Water is an essential ingredient for life on Earth. If we want to find life on another world, we look for the presence of liquid water. We know there is liquid water on other worlds, but the question remains whether our planetary sibling, Mars, has liquid water. 

We have long known that Mars was once a wet world. Isotope measurements show Mars was an ocean world in its youth. Rain fell from the sky, and rivers flowed. But today Mars is cold and dry. Ice can be found on its surface, but long gone are the Martian rivers and seas. With weak gravity and little magnetic field, the red planet simply couldn’t hold a rich atmosphere, and so the liquid water evaporated and froze.

But it’s possible that some water might exist beneath its surface, and ice in its soil might be able to liquify under the right conditions. There is evidence to support this idea, such as through recurring slope lineae. These dark lines along the sides of hills look similar to what you would see if you scooped your hand through wet sand. Rather than a rich flow of water, it is a damp seeping through the sand. Looking at the images, it certainly looks like a liquid flow, but astronomers know the risk of simply assuming something to be true based upon appearances. If it looks like liquid water it’s worth a further look, but appearances are not proof.

In 2015 there was a great deal of excitement about these lineae when astronomers found evidence of perchlorate salts within them. The salts could mix with water to created a brine, and this brine could remain liquid even at the temperature and pressure of Mars’ summer weather. In the winter of Mars it would freeze, and this would explain why the recurring slope lineae seem to appear seasonally in the summer.

But while liquid water is a good explanation, it isn’t the only possible explanation. This is why the discovery was met with cautious optimism. Now it seems it was good to be cautious. New observations from the Mars Reconnaissance Orbiter point to a dry answer. Careful analysis shows that the lineae occur along steeper slopes, but stop when the slope reaches a shallow enough angle of incline. This critical incline is consistent with the critical incline seen in flows of dust and sand. It’s similar making a hill out of sand. It’s easy to build a shallow hill by piling on ever more sand, but if you try to make a steeper hill the sand topples down to the shallow angle.

New observations of recurring slope lineae show they behave more like dust than water. Credit: NASA/JPL/University of Arizona/USGS

Liquid water doesn’t have such a critical angle. If the lineae were due to liquid water, you might see it stop at a similar distance from its source, but water can still flow down shallow hills. So it seems this is a dry flow of dust and sand rather than a flow of water brine. If that’s the case, it could mean that Mars is very dry indeed. Perhaps too dry to support life.

As with earlier evidence, we should still be a bit cautious. There might still be liquid water on Mars in some places. The question of liquid water on Mars is so important that we want to be careful about both the evidence and our conclusions. It’s also the best way to do science if you want to get it right.

Paper: Colin M. Dundas, et al. Granular flows at recurring slope lineae on Mars indicate a limited role for liquid water. Nature Geoscience doi:10.1038/s41561-017-0012-5 (2017)

The post Getting It Wrong And Getting It Right appeared first on One Universe at a Time.

The origin of Earth’s water is a bit of a mystery. While water is common in our solar system, it’s much more common in the outer solar system, such as Jupiter’s moon Europa or Saturn’s moon Enceladus. Most of the worlds of the inner solar system are fairly dry. So how did Earth come to have large oceans on its surface?

There are two main ideas on the origin of Earth’s water. One is that Earth’s water was locked up in the original rocks and dust that formed our planet. As the material collapsed under its own gravitational weight, water was released and eventually formed the oceans we have now. The other idea is that any water in the original material escaped early on, and the current water of Earth came to our planet through the bombardment by asteroids and comets. Evidence that Venus and Mars were also wet in their early history points toward a formation origin of water, but there has generally been more evidence to support the bombardment model.

Three isotopes of atomic hydrogen. Credit: Dirk Hünniger, CC BY-SA 3.0.

This evidence comes through what’s known as the deuterium/hydrogen (D/H) ratio of Earth’s water. Deuterium is an isotope of hydrogen that has a nucleus of a proton and neutron, rather than the single proton of regular hydrogen. Chemically it reacts in the same way as hydrogen, but since it is heavier than regular hydrogen there are slight differences. For example, when a deuterium atom is part of a water molecule, the extra mass means it doesn’t evaporate as readily as regular water. Deuterium water is more likely to form in space than in the gravitational field of a planet, so the D/H ratio of water tells us about the origin of that water.

The D/H ratio for Earth’s oceans is about 150 parts per million, which is similar to that of chondrite asteroids. This would seem to support the bombardment model. But a new paper argues that such a conclusion is too simplistic. Our oceans cycle between the surface and interior of Earth, which could affect the D/H ratio. In this paper the team looked at rocks from Earth’s mantle, and they found that the water contained within these rocks has a much lower D/H ratio than that of our oceans. This suggests that mantle water formed locally rather than through astroid bombardment.

