Sunday, 8 March 2015

The Atacama Large Millimetre/Sub-Millimetre Array (ALMA)

Our flight to northern Chile and stay in San Pedro de Atacama was designed so we could acclimatise to the high altitude and arid conditions of the high desert before visiting the radio observatory at 5050 m on the Atacama plateau.  ALMA is the world’s most sophisticated observatory at these wavelengths, a truly collaborative project between Europe, Japan, America and Chile.  It works by having fifty 12-m antennae (the Main Array) with variable separations in between.  The different baselines allow you to take a Fourier Transform of the sky, correlating the signals from each antenna to provide a spatial resolution far superior to what you can achieve from a single dish in isolation.  The sensitivity to a particular spatial scale depends on the length of the baseline.  Short baseline configurations (150-m) provide access to large spatial scales; long baselines (up to 15 km) provide access to the smallest spatial scales. Twelve additional 7-m antennae (the Compact Array) provide very short baselines for the largest spatial scales.

Within each antenna is a series of receivers, or bands, which determine what wavelengths can be observed.  At the moment bands 3-9 are available to users, providing wavelengths from 420 µm to 3.6 mm (84-720 GHz).  This should extend up the 950 GHz when band 10 is offered, and maybe down to 30 GHz with future receiver development.  Measurements in these spectral bands, at a variety of spectral resolutions, are then fed by cables to the main correlator building a few metres away from the antennae, using the Fourier Transform to assemble an image of the sky with unprecedented spatial resolution.


The ALMA site offers the best observing conditions in Chilean winter (July to November), so the more challenging configurations are used then.  In February the conditions are usually hazardous with extreme snowfall, meaning that the antennae must be oriented so that snow does not accumulate inside the dishes.  Conditions below the snow line can be so wet that flash flooding can destroy the access roads.  Thankfully, by the time of our visit in March, conditions were cold and clear again and excellent for some astronomy tourism.

Extreme Tourism at 5 km 


On Saturday morning we met a bus at 7am for the drive out to the OSF (Observer Support Facility) at an altitude of 2500m.  The sun was rising over the volcano to the east as we drove the dirt road to the facility, a cluster of buildings featuring the main control room and data banks, in addition to the hangers and workspaces of the contractors from Japan, the US and Europe responsible for constructing and delivering the 66 antennae (now empty as their work is done).  Construction was still taking place for a permanent visitor quarters, with temporary buildings housing the astronomers.  We had to each undergo a compulsory medical exam (blood pressure and O2 levels) and safety video, being supplied with small oxygen canisters to use should we feel dizzy at high altitude.  This is a serious medical screening - one of our number didn’t pass and had to remain at the OSF.

We ascended the switch-backed road through the mountains, watching as the dry slopes gave way to some green vegetation and cacti, allowing the grazing donkeys and llama (vicuna) to survive despite the seemingly hostile conditions.  The landscape was not as volcanic or young as that on Mauna Kea, this is a more ancient geology.  We all started to feel lightheaded, but cheered as we passed the 4200-m mark (the height of the Mauna Kea observatories).  For many of us, this marked the highest point we’ve ever been to in our lives.  As we crossed the mountain pass, the plateau opened up before us and the ALMA array came into view.  A small Japanese observatory could be seen on one of the high peaks, which must be one of the highest manned observatories in the world.

We had about half an hour to wander amongst the antennae, which was in a compact configuration after the February snows and undergoing engineering work to prepare for more science.  Eric Villard served as an excellent tour guide, showing the differences between the US and European antennae designs.  As we watched, one of the antennae rotated around silently, controlled by some unseen operator down at the OSF.  The thin air (0.5 bar) and lack of O2 at 5050 metres above sea level does strange things to the brain, an almost drunken experience as we posed for photos with the array in the background.  All of us took occasional puffs from the oxygen canisters if we felt any dizziness, but thankfully I didn’t experience any of the headaches or nausea sometimes associated with high altitude.

We then went inside the correlator building, seeing the banks and banks of computers and hard drives required to bring together the signals from each of the individual antennae.  Then it was time to begin our 45 minute descent back to the OSF for lunch and then on to Calama, drinking in the thicker atmosphere and feeling tired.  It had been a tremendous experience, not only because I now know more about the challenge of radio interferometry for astronomy, but also because of the extreme environment we’d been lucky enough to visit.  Very few humans get this opportunity, so I’m extremely grateful to ESO and the organisers of #planets2015 for the chance!

