Solar System - Mars

[Airmid]

Lately I've been reading about the formation of the solar system and so many questions presented themselves! Here's one: how come Mars is so small?

 

Common sense tells me that the further from the Sun, the larger a planet can be. Please tell me if my intuition is right!

Yet Mars is a lot smaller than Earth. One possible explanation I came across has to do with Jupiter. I read that the presence of Jupiter probably inhibited the formation of a planet in the asteroid belt: if a planet would have formed there, Jupiter's gravity would have pulled it out of its orbit. Would the same have been the case if Mars had grown somewhat larger?

 

Another thing I don't understand is how Mars' orbit can be stable. It could be that my basic understanding of gravity is lacking. The way I see it, all planetary bodies in our solar system are falling towards the Sun. The orbital speed they have is crucial there: if a body moves too slow, it will eventually collide with the Sun, and if it is too fast, it will drift away from the Sun. Correct? If so, is there a formula that gives the relationship between mass, distance from the Sun, and orbital speed? If so, does Mars conform to this relationship, or is it actually Jupiter that keeps Mars in orbit?

 

Sorry for asking so many, probably basic, questions!

Airmid.

[Y-S]

well i mean there are other planets that's smaller than mars..........

[Airmid]

well i mean there are other planets that's smaller than mars..........

 

Officially..... only one: Mercury. But Mercury's size makes sense to me, because it is so much closer to the Sun, and the Sun would have "eaten up" most of the potentially planet-forming dust. (Again, I could be very wrong.) Beyond Jupiter, planets are getting smaller again, which makes sense to me too, because the planetary dust cloud would have been much thinner in the perifery. But Mars seems to be the odd one out. Of course, there's the asteroid belt beyond Mars, so the mass was available in that area of the solar system.

 

By the way, I found the orbit formula and found that distance from the sun and orbital speed are directly related; and that the mass of the object plays no role in the formula. Is this because the planets are so light compared to the Sun?

 

Airmid.

[J'Dona]

But Mercury's size makes sense to me, because it is so much closer to the Sun, and the Sun would have "eaten up" most of the potentially planet-forming dust.

Don't forget, many extrasolar planets being discovering now (most in fact) break this trend. Some planets with many times the mass of Jupiter have been found so close to their star and moving so quickly that they complete an orbit in a matter of days, like this one.

 

You originally say: "Common sense tells me that the further from the Sun, the larger a planet can be." Do you mean this in that there is more space for it to fit without interfering with other planets? Because aside from that effect, which is usually negligable (but still possible as in the case of Jupiter and the asteroid belt), there isn't much of a relationship at all: small bodies can exist anywhere from very close (like Mercury) to beyond the orbit of Neptune, and gas giants can be extremely close, middle-of-the-range, or so far and so massive that they undergo nuclear fusion and we have a binary star system. We could try to deduce trends of planet formation based upon our own solar system, but as soon as other systems enter the frey those predictions would probably fall apart.

 

Another question you might ask is: if the distribution of dust and gas in the proto-solar system was smooth and exactly proportionalo to distance from the sun, so that we could make assumptions based upon it alone, then why did planets form at all?

[Janus]

 

By the way, I found the orbit formula and found that distance from the sun and orbital speed are directly related; and that the mass of the object plays no role in the formula. Is this because the planets are so light compared to the Sun?

 

Airmid.

 

Yes it is. Or more correctly, if the mass of the planet is very small as compared to the mass of the Sun then the contribution of the planet's mass can be safely ignored.

 

When the planet's mass becomes large enough you need to use the following formula to get an accurate answer.

 

 

 

[math] P = 2\pi\sqrt{\frac{a^3}{G \left(M_1 + M_2\right)}} [/math]

 

How large is "large enough" depends on what degree of accuracy you need.

 

For instance, with the Earth's orbit, ignoring Earth's mass causes a difference of only about 70 seconds or about 0.00015%. If you needed to calculate Earth's orbital period finer than this then you need to include its mass. (However, in this case you also need to know the Earth's average dist from the Sun, the mass of the Sun, etc to equal accuracies first)

[jackson33]

Lately I've been reading about the formation of the solar system and so many questions presented themselves! Here's one: how come Mars is so small?

 

Common sense tells me that the further from the Sun, the larger a planet can be. Please tell me if my intuition is right!

