The answer may be that virtually all the water on Mars is now locked in the permafrost layer under the surface, with more contained in the planet's polar caps.

答案可能是，现在火星上几乎所有的水都被锁在地表下的永久冻土层中，更多的水被包含在这个星球的极地冰帽中。

youdao

These glaciers are the largest body of ice outside the polar caps.

VOA: standard.2010.01.23

Warmer temperatures cause water to expand,a phenomenon called thermal expansion, and cause glaciers and polar ice caps to melt.

VOA: standard.2009.11.09

Some of it has gone to make the polar caps.

BBC: Q&A: Liquid water on Mars

The reason for the excitement is that, although the existence of ice on Mars has been known since 1666, when the planet's polar caps were first seen, the low temperatures and air pressure there mean that water, even if heavily salted, is likely either to freeze or to evaporate.

ECONOMIST: Water on Mars

Yet while most people have read of the threat of rising sea levels and melting polar caps, or experienced a nagging guilt when booking a cheap flight or buying exotic fruit imported from the other side of the planet, hitherto global warming has largely been something we are happy to leave to the scientists to worry about.

CNN: Six degrees of devastation