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The Planetary Society BlogBy Emily LakdawallaALICE at work at JupiterMar. 21, 2007 | 12:34 PDT | 19:34 UTC
Yesterday the New Horizons science team released a nice graphic showing one example observation by the ALICE instrument of the Jupiter system. The observation was made on February 4, 2007, about four days before the spacecraft's closest approach. What I like about this graphic is that it effectively communicates where ALICE's detector was looking when it acquired three spectra at once.
ALICE has a keyhole-shaped field of view with a box-shaped component and a slot-shaped component. The box-shaped component is typically used for stellar occultations (the large area helps them make sure that even with slightly incorrect pointing the star being observed will definitely be within the field of view), and the slot is used for "airglow" observations, where the instrument measures ultraviolet emissions from atmospheric gases or ions. At each of 22 positions within the slot, Alice acquires an ultraviolet spectrum showing the brightness of emissions detected at each position. For this airglow observation, the slot was aligned so that it sampled three different targets in the Jupiter system: Io (or, more accurately, its atmosphere); the "plasma torus" of ions that surrounds Io's orbit, and the atmosphere of Jupiter. All three spectra are shown here. Spectra tend to look like a bunch of "squiggly lines" to the uninitiated. In fact, I've heard that exact phrase used very frequently by non-spectroscopists to dismiss spectral measurements as being boring; spectroscopists also use the phrase self-deprecatingly. But there is worlds of meaning in those squiggly lines. In this example, scientists can read information on the composition of the atmospheres and plasma torus from the positions of peaks in the spectra. For example, the high peak at 1216 Ångstroms in the Io and Io torus spectra come from atomic hydrogen. (This is the so-called Lyman-α wavelength, which is the wavelength of photons emitted from atomic hydrogen when it transitions from the excited [n=2] state to the ground state.) The shorter peaks at 1026 and 973 also come from atomic hydrogen. A peak at 834 comes from O+, and according to Alan Stern, there are "lots of O and S ions in the torus and O as a neutral in the Io spectrum." As with most spacecraft, New Horizons' optical instruments (the ones that measure electromagnetic radiation like ultraviolet, visible, and infrared light) share the same boresight, so when one of them is making an observation, the others can "ride along" to make observations of the same target. Just for fun, here's a photo that the LORRI camera took at exactly the same time. It was pointed at the position of the red cross mark on the diagram above, which is to say at Io.
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