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OFJ Field Journal from Glenn Orton - 11/5/95


I'm in Hawaii now, but I urge you to drop any mental images of palm trees, high surf and flower leis.

I'm at the 13,800 foot summit of Mauna Kea, working at the NASA Infrared Telescope Facility (IRTF), trying to get a glimpse of Jupiter. Since the Project adapted a very conservative policy regarding data, so am I. I'll consider that any image of Jupiter I get may be the last one before the Probe entry on Dec. 7. This is not so rabid as you might expect, as winter storms here have been known to drop several feet of snow on the summit and effectively close it down for weeks at a time.

Unfortunately, this is not a good observing run to enforce that policy. Since Nov. 2, we've been under the outer edge of a mid-Pacific stationary low system, characterized by cirrus/stratus clouds of various thicknesses. Worse yet, the air around us is much wetter than usual, so when afternoon winds push by Mauna Kea, they upwell moist air which forms a cloud right over the summit! Nov. 2 we watched snow fall in the first winter major storm. Yesterday afternoon (our scheduled observing time is 3 - 8 PM: you can observe in the infrared in the daytime) was the first time we had any chance to observe anything at Jupiter, and that was simply through relatively thin cirrus. We finally got Jupiter above the horizon at 3 airmasses (where one airmass is what you look through straight overhead). Seeing was awful; we could recognize that Jupiter was round and may have banded structure - and that was IT! We also couldn't see anything near wavelengths of 7.9 microns and 17-23 microns which are important to us as we use them to determine the temperatures across the planet: they are unfortunately also close to spectral regions of water absorption.

When things get cooler, after the sun sets (with Jupiter soon following), much of the cloud cover dissipates and it becomes thinner - nature is perverse sometimes. But this has let us get some near-infrared images on Nov. 3 and thermal (mid-infrared) images on Nov. 4 of Saturn.

As I write this, the clouds are as thick as ever, but with occasional holes. We call them "sucker holes", because we get so desperate that we get suckered into trying to grab something really fast through them...but they're always here and gone too fast for much meaningful work.

Nov. 5 (later the same day), Nov. 6 (UT)

We got up to the summit today by about 2 PM, driving through some thick cumulus clouds on the way. When we got to the top, half the sky was blue! Unfortunately, Jupiter was in the wrong half!

By 3 PM, however, winds had moved all the clouds below Jupiter. After some momentary problems setting up the equipment, we moved on rapidly. We moved around trying to find a very bright infrared star, Spica (alpha Scorpii, "alpha Sco"). After a few minutes we found it, and moved to the correct beam (note that, while we can see during the day in the infrared, the guide camera is in the visual range and has the same problems your eyes do finding any star during the daytime).

The small infrared signal of Jupiter is added to the top of the infrared heat emitted by the atmosphere itself. We detect that small signal by:

  1. 'chopping' the telescope secondary mirror on and off the object, as often as several times a second,
  2. 'nodding' the telescope onto and off the object once every several seconds, and
  3. all of the above.

By working with the complicated option (3), we obtained the most stable image and compensated for variations of the sky emission on a couple of different time scales. To our great delight, we found light beaming through all those heretofore awful wavelength regions filled with gaseous water absorption in our own atmosphere. The humidity had dropped to 70% and was still falling, the wind also dropped to 10 mph so the atmosphere was relatively calm and the images quite steady.

We moved to Jupiter.




(Hey, it was Sunday, so permit me some genuflectual exaltation! :) )

Jupiter was nothing less than wonderful! Belts, zones and lots of detail we had give up for lost. We centered on the planet while keeping the filter at 8.57 microns, a wavelength relatively free from the effects of our terrestrial atmosphere absorption and sensitive to variations of Jupiter's cloud thicknesses. Absolutely fabulous!

We moved to the water-sensitive but important 7.85 microns where we could map methane (CH4) emission and Jupiter's stratospheric temperatures. Double fabulous!! This is not a region in Jupiter's spectrum where one can get a great deal of "signal," but we could tell from even one of the 20 or so images we'd later process that we could see waves in Jupiter's atmosphere, other features we didn't previously suspect were there, and two warm bands on either side of the equator (by some 10 - 15 degrees latitude).

Then the VERY water sensitive 17- to 20-micron region which was also important to determine Jupiter's tropospheric temperatures. Triple fabulous!!!!!!

We went back to the star alpha Sco, after working on Jupiter for 40 minutes through a variety of wavelengths. We measured alpha Sco as it got closer and closer to the horizon so that we could use this behavior to determine how Jupiter's own infrared brightness was dropping as it was seen at various angles in the sky. We also took a little time to focus VERY carefully, then went back to Jupiter.

Quadruple fabulous!!!!! We were picking up such a huge wealth of detail, that this was going to become one of our very best observing nights (uh, days ??) However, we realized, that the Probe entry site had already rotated out of view. Well, it would just have to be fabulous to triple fabulous. The higher spatial resolution (allowing us to see small details in Jupiter's atmosphere), however, was showing us that we had lots of wave structure in the temperature field near the equator, and that might just show up as waves in the vertical temperature structure that the Probe Atmospheric Structure Instrument (ASI) would detect. This would also be true further from the equator with the determination of the temperature structure in the stratosphere from the radio occultation experiment.

Jupiter was now lower in the sky (at 2.5 air masses) and sinking fast. I gambled on changing instruments to try to get our last glimpse of this region in reflected sunlight and very high spatial resolution at a wavelength of 5 microns, which is sensitive to deep cloud structure. We made the change in 15 minutes, got back to Jupiter, now even lower (at 3.5 airmasses), and got further observations using three different wavelengths, each one showing us Jupiter's cloud structure at different heights. All the observations were between 3.8 and 4.4 airmasses - not all were great, but we would select the best of many repeated images. We stopped when the telescope hit the limits of its ability to track low in the sky. We had an interesting time then; the triggering of the limit switch set off a newly installed high-tech fire alarm. We went off searching for bogus fires in about 5 different places simultaneously ... just in case!

Back to the mid-infrared camera, we completed calibrations using a star, gamma Aquillae ("gamma Aql") which is used as a standard flux reference, so we could later reference Jupiter's emission to the known flux of this star (flux is how much energy of a given wavelength hits an area over a given time; this can be measured, for example, as watts per square meter).

With a gap between then and when we wanted to make observations of gamma Aql again at a somewhat lower airmass, we moved around to Saturn. We had focused with great care on gamma Aql, and discovered that Saturn was splendid. Just as we got images at 12.2 microns, where ethane (C2H6) is emitting radiation from Saturn's stratosphere. Gorgeous sight! No rings (they were just edge-on to the Sun recently, and so were quite cold), but lots of banded structure, and east-west variations: a dark equator and a hot south pole. Looking on at that time were Dr. Don Hall, the head of the University of Hawaii's Institute for Astronomy, who oversees many of the managerial operations at ALL the Mauna Kea telescopes. With him were four Asian men whom I first through might be visiting Japanese, as Japan is building a large telescope barely 1/4 mile from the IRTF. But, no, they were members of Hawaii's legislature, taking a tour! Sometimes nature isn't that perverse, after all!

By 7:52 PM, we wrapped up what we would do and handed the telescope over to the next observer. I and my colleague, Dr. Jose Luis Ortiz, were delighted. It was a time for champagne, which you might rightly guess is strictly forbidden (as is any other alcoholic drink) at the summit. It was his first good quality night at the summit of Mauna Kea.

So I started transferring the data to JPL electronically and went into the "day room" to have my cold supper: the mircowave was on the fritz.



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