Friday, 6 March 2015

San Pedro de Atacama

The planetary science workshop ended on Thursday evening, but eighteen of us stayed on in Chile with the opportunity to visit the Atacama Large Millimetre/Sub-Millimetre array (ALMA), high in the Atacama desert in northern Chile.  This was a chance not to be missed, both for the otherworldly environment and the prospect of glimpsing the worlds most sophisticated radio observatory.

We departed Santiago for the two hour flight to Calama, cruising to the west of the Andes and passing the tallest peak in the whole of South America (Aconcagua), 7000 m high and in neighbouring Argentina.  The landscape below was dry and desolate, punctuated by extensive mining works for copper and lithium (apparently Chile is one of the largest global exporters of lithium, making money out of its use in electronics all over the world).  The Atacama is the world’s driest non-polar desert, with some rain gauges having never received any rain, period.  And you could really tell from the air.  Calama itself was a dust bowl, and the rich salt lakes covered the land right up to the mountains (extinct and ancient volcanoes).

A 90-minute bus took us from Calama to San Pedro de Atacama, a small town grown around an oasis in the desert.  We passed through the study Valle de la Luna, with sedimentary rocks thrust and turned on their side to produce a vast red canyon of jutting rock formations.  San Pedro was a really striking experience, as every building was made of the red-coloured adobe, including our hotel the Casa de Don Thomas.  The streets were largely unpaved and prone to dusty breezes flowing through, stray (but not aggressive) dogs roamed everywhere, and waterways criss-crossed the town for use in irrigation.  The village had a central square, featuring an adobe church that was undergoing renovation, a museum containing archaeological remains of the Chilean peoples who first lived here, and a relaxed bar with tables spilling into the square.  There’s one long main street, featuring endless small restaurants, souvenir shops and excursion organisers (from here tourists can visit a geyser field, salt lakes, flamingo reserves, or go sand boarding).

Sadly I had no time to enjoy the resort, only having a couple of hours to stroll the main street and dine in the Adobe restaurant.  Others stayed on for a few extra days following our trip to ALMA, but for me it was time to leave Chile behind.  I returned to Santiago on Saturday night for a brief stay in an airport hotel, then an early morning flight back to London via Sao Paolo.  Incidentally, the 4-hour stop in Sao Paolo on the Tropic of Capricorn was my first ever trip to Brazil!  One day I’ll have to come back….

Thursday, 5 March 2015

ESO Planetary Sciences Workshop 2015

The purpose of my visit to Santiago this spring was to present an invited talk at the ESO “Planets 2015” workshop, known as a “Joint Venture in Planetary Science” between space-based and ground-based observatories.  Organised by Eric Villard (ESO and ALMA) and Olivier Witasse (ESA, and the new project scientist for JUICE), this meeting brought around 80 planetary scientists together for four days in the ESO Vitacura office.  It was a wonderful opportunity to meet people and forge new collaborations, and certainly one of the best meetings I’ve been to in a long time.  Several of us were live-tweeting the meeting, so the highlights can be found there, or via this link on storify.

Science Sessions

Following a keynote talk by Mike Mumma from Goddard Center for Astrobiology, the days were subdivided into science sessions and facility sessions, punctuated by healthy coffee breaks and lunch sessions sat in the gardens surrounding the ESO office.  Monday covered giant planets, where I delivered an overview talk on synergistic studies of dynamics, chemistry and origins from spacecraft and telescopes (I’ll try to summarise that at some point), Imke de Pater revealed gorgeous VLA images of Jupiter; Gordy Bjoraker showed high-resolution 5-µm spectra of Jupiter and Saturn; and Ted Kostiuk gave an overview of atmosphere-auroral interconnections via infrared spectroscopy.  Tuesday’s science session covered terrestrial planet atmospheres (including a Doppler velocimetry technique to measure winds using visible spectroscopy), focussing on Venus and Mars, but with some fascinating ALMA results on Titan (Cordiner) and Io (Moulet), and Katherine de Kleer’s long-term program to monitor Io’s volcanic activity in the L and M band (3-5 µm) using Keck and Gemini.


