Ice

Ice is a comparatively rare gameplay element in the Metroid series. Composed of frozen Water like in the real world, it is mostly found in sub-zero locations, such as Phendrana Drifts, Arcterra and the Cryosphere.

Ice is sometimes mixed with another compound to create the much more durable Frigidite.

As an environmental feature
In Metroid Fusion, ice is purely for cosmetic purposes to distinguish ARC from other zones, though the area's supercooled atmosphere could harm her prior to obtaining the Varia Suit. Ice was also used to keep Ridley's body preserved.

Metroid Prime and Other M made ice a more prominent part of the gameplay, as ice walls blocked off upgrades in Phendrana (requiring the Plasma Beam) in the former, and both games had Stalactites hanging from the ceiling that could be dropped to kill enemies and create platforms via Missile explosions.

Underwater sections of Phendrana also contain fish frozen behind ice walls in the caves.

In combat
Samus often uses ice in her attacks. The Ice Beam is one of the most useful and potent weapons in her arsenal, and is essentially the only way to destroy the titular Metroids. Their vulnerability to ice also drove much of the plot for Other M.

Beyond the Ice Beam, Samus has also equipped herself with the Ice Missiles and Diffusion Missiles, with a similar effect.

Both the beam and missiles can freeze enemies solid into blocks of ice, allowing Samus to use them as platforms or shatter them instantly.

Fellow Bounty Hunters Noxus and Rundas use ice in their attacks as well, with Noxus wielding the Judicator and Rundas being capable of spawning ice to "surf" on as well as creating shields and projectiles out of the substance.

The Metroid Prime shifts between elemental forms. In its ice form it can fire a freezing beam that encases Samus in ice until she jumps or Bombs repeatedly, and can also summon slow-moving ice balls to hover toward her. It is also capable of launching freezing waves of ice in any form.