Valley network (Mars)
[3] Objections chiefly arise from repeated results from models of martian paleoclimate suggesting high enough temperatures and pressures to sustain liquid water on the surface have not ever been possible on Mars.
[4] The advent of very high resolution images of the surface from the HiRISE, THEMIS and Context (CTX) satellite cameras as well as the Mars Orbital Laser Altimeter (MOLA) digital terrain models have drastically improved our understanding of the networks in the last decade.
[1] The form of the tributary valleys is commonly described as "stubby" or a similar term, implying short lengths away from the trunk streams and amphitheater-like terminations at their heads.
[1][5] Many authors have described the drainage density of the networks as typically much lower than would be seen on Earth,[6][7][8] though the extent to which this may be an artifact of image resolution, landscape degradation or observer bias has also been raised in the literature.
Due to the later deposits on Mars, however, in almost all cases it is unclear whether the valley floors contain individual channel structures or whether they are fully inundated in flow events.
The concentration of the valleys in the Noachian-age southern highlands and their sparsity on the northern Hesperian plains, circumstantially combined with independent estimates of a multi-order of magnitude decrease in global martian erosion rates at the end of the Noachian,[12] probably indicates that most of the networks were cut during this early interval.
Processes as diverse as glaciation, mass wasting, faulting, and erosion by CO2, wind and lava have all been invoked at some point in the formation of some networks, and may play important roles locally in some regions on Mars.
[13] Independent lines of evidence also suggest the existence of liquid water at or very near the surface at various times in martian history, for example, evaporites at Meridiani Planum and pervasive aqueous alteration of rocks in the Columbia Hills, both investigated by the Mars Exploration Rovers.
[19] However, without some recharge mechanism for the putative aquifers producing this seepage, i.e., a hydrologic cycle of some kind, it is extremely unlikely that enough water could seep to cut all of the valleys formed in the Noachian.
This mechanism is appealing as it requires little conjecture about radically different past climate, and fits well with independent theories on the origins of the martian outflow channels at chaos terrains as major aquifer breaches.
[23][24] However, this version struggles to explain the longer, larger valley networks - if water flows hundreds or thousands of kilometers away from the heat source, ground will again be frozen and recharge will not be possible once again.
This precipitation may have occurred as rain or snow (with subsequent melt on the ground), but either demands a significantly more humid, and thus warmer and thicker, atmosphere than presently exists.
[26] Solutions to this apparent contradiction may include exotic mechanisms that do not require a sustained CO2-H2O greenhouse, such as episodic heating due to volcanism or impacts.
This CO2-H2 greenhouse has been subsequently found to be even more effective than originally demonstrated in Ramirez et al. (2014),[28] with warm solutions possible at hydrogen concentrations and CO2 pressures as low as 1% and 0.55 bar, respectively.
