Martian Atmosphere Supersaturated with Water?

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Last week, scientists announced findings based on data from the SPICAM spectrometer onboard ESA’s Mars Express spacecraft. The findings reported in Science by Maltagliati et al (2011), reveal that the Martian atmosphere is supersaturated with water vapor. According to the research team, the discovery provides new information which will help scientists better understand the water cycle and atmospheric history of Mars.

What processes are at work to allow large amounts of water vapor in the Martian atmosphere?

The animated sequence to the left shows the water cycle of the Martian atmosphere in action:

When the polar caps of Mars (which contain frozen Water and CO2) are warmed by the Sun during spring and summer, the water sublimates and is released into the atmosphere.

Atmospheric winds transport the water vapor molecules to higher altitudes. When the water molecules combine with dust molecules, clouds are formed. If there isn’t much dust in the atmosphere, the rate of condensation is reduced, which leaves water vapor in the atmosphere, creating a supersaturated state.

Water vapor may also be transported by wind to the southern hemisphere or may be carried high in the atmosphere.In the upper atmosphere the water vapor can be affected by photodissociation in which solar radiation (white arrows) splits the water molecules into hydrogen and oxygen atoms, which then escape into space.

Scientists had generally assumed that supersaturation cannot exist in the cold Martian atmosphere, believing that any water vapor in excess of saturation instantly froze. Data from SPICAM revealed that supersaturation takes place at altitudes of up to 50 km above the surface when Mars is at its farthest point from the Sun.

Based on the SPICAM data, scientists have learned that there is more water vapor in the Martian atmosphere than previously believed. While the amount of water in Mars’ atmosphere is about 10,000 times less water vapor than that of Earth, previous models have underestimated the amount of water in the Martian atmosphere at altitudes of 20-50km, as the data suggests 10 to 100 times more water than expected at said altitudes.

“The vertical distribution of water vapour is a key factor in the study of Mars’ hydrological cycle, and the old paradigm that it is mainly controlled by saturation physics now needs to be revised,” said Luca Maltagliati, one of the authors of the paper. “Our finding has major implications for understanding the planet’s global climate and the transport of water from one hemisphere to the other.”

“The data suggest that much more water vapour is being carried high enough in the atmosphere to be affected by photodissociation,” added Franck Montmessin, Principal Investigator for SPICAM and co-author of the paper.

“Solar radiation can split the water molecules into oxygen and hydrogen atoms, which can then escape into space. This has implications for the rate at which water has been lost from the planet and for the long-term evolution of the Martian surface and atmosphere.”

However, water vapour is a very dynamic trace gas, and one of the most seasonally variable atmospheric constituents on Mars.

Source: ESA/Mars Express Mission Updates

How Water Protected Our Molecules

One would think that crafting a shield out of water wouldn’t do much good (not in medieval combat re-enactments, anyways). But that’s precisely what the molecules in the early Solar System – some of the same ones that you are made out of today, perhaps – may have done. In their case, protection from broadswords wasn’t as much of a concern as the effects of ultraviolet radiation from the Sun.

UV light is pretty hard on molecules because it readily breaks them up into their constituent parts. Larger organic molecules that coalesced in the dusty disk out of which our planets formed billions of years ago would have been broken apart by the Sun’s rays, but calculations by two astronomers at the University of Michigan show that thousands of oceans worth of water present in a protoplanetary disk can shield other molecules from being broken up.

Edwin (Ted) Bergin and Thomas Bethell, both of the Department of Astronomy at the University of Michigan, calculated that in Sun-like systems the abundance of water early on can absorb much of the ultraviolet light from the central star. By shielding other molecules from being broken up, they continue to persist in the later stages of the disk’s development. In other words, these molecules hang around until the formation of planetesimals and planets, and this mechanism could have been guarded the constituents of life from the ravages of the Sun in our own Solar System.

Circumstellar disks modeled by Bergin and Bethell in their paper include DR Tau, AS 205A and AA Tau.

Bergin told Universe Today, “At present there have been upwards of 4 systems with water vapor observed.  All are consistent with our model. I understand that there are numerous other detections of water vapor by Spitzer but these have yet to be published. The water vapor that we see is continually replenished by high temperature chemistry in these systems, so you would not see any degradation.”

