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:

• The Uranus Pathfinder Mission 2010: http://www.mssl.ucl.ac.uk/planetary/missions/uranus/
• Experimental Astronomy Article on Uranus Pathfinder (2012).