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

Ozone hole might slightly warm planet

A map of ozone concentrations in the Southern Hemisphere shows thinning of the ozone layer over the South Pole. 

This region of reduced ozone, which is called the "ozone hole," causes changes in wind patterns and cloud cover. 

Credit: NASA

A lot of people mix up the ozone hole and global warming, believing the hole is a major cause of the world's increasing average temperature. Scientists, on the other hand, have long attributed a small cooling effect to the ozone shortage in the hole.

Now a new computer-modeling study suggests that the ozone hole might actually have a slight warming influence, but because of its effect on winds, not temperatures.

The new research suggests that shifting wind patterns caused by the ozone hole push clouds farther toward the South Pole, reducing the amount of radiation the clouds reflect and possibly causing a bit of warming rather than cooling.

Kevin Grise

"We were surprised this effect happened just by shifting the jet stream and the clouds," said lead author Kevin Grise, a climate scientist at Lamont-Doherty Earth Observatory of Columbia University in New York City.

Grise notes this small warming effect may be important for climatologists trying to predict the future of Southern Hemisphere climate.

The work is detailed in Geophysical Research Letters, a journal of the American Geophysical Union.

Each ozone molecule consists of three oxygen atoms bound together. These ozone molecules gather in the lower portion of the stratosphere about 20 to 30 kilometers (12 to 19 miles) above the ground—about twice as high as commercial airliners fly.

Thankfully for the living things below, this layer of ozone shields Earth from some of the hazardous ultraviolet radiation barraging the atmosphere. Unchecked, these ultraviolet rays can cause sunburns, eye damage and even skin cancer.

In the 1980s, scientists discovered thinning of the ozone layer above Antarctica during the Southern Hemisphere's spring months.

The cause of this "hole" turned out to be chlorofluorocarbons, such as Freon, from cooling systems, aerosols cans and degreasing solvents, which break apart ozone molecules.

Even though the1987 Montreal Protocol banned these chlorofluorocarbons worldwide, the ozone hole persists decades later.

More information: The Ozone Hole Indirect Effect: Cloud-Radiative Anomalies Accompanying the Poleward Shift of the Eddy-Driven Jet in the Southern Hemisphere, in Geophysical Research Letters. onlinelibrary.wiley.com/doi/10.1002/grl.50675/abstract