With the recent images from New Horizons, my question is simple. How do we know these mountains are water ice? The linked article doesn't have much detail on how water ice could make 10,000m+ mountains.
Like the asteroid belt, it consists mainly of small bodies, or remnants from the Solar System's formation. Although many asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles (termed "ices"), such as methane, ammonia and water.
The average surface temperature on Pluto is $40$ $K$, $-233$ $^oC$.
At temperatures that cold, ice behaves like a rock, and can actually hold on to the topography and texture that we see on rocks on Earth.
During the formation of the solar system,
The inner Solar System, the region of the Solar System inside 4 AU, was too warm for volatile molecules like water and methane to condense, so the planetesimals that formed there could only form from compounds with high melting points, such as metals (like iron, nickel, and aluminium) and rocky silicates. These rocky bodies would become the terrestrial planets (Mercury, Venus, Earth, and Mars).
The giant planets (Jupiter, Saturn, Uranus, and Neptune) formed further out, beyond the frost line, the point between the orbits of Mars and Jupiter where the material is cool enough for volatile icy compounds to remain solid. The ices that formed the Jovian planets were more abundant than the metals and silicates that formed the terrestrial planets, allowing the giant planets to grow massive enough to capture hydrogen and helium, the lightest and most abundant elements.
Being beyond the orbit of Neptune, Pluto and the Kuiper belt are also well beyond the frost line. This is how scientists know Pluto is largely composed of frozen volatiles and that its mountains are made of water ice.
From the article you linked to:
The mountains are probably composed of Pluto’s water-ice “bedrock.”
Although methane and nitrogen ice covers much of the surface of Pluto, these materials are not strong enough to build the mountains. Instead, a stiffer material, most likely water-ice, created the peaks. “At Pluto’s temperatures, water-ice behaves more like rock,” said deputy GGI lead Bill McKinnon of Washington University, St. Louis.
emphasis mine. For now, water ice is our best guess based on the elements we know exist op Pluto and their properties. Confirmation should come from RALPH spectroscopic measurements of the region.
Planetary geologists have studied the behavior of various compounds at the temperatures found on Pluto, and:
Water ice is the only ice detected on Pluto that is strong enough at Plutonian temperatures to support such heights.