In systems like the Solar System, planets form out of a disk of dust and gas that surrounds the young star. This large, flat disk later solidifies into planets, comets and asteroids. Near the center of the disk, between 1 and 5 astronomical units, warm water vapor in the disk could “protect” molecules inside this layer from being broken apart by UV light.

H2O breaks down when exposed to UV light into hydrogen and hydroxide. The hydroxide can be further broken down into oxygen and hydrogen atoms. But water, unlike other molecules, reforms at a quick pace, replenishing the shield of water vapor.

Smaller dust grains within the disk capture some of the UV radiation in the early formation periods of a protoplanetary disk. Once these dust grains start to snowball into bigger pieces, though, the UV light filters through and breaks apart molecules in the inner portions of the disk, where planets are in their early stages of formation.

The previous model for how organic molecules persisted past this point suggested that comets from the outer portion of the disk somehow fall into the center, releasing water to absorb the harmful radiation. But this model didn’t explain the hydroxide measurements for the disks so far observed.

If enough water is present, which seems to be the case in a handful of disks observed by the Spitzer Space Telescope, these other molecules remain intact, and as a bonus the water present in the interior portions of the disk also sticks around.

Bergin told Universe Today, “There are other molecules that can shield themselves – CO and H2 – but these cannot shield other molecules as well (because they capture only a fraction of the spectrum of light). Water is the only one with a strong formation that can compensate for destruction. It then provides the full shielding for other species. It is unlikely that another molecule will do this.”

This mechanism would only protect water vapor and other molecules in the inner part of the disk, closest to the star.

“This will likely be active in the inner few AU — at some point say between 5-10 AU it will become inactive and things will be inhospitable for various species [of molecule],” Bergin said.

So, where does all of the water go once the planets form? The vapor closest to the star – within about 1 AU – eventually gets broken down by the starlight into hydrogen and oxygen. At about 3 AU from the star, the water could constitute part of the planets and asteroids that form in that region. It may have been such asteroids that carried water to the surface of the Earth during its early formation, filling up our oceans. Outside of this region, H2O is broken down into hydrogen and oxygen and blown into space, said Bergin.

When asked whether this protective shield of water was present in our own Solar System, Bergin answered, “When we say that there were thousands of oceans of water vapor in the habitable zone, we mean around Sun-like stars.  Presumably this was present around our Sun as well.”

Source: Physorg, Science, email interview with Ted Bergin

Astronomers Could Detect Oceans on Extrasolar Planets

Imagine if astronomers could tell the difference between Earth-like extrasolar planets just by seeing the reflected light from their oceans? That sounds like science fiction, but a team of researchers have proposed that it’s really possible to detect the shape of the light curve glinting off an extrasolar planet and know if it has oceans.

This ground-breaking (water splashing?) idea was written in a recent journal article by D.M. Williams and E. Gaidos, entitled Detecting the Glint of Starlight on the Oceans of Distant Planets published January, 2008 in the Arxiv prepress e-Print archive.

The article describes the methods astronomers could use to detect the glint, or water reflection, from the “disk-averaged signal of an Earth-like planet in crescent phase.” They used the Earth as an example, and generated a series of light curves for a planet with our orientation and axial tilt.

They calculated that planets partially covered by water should appear much brighter when they’re near the crescent phase because light from the parent star reflects off the oceans very efficiently at just the right angles. By watching an extrasolar planet move through its orbit, its light curve should give off the telltale signature that there are oceans present.

According to their calculations, this method should work for about 50% of the visible planets. Furthermore, it should be possible to measure the ratio of land to water, and even get a sense of continents.

In order to test their theories, they’re planning to use remote observations of Earth, using interplanetary spacecraft. This will demonstrate if Earth can be observed at extreme phase angles—orbiting spacecraft around or on route to Mars.

And then the upcoming planet hunting missions, such as Darwin and the Terrestrial Planet Finder (if it ever gets completed) should be able to make the direct analysis of Earth-sized worlds orbiting other stars. Just by measuring the brightness, they should know if there are oceans, boosting the prospects for life.

Original Source: Arxiv