Re: [meteorite-list] CORRECTIONS TO Dwarf Planet 'Becoming A Comet'
Hi, Just to clear up a few things: EL61 is a big rock with a thin layer of snow. But it's so big and there's so much snow on a surface that size, that it amounts to all those Hale/Bopps. Some of my arithmetic last night was wrong (note to self: put on glasses, use calculator, dummie!), but the correct figures don't change the picture for the better. EL61's surface area is hard to calculate, given its odd shape, but it's about 9-10 million km^2. How do you measure the surface of that tri-axial shape? It would be just under 8 million km^2 if it were an efficient sphere, or 13 million km^2 if it were a box. EL61's shape has more surface than a sphere of the same volume, but less than the box it would fit in. Theories of packing efficiencies say a long egg is about 83% efficient, which would make EL61 about 10 million km^2. A Hale/Bopp sized object is about 33,500 km^3, so every kilometer of depth of ice on EL61 is equal to 298.5 Hale/Bopp sized objects. Every ten feet of snow on EL61 is equal to one Hale/Bopp. And every 45 cm of depth is a layer of material equivalent to the entire mass of the interplanetary dust of the Zodaical Light. If the ice on EL61's surface was one kilometer deep, its total volume would be the equivalent volume of a 270-kilometer diameter comet, a pure iceball. If the ice is ten kilometers deep, it would be the equivalent volume of a 580-kilometer diameter iceball. As huge as that volume of ice is, it's nothing compared to the total volume of 2003 EL61, which is 1,760,000,000 km^3. Another critical factor is that those volatiles are all spread out on a vast surface with the maximum ability to intercept the sun's rays, a surface of a body that is spinning so fast that every point crosses the nightside in 2 hours, guaranteeing full exposure of most of the surface and an average exposure of 50% everywhere. This is less mass than I calculated (too quickly) last night, but still more than enough to produce the results I described. I hate when you're off by more than an order of magnitude too big, but when you discover and correct it, the results are just as lousy and discouraging as before. Really annoying, and just as dangerous. Could the ice on EL61 just be very shallow, so that there's no big deal? The only source of crystalline ice (snow) on such a world is water geysers which must be driven by internal heat and pressure, like Enceledus, the moon of Saturn. This argues for some good depth of ice to reach or generate that heat and pressure. With an albedo of 0.70, the surface of 2003 EL61 is a mixture of new-fallen crystalline snow (albedo 0.90) and older icy surfaces (albedo 0.67). This suggests about 20% of 2003 EL61's surface is new fallen snow. Crystalline ice (snow) has destinctive spectral characteristics that ice (old and solid) does not. EL61's got it; its moons do not. If 20% of EL61 is covered with new snow, that suggests a fair rate of geological activity. If we had an orbiter watching it, it would probably find an active geyser or two going at any one time... Brown is using the term become a comet to describe the appearance, not the character, of 2003 EL61 if it entered the inner solar system. It confuses the listener, because he doesn't mean it IS a comet. What a comet is, is in flux right now, with all the recent missions and recoveries going on. They do not appear to be the traditional dirty snowball, but much more asteroidal. Conversely, we keep finding asteroids that may be comets. Ultimately, I think all the Small Bodies are similar with a range of volatiles that is not as wide as we thought. Comets are rockier; asteroids are wetter, than believed. The difference may be between hot and cold asteroids, rather than asteroids and comets. Sterling K. Webb --- - Original Message - From: Rob McCafferty [EMAIL PROTECTED] To: Sterling K. Webb [EMAIL PROTECTED]; meteorite-list@meteoritecentral.com Sent: Saturday, February 03, 2007 4:25 PM Subject: Re: [meteorite-list] Dwarf Planet 'Becoming A Comet' (2003 EL61) Apologies for taking selected bits. Hope it's not out of context. --- Sterling K. Webb [EMAIL PROTECTED] wrote: ''2003 EL61 is a very bright body, reflecting 70% of the light that falls on it, and it is indeed, as you would suspect from this brightness, covered with water ice. BUT, it's not old water ice, but new, freshly fallen crystalline ice, otherwise known on our planet as snow'' Curiously, Halley's comet has an abledo of less than 4%, less than that of coal or black velvet. While Halley is not necessarily typical of comets, it is agreed that comets are very dark objects. Nucleus[nuclei] sizes have been estimated by removing modelled coma brightnesses from Hubble images and for nearby comets radar measurements seem to confirm the low albedo. Cometary dust may begin as silicate grained materials mantled with organic matter. To
