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First Observation Of Aurora On Jupiter
Doctor Fishboy writes: "Aurora are not restricted to our planet -- BBC news reports the sighting of Aurora on Jupiter for the first time. It looks as if it was a happy coincidence that the Hubble Space Telescope was looking at Jupiter when the aurora happened."
Aurora have been visible on jupiter for a long time, at least galileo has been imaging them for 5 years. Furthemore ground based te4lescopes have observed the aurora - although not in the same detail as the HST.
What's really interesting is the observation of a flash in the images lasting about 5 minutes and having an estimated energy of ~10^17 - 10^18 joules - 20 -200 megatonnes. This story neglects to mention this observation, but I've seen it elsewhere. Some people think this is an auroral flare, while others think it make be a small meteor impact (it'd have to be really small - impact energies at jupiter are huge)
Very good question! Let's take a look at how we know the Earth has a metallic core; after all, we've never been there, either, have we?
Well, let's see. How did we first measure the Earth's size, and how did we learn it was round? A Greek mathematician (Eratosthenes?) looked down a well at noon on a particular day and discovered the sun cast no shadow in the well; i.e., it was directly overhead. The next year, he did the same experiment in a different city; lo and behold, the sun cast a shadow in the well. Eratosthenes had a good idea (based on other evidence I won't go into) that all rays from the sun were parallel; so, if the sun cast no shadow in the well of one city, but did in another, that meant the Earth had to be curved.
And thus, Eratosthenes measured the size of the Earth, with remarkable accuracy, by watching rays from the sun, paying attention, and knowing basic geometry.
Then, Sir Isaac Newton invented Newtonian physics a few thousand years later. (Newton didn't invent physics; Eratosthenes, for instance, was one hell of an experimental physicist!) With Newton came the Newtonian Theory of Gravity, which correlated mass to gravitational force.
Then a lot of other physicists began figuring out what that gravitational force was. The numeric term, as it turns out, is 6.67*10^-11, if I remember correctly--so weak that it was hell to measure. But they did measure it, and before long they measured it accurately.
Now that they knew how strong the Earth's gravitational field was (Galileo demonstrated this by rolling balls down an inclined plane), and how much gravitational energy was present per unit mass/distance-squared, and how large the Earth was, they were able to compute the mass of the Earth.
(Do the math for yourself. The force of gravity is 9.8033m/s^2. The gravitational constant's numerical term is 6.67 * 10^-11. The radius of the Earth is 6.378 * 10^6 meters. The equasion is
- Gf = (Gc * M1 * M2) / (d^2),
... where Gf is the gravitational force, Gc is the gravitational constant, M1 and M2 are the masses being attracted to each other, and d is the distance between them. The algebra involved is left as an exercise for the reader.)... Doing this equasion by hand takes about five minutes. Less if you're handy with exponentials. When all is said and done, you have the Earth's mass: 5.98 * 10^24 kilograms.
Now, assuming the Earth is a sphere (it's not, but close enough), it has a volume given by pi multiplied by the cube of the radius. Do the math, and divide the mass by the volume to get the density of the Earth.
You get about 7.3 metric tons per cubic meter.
Now go out and get yourself a box a cubic meter on a side. Pick up a shovel and start digging, throwing everything you find into the box until it's full. Now weigh it. You get much, much less than 7.3 metric tons per cubic meter.
Conclusion: what's deep inside the Earth must be much,
And then you share your results with the chemists.
And soon, after a few more back-of-the-paper-napkin calculations, you're able to show that the center of the earth possesses a metallic core--either that, or else something else extremely exotic which would provide the mass you need. And since the simple explanations tend to be the correct ones (Occam's Razor), the vast majority of scientists today believe the earth has a metal core.
Now, what did you need to do all this, to come to all these conclusions in which you have so much confidence?
- Geometry had to be invented.
- Eratosthenes had to look down a well.
- Newton had to invent calculus.
- Newton had to invent the theory of gravity.
- Somebody had to measure the gravitational constant.
... That's it.That's the beauty of science. Just by thinking logically, by searching and striving for the simplest answer, you can come to absolutely breathtaking discoveries about the world using nothing more than your brain and a couple of discoveries from other people.
Free your mind... and the Cosmos will follow.
Item 5 above is "measure the gravity of the planet." For Earth that's easy to do - drop something. For Jupiter, we can do it by measuring the orbit of something going around Jupiter. It's got a whole mess of moons which are nice and visible, so this is relatively straightforward. But you can get *much* better data if you send a spacecraft there. As the spacecraft flys by or orbits the planet, its course will be deflected by Jupiter's gravity. Carefully measuring the spacecraft's course (which you do anyway for navigation purposes) lets you measure the gravity of Jupiter, and more importantly, how it differs from place to place
Turns out Jupiter is not a sphere. None of the planets are - they all spin, so they're all flattened by rotation. (Take a look at pictures of Jupiter - it's *visibly* about 10% wider than it is tall. Spinning a planet 320 times larger than the Earth around once every 10 hours will do that to ya.) Now maybe you've heard of a physics theorem that if you have a spherical object, its gravity is the same as that of a point source at the center. You couldn't figure out the internal structure of a sphere based on its gravity because it's so symmetric. But Jupiter's not a sphere, and so the gravitational potential varies from place to place in such a way that you can calculate backwards from it and get the densities at different places inside of the planet. Voila, we know it's mostly atmosphere, turns to metallic hydrogen a few thousand km down, and probably has a rocky core about 6-10 times larger than the Earth.
Aurorae have been observed on Jupiter for years (Even Voyager detected them 25 years ago!). Galileo and Hubble have been taking pictures of them for years now.
What is new is the observation of an auroral "flare", a sudden and significant increase in auroral activity. Such flares are quite rare on earth, and probably on Jupiter too, so it's quite a lucky streak for Hubble to be observing at just the right time.
In fact, only a couple of months ago, Astronomy Picture of the Day released this:
http://antwrp.gsfc.nasa.gov/apod/ap001219.html
Bart Declercq
Jupiter has the largest magnetic field of any object in the solar system (except the sun). It is so large that if it were visible in the sky, it would stretch some 20-30 times the diameter of the moon. Jupiter itself is layered, consisting of a thick outer layer of gas (predominantly Hydrogen & Helium) about 35.000km thick, within this, there is a "liquid" zone (although at the incredible pressures there, the distinction between liquid and gas is somewhat (pardon the pun) hazy.) This zone is about 25.000km thick. Inside this there is a solid ball of mostly metal which is about 20.000km in diameter. The magnetic field is actually produced in the liquid layer, parts of which are in a state called "metallic hydrogen", where hydrogen behaves as a conducting metal, creating one ginormous electromagnet.
For some great pictures and a more detailed explanation, try here.
--Shoeboy
--Shoeboy
(posting anonymously to preserve my precious karma)