www.sciam.com/article.cfm?id=the-genesis-of-planets