Re: [meteorite-list] CORRECTIONS TO Dwarf Planet 'Becoming A Comet'
Fascinating reading. I can't help but feel that the use of the word comet is typical of scientists attempts to engage the layman. In this case, I accept the term 'becoming a comet' in a context of trying to engage the common plebian [we call them 'the Sun' Readers in the UK] The average Sun reader thinks they know what a comet is. If they are paying taxes for scientists to explain stuff then they want it in terms they think they understand. If the scientist try to get all superior then it's not on. That'd suggest the scientist have one up on the guys who pay their salaries and that's a no-no. I accept that 2003 EL61 is a fascinating object but a comet it ain't. Again I refer to the non-typical Halley as an example. The density of this comet is .25g/cm^3. Much less than 2003EL61. With an essentially rocky core, this is surely a different class of object. Fascinating discussion. [I continue this until we can establish once and for all whether CI or CM are cometary] Rob McC --- Sterling K. Webb [EMAIL PROTECTED] wrote: Hi, Just to clear up a few things: EL61 is a big rock with a thin layer of snow. But it's so big and there's so much snow on a surface that size, that it amounts to all those Hale/Bopps. Some of my arithmetic last night was wrong (note to self: put on glasses, use calculator, dummie!), but the correct figures don't change the picture for the better. EL61's surface area is hard to calculate, given its odd shape, but it's about 9-10 million km^2. How do you measure the surface of that tri-axial shape? It would be just under 8 million km^2 if it were an efficient sphere, or 13 million km^2 if it were a box. EL61's shape has more surface than a sphere of the same volume, but less than the box it would fit in. Theories of packing efficiencies say a long egg is about 83% efficient, which would make EL61 about 10 million km^2. A Hale/Bopp sized object is about 33,500 km^3, so every kilometer of depth of ice on EL61 is equal to 298.5 Hale/Bopp sized objects. Every ten feet of snow on EL61 is equal to one Hale/Bopp. And every 45 cm of depth is a layer of material equivalent to the entire mass of the interplanetary dust of the Zodaical Light. If the ice on EL61's surface was one kilometer deep, its total volume would be the equivalent volume of a 270-kilometer diameter comet, a pure iceball. If the ice is ten kilometers deep, it would be the equivalent volume of a 580-kilometer diameter iceball. As huge as that volume of ice is, it's nothing compared to the total volume of 2003 EL61, which is 1,760,000,000 km^3. Another critical factor is that those volatiles are all spread out on a vast surface with the maximum ability to intercept the sun's rays, a surface of a body that is spinning so fast that every point crosses the nightside in 2 hours, guaranteeing full exposure of most of the surface and an average exposure of 50% everywhere. This is less mass than I calculated (too quickly) last night, but still more than enough to produce the results I described. I hate when you're off by more than an order of magnitude too big, but when you discover and correct it, the results are just as lousy and discouraging as before. Really annoying, and just as dangerous. Could the ice on EL61 just be very shallow, so that there's no big deal? The only source of crystalline ice (snow) on such a world is water geysers which must be driven by internal heat and pressure, like Enceledus, the moon of Saturn. This argues for some good depth of ice to reach or generate that heat and pressure. With an albedo of 0.70, the surface of 2003 EL61 is a mixture of new-fallen crystalline snow (albedo 0.90) and older icy surfaces (albedo 0.67). This suggests about 20% of 2003 EL61's surface is new fallen snow. Crystalline ice (snow) has destinctive spectral characteristics that ice (old and solid) does not. EL61's got it; its moons do not. If 20% of EL61 is covered with new snow, that suggests a fair rate of geological activity. If we had an orbiter watching it, it would probably find an active geyser or two going at any one time... Brown is using the term become a comet to describe the appearance, not the character, of 2003 EL61 if it entered the inner solar system. It confuses the listener, because he doesn't mean it IS a comet. What a comet is, is in flux right now, with all the recent missions and recoveries going on. They do not appear to be the traditional dirty snowball, but much more asteroidal. Conversely, we keep finding asteroids that may be comets. Ultimately, I think all the Small Bodies are similar with a range of volatiles that is not as wide as we thought. Comets are rockier; asteroids are wetter, than believed. The difference may be between hot and cold asteroids, rather than asteroids and comets. Sterling K. Webb
