Image by NASA/JPL/MSSS
The question is,... What makes anyone still think the erosion features seen on Mars required liquid water for their formation, and if there ever was enough liquid water to generate them, where are the rest of the obligatory landforms that have to go with it?
I do not believe in long-vanished Martian rivers or oceans, despite many recent papers and press releases to the contrary, and suspect they will go the way of the infamous canals, to the embarrassment of many.
I accept that there is probably some frozen water at the poles, and that there are trace amounts of water in the atmosphere, and probably about 2% water in the soil, causing oxidation and perhaps forming laterites.
My contention is that Mars is nevertheless a very dry planet and probably always was. The old geological adage, that "the present is the key to the past" though not strictly accurate, proved very useful in unravelling Earth's geological history. Maybe it can profitably be applied to Mars also.
The surface features there can perhaps be explained without inventing a different past atmosphere, and without invoking huge flows of liquid water. If you want to count angels dancing on pinheads, first prove there are such things as pins. Minor technical details can follow.
The overall explanation may lie in the fact that Mars has an active wind-driven sedimentary system, working now just as it has for aeons. It differs in several ways from aeolian erosion systems operative on earth, since for starters the Martian wind is 95 percent carbon dioxide at 7 millibars, not 80 percent nitrogen at 1000 millibars, as here, and the surface gravity is only 0.38 that of Earth.
The geological effects of these differences are difficult to envisage, but very thin air can move at high velocities. High-level jetstreams above Earth reach speeds of 320 km per hour. Wind velocities on Mars are largely unknown, but it is quite clear, from the fact that dunes are to be seen all over the unfrozen parts of the planet, that the Martian atmosphere is capable of moving large quantities of fine material.
In the absence of liquid water and of the isostatic rebound, subduction and mountain-building which are driven on Earth by plate tectonics, the effect and power of the Martian wind would have been much greater than anything geologists have observed on Earth. Applied constantly over billions of years to an otherwise almost stable surface, it would have been quite adequate to produce all the erosion features observed.
There are serious geological problems with the notion that Mars once supported rivers and oceans of water, and that the outflow channels and valley networks there are the result of enormous river flows.
Liquid water does not exist on the surface now in any appreciable quantity, and cannot be supported by the present climate, as temperatures and pressures are too low. So a very different model encompassing a long-gone wetter climate has been invoked.
The atmosphere is mainly CO2, so that if there ever was a lot of liquid water on the surface there should now be a lot of carbonate rocks such as limestone. They should be easily evident, being very white when pure and hence having a very high albedo, but they have not been found.
The drainage features supposed to be evidence of the past wetter regime are too fresh to have been inactive for millions of years, let alone the billions currently suggested.