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The Hubble Space Telescope Looks at Saturn

April 22, 2013

Hubble image of Saturn's Aurora
This movie shows a concatenation of images, hotly downloaded of the telescope today, April 22, 2013. While the frame rate is constant, the movie is time-lapse but by varying degrees. Click on the image for the movie. Credit: NASA/ESA/Leicester
I sometimes get asked why I wanted to be a scientist. Couldn’t I have made myself useful and gone into finance instead? The answer, of course, is the thrill of discovery, encapsulated in this moment when I have on my computer a file newly downloaded from one of humanity’s most important scientific tools: the NASA/ESA Hubble Space Telescope. Not a single soul in the history of mankind has ever known the snippet of scientific knowledge that I alone now perceive, glowing from my screen in the form of a mint-condition image obtained by the famous telescope. And, once written down in a scientific paper, this snippet will enter the literature for posterity to recognize as my own small contribution to the world’s scientific endeavor. This is why I love science.

It’s only a one-megapixel photograph, but that doesn’t matter since the detector* has the Hubble Space Telescope bolted onto it. The image is taken in the ultraviolet, the type of light that lies just off the purple edge of a rainbow, and which in very moderate doses gives us a pleasant suntan. In fact, most cosmic ultraviolet light is absorbed by the Earth’s atmosphere, which is one of the reasons why the Hubble is a space telescope and not a ground telescope. Located in low Earth orbit, the observatory is able to capture and record some of this light before it is devoured by the Earth’s atmosphere. The ultraviolet light in the case of this particular image was emitted from Saturn, roughly an hour before it was detected by Hubble, and is known as aurora.

Aurora as seen from the International Space Station
Aurora Underfoot: screen capture from video of aurora as seen from International Space Station (ISS) in September 2011. Credit: NASA/ISS
Most people are familiar with the northern lights, or aurora, on Earth, or at least familiar with photographs of an eerie emerald glow above snowy landscapes; less common is the knowledge that this phenomenon also occurs on other planets, and that the patterns in the glow help us to understand how the magnetic fields of the planets are being stirred about. Without breaking more of a sweat than required to get above our atmosphere, we can sit back and let the photons bring the information to us. Auroras are caused when electrically charged particles trapped on the planet’s magnetic field are fired along the field toward the atmosphere, causing it to glow in the manner of an old cathode ray tube television set. Saturn’s auroras shine brightly in the ultraviolet since the planet is made principally of hydrogen, whereas the Earth’s green, red, and magenta auroras are caused by oxygen and nitrogen. Auroras shine near both the north and south magnetic poles due to the bar-magnet shape of the planet’s magnetic field, and one of the reasons why we are interested in observing Saturn now is that the planet’s season has prevented a decent view of the planet’s north pole from Earth, until now.

While we can tell a great deal from the auroral emissions, we can get a full picture by sending a robot to the planet in order to metaphorically kick the tires, and actually measure the magnetic field and charged particles in situ, as is presently being accomplished by the NASA/ESA Cassini orbiter. These two methods of study complement each other - one provides ground truth, the other a global view of the magnetosphere. Cassini is also examining the auroras in different wavelengths, i.e. colors, notably in the infrared, which lies off the opposite end of the rainbow to ultraviolet and is omitted by a special, ephemeral arrangement of three hydrogen nuclei (or protons, to you and me) that tells us about the temperature and density of Saturn’s upper atmosphere. Together, this battery of instruments trained on Saturn presents a powerful way of doing science.

So, what does Hubble show us? This movie shows a concatenation of images, hotly downloaded of the telescope today. While the frame rate is constant, the movie is time-lapse but by varying degrees. Hubble’s operational time is divided into units of orbits, scheduled usually one per day in this particular program, and each orbit is roughly 90 minutes in length, during which Saturn is visible for 50-odd minutes. Each exposure, i.e. movie frame, is 100 seconds long, and the abrupt changes in the brightness of the disc are caused by the use of different filters. The majority of the field of view is filled with Saturn’s disc and its rings, shining with reflected sunlight, and the occasional moon saunters past on the orbital merry-go-round. However, the science lies in the shimmering glow near the planet’s north pole. The auroras are initially bright, and cover a large region on the dawn side (i.e. left from this vantage point). This is a characteristic signature that Saturn’s magnetic field has recently been walloped by a strong burst in the solar wind, a stream of charged particles that flows away from the sun at roughly a million miles per hour. Over the next few days the auroras dim and slowly brighten again, forming a ring surrounding the pole with individual rotating blobs**, which is the more typical state. The way the blobs move, along with Cassini’s simultaneous measurements of the magnetic field and particles, and the relative brightness of the different wavelengths of auroral light, will shed light on what Saturn’s magnetic field is presently spending its time doing
We will be obtaining more images like these over the coming days, and we’ll be able to produce more movies from them. Let us know what you think!

*The Advanced Camera for Survey Solar Blind Channel
** The technical term is indeed “blobs”

Dr. Jonathan Nichols
Lecturer and STFC Advanced Fellow in Planetary Auroras
Principal Investigator for the HST Saturn Aurora Observation Campaign

Radio and Space Plasma Physics Group
Physics and Astronomy Dept.
University of Leicester

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