Why is there no liquid water on Mars at present?
Here on Earth, we have water as liquid in the oceans, in solid form (ice) in the ice caps and glaciers, and as vapor in the atmosphere. On Mars, the story is different.
The figure to the right shows a phase diagram of water. On this diagram, we can see that on Earth, both the surface temperature and pressure are high enough for water to exist in both solid, liquid and vapor phases. This is different on Mars: the low pressure and low temperatures do not allow water to be stable in the liquid phase. Therefore, water on Mars is usually only stable as ice on the surface and as vapor in the atmosphere.
However, the diagram also illustrates that the typical pressure and temperature on the surface of Mars are relatively close to the point where water is stable as liquid, solid and vapor (called the triple point). This means that if the temperature and pressure increase for a short period of time, for example on a warm summer day, it is possible for ice to melt instead of going directly from ice to vapor.
An example of this can be seen on the picture below. The two pictures show the same crater side with 6 years in between. In those years, a new pattern formed on the side of the crater, probably originating from subsurface ice melting and running down the crater slope.
You can read more about the newly discovered patterns on the Malin Space Science Systems website.
There is much evidence that liquid water once existed on Mars, both of rivers and lakes or oceans. The figure to the right shows a picture of a flow pattern, created by small rivers that turn into bigger rivers (a so called dendritic network) on the surface of Mars. Most of these features were created long time ago (over 3 billion years ago) when Mars had a different climate with a thicker atmosphere providing the necessary pressure and temperature conditions for liquid water to permanently exist at the surface.