On Wednesday we venerated into the realm of asteroids, TNOs and comets, including radar observations of asteroids where Benner produced 3-D printed versions of asteroid Bennu (the destination for the OSIRIS-REx mission, due to launch in the next couple of years) showing an equatorial bulge and distinct ‘boulder’ on the surface.  We heard about the jovian trojans and hildas, asteroid families that I know very little about; the discoveries of rings around the centaurs (Chariklo and Chiron); and the prospects for detailed studies of Trans-Neptunian objects.  I learned that many of the Kuiper Belt objects featured small satellites, whose names were completely new to me.  Finally, on Thursday we ventured briefly into exoplanets and planetary formation.  The science sessions had covered a very wide range, and I felt I learned the most from the review/overview presentations rather than the more detailed science talks.  If this meeting were to happen again, a stronger emphasis on review and forward thinking, rather than focussing on your own research, might be the way to go.

Facility Sessions

In contrast, I got a lot more out of the four facility sessions.  As we were sat listening to the presentations on the various observatories, I could see many people in the audience thinking of new ways to study their fields, me included.  Monday afternoon served to pique my curiosity about ALMA, as Eric Villard presented the capabilities of ALMA for planetary science (more on that in a later blog post).  We heard talks on:



  •    The Mauna Kea sub-millimetre valley (SMA, JCMT and CSO) by Mark Gurwell; 
  •    Eliot Young reviewed NASA’s ideas for balloon-borne planetary observatories;
  •    The US NRAO (National Radio Astronomy Observatory) facilities, including the VLA, VLBA and Green Bank Telescope (Butler);
  •    The SOFIA observatory and its instrument suite (Reach);
  •    Ground-based support for Cassini and Juno (Orton);
  •    Use of the deep space network for both telemetry and science via radio link (Lasio);
  •    Instrumentation roadmaps and plans for the Paranal observatory (VLT) in contrast to Keck and Gemini (Dumas);
  •    Plans for the NASA Infrared Telescope Facility (Tokunaga);
  •    The capabilities of the Large Binocular Telescope and Interferometer (LBTI) for high angular resolution studies (Conrad);
  •    IRAM for millimetre studies (Boissier);
  •    The prospects for the James Webb Space Telescope (JWST) for solar system science (Stansberry).


Having experts in these facilities in the same room as the science users proved to be an excellent idea. I had so many useful discussions over coffee and lunch that my to-do list is now enormous.  ALMA, although heavily oversubscribed, is particularly exciting and it would be great to use it for giant planet science.  I got to talk to the instrument scientists in charge of the VISIR renovation and recommissioning (a work-horse of mine for infrared imaging and spectroscopy) and hopefully instilled an excitement for looking at Jupiter and Saturn soon (time awarded in the next semester).  I discussed our SOFIA/FORCAST data on Jupiter with SOFIA specialists, which we’ll use to study deep circulations.  I met with Japanese colleagues working on Subaru/COMICS Saturn imaging (which I actually acquired, along with Glenn Orton, from the summit of Mauna Kea in January 2008), investigating the changing brightness of Saturn’s rings as a function of season.  I met others working on VLT/SPHERE observations and struggling (as I am) with data reduction.  I caught up with a colleague working on exoplanet observations (who just happened to be in Santiago on his way to the VLT), and with colleagues working on Cassini/CIRS, and had lengthy discussions about organisation for ground-based supporting observations for the Juno mission.  On that topic, one of my first tasks when I get back to Oxford is to draft a white paper to try to convince observatories to support this mission.

The last day of the meeting featured a talk from Will Grundy on the New Horizons mission, particularly the heroic ground-based efforts from John Spencer and other to identify a suitable KBO target for a second flyby after Pluto.  Two candidates were found (PT1 and PT3), ultimately using lots of Hubble time.  But the idea that the Pluto encounter is already underway, and that this whole system will be gradually revealed in glorious detail over the next few months, is breathtaking.  History in the making, and a great way to end the meeting.