Yet Mars is a lot smaller than Earth. One possible explanation I came across has to do with Jupiter. I read that the presence of Jupiter probably inhibited the formation of a planet in the asteroid belt: if a planet would have formed there, Jupiter's gravity would have pulled it out of its orbit. Would the same have been the case if Mars had grown somewhat larger?

 

Another thing I don't understand is how Mars' orbit can be stable. It could be that my basic understanding of gravity is lacking. The way I see it, all planetary bodies in our solar system are falling toward the Sun. The orbital speed they have is crucial there: if a body moves too slow, it will eventually collide with the Sun, and if it is too fast, it will drift away from the Sun. Correct? If so, is there a formula that gives the relationship between mass, distance from the Sun, and orbital speed? If so, does Mars conform to this relationship, or is it actually Jupiter that keeps Mars in orbit?

 

Sorry for asking so many, probably basic, questions!

Airmid.

 

gravity has a lot to do with total mass and speed of objects. our solar system is 99.80% the sun and all else orbits the sun. during formation what produced the sun had debris. has the sun cooled and took shape, everything not traveling certain speeds either fell into the sun of left the system, for awhile no doubt mass made some difference on which. our moon, though in duel orbit (earth and sun) both travel at about 20 MPS in there orbits. things closer to the sun travel much faster and the further you go outward, matter orbits at slower speeds. down to 3 mps for Pluto. there is no direct link to mass or what your thinking (pull of mass on gravity).

 

actually all matter, planets-moons-asteroids and comets are in a form of free fall toward the sun. much as what we send into orbit, will eventually fall back to earth. your next question would be the moon, but this has duel orbit and has been leaving earths orbit a couple inch per year.

 

there are many that question the accuracy of gravity as it reacts different outside our solar system. my guess it lies in speed of all things and a total missing factor. we travel the sun at a very fast pace, the sun orbits the core of the Milky Way at even faster speeds, our little four galaxy cluster travels as well. some think dark matter is the missing factor. my though is somewhat different.

[Edtharan]

Common sense tells me that the further from the Sun, the larger a planet can be. Please tell me if my intuition is right!

Just because something is a certain way, doesn't mean that it has to be that way in all cases.

 

What I am saying is that just because in our solar system there is a general trend for the planets to start small, get bigger and then taper off to small(ish) again doesn't mean that it has to be that way.

 

Planet formation (as far as we understand it) is a very chaotic process. There is fairly good evidence that a "planet" around the size of Mars (although not Mars it's self), collided with the Earth. This evidence is the Moon. The Moon is thought to be a chunk of Earth that got smashed out during this collision and is backed up by the composition of the rocks (they are almost identical to the rocks of earth, only that they have very little water in them).

 

So here we have an entire planet that was "whizzing" around the solar system and has now left it. This means that the position of a planet does not necessarily have to be where it formed.

 

This means that the current positioning of a planet within the solar system is not necessarily where it originated from. Also, we are also fairly certain that the current positions of the planets will not be the final positions of the planets, there is even a distinct (and fairly likely) chance that the solar system will loose one of the inner planets (possibly Mars or even Earth), although not for quite some time yet (different models give different answers for the time but it could be around 10 billion years - and note that the earth is only 4.5 billion years old).

 

The size of a planet is dependent on the amount of matter available in the area where it originated which is influenced by the gravitation of the sun and any other planets forming nearby.

 

It is hard to give a definitive answer to how and where Mars formed as, to put it bluntly: We don't exactly know. But it most likely formed not too far (that is not in the outer solar system) from where it is today.

[Airmid]

Thanks folks, for putting me right. It took me a while to wipe out my misconceptions about gravity. It's amazing how important gravity is when you're trying to understand our solar system; not only for the formation of planetary bodies, but also for atmospheres and the "layered" composition of the planetary bodies. I think I'm on the right track now!

 

Don't forget, many extrasolar planets being discovering now (most in fact) break this trend. Some planets with many times the mass of Jupiter have been found so close to their star and moving so quickly that they complete an orbit in a matter of days, like this one.

Wow, that's an awesome example, especially because that sun is so very close in mass to our Sun! Do you have any idea how this sun compares in heat output to ours, or what the temperature of this Jupiter-like planet is?

 

You originally say: "Common sense tells me that the further from the Sun, the larger a planet can be." Do you mean this in that there is more space for it to fit without interfering with other planets? Because aside from that effect, which is usually negligable (but still possible as in the case of Jupiter and the asteroid belt), there isn't much of a relationship at all: small bodies can exist anywhere from very close (like Mercury) to beyond the orbit of Neptune, and gas giants can be extremely close, middle-of-the-range, or so far and so massive that they undergo nuclear fusion and we have a binary star system. We could try to deduce trends of planet formation based upon our own solar system, but as soon as other systems enter the frey those predictions would probably fall apart.

Originally this was based on two ideas: the idea that heavier planets would be attracted more intensely by the Sun (I know now that this is neglegible); and the availability of planet-building material. The center of the contracting dust disk would have been much denser than the perifery, but the emerging Sun would have vacuumed out the center much more than the perifery, so less material would remain to form planets close to the Sun. Again, I know now that this idea is largely wrong too.

 

Another question you might ask is: if the distribution of dust and gas in the proto-solar system was smooth and exactly proportionalo to distance from the sun, so that we could make assumptions based upon it alone, then why did planets form at all?

I would say now that the distribution of matter in the proto-solar system isn't really important; what matters more is the distribution of the velocities of the particles and molecules. Would you say I'm closer to the mark this time?

 

Planet formation (as far as we understand it) is a very chaotic process. There is fairly good evidence that a "planet" around the size of Mars (although not Mars it's self), collided with the Earth. This evidence is the Moon. The Moon is thought to be a chunk of Earth that got smashed out during this collision and is backed up by the composition of the rocks (they are almost identical to the rocks of earth, only that they have very little water in them).

*nods* There's pretty convincing evidence. Another good bit of evidence is the low iron content on the Moon.

 

So here we have an entire planet that was "whizzing" around the solar system and has now left it. This means that the position of a planet does not necessarily have to be where it formed.

 

This means that the current positioning of a planet within the solar system is not necessarily where it originated from.

I have been reading that the water content of a planetary body gives us a pretty good idea where it has been formed. Would you agree with that? Also, is there evidence that Mars has experienced a similar violent past?

 

Thanks again, folks!

Airmid.

[Edtharan]

*nods* There's pretty convincing evidence. Another good bit of evidence is the low iron content on the Moon.

Actually, the rocks of the moos most closely resemble the rocks of the Earth's Mantle.

 

have been reading that the water content of a planetary body gives us a pretty good idea where it has been formed. Would you agree with that? Also, is there evidence that Mars has experienced a similar violent past?

Yes, the water content can be used as a rough guide to where the planet formed. But it by no means an absolute guide.

 

As for Mars experiencing a violent history, nearly all planets experienced the "Late Heavy Bombardment". This was a period of time around 4.5 billion years ago where they was a "Late Heavy Bombardment" of the detritus from the planetary formations (we call these asteroids and comets).

 

So, yes, Mars did experience a violent period of asteroid and cometary activity around the same time that the earth did, but it is unlikely that it was hit by another "planet" (or planet like body) how Earth was.

 

The main reason for this is there is no large Moon. Mars only has the two small Moons (most likely captured asteroids or comets) and they are of different composition to Mars it's self.

[young-albert]

i think your idea "Common sense tells me that the further from the Sun, the larger a planet can be" is wrong as then..what will you say about pluto??

[Edtharan]

I have been reading that the water content of a planetary body gives us a pretty good idea where it has been formed.

I just remembered another thing about Mars. Mars has no ozone layer. This means that a lot of UV radiation will reach the surface. Water, in the presence of UV radiation can break down into Hydrogen and Oxygen.

 

The lighter a planet its the harder it is for it to retain lighter gasses. This is because of equation E=M*V (and V=E/M). So a big, slow gas molecule will hit a lighter gas molecule and give it a much higher velocity.

 

If this velocity is greater than the escape velocity of the planet, the gas molecules will escape and never return. For Earth gaseous Hydrogen will easily escape, so we wouldn't expect much (if any) gaseous hydrogen in our atmosphere (and what a surprise, we don't find it ).

 

For Mars, iirc, it is too light to even retain Oxygen, so any water on Mars would be broken down into Hydrogen and Oxygen Gas that would then escape into space (water under the surface and protected form UV radiation is another matter altogether).

[weknowthewor]

some factors affecting the size r gravitational force, temperature, amount of water, presence of ozone layer, etc..