Sunday, 3 February 2013

First Night on Sky: Jovian Waves and Saturn's Composition

The view of Mauna Kea from Saddle Road,
with one of the domes visible on top.
Awake at 4.30, quick dinner in HP then a steep drive up a dusty road to the summit of Mauna Kea.  Beautiful views over the saddle to Mauna Loa, and a wonderful clear blue sky as the sun sets over the clouds below us, but we all disappear into the Infrared Telescope Facilities operating room for the next 12 hours.  The transition from sea level to the summit is pretty tough to do in a day, and I spend the first hour or two with a metallic taste in the back of my mouth, a sign that my brain isn't getting it's usual oxygen rush!  But once you get used to it, stay hydrated and start thinking about the problems at hand, it's really not that hard.

Jupiter's Waves and Aurora

The run actually started yesterday, on February 1st, with Tommy Greathouse's program of Jupiter observations.  TEXES is an exceptionally high-resolution spectrometer (I'll try to blog about the instrument at some later point), and measures spectra in very small chunks.  So we pick a couple of ranges where we know that (i) the spectral lines from Jupiter are strong and useful for probing the atmospheric temperatures; and (ii) the atmosphere of our own planet is transparent enough for us to see through without hindrance.  We pick lines of ethane and methane, as this permits us to measure the temperatures of Jupiters stratosphere.  In addition, the smooth continuum spectrum in between the forest of ethane is actually sensitive to the tropospheric temperature, so we can map both troposphere and stratosphere at the same time.

Looking at the IRTF secondary mirror (dead centre).

Tommy's program is to get a full 360 degrees of longitude on Jupiter to search for wave activity in the jovian tropics.  There's all sorts of waves here, including horizontal slowly moving waves that could be related to weather events below, plus vertically propagating waves known as Jupiter's quasi-quadrennial oscillation by analogy to the quasi-biennial oscillation we have in Earth's atmosphere.  TEXES can see all of them, and map them out in three dimensions (i.e., horizontally, and with height in the atmosphere above the jovian cloud tops).  As Jupiter rotates once every ten hours, but is only available to the telescope for a few hours in a night (6 hours at the moment), we need at least two consecutive nights to map a complete longitude circle.  So they started yesterday while I was sleeping in Hilo, and we continued tonight under stunning weather conditions - clear skies, low humidity, low winds, just a perfect observing run.

A view inside the IRTF control room, where
our entire night was spent

The ethane and methane emission maps were built up by scanning the instrument over the planet, aligning the entrance slit north to south, and then stepping it from east to west, taking an observation every 0.7 arcseconds (Jupiter is 43 arcseconds across right now) to get the longitude coverage.  We also step from north pole to south pole to get latitudinal coverage, meaning that we get the tropical and equatorial oscillations that we're after, plus glimpses of the hotspots associated with heating in Jupiter's auroral regions.  We clearly saw the high temperatures of first the south, then the north auroral hotspots.  This dataset will now be reduced to track all this wave activity, and we'll repeat the observations in a week or so to see if any of these features are moving and shifting with time.  So from 6pm to midnight we got plenty of excellent Jupiter data, moving back and forth as the planet rotated beneath us, now it's time for come caffeine until Saturn rises at 3am.....

Saturn's Composition

Once Jupiter was setting into the murk at around midnight, we spent a few hours observing starburst galaxies to study gas flow velocities, before getting to where I really wanted to be.  My proposal to come to Hawaii had centred around capturing Saturn's beacon (see earlier post), but as that won't be on the right side of Saturn until tomorrow night, we had plenty of other projects we could execute.  My top priority addresses the question of Saturn's ammonia abundance.  The Cassini spacecraft struggles to measure ammonia in the mid-infrared because of a low sensitivity in the required spectral regions.  TEXES, on the other hand, has no such problems.  So we tried something that had been rarely done before, choosing the lowest resolution mode of TEXES so that we captured as wide a spectral region as possible.  This works nicely for tropospheric features like Saturn's ammonia (the key species forming its clouds and hazes) and phosphine (a gas thought to be dredged up from deep within the troposphere, beneath the clouds), as they both have extremely broad lines.

It took us a little playing to make sure the spectral setting was right, and took us simultaneously acquiring, reducing and modelling the data, all at 13,000 ft and at 3am in the morning, to make sure we were getting the right thing.  But when we started taking data, we realised we could see lines of both phosphine and ammonia straight away.  Furthermore, by observing for as long as we could as Saturn rose in the sky, we could separate the signal from the noise sources to get a beautifully clean looking spectrum of ammonia and phosphine lines.  The weather conditions remained perfect for this, as we just added more and more data racing until 6am in the morning.
Tommy Greathouse and Glenn Orton controlling
the TEXES instrument and telescope.

Having a clean spectrum will be a huge step towards one measurement I've been after for ages now, and Cassini simply can't do it.  That's measuring the relative abundances of two versions of ammonia, NH3, in Saturn's atmosphere.  One version has a slightly heavier isotope of nitrogen than the other.  The trouble is, the amount of the heavier isotope is around a thousand times less than the lighter isotope.  Hence the need for a really good spectrum, to see the heavier isotope.  If we can measure that ratio, it tells us something about how the planet formed, particularly where it got all of its nitrogen from.  It's been done on Jupiter by Cassini in 2000, but there's so much more ammonia there than on Saturn, where the spectral features are normally all completely obscured by phosphine.  So, I've now got my work cut out for me, but my first night of TEXES observing has provided enough spectral data for some fascinating science.

Now my sleep-deprived brain needs to shut down and sleep.....  it's 5.45am, we're moving the telescope to zenith and closing up.  Superb night.

A long exposure view back across at the Keck telescopes, with Jupiter setting in the east.
You can just see Subaru in the distance too.

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