Recently I have been looking into planetoids in our asteroid belt and I have found one that caught my interest, Ceres. One of the main points that was said about it was that it had a subterranean ocean. But I'm puzzled as to how astronomers can come to this conclusion. Any explanations would be much appreciated.
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$\begingroup$ Not a complete answer, but density tells us some of that. It's easy enough to calculate density if you can measure the size and gravitational field of an object. Ceres has no significant ice on it's surface cause it's too close to the sun and ice sublimates but a rough estimate of its ice content can be estimated by it's density. Ceres density if 2.16 suggests it's about half ice. en.wikipedia.org/wiki/Ceres_%28dwarf_planet%29 I'm less clear on exactly how they determine liquid vs solid. $\endgroup$– userLTKCommented Oct 23, 2015 at 20:44
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$\begingroup$ So deducing whether the planet has a subterranean ocean by density? Hmm, that sounds like an interesting method. $\endgroup$– The Contextual PathCommented Oct 25, 2015 at 2:10
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$\begingroup$ @Perry_Steven, it's an incomplete method, but it's a starting point. One downside is, we have a hard time getting a good size estimate without getting pretty close. We still don't have a good estimate for the size of several dwarf planets. $\endgroup$– userLTKCommented Oct 25, 2015 at 3:24
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2 Answers
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How they have concluded that there could b.e a subterranean ocean on Ceres is by Spectralscopy.
Spectral Signature can be summarised to like this:
Different elements emit different emission spectra when they are excited because each type of element has a unique energy shell or energy level system. Each element has a different set of emission colors because they have different energy level spacings. We will see the emission spectra or pattern of wavelengths (atomic spectra) emitted by six different elements in this lab. We will then identify an unknown element by comparing the color of the unknown with the flame color of our knowns.
And another:
When something is hot enough to glow (like a star), it gives you information about what it is made of, because different substances give off a different spectrum of light when they vaporize. Each substance produces a unique spectrum, almost like a fingerprint.
So how the scientists would have concluded that there was water because of the water vapours in the atmosphere. It gives off a certain wavelength that could be compared the closest element that gives off the same wavelength which is water.
Then the scientists would have used Galileo's magnetometer(an instrument which measures the strength and direction of magnetic fields) to conclude that there could be an ocean on the dwarf planet. The strength and response of the induced field would tell the scientists a rough estimate of the dwarf planet's sub surface which in return they could deduce the fact the there is a large amount of water present which equals an ocean.
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1$\begingroup$ @CipherBot: Sorry but this is wrong. While spectroscopy helps you to deduce that there is water, that doesn't mean there's a subterranean ocean. Those you find by measuring induced magnetic fields that supposedly come from solved, moving ions in the subterranean ocean. $\endgroup$ Commented Oct 23, 2015 at 9:01
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$\begingroup$ Must have pressed the wrong button and discarded my edit. Well this is the full answer. Apologies for any inconvenience. $\endgroup$ Commented Oct 23, 2015 at 9:22
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1$\begingroup$ @CipherBot: Your answer still doesn't establish how the presence of water can be detected by a magnetometer.. Sorry to be so picky, but I think this is a good question and deserves a detailed answer. Shall I write one? $\endgroup$ Commented Oct 23, 2015 at 9:25
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$\begingroup$ @AtmosphericPrisonEscape sure you can. $\endgroup$ Commented Oct 23, 2015 at 10:05
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$\begingroup$ Well, I think it now answers my question. Thanks again for your input CipherBot, AtmosphericPrisonEscape, and UserTLK. It's much appreciated. $\endgroup$ Commented Oct 27, 2015 at 6:30
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From what I have read including this and this and this, some models predict a layer of liquid water. The presence of water (liquid or ice) is predicted based on the density measurement of 2.1 g/cm3 which is a little more than Ganymede, Titan, and Callisto and considerably less than Europa. Early in Ceres history it's interior would have been hotter due to the residual heat from formation and decay of radioactive elements and there would have been a large amount of liquid water. As Ceres cooled with age by way of convection of heat to the surface and lessening radioactive decay, most or all of the subsurface liquid water would have frozen.There may still be a layer of liquid water due to some remaining interior heat and the mixing of materials such as ammonia in the water that would reduce the freezing temperature. As far as surface water vapor detection goes, there is a chance that there is some "cryovolcanism" (water volcanoes) on Ceres (like Enceladus and Europa), which would indicate subsurface water, but it is thought to be from ice sublimation on the surface similar to what happens on comets. According to one of the articles I read above, scientists believe that when Ceres is closest to the sun this sublimation occurs and when farther away it doesn't.
answered Oct 31, 2015 at 14:17