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

1. Oct 6, 2008

### Bjarne

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

2. Oct 6, 2008

### granpa

because its mostly iron?

3. Oct 6, 2008

### 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.

4. Oct 6, 2008

### granpa

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

5. Oct 7, 2008

### 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?

6. Oct 7, 2008

### 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 isnt due to pressure.

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

Last edited: Oct 7, 2008
7. Oct 7, 2008

### granpa

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.

8. Oct 7, 2008

### Bjarne

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?

9. Oct 7, 2008

### granpa

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

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

10. Oct 7, 2008

### Bjarne

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

11. Oct 7, 2008

### granpa

um...inside stars.

12. Oct 7, 2008

### Staff: Mentor

13. Oct 7, 2008

### 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?

14. Nov 11, 2008

### 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?

15. Nov 12, 2008

### Chaos' lil bro Order

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

16. Nov 12, 2008

### Algr

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.

17. Dec 11, 2008

### Widdekind

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:
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.

18. Dec 11, 2008

### Widdekind

Here is another, relevant thread:

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.

Last edited by a moderator: Apr 24, 2017
19. Dec 12, 2008

### 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*)

20. Dec 12, 2008

### Widdekind

In their inner disks, Stars vaporize even heavy metals: