How can "Geysers" on Europa reach heights of 100km? After seeing today's Washington Post's article NASA just saw Europa’s geysers erupting into space. Again., and especially the phrase "geysers erupting into space" I checked and Europa has a surface gravity of about 1.3 $m/s^2$, which is substantial - almost the same as the Moon's at about 1.6 $m/s^2$!

The BBC's website article Europa moon 'spewing water jets' says


  "Further evidence has been obtained to show that Jupiter's icy moon Europa throws jets of water out into space."


and 


  "The suggestion is that the jets reach several hundred kilometres in height before then falling back on to Europa."


On earth, pressure can come from geological hydrostatic forces and by steam produced by geothermal heating, but they rarely rise beyond a few dozen meters. That would be perhaps 100 meters in Europa's gravity.

The images shown of the water above Europa show heights of 100 kilometers and more. 

Unlike Earth rock being heavier than water, Europa's ice crust floats. What could be sources of hydrostatic pressure or pressurized steam near the surface that can create such "geysers" or "jets" that can project water to such heights in this gravity?



above:  The geyser Strokkur in Iceland, Earth, from here.
Technically those aren't "geysers" on Europa, they're cryovolcanos.    Though that definition may be a bit imprecise as well, but the 100 km eruptions on Europa probably have more in common with volcanic eruptions on Earth than geysers.   Europa undergoes significant tidal flexing which, in combination to it's solid icy crust, could be compared to explosive volcanic eruptions on Earth and not the cyclic geysers, which are far smaller.  

Volcanoes on Earth can shoot material high into the atmosphere on Earth but volcanic ash isn't a good example because it's not so much "shot" into the air as it rides rising hot air to altitudes 30 and 40 KM high.

Magma fountains on Earth can reach a few hundred meters and Tephra (anything larger than Ash, from pebbles to large rocks/boulders) can be found as far as a mile from a volcano and can exit the volcano at several hundred meters per second.   From the Tephra article:


  Blocks and bombs as large as 8-30 tons have fallen as far away as 1 km
  from their source (Bryant, 1991). Small blocks and bombs have been
  known to travel as far away as 20-80 km (Scott, 1989)! Some of these
  blocks and bombs can have velocities of 75-200 m/s (Bryant, 1991)


Europa's Tidal-flexing can cause significant pressures within it's icy crust.  See here.  and here. 

So, if we draw an Earth-Europa comparison, and assuming my math is right, using 1/2 at^2, all you'd need for material to reach 100 km on Europa is an exit velocity from the ground of a bit over 500 meters per second.   Or, some 20%-30% faster than a 22 caliber bullet.   That's faster than material usually leaves an Earth Volcano, but not all that much faster.   When frozen, Europa's outer icy crust is probably very hard and brittle.  In pressure spots where the pressure exceeds the structural integrity of the icy crust, it's not all that surprising that the results are pretty explosive, leaving the surface at 500 meters per second.  

Ice is very strong and brittle and when cold enough, where the Earth's crust, while rocks are quite brittle and strong, the crust overall can also be quite malleable, especially a few miles down in hotter temperatures under an erupting volcano.  

Very cold ice is resistant to cracking, but under enough pressure, when it finally does crack it can release explosive energy.   Ice cracks in water due to differential expansion, (Silicate rock doesn't expand with temperature nearly as much as frozen ice does), and while the cracking of an ice cube dropped into water might not seem like much, a big enough ice cube dealing with enough tidal heating can have explosive results.  

While this fun little video might seem unrelated (turn down your volume before watching), it demonstrates the explosive force that rapid warming can have on a hard block of ice.  This isn't steam pressure, as the steam is all escaping.  This is the inside of the block expanding faster than the outside can and the result is impressive.   https://www.youtube.com/watch?v=epkRd-w3TGw   So, with that in mind, explosive energy from a hard, cold 10 mile thick icy surface under the influence of tidal flexing shouldn't be all that surprising.