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The Planetary Society Blog

By Emily Lakdawalla

ALICE at work at Jupiter

Mar. 21, 2007 | 12:34 PDT | 19:34 UTC

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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.

Click to enlarge >

An ALICE observation of Jupiter and Io
Credit: NASA / JHUAPL / SwRI

ALICE is an ultraviolet spectrometer, meaning that it gathers incoming light in ultraviolet wavelengths (shorter wavelengths than are visible to human eyes) and splits it, using a prism, into tiny slices of light in narrow wavelength ranges, then measures how bright the light is in each wavelength slice. The measurements from spectrometers are usually displayed as spectra, graphs representing the brightness of light at each wavelength. There are three spectra shown on this graphic, all of which could be acquired at the same time because of careful positioning of ALICE's field of view.

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.

Io from New Horizons
New Horizons took this picture of Io on February 24, 2007 at 13:50. The image is centered on Io's leading hemisphere. An isosceles triangle of three tiny dark dots in the center of the disk are volcanoes called (clockwise from top) Gish Bar, Hi'iaka, and Sigurd Paterae. A larger right triangle of larger dots is formed by (clockwise from top) Shango, Shamshu, and Itzamna Paterae. Credit: NASA / JHUAPL / SwRI