Thursday 31 January 2013

Saturn's Hexagon Viewed from the Ground

January 26th 2013 image of Saturn obtained by husband and wife team,
Darryl Pfitzner Milika and Patricia Nicholas 
from South Australia, (full details given below).  
Vertices of Saturn's north polar hexagon can be clearly 
seen (north is downward). (Credit: Milika & Nicholas, all 
images reproduced with their permission).
For the first time, amateur astronomers are capturing spectacular images of Saturn's bizarre north polar hexagon.  Back in 2007, the Cassini spacecraft was in its first high inclination phase around Saturn, and gave us our first glimpse of Saturn's north pole, hidden from Earth's view in the darkness of winter.  We used thermal infrared imaging from the Cassini/CIRS (Composite Infrared Spectrometer) instrument to measure the emission from the polar atmosphere, and observed the bizarre hexagonal wave feature that borders the north polar region at 78.7N planetographic latitude (Fletcher et al., 2008, doi:10.1126/science.1149514, and see the JPL story on 'Hot Cyclones on Both Poles of Saturn').  At the same time, Cassini/VIMS (Visual and Infrared Mapping Spectrometer) observed the hexagon in Saturn's deep clouds, also in thermal infrared light (Baines et al., 2009, doi: 10.1016%2Fj.pss.2009.06.026), showing that the wave is an extended feature from the deep clouds all the way up to the top of the troposphere.

This was a rediscovery of the hexagon, first observed in reflected sunlight by Voyager in the early 1980s (Godfrey et al., 1988,  doi:10.1016/0019-1035(88)90075-9) and later in 1990/91 by professional astronomers using the 1.04-m diameter Pic du Midi observatory (Sanchez-Lavega et al., 1993, doi:10.1126/science.260.5106.329).  Those reflected sunlight observations were in northern summertime conditions, whereas Cassini was viewing the hexagon during northern winter, confirming that this is a long-lived wave, present for at least a Saturnian year and relatively unperturbed by the relentless march of Saturn's seasons.  The origins of the wave are still in doubt, and I won't try to cover the competing theories here (natural instabilities in Saturn's westward jets, interactions with large long-lived vortices, etc.).

Saturn passed through the northern spring equinox in August 2009, such that the north pole has been slowly emerging into spring sunshine ever since.  In a talk at the Division of Planetary Science meeting in Reno last year, I pointed out that amateur observations of the planets have reached such a high quality that they would now be able to map Saturn's hexagon from a backyard telescope.  We even talked about a competition for the first observer to do so, but we're too late!  Although it's very early in the 2013 apparition of Saturn (i.e., it's still very low in the sky, particularly for European observers), husband and wife team Darryl Milika and Patricia Nicholas have been able to capture this stunning image of Saturn from down in Oz.  In Darryl's words, "I’m really happy that I could be so lucky as to grab such an image of Saturn so early in the apparition…"  Their observations were taken  just outside a little hamlet called “Palmer” on the edge of the Murray Mallee, a wide belt of Mallee tree country bordering the Murray River about 1.5 hours from Adelaide.

Milika & Nicholas' image of Saturn from January 26th 2013.
This is effectively 'upside down', with the north pole
and the hexagon at the bottom of the image.
(Credit: Milika & Nicholas)

They were using a Celestron C14, observing Saturn at an elevation of 51.8 degrees above the horizon.  They were testing out a new CMOS-based planetary camera, ZW Optical’s ASI120MM, which has very high sensitivity and low noise, helping them to capture these crystal-clear images.  Darryl tells me that they were very lucky to get these images, as clouds were racing in from the west and about to drown them out - they got one RGB set (and just a partial second set) before the clouds 'completely torpedoed their early-morning session'.  

At the urging of users on the Cloudy Nights forum, they used the free WinJUPOS software to create a polar projection of his image, proving that they observed at least three, if not four, of the hexagon vertices.

Polar reprojection of Milika's image using the
WinJUPOS software, RGB colour image on the
left, and just the red channel on the right. (Credit: Milika & Nicholas)

Another version of the polar reprojection of Milika's image using the
WinJUPOS software (Credit: Milika & Nicholas)

This might not be the first amateur image of a hexagon vertex, but it's certainly the first amateur polar projection I've seen of this sort, and it's absolutely stunning.  It certainly whets the appetite for what's to come in the next few months as Saturn reaches opposition in April.  I expect we'll get plenty more hexagon images, and the next challenge is to map Saturn over a full rotation (maybe over sequential nights) to produce a complete map of the northern springtime pole!  From Darryl:  "Personally I’m hoping for better seeing to go with the fact that Saturn will increase in size a couple of arcseconds, go from 0.6 to 0.3 magnitude and rise 20 degrees higher down here in Oz over the next 3 months."  I for one can't wait to see the results, particularly as we'll be able to track the longitudinal locations of the vertices over time to determine how the hexagon is moving (some suggest it's completely stationary in 'System III' longitude).

Saturn's hexagon as viewed by Cassini (Credit: NASA/JPL/SSI),
colour processing by Jason Major. 

While these efforts are underway on Earth, Cassini is back in high inclination orbits, and is taking the opportunity to gaze down at both poles using a broad variety of wavelengths (infrared included).  Back in November 2012 we were treated to high resolution views of the north polar vortex and hexagon, described here.  Cassini will continue to scrutinise the polar atmospheres for the next few years, through to the summer solstice in 2017, to see how all of these dynamic features vary over Saturn's year.

Friday 25 January 2013

Saturn's Beacon Two Years On

Last year we published a long paper which tracked the aftereffects of Saturn's gargantuan springtime storm that erupted in 2010.  The churning tropospheric storm created dramatic changes in visible light (a bright white storm system looping around the planet), but in infrared light we detected an enormous anticyclonic vortex high in the stratosphere, a large swirling airmass some 80 K warmer than its surroundings.  This stratospheric vortex, somehow generated by the churning storm below,  was moving slowly westward and cooling with time.  Nicknamed the beacon, the high temperatures within its peripheral jet presented a unique opportunity for the detection of species that are usually too faint to see.  The high temperatures made all these spectral fingerprints more prominent, allowing us to study gases that we don't normally see (e.g., ethylene).

Although our paper only covered dates through September 2011, Cassini has continued to monitor the location of the beacon as it slowly decays away, back to the normal background state. I'm heading out to Hawaii next week to use an extremely high-resolution infrared spectrometer (TEXES) to study the detailed chemistry in the heart of the beacon, so I thought it best to check that this swirling vortex is still there!  One particular observation by the Cassini Composite Infrared Spectrometer (CIRS) cut right through the centre of the feature at Saturn's northern mid-latitudes, and found the beacon at 170W longitude (System III) on January 5th 2013.  The beacon has continued to cool, now showing peak brightness temperatures of about 160 K in the methane emission band at 7.7 ┬Ám.  It's westward motion continues to be constant at around 3.0±0.05 degrees of longitude per day, a velocity of 31.3±0.5 m/s towards the west.  It's longitude in System III can be roughly obtained by the following formula, although it extrapolates over a large date range:
Lon = -248.23 + 3.01 * (Date - 2011-01-01)
... i.e., using the difference between today's date and January 1st 2011 (roughly when we first saw the beacon).

Cassini/CIRS observation on January 5th showing the location of the beacon.
Extended version of our beacon-tracking figure, showing how the westward motion is fairly constant with time.
In preparation for the TEXES observations from NASA's Infrared Telescope Facility, I calculated the times when the beacon will be within 20 degrees of the central meridian of the planet (and when Saturn is actually visible with airmass < 2.0).  This will occur as follows.  IRTF operates between approximately 18:00-06:00 HST (04:00-16:00 UT).  Saturn time is scheduled from 02:00-06:00 HST (1200-1600 UT), which means we'll currently only see the beacon on February 4th at around 14:00 UT (04:00 HST).  The beacon will be near 251 W on February 1st, moving to 277 W by February 11th 2013.

2013-Feb-01 10:40-11:40 UT (Could see tail end if starting at 12:00 UT)
2013-Feb-02 08:00-09:00 UT
2013-Feb-03 06:20-06:30 UT (High airmass)
2013-Feb-04 13:30-14:30 UT (Best Observation)
2013-Feb-05 11:00-12:00 UT  (Could see tail end if starting at 12:00 UT)
2013-Feb-06 08:20-09:20 UT
2013-Feb-07 06:00-06:40 UT (High airmass)
2013-Feb-08 13:50-14:10 UT (High airmass)
2013-Feb-09 11:10-12:10 UT  (Could see tail end if starting at 12:00 UT)
2013-Feb-10 08:30-09:40 UT
2013-Feb-11 06:00-07:00 UT
2013-Feb-12 14:00-14:10 UT (High airmass)

