Hi there! Well, we already knew that there has been liquid water on the surface of Mars, a long time ago. Indeed, the space mission Mariner 9 imaged valley networks in 1972. Since then, several missions refined the data. The study I present today, Estimate of the water flow duration in large Martian fluvial systems, by Vincenzo Orofino, Giulia Alemanno, Gaetano Di Achille and Francesco Mancarella, uses the most recent observations to estimate the length and depth of former Martian rivers, and their duration of formation and erosion. This study has recently been accepted for publication in Planetary and Space Science.
Evidences of liquid water in the past
The current atmosphere of Mars is pretty thin, its pressure being on average 0.6% the one of the Earth. Such a small atmospheric pressure prevents the existence of liquid water at the surface. Water could survive only as ice, otherwise would be just vaporized. And ice water has been found, particularly in the polar caps. But if the atmosphere were thicker in the past, then liquid water would have survived… and we know it did.
We owe to Mariner 9 a map of 85% of the Martian surface, which revealed in particular river beds, deltas, and lake basins. The study we discuss today focused on valley networks, which are particularly present in the southern highlands of Mars. These valleys are typically less than 5 km wide, but may extend over thousands of kms, and they reveal former rivers.
The history of these rivers is inseparable from the geological history of Mars.
The geologic history of Mars
We distinguish 3 mains eras in the geological history of Mars: the Noachian, the Hesperian, and the Amazonian.
The Noachian probably extended between 4.6 and 3.7 Gyr ago, i.e. it started when Mars formed. At that time, the atmosphere of Mars was much thicker that it is now, it generated greenhouse effect, and liquid water was stable on the surface. It even probably rained on Mars! During that era, the bombardment in the inner Solar System, including on Mars, was very intense, but anyway less intense than the Late Heavy Bombardment, which happened at the end of the Noachian. Many are tempted to consider it to be the cause of the change of era. Anyway, many terrains of the south hemisphere of Mars, and craters, date from the Noachian. And almost all of the river beds as well.
After the Noachian came the Hesperian, probably between 3.7 and 3.2 Gyr ago. It was a period of intense volcanic activity, during which the bombardment declined, and the atmosphere thinned. Then came the Amazonian, which is still on-going, and which is a much quieter era. The volcanic activity has declined as well.
So, almost all of the valley networks date from the Noachian. Let us now see how they formed.
Use of recent data
We owe to the space missions accurate maps of Mars. From these maps, the authors have studied a limited data set of 63 valley networks, 13 of them with a interior channel, the 50 remaining ones without. The interior channel is the former river bed, while the valley represents the area, which has been sculpted by the river. The absence of interior channel probably means that either they are too narrow to be detectable, or have been eroded.
These valley networks are located on sloppy areas, most of them close to the equator. The authors needed the following information:
- eroded volume,
- valley slopes,
- width and depth of the interior channel.
To get this information, they combined topographic data from the instrument MOLA (for Mars Orbiter Laser Altimeter) on board Mars Global Surveyor (1997-2006) with THEMIS (THermal Emission Imaging System, on board Mars Odyssey, still operating). MOLA permits 3-D imagery, with a vertical resolution of 30 cm/pixel (in other words, the accuracy of the altitude) and a horizontal one of 460 m/pixel, while the THEMIS data used by the authors are 2D-data, with a resolution of 100 m /pixel. When the authors judged necessary, they supplemented these data with CTX data (ConTeXt camera, on board Mars Reconnaissance Orbiter, still ongoing), with a resolution up to 6 m/pixel.
These information are very useful to estimate the formation time and the erosion rate of the valley network.
Dynamics of formation of a river bed
They estimated these quantities from the volume of sediments, which should have been transported to create the valley networks. The idea is, while water is flowing, assisted by the Martian surface gravity (fortunately, this number is very well known, and is roughly one-third of the gravity on Earth) and by the slope, it transports material. The authors assumed in their calculations that this material was only sediments, i.e. they neglected rock transport, and they did the maths.
Several competing models exist for sediment transport. This is actually difficult to constrain, given the uncertainties on the sediments themselves. Such phenomena also exist on Earth, but the numbers are very different for instance if you are in Iceland or in the Atacama Desert.
It also depends on the intermittence: is the water flow constant? You can say yes to make your life easier, but is it true? On Earth, you have seasonal variations… why not on Mars? A constant water flow means an intermittence of 100%, while no water means 0%.
And keep also in mind that the water flow depends on the atmospheric conditions: is the air wet or pretty arid? We can answer this question for the present atmospheric conditions, but how was it in the Noachian?
No icy Noachian
And this is one result of the present study: there must have been some evaporation in the Noachian, which means that it was not cold and icy. The authors show that such a Noachian would be inconsistent with the valley networks, as we presently observe them.
However, they get large uncertainties on the formation timescales of the valley networks, i.e. between 500 years and almost twice the age of the Solar System. They have anyway median numbers, i.e.
- 30 kyr for a continuous sediment flow,
- 500 kyr with an intermittence of 5%,
- 3 Myr with an intermittence of 1%,
- 30 Myr with an intermittence of 0.1%.
And from the data, they estimate that the intermittence should be in the range 1%-5%, which corresponds humid (5%) and semiarid/arid environments. This is how they can rule out the cold and icy Noachian.
The study and its authors
- You can find the study here. And now the authors:
- The website of Vincenzo Orofino,
- the Google Scholar profile of Gaetano Di Achille,
- and the ResearchGate profile of Francesca Mancarella.