Thursday, May 29, 2014

The paradigm of the snow line (1)

 
Saturn's Rings (photo from NASA's Cassini probe)

Saturn is very unlike the Earth. For one thing, its rings are made of almost pure water ice. Rings like that wouldn't last long in our neck of the Solar System. In daytime, the surface of the Moon can reach over 100 degrees Celsius, the boiling point of water on earth at sea level (at night, it gets below -200 Celsius). Water ice exposed to sunlight here wouldn't last very long. But out where Saturn is, water ice persists through the centuries that we've observed it. 

The Sun--with the help of atmospheric greenhouse gases keeping the average temperature of the Earth above freezing--keeps things toasty here near Earth. Exposed ice out from the Sun, past Mars, and into the Asteroid Belt does not last very long. Frozen comets traveling near the sun thaw and spew off water vapor and dust. However, once we pass a certain point in the Asteroid Belt, water ice is finally stable. This is the snow line; it is perhaps one of the simplest ideas but most profound-in-result structures in the Solar System. 

The snow line in our solar system isn't really that much different from snow lines on earth. Near the equator, the snow line is far up in elevation above the heat radiating surface and above the greenhouse gases that hold heat in. The further from the equator that you go, the lower the snow line gets until "up" in Greenland and "down" in Antarctica, the snow line is practically at sea level. This is the land of perpetual ice where the average temperature is below freezing. 

In space, the snow line seems set where it is between Mars and Jupiter. However, as the Solar System has changed from a disk of gas and dust to the aged system of today, the snow line's distance from the sun has changed through time. Even now, it slowly is moving outwards from the Sun as it slowly gets brighter (since the Cambrian explosion of life, the Sun has gotten about 10% more luminous).

Why do we care where the snow line is, that its location has changed through time, and that it exists at all? Why am I calling this blog "The paradigm of the snow line" (besides the rhyming of the words)? It seems mundane that the further that one goes from a heat source, the colder it becomes, correct?

In one way, the snow line defines the make up of our solar system. The gas giants--Jupiter, Saturn, Uranus, Neptune--are very different from the smaller terrestrial planets. The asteroid belt straddles the snow line, which is fortunate as tidal forces from the gas giants kept larger bodies from forming and melting, which preserved the very material that makes up the planets for us to study. 

In the next few posts, except when news interrupts the process, I want to explore the snow line and its ramifications. I'll get into ideas about the pre-solar nebula, how planets formed, and the role of the snow line in that process. What did the snow line have to do with where gas giants are located? When we see a gas giant planet very close to a star, does that mean that the snow line didn't affect its formation? Or does that say something about that solar system that is very different from our own, a place where huge planets can spiral inwards and probably destroy terrestrial planets that may have been there? 

This is a big topic, but understanding it is as big a thing as it is to understand that, on Earth, the mantle convects and drives plate tectonics, a mechanism that explains or affects the majority of processes that occur on Earth's surface. The snow line isn't a mechanism; it's simply a place that the defines the stability of water. But it's more than saying that "here Saturn's rings are stable" and "here they are not stable". Without the snow line, would Saturn itself exist?

Tuesday, May 6, 2014

Space Coasts

Ganymede, in natural light. Image captured by NASA’s Galileo spacecraft, 1996. From https://solarsystem.nasa.gov/multimedia/display.cfm?IM_ID=9003



Space coasts

Recently I was in central Florida, east of Orlando, to visit family. This area is near Cape Canaveral and the Kennedy Space Center. The barrier island has a long sandy beach where waves break onto fossil-filled ledges at low tide, a Walmart, low slung condos, schools of fish playing under piers, small beach bars, Patrick Air Force Base, and Space Coast Credit unions sprinkled up and down highway A1A. This is America’s Space Coast. It’s small town; it’s high tech.  From Cape Canaveral, SpaceX just tested a new rocket type where the first section, can make its way back to the surface, hover, and land .

In the Solar System, there are no other Space Coasts. Earth is in the “sweet spot”, the habitable zone around our star where water can exist as a liquid. Some scientists looked at the topography of Mars and say that there used to be similar coastlines there surrounding a northern ocean, although it remains to be proven.  Like Earth, Mars is also in the habitable zone, but most of its atmosphere bled into space long ago and liquid water is rare and ephemeral.   

Even though we know of no other surface water oceans (Titan, a moon of Saturn, has big lakes of liquid methane), there is evidence for layers of liquid water inside a growing number of bodies in the Solar System. Europa, a moon of Jupiter, has been long thought to have an ocean under its icy surface—if you ever watched the 1960s classic movie 2001, then you know its significance. Other bodies which may have of water within it include the dwarf planet Ceres with its water geysers, Jupiter moons Ganymede and Callisto (in addition to Europa), Saturn moons Titan and Enceladus, and, perhaps, Neptune’s Triton. Ganymede is not a small body—it is larger than Earth’s moon and the planet Mercury.

Cartoon of the results of Vance and other authors showing the interlayering of liquid water and different ice types or polymorphs. From http://www.huffingtonpost.com/2014/05/03/jupiter-moon-ganymede-ocean-life_n_5252853.html
In the journal Planetary and Space Science, Vance and other authors have modeled water’s behavior inside Ganymede. The authors, assuming that water is brackish with salts like magnesium sulfate (Epsom salt), calculate that Ganymede may have more than one ocean layer sandwiched between layers of water ice (see Figure). These ice layers are most likely different versions of the ice that we are familiar with. On the figure Ice I is ice that we are familiar with, ice that floats on water. Higher ices are stable at in the high pressures deep in the Ganymede ocean or in terrestrial lab experiments.


One of the more interesting findings is that liquid water, like on Earth, may lie on top of silicate rock deep within Ganymede. This is important as early terrestrial life may have begun in this setting on Earth, where some minerals provided the fuel to run chemical reactions.

Unlike Florida’s Space Coast, there are no palm trees blowing in the breeze and no waves breaking in the distance. On Ganymede, the meeting of ocean and land occurs in the dark at the bottom of a multi-layered ocean. But does life gather at this interface at the Ganymede equivalent of a beach-side bar? Without visiting, we won’t be able to prove if oceans or a tacky t-shirt shop can be found.

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