There’s still a number of unanswered questions. This latest work doesn’t disprove the bombardment model, and it’s possible that our water came from a number of sources. Further study on both fronts is needed to resolve this mystery.

Paper: Lydia J. Hallis, et al. Evidence for primordial water in Earth’s deep mantle. Science, Vol. 350 no. 6262 pp. 795-797 (2015) DOI: 10.1126/science.aac4834

The post Earth’s Oceans Have Always Been Local appeared first on One Universe at a Time.

At the dawn of the 20th century, Percival Lowell saw canals on Mars. This network of dark bands across the martian surface must surely be signs of open water. Their geometric patterns hinting that they were built by alien life. Subsequent observations proved Lowell wrong, and it serves as a cautionary tale about making unsubstantiated claims. With that in mind, let’s look at yesterday’s NASA announcement of liquid water on Mars.

Lowell’s sketches of the martian canals.

The history of our search for water on Mars is a bit convoluted, and like many discoveries in science it has happened in small steps rather than a revolutionary discovery. We’ve known that water exists on Mars for some time. First hinted by the discovery of martian polar caps, and then later when detailed observations of the northern ice cap confirmed that it was largely water ice. We’ve also found surface on other parts of Mars, and ice in the martian soil.

Water ice on the surface of Mars.

But it’s always been frozen water or water vapor, and that has to do with the temperature and atmospheric pressure on Mars. The martian surface can vary in temperature from -80 to 10 Celsius  (-110 to 50 F), and atmospheric pressure is 0.06% that of Earth at sea level. As a result, pure water ice sublimes directly to vapor, just as dry ice does on Earth. Just as our atmosphere isn’t thick enough to allow frozen carbon dioxide to melt, the atmosphere of Mars makes it impossible for liquid water to form. So we know that even a small lake of water on Mars was impossible. Mars was once a wet world, but it is now a desert planet.

Recurring slope lineae. Credit: NASA / JPL / UA / Emily Lakdawalla

But there has been some hints of liquid water appearing occasionally on the surface. The most tantalizing bit of evidence has been recurring slope lineae (RSL). They look similar to what you might see if you scoop your hand in wet sand. Observations of RSL have found that they are seasonal, and are most present when the surface temperature is warmest. This screams liquid water, but we don’t want to fall into Lowell’s trap of over extrapolating. “Warm” is a relative term here, and the atmospheric pressure is still far to low for RSLs to be caused by pure water.

Which brings us to yesterday’s announcement. Spectroscopic observations of RSLs from the Mars Reconnaissance Orbiter have found evidence of perchlorate salts in those regions. These are water soluble, and could create a briny water that could (temporarily) maintain a liquid state. This wouldn’t be a flow of water, but would be enough to wet the soil like a damp sponge. It’s possible that the salts would attract water moisture out of the air and dissolve to create a brine.

This is the best evidence we have of liquid water on Mars. If by liquid you mean damp soil. But even that is enough to get some folks excited about the possibility of life currently existing on Mars. Damp sand and soil on Earth is filled with life, and if we want to look for life on Mars, visiting a region of RSLs would be a great place to look. But we don’t want to assume too much. The possibility of life is very different from the confirmation of life.

Paper: Lujendra Ojha, et al. Spectral evidence for hydrated salts in recurring slope lineae on Mars. Nature Geoscience doi:10.1038/ngeo2546 (2015)

The post The Edge of Wetness appeared first on One Universe at a Time.

Our home planet is exceptional for being a warm rocky planet with plenty of water on its surface. There have been several proposed origins for Earth’s water, such as that it originated within the primordial materials of our planet, or that it was brought by cometary or meteor impacts. We know from measuring isotopic ratios in Earth’s water that much of it was formed before even our solar system, and that it seems to match water found in certain asteroids. So the most popular model for Earth’s water is that it was brought to our planet by asteroid-like meteors. But did asteroids in our young planetary system really have enough water to produce our oceans?

It would seem the answer is yes, and new research tends to support that conclusion. In this work, the authors looked at the spectra of a white dwarf and found evidence of hydrogen at oxygen in its atmosphere. A white dwarf is a sun-like star that has reached the end of its life. After is has consumed much of its hydrogen, fusing it to heavier elements like helium and carbon) it collapses under its own weight until it is roughly the size of Earth (but still the mass of the Sun). Because a star will enter a red giant stage before collapsing to a white dwarf, most of the lighter elements like hydrogