by Chris Peterson » Fri Jul 17, 2015 12:50 am
jluetjen wrote:Chris Peterson wrote:APOD Robot wrote:Suggesting surprising geological activity, they are also likely young with an estimated age of 100 million years or so based on the apparent absence of craters.
Actually, I think the estimate is for a
maximum age of 100 million years, with no bounds yet on how much younger the surface might be (at least until some craters are observed).
Actually, I was thinking the opposite. The age might be greater than that. My reasoning goes like this:
1) The above statement presumes that the crater creation rate would be the same across the solar system. Is this a good assumption?
The assumption about uniform crater creation rate is false, as is your assumption that this applies to the above statement.
2) My understanding is that the craters formed on the inner planets were formed from asteroids, comets and other debris that were displaced from their orbits by the gas giants, either in their original orbits or as the gas giants migrated to other orbits. Alternatively passing stars might also disturb objects in the Ort cloud sending them into the inner solar system
Generally, yes, but why do you speak in the past tense? This is an ongoing process, and it isn't limited to the inner system. Plenty of material is jettisoned into orbits that extend well beyond Pluto, as well.
3) Displaced asteroids and comets can generally get displaced two ways -- inwards or outwards. We'll ignore those objects that get displaced off of the plane of the planets' orbits.
Although there is a concentration of material near the ecliptic plane (or the invariable plane- for our purposes they are essentially the same), there is also a great deal of material orbiting at high inclinations. This material also contributes to collisions.
3a) Inwards: A percentage of asteroids and comets displaced inwards will strike an inner planet or other object creating a crater. As things fall inwards towards the sun, the probability of hitting something would increase. This is because a given arc of orbit will cover less space the closer you get to the sun. To put it differently, if asteroids and comets fall inward in a random distribution, would more of them strike Mercury than Mars for the simple reason that the probability of the falling object and Mars being in the same place and time would be much smaller than in the probability of Mercury and the object? A different way of visualizing it (forgive me, I'm struggling a bit to explain my concept) is that if any portion of Mercury's orbit has equal probability of having a piece of debris in it, (and the same applies to Mars), than all of the space debris falling into Mercury's orbit will be spread out over an area only 27% as large as Mars's, so all other things being equal, Mercury might have ~3x the craters of Mars.
3b) Looking in the other direction, debris scattered outward would be spread over a larger area, and Pluto is a fairly small object, so the probability of it hitting one of these objects would be quite a bit less than the probability of one of the inner planets being hit. In addition to the area covered by Pluto's orbit being quite a bit larger, Pluto is also a fairly small object, the chances of a piece of debris and Pluto being in the same place and time would be quite a bit less.
Yes. But this is well understood, and accounted for in using crater density to estimate surface age.
Given this logic, the we might encounter fewer and fewer craters on objects the further out that you go, to the point were Kuiper belt objects may only have craters on them from random collisions with other Kuiper belt objects, even though the surfaces might be very old and undisturbed.
KBOs still lie inside the Oort cloud, and will be subject to collisions over millions of years. I think the estimate of 100 million years for the maximum surface age is based on a reasonable model of cratering rate
for a body the size of Pluto, in Pluto's orbit. The same formula isn't used here as would be used with, for example, Mars.
[quote="jluetjen"][quote="Chris Peterson"][quote="APOD Robot"]Suggesting surprising geological activity, they are also likely young with an estimated age of 100 million years or so based on the apparent absence of craters.[/quote]
Actually, I think the estimate is for a [i]maximum [/i]age of 100 million years, with no bounds yet on how much younger the surface might be (at least until some craters are observed).[/quote]
Actually, I was thinking the opposite. The age might be greater than that. My reasoning goes like this:
1) The above statement presumes that the crater creation rate would be the same across the solar system. Is this a good assumption?[/quote]
The assumption about uniform crater creation rate is false, as is your assumption that this applies to the above statement.
[quote]2) My understanding is that the craters formed on the inner planets were formed from asteroids, comets and other debris that were displaced from their orbits by the gas giants, either in their original orbits or as the gas giants migrated to other orbits. Alternatively passing stars might also disturb objects in the Ort cloud sending them into the inner solar system[/quote]
Generally, yes, but why do you speak in the past tense? This is an ongoing process, and it isn't limited to the inner system. Plenty of material is jettisoned into orbits that extend well beyond Pluto, as well.
[quote]3) Displaced asteroids and comets can generally get displaced two ways -- inwards or outwards. We'll ignore those objects that get displaced off of the plane of the planets' orbits.[/quote]
Although there is a concentration of material near the ecliptic plane (or the invariable plane- for our purposes they are essentially the same), there is also a great deal of material orbiting at high inclinations. This material also contributes to collisions.
[quote]3a) Inwards: A percentage of asteroids and comets displaced inwards will strike an inner planet or other object creating a crater. As things fall inwards towards the sun, the probability of hitting something would increase. This is because a given arc of orbit will cover less space the closer you get to the sun. To put it differently, if asteroids and comets fall inward in a random distribution, would more of them strike Mercury than Mars for the simple reason that the probability of the falling object and Mars being in the same place and time would be much smaller than in the probability of Mercury and the object? A different way of visualizing it (forgive me, I'm struggling a bit to explain my concept) is that if any portion of Mercury's orbit has equal probability of having a piece of debris in it, (and the same applies to Mars), than all of the space debris falling into Mercury's orbit will be spread out over an area only 27% as large as Mars's, so all other things being equal, Mercury might have ~3x the craters of Mars.
3b) Looking in the other direction, debris scattered outward would be spread over a larger area, and Pluto is a fairly small object, so the probability of it hitting one of these objects would be quite a bit less than the probability of one of the inner planets being hit. In addition to the area covered by Pluto's orbit being quite a bit larger, Pluto is also a fairly small object, the chances of a piece of debris and Pluto being in the same place and time would be quite a bit less.[/quote]
Yes. But this is well understood, and accounted for in using crater density to estimate surface age.
[quote]Given this logic, the we might encounter fewer and fewer craters on objects the further out that you go, to the point were Kuiper belt objects may only have craters on them from random collisions with other Kuiper belt objects, even though the surfaces might be very old and undisturbed. [/quote]
KBOs still lie inside the Oort cloud, and will be subject to collisions over millions of years. I think the estimate of 100 million years for the maximum surface age is based on a reasonable model of cratering rate [i]for a body the size of Pluto, in Pluto's orbit[/i]. The same formula isn't used here as would be used with, for example, Mars.