NASA Mars HiRise: Study of Aeolis Dorsa and Halcyon times

This NASA image obtained by the Mars HiRISE camera March 13, 2014 shows a sand dune field in a Southern highlands crater on Mars

Cold and dry today, Mars was previously warm and wet but possibly only at intervals, a study published on Sunday suggests.

Scientists have long puzzled over what happened to the water, the precious stuff of life, on the Red Planet.

Unmanned spacecraft have sent home tantalising images of gouged canyons, valleys and sedimentary deltas, while landers have found hydrous rocks, all suggesting Mars at one time hosted hundreds of kilometres (miles) of rivers and lakes.

Today, though, Mars is too cold and the pressure of its carbon-dioxide atmosphere way too low for liquid H2O to exist. If you tried to pour water on its surface, it would simultaneously freeze and vapourise.

So when did Mars host liquid water? And what happened to it?
In a study published in the journal Nature Geoscience, planetary geologist Edwin Kite of the California Institute of Technology takes a new stab at the riddle.

Edwin Kite

Kite and his team measured craters, left on the Martian surface by asteroid collisions, to gain an idea of its past atmospheric pressure.

The principle behind their calculation is this: the thicker the atmosphere, the bigger the space rock has to be to survive the friction of contact with it.

Conversely, a thinner atmosphere means that smaller rocks are able to survive the descent and whack the surface.

Aeolis Dorsa

Kite's team looked at 319 craters in Aeolis Dorsa, a 3.6-billion-year-old region that shows evidence of past rivers to get an indication.

Mystery of flowing water They calculated that these craters were formed when Mars had atmospheric pressure of up to 0.9 bar.

This pressure is 150 times greater than that of today and intriguingly close to that of the water-rich Planet Earth at sea level.

The bad news, though, is that Mars is far more distant from the Sun than Earth and at that far-off time, our star was much less bright than now.

As a result, Mars would have required pressures of at least five bar for its surface to keep above the freezing point of water. It seems to have lacked a long-lasting thick atmosphere during its river period.

"If Mars did not have a stable multi-bar atmosphere at the time that the rivers were flowing—as suggested by our results—then a warm and wet CO2/H2O greenhouse is ruled out, and long-term average temperatures were most likely below freezing," said the study.

Sanjoy Som

This throws up other possible explanations for the water, said Sanjoy Som of NASA Ames Research Center in a commentary published in the same journal.

One is that the water was high in acidity and salt content, giving it a lower freezing point and enabling it survive as a liquid in lower air pressure.

Another is that greenhouse gases from volcanic eruptions helped Mars, for a while, to have a denser atmosphere that enabled the water to flow.

Another possibility is "transient intervals" of denser atmosphere caused by the planet's tilt, said Som.

Like a child's top that is slightly off centre, Mars tilts slowly around its axis of spin.

It takes 120,000 years to complete one axial revolution, a timescale that leads to major changes in the amount of sunlight reaching its poles, whose water either froze to form ice-sheets or warmed to "reinflate" the atmosphere and form rivers that flowed at kinder times.

More information: Nature paper: Low palaeopressure of the martian atmosphere estimated from the size distribution of ancient craters, www.nature.com

Svalbard's Frozen antenna field sounding out the Tropopause

(Image: Vincent Fournier)

Svalbard - the most northerly outpost of Norway and home to some 3000 polar bears - is an isolated group of glacier-covered islands deep within the Arctic Circle.

As we know only too well, glaciers haven't been faring well in recent years.

But it isn't the state of the ice that is being monitored here.

Instead, the vital climate change research being carried out is aimed high in the sky, to the mysterious region of the atmosphere known as the tropopause.


Stand on the surface of the Earth and your head is in the troposphere. Near the poles the troposphere is at its thinnest: about 9 kilometres thick. At the equator it is more like 17 kilometres.

Above it is the stratosphere, and at the intersection between the two is the tropopause.

Monitoring the region is the SOUSY Svalbard Radar (SSR), an array of 96 Yagi-UDA antennas in Adventdalen on Spitsbergen Island, shown in the photo.  

SOUSY stands for sounding system: its job is to sound out the activity of the atmosphere, by measuring phenomena such as gravity waves and air turbulence.

We need to know what the tropopause is doing because greenhouse gases have very different effects above and below it.

"While increasing greenhouse gas concentrations contribute to warming the troposphere, the very same gases diffuse into the middle atmosphere where they act as refrigerants and therefore cause 'global cooling'," says Chris Hall of the University of Tromsø, Norway, who works on the SSR.

Since the altitude of the tropopause is affected both by warming from below and cooling from above it is particularly sensitive to radiative forcing, the amount of solar energy trapped in the atmosphere.

"The SOUSY radar monitors parameters such as tropopause height 24/7," says Hall. "A long-enough time series built up by this instrument helps us monitor and understand climate change in new ways."

Climate Change and the Greenhouse Effect