Monday, 2 March 2015

Santiago de Chile

I’m back on the road again, but this time for a pretty exciting trip - my first ever excursion to South America, travelling to Santiago, Chile as an invited speaker at ESO’s Planetary Science workshop.  This four-day event is intended to explore scientific overlaps between ground-based observatories and spacecraft exploring our solar system, so it was very easy to say yes to the invitation.  At the end of the week, several of us are flying to San Pedro de Atacama for a trip to ALMA, which promises to be a highlight of the trip.  I’ll be talking on Monday (Day One) about synergistic investigations of giant planet dynamics and chemistry (touching on planetary impacts, storms and global circulation), but I need to get there first.

Flight to Santiago

The trip to Chile involves an evening departure from London, a couple of hour hop over to Madrid, before catching a packed Iberia flight direct to Santiago.  I’m told it’s almost always an overnight flight from Europe, a trip of 6500 miles and 12.5 hours from 52N to 33S, landing in the early morning.  Santiago is the Chilean capital, nested between the Andes to the east and the coastal cordillera to the west, on the Panamericana highway.  My first view of the Andes came as we descended into the early morning Santiago, snow-capped despite the 30-degree summer temperatures of early March.  Escaping the airport was an ordeal - hundreds of people cram into the arrivals hall, screaming ‘Taxi taxi’ at you.  I’ve heard many horror stories of people being ripped off at this point (indeed, some of my colleagues accepted a ride only to wait hours for the van to show up), but I’d been advised to seek out one company, Transvip, to get me to ESO.  They were even expecting me, which made life much easier.

Vitacura

The taxi wound its way from the airport west of the city, spending a long time in extensive underground tunnels before emerging in the shiny and modern Vitacura district to the northeast of the city centre.  Sometimes referred to as ‘Sanhatten’ for its upmarket hotels, skyscrapers and restaurants, this is the home of ESO’s Vitacura office, next to the bicentennial park (Chile declared independence from Spain in 1810).  I stayed in the Hotel Director, along with may of the other conference participants, and we dined every night in various restaurants around the Alonso de Córdova.  I’ve never been been to a conference where I’ve eaten so consistently well every evening, with memorable locations including al fresco dining on fresh sea bass and seafood salad, and the Noi Hotel’s rooftop bar, where we drank pisco sours and Krass beer while the (upside) moon rose over the Andes.  Being with a bunch of astronomers, we spent most of the evening gazing upwards to the southern constellations, and remarking how strange Orion looked from the southern hemisphere perspective.

Exploring Centro and Santa Lucia Hill - Sunday

Given the packed conference schedule, I decided to go exploring in central Santiago on Sunday afternoon, despite the lack of sleep on my overnight flight.  A 30-minute walk down Av. Vitacura took be to the Tobalaba metro station, where I used my finest Spanish to figure out how to buy the ‘Bip!’ card to use the metro system.  Centro was 7 stops and 2 changes away from Vitacura, but it took no time at all and I emerged on the Plaza de Armas to explore the 19th century buildings and squares.  The Plaza is the heart of the city, containing hundreds of palm trees and bordered by the Catedral Metropolitana, built between 1748 and 1800.  I explored the Cathedral, including the subterranean crypt where former bishops are buried, and wandered north to the Mercado Central fish market, a bustling combination of smelly fish stalls and crowded eateries.  The pedestrian streets were packed with sunday-afternoon shoppers, and the whole place felt vibrant and friendly.

I strolled south east through the Centro district to find the Cerro Santa Lucia, a rocky hill that was transformed in the 19th century into a landscaped park laced with trails and steep stairs to reach the summit.  I climbed through the flowery gardens to fine Torre Mirador, a red brick tower at the hilltop providing excellent views of the surrounding city, including a view back towards ‘Sanhatten’ and the snowcapped Andes in the distance.  With that, it was time to descend to the Santa Lucia metro station and return to Vitacura in time for the conference welcoming reception (and a few pisco sours) at the ESO Vitacura office.

With all this exploring, I couldn’t figure out why my sense of direction was so screwed up, until it hit me.  The sun is in the wrong place.  This simple realisation made me laugh out loud - my northern-biased brain is so hardwired to use the sun’s southerly position to allow me to navigate, that the sudden shift into the southern hemisphere completely messed up my internal compass!

