May 14, 2008

Interviews: Water on Mars

Even in the clearest, bluest sky on Earth, there is still water vapor in our atmosphere. If you could condense all the water vapor out of the atmosphere above you, it would form a layer of water two centimeters deep. On Mars today, there is also water vapor in the atmosphere but it would create a layer just 10 micrometers thick.
As on Earth, this water is constantly moving through a cycle of condensation and evaporation. When it condenses, it falls to the surface. When it evaporates, it re-enters the atmosphere and is blown by the winds around the planet before condensing and starting the cycle over again. Regardless of the apparent paucity, the constant movement of water through the Martian atmosphere has an important effect on the Martian climate.

Mars Express carries three instruments, PFS, SPICAM and OMEGA, which allow planetary scientists to study the water cycle of Mars in unprecedented detail. With their ocean of new data, scientists are building a multifaceted story of the martian water cycle. It is strikingly similar to Earth in some respects and exotically different in others.

We took the chance to question some experts on this subject while they were at the Mars water cycle workshop in Paris, France.

The martian water cycle and climate

With Robert Haberle, NASA Ames Research Center and Oleg Korablev, Space Research Institute, Moscow, Russia.
Q. How would you describe the martian water cycle?
Robert Haberle, NASA Ames Research Center [RH]: It is much less intense than on Earth. Nevertheless, water vapor content varies substantially in the atmosphere of Mars, both with the season and by geographical location. What we have to do is measure the amount of water vapor, then work out where it is coming from and where it is going. 

Q. What effect does water vapor have on the martian atmosphere?
Oleg Korablev, Space Research Institute, Moscow, Russia [OK]: Water vapor in the atmosphere of Mars is a signature of water being transported around the planet. Water ice exists on Mars in the polar caps and within the soil. If it is going to move from one place to another, it must evaporate or sublimate and enter the atmosphere. Once in the atmosphere, it can make clouds and these clouds reflect solar radiation back into space. So the clouds have a cooling effect on the martian climate.

Q. There are observations of water vapor that stretch back to NASA's Viking missions in the 1970s, how do the new Mars Express res
RH: They show a drier water cycle than previously. This means that there could be significant annual variations and, if true, that could be very interesting. If we believe the results as they stand, they tell us that, at the time of the Viking missions, there was almost twice the quantity of water in the atmosphere over the north pole of Mars as there is now.

Q. Is there any doubt about the older results?
OK: The spectroscopic database used to interpret the Viking results was incomplete. This may mean that the analysis over-estimated the wetness of the atmosphere. It is too early to be certain about this but it looks as though a re-analysis using modern techniques may make them more consistent with the Mars Express results. This would suggest a more stable martian water cycle. Scientifically, that would be less fascinating but probably more reasonable.
Q. What are the major sources of water on Mars?
RH: That's the $64,000 question! The north polar cap must be a major water source. By the summer, the carbon dioxide ice at the pole has evaporated away revealing a layer of water ice beneath. This can then sublimate into the atmosphere. At the south, the carbon dioxide ice never completely goes away, so the water there remains trapped. Computer models suggest that the sublimation and condensation of the north pole is responsible for the major features of the martian water cycle.

Interview: Frost and ice below the martian surface

With Bill Feldman, Planetary Science Institute, Arizona, USA and Bernard Schmitt, Laboratoire de Planetologie de Grenoble, France.

Q. Where can we expect to find water ice on Mars?
Bill Feldman, Planetary Science Institute, Tucson [BF]: Water ice is never stable within 45º of the equator. Poleward of 50º in both hemispheres, ice can be stable under the surface. In the north, ice plains have recently been mapped in a ring around the pole at these latitudes. Once you get beyond 85º, ice can exist on the surface. At the north pole there is a 2-3-km-thick layer of water ice on the surface. It is similar at the south pole, but with carbon dioxide over it.

Q. When was the ring of water ice surrounding the north polar cap discovered?
BF: It was discovered by the neutron spectrometer onboard NASA's Mars Odyssey in 2002. The instrument actually shows that lots of hydrogen is present in the martian soil. About 45% of any planetary surface is composed of oxygen because silicon dioxide is one of the main components of rock. Because a water molecule contains two hydrogen and one oxygen atoms, it implies that there must be lots of water ice there.

Q. Are there other seasonal sources of water in addition to the north polar cap?
Bernard Schmitt, Laboratoire de Planetologie de Grenoble, France [BS]: Earlier, we thought that the seasonal carbon dioxide layer at the north polar cap had to sublimate before the water ice there was exposed. So, water couldn't start entering the atmosphere until the late spring. But sublimation of seasonal deposits of carbon dioxide releases some millimeters of water frost that form a moving ring of ice on the surface around the pole, starting at low latitude of about 50°. It means that water trapped in these deposits sublimates into the martian atmosphere starting in the early spring. It changes the details of the water cycle but not the global picture.
Q. What does Mars Express allow us to do?
BS: The OMEGA instrument shows us where and when we can find ice on the martian surface. It can also tell us what is water and what is carbon dioxide. This is important because carbon dioxide ice is at 145 K (128° below zero), so as soon as water comes into contact with carbon dioxide ice, it becomes trapped. The south polar cap is a major trap for water vapor on Mars. Once it touches the carbon dioxide there, it cannot escape again. OMEGA is also showing us minerals that have adsorbed water, so water molecules stick to the surface of the minerals in the uppermost layers of Mars. So these, too, can release water into the atmosphere in the summer when they have been heated up.
Q. Where next for this research?
BF: We need rovers and landers to chemically analyze the hydrated minerals. Currently the NASA rovers are doing this. Computer models can tell us how stable these minerals have been in the past. There is a lot of work still to do before we fully understand the martian water cycle and the past climate of the planet.


Image Caption: This HRSC image provides a perspective view of residual water ice on the floor of Vastitas Borealis Crater on Mars. The image is centered at 70.17º North latitude and 103.21º East longitude. Credits: ESA/DLR/FU Berlin (G. Neukum)


On the Net:

NASA Ames Research Center

Space Research Institute

Planetary Science Institute

Laboratoire de Planetologie de Grenoble