This updates the estimates of the beacon positions from the paper we published in 2012 (where x is the date minus January 1st 2011):

Phase 1 (B1):  2.74±0.11 deg/day (lon=2.74*x - 40.78)
Phase 1 (B2):  0.59±0.06 deg/day (lon=0.59*x +224.32)
Phase 2 (day 110-180):  1.61±0.20 deg/day (lon=1.61*x+92.12)
Phase 3 (day 180-300):  2.40±0.29 deg/day (lon=2.40*x -58.6)
Phase 4 (day 300+): 3.01±0.05 deg/day (lon=3.01*x - 248.23)

Tuesday 22 January 2013

Jupiter's Weather in 2012 - BAA Report

John Rogers of the British astronomical society has just released an interim report describing the changes seen in Jupiter's weather systems over the past few months, since August of 2012. You can find the pdf file at the following link:

John has again done an incredible job of compiling a huge number of images from amateur Jupiter observers into one place, describing each region of the planet in turn. In the middle of 2012 Jupiter's northern hemisphere had a particularly dramatic appearance, but it sounds like the activity, known as revivals of the northern equatorial and temperate belts, seems to have calmed down. I was also particularly interested in the red band that had been spotted around the equator, something that hadn't been seen before, but now appears to have broken up somewhat. Finally, the report describes the close encounter between the great red spot and oval BA that occurred last autumn, leaving dramatic after effects in the planets southern tropical regions.   

Thursday 3 January 2013

Studying the Mysteries of the Outer Solar System

In 2012 I was awarded a Royal Society University Research Fellowship (URF), to continue my research at Oxford.  Part of the application process is to write a simple overview of the work I intend to accomplish, and I've decided to publish it here as a general introduction to my giant planet research.

Exploring the Mysteries of the Giant Planets in our Solar System and Beyond

The frigid outer reaches of our solar system is the realm of the giants.  Four enormous balls of gas and ice move slowly along their orbits, taking decades to complete one lap of the Sun.  Observations of these worlds with telescopes and robotic spacecraft reveal incredibly dynamic planets, with alien weather systems in churning atmospheres, spectacular storms larger than our entire planet and a diverse range of cloud colours.  The giant planets are a paradise for planetary scientists who seek to understand the atmospheric physics and chemistry at work throughout our solar system, in the hope that we might better understand our own terrestrial atmosphere and the emergence of life.  Furthermore, the giant planets contain in their compositional make-up a unique record of the solar system’s early history, as the dust, ice and gas that collapsed to form the planets billions of years ago is forever stored within their churning atmospheres.

I use powerful ground-based and space-borne telescopes to study these worlds, in addition to the most sophisticated interplanetary spacecraft ever built, Cassini, which is orbiting Saturn and returning spectacular images from a billion kilometres away.  Our exploration of the outer solar system is still in its infancy, and yet our understanding is being used to interpret the very first studies of planets around other stars (exoplanets).  There’s an enormous potential for new discoveries at the cutting edge of planetary astronomy: what might these places be like, and is the structure of our solar system (habitable worlds in the inner system and cold giants in the outer system) commonplace?  These questions go to the very heart of modern planetary science, and our quest to understand the bewildering array of atmospheres in our galaxy.  

The research I propose will address two connected questions – how and why do the giant planets vary in their appearance (i.e., what drives the huge storms that we see, with their changing colours, clouds and temperatures); and what can the chemical composition of these worlds tell us about their origins, both in our solar system and around other stars?  I’ll be using data from a variety of sources, from observatories in Hawaii and Chile, to telescopes in Earth orbit and the Cassini spacecraft at Saturn.  Computer models will replicate the spectrum of light observed from each planet, and allow us to measure the temperature, gaseous composition and clouds within their atmospheres.  Ultimately I want to answer these questions by designing new exploration missions here in the UK, to fire the imaginations of the next generation of scientists and engineers. 

The presence of an atmosphere around our planet, protecting us from solar radiation and keeping the temperatures in the habitable range, was essential for the development of life.  The immense gravity of the giants shields our world from catastrophic impacts with solar system debris.  By understanding how this planetary system formed and how atmospheres evolve and change at different distances from the Sun, I hope to gain a better understanding of the origin of Earth and its protective atmosphere.   If we can understand our own origins, this will be a step towards answering the ultimate question for planetary science – are habitable terrestrial environments common in our galaxy, or are we alone?