Tarapaca Vineyard - Wednesday

Our conference dinner was organised in the gorgeous surrounds of the Tarapaca winery in the Maipo Valley.  Nestled amongst the vines was a small hotel, with rooms named for the grape varieties grown there. We were given a tour of the fermentation plant and wine cellar, and stood  beneath the trees to sample various varieties including a rather tasty carmenere.  After presentations from NASA and ESA representatives on their hopes for greater collaborations between space and ground-based facilities, dinner was a vast barbecue served in the grounds.

Cerro San Cristóbal and Barrio Bellavista - Thursday

I managed to get back into Santiago on Thursday evening, using expensive taxis (we were ripped off by the Hotel) to reach Barrio Bellavista to ride the funicular railway to to the top of San Cristobal hill.  A 14m-high statue of the Virgen de la Inmaculada Concepción sits at the top of the hill, overlooking the city below, and an easy climb from the railway station provided great views in all directions around the city.  In the evening light Santiago is a very hazy place, apparently due to the fact that the city sits in a basin between mountain ranges, and with no sea breeze to clear out the smog.  The Andes could just be seen in the distance, although photographs didn’t really do them justice.

A stroll down Pio Nono brought us past endless lines of noisy bars, filled with students from the nearby university.  Searching for food in the Barrio, we stumbled across the Patio Bellavista, a courtyard of upmarket eateries, and I ate lamb shank drizzled in a syrah and smoked bacon sauce, a superb end to our brief excursion to Santiago.


Thursday, 9 October 2014

Q&A: Jupiter's Great Red Spot

1. When was the GRS first observed and what were its dimensions at this time, please?

John Roger’s excellent book has the history of the GRS:  it its likely that it’s not the same spot as observed in the 17th century by Cassini, but the first definitive observations of ‘our’ GRS came in the 19th century - observers started to draw the ‘hollow’ around the GRS in 1831.  The vortex within the hollow seemed to come and go with time, until the 1870s when observers first started to draw a large red oval within the hollow.  It covered about 34 degrees of longitude between 1879 and 1882.


2. What are its dimensions today?

Hubble images in 2014 showed the GRS to be about 10,000 miles in east-west distance.  See the press release by  Simon et al. : http://www.nasa.gov/press/2014/may/nasas-hubble-shows-jupiters-great-red-spot-is-smaller-than-ever-measured/#.VCz_Ri5dVR4


3. Would it be fair to say that the GRS consists mostly of hydrogen gas with a colossal cloud – mainly ammonia ice, plus something, probably phosphorus, making it reddish?

Well, hydrogen and helium are everywhere on the giant planet, so that does’t really distinguish it from the rest of the atmosphere.  Instead, the GRS is a region of unusual clouds and chemicals, entrained by a peripheral collar of winds rotating anticlockwise.  The composition of those clouds are largely unknown, but likely to be a combination of nitrogen, sulphur and phosphorus compounds and ices, possibly coated in hydrocarbons (and possibly nitriles) raining down from above. The source of the red colour remains a mystery - the compound has to be strongly blue absorbing, but it’s identify is unknown.  Read my blog about it here: http://planetaryweather.blogspot.co.uk/2014/01/what-gives-jupiter-its-colourful-stripes.html



4. Would it also be correct to say that the revolving storm is stirred by the planet’s rotation and that it has raged for more than three centuries?

The formation of vortices is certainly related to the rapid rotation of the planet, as eddies are spawned by the unstable jets, interact and merge to provide energy to continually power the GRS.  It’s longevity is unclear - it’s been there since at least the nineteenth century, and there’s the suggestion that large storms might be commonplace in this region of Jupiter’s atmosphere.  I.e., if our GRS eventually dissipates, maybe another will form at a similar latitude.



5. Has the GRS always been oval shaped? And has it always been wider across (ie from east to west) rather than "higher" (ie from north to south)?

The early observations show a very elongated oval, which is shrinking steadily in east west extent.  The shrinkage has been known for years, but amateur and professional data suggest that it’s now accelerating rapidly.


6. Is the GRS showing signs of becoming more circular?

That’s certainly what it looks like!


7. There have been some suggestions that the anticyclone storm is shrinking at the rate of about 1000 kilometres a year. Is this correct?

Not according to the Hubble and Voyager measurements, which show a decrease from 14,500 miles in 1979 to 10,250 miles in 2014.


8. If the shrinkage continued at this rate, does this imply that the GRS would have disappeared by the year 2030, or thereabouts, please?

That’s very hard to say, as it depends on the reason why the GRS is shrinking, and whether any particular aspect ratios (i.e., ovals or circles) are more stable than others.  We certainly live in an interesting time.


9. Lastly, is it conceivable that the shrinkage will stop at some stage and perhaps start increasing in size again?

Absolutely.  If the GRS is maintained by swallowing up smaller storms and eddies that have the misfortune to be at the same latitude, then large storms (such as those seen in 2010-11 when Jupiter’s faded SEB revived) could feed more energy into the GRS  and help it to grow again. We’ll have to watch what’s going on very carefully.

Friday, 29 August 2014

Missions to the Ice Giants

The past several years has seen a resurgence in interest in exploration of Uranus and Neptune as the next logical step in the exploration of the Outer Solar System.  In the USA, NASA's 2009 planetary decadal survey ranked Uranus as a high priority (following Mars and Europa) for a future flagship class mission, despite the low likelihood of billion-dollar missions in NASA's present roadmap.  In Europe, teams of scientists have proposed Uranus missions as part of the 2010 Cosmic Vision call for Medium-class missions (Uranus Pathfinder for M3) and the 2013 call for Large-class mission science themes (L2 and L3 launch slots).  Although none of these mission concepts have been accepted to date, they have served to raise the profile of the ice giants as key destinations for future exploration, ticking all the right boxes within ESA's Cosmic Vision programme.  The momentum has been maintained on both sides of the Atlantic via international workshops (Paris 2013 and Maryland 2014) dedicated to the exploration of the ice giants.  And with the recent announcement of opportunity for ESA Medium class missions (M4) for launch in 2025, there's never been a better time to consider the merits of exploring the ice giants.

Uranus as seen by Keck in 2012, via the Planetary Society:
Credit; NASA / ESA / L. A. Sromovsky / P. M. Fry / H. B. Hammel
 I. de Pater / K. A. Rages
Exploration of Uranus and Neptune is a gaping hole in our current exploration of the solar system, having been visited only once by Voyager 2 in the late 1980s.  This was just a flyby mission, compared to the detailed orbital reconnaissance of Jupiter (Galileo, Juno and the upcoming ESA JUICE mission) and Saturn (Cassini).  Although Voyager 2 made history, and provided humankinds' first close-up views of these distant worlds, that's now been a quarter-century ago, using technology developed in the 1970s.  Even if construction of a new mission began today, it's likely that half a century will have gone by since the heady days of the Voyager flybys before we arrive at Uranus or Neptune.  Just imagine what we could accomplish with a sophisticated 21st-century spacecraft, orbiting these worlds for the first time!

But why bother with these distant balls of gas?  That's a question that comes up so often I'm trying to organise my thoughts into compelling reasons, heavily biased towards my atmospheric science perspective.  Below I don't even mention their bizarre magnetospheres (completely tilted by 50-60 degrees from the rotation axes), diverse satellite systems and unusual rings, which serve to make them even more compelling destinations.

1.  What makes the Ice Giants special, and different from the Gas Giants?
Some of the latest surveys of exoplanetary systems seem to suggest that planets of Uranus/Neptune size (i.e., 14-18 Earth masses and around 4 Earth radii) might be commonplace, the missing link between the larger gaseous worlds like Jupiter and Saturn and the smaller terrestrial worlds (Earth and Super-Earth sized planets).  If that turns out to be true, once observational bias in the exoplanet samples are resolved, then we might have two great examples of the most common planetary type here in our own solar system.  The major difference between the two categories (gas and ice giant) are caused by their origins - by forming more slowly in the distant solar system, the ice giants couldn't suck up as much of the primordial hydrogen and helium as the gas giants, making them smaller and relatively more enriched in heavy elements.

Their size, rotation (16-17 hours), composition and low temperatures then account for the broad observational differences - blue colours due to long paths through red-absorbing methane gas; fewer zonal jet streams and cloud outbursts than their giant planet cousins (mainly one westward jet at the equator and a prograde jet encircling each pole), etc.  But this is a very basic picture, and any future mission would hope to construct a full three-dimensional understanding of an ice giant atmosphere, from the tenuous thermosphere down to the cloud decks of ices (methane, ammonia, etc.) and into the deeper interior, to test our understanding of the physics and chemistry of planetary atmospheres under these extreme conditions.
Neptune from Voyager 2, from the Planetary Society Blog,
with credit to NASA / JPL / Björn Jónsson 


2. Why do Uranus and Neptune appear so different, despite their shared origins?
If Uranus and Neptune formed at about the same time in the solar nebula, at about the same temperatures, accreting material from the same icy reservoirs and then undergoing the same thermal evolution, why do they look so different today?  Neptune has some of the most powerful winds in the solar system, with cloud features shifting and evolving over hourly timescales, despite its great distance (30AU) from the Sun.  Uranus, on the other hand, appears sluggish most of the time, with the occasional convective outburst that can be seen punching through the overlying hazes that cause the blue-green fuzzball appearance in visible light.  Neptune has an 'Earth-like' axial tilt subjecting it to 'normal' seasons over its 165-year orbit (summer solstice was in 2005), whereas Uranus has been completely bowled over onto its side, moving on its 84-year orbit like a spinning top on its side.  From the atmospheric science perspective, Uranus really is the oddball of the solar system, rotating on its side so that its poles are subjected to 42 years of summer sunlight then 42 years of winter darkness (the last equinox was in 2007).  That should set up the most extreme seasonal changes of any planet in our solar system, but we don’t really have a good understanding of how the atmosphere responds to those vast changes in sunlight.  Maybe some cataclysmic event deep in Uranus' past, such as a collision with another forming planet, completely bowled the planet over onto its side.

Can such an event explain the fact that Uranus is so sluggish, whereas distant Neptune is so active?  Maybe.  All the giants glow in infrared light, emitting more heat energy than the light they receive from the Sun.  They have their own internal heat, powered by slow gravitational contraction and possibly helium settling on Jupiter and Saturn.  But Uranus has no heat source that we can detect.  So is there something weird about the interior and atmosphere that prevents broad convection, and traps that old heat inside?  Or was all the heat lost catastrophically, maybe connected to whatever cataclysmic impact knocked the ice giant over onto its side?  Again, Uranus’ lack of internal energy makes it stand out as one of the strangest targets in our solar system from an atmospheric perspective, and maybe that’s why we see so few discrete clouds and storms.  Neptune, on the other hand, has the strongest internal heat of any giant planet, so this is likely powering the meteorology with only a small amount of help from the Sun.  It is these stark differences between the two ice giants, despite their similar composition and origin, that make them so tantalising.

3.  What can the ice giants tell us about the evolution of the outer solar system?
Due to their vast size, the giant planets lock away the fingerprints of formation in their chemical soup.  Once the gases, ices and rocks are accreted by the forming planets, they find it rather hard to escape again, being forever locked away in the planets we know today.  Of course, there might be significant reprocessing of the material - heavier stuff settling downwards to potentially form a core; chemical reactions and cloud formation locking certain species away.  But the bulk composition still contains the balance of elements and isotopes that must have been present in the solar system 4.5 billion years ago.  Simply put, comparisons of giant planet composition can help us understand how they formed and evolved.  But precise comparisons need accurate measurements, and we only really have that from Jupiter's Galileo probe in 1995.  A probe entering an ice giant world to sniff out the chemical composition (particularly water, Nobel gases and the carbon, sulphur and nitrogen chemicals below the main cloud decks) would provide a paradigm shift in the understanding of these worlds.  Hopefully any future mission to orbit an ice giant would take an entry probe along with it.

Uranus through the wavelengths, showing the capabilities of a sophisticated orbiter.

So think of Uranus and Neptune as the missing links, helping us to explain the story of our own solar system, and connect us to the most common types of planets throughout our galaxy.

Further Reading:




Monday, 25 August 2014

Uranus on BBC Future

Back at the beginning of August, Chris Arridge and I were contacted by Richard Hollingham, a science journalist and presenter of the Space Boffins podcast.  He wanted to write an article on the renewed interest in the exploration of Uranus, and here's the result (although a shame they couldn't include some of the great Voyager or ground-based images of the planet!).

http://www.bbc.com/future/story/20140822-the-mission-to-an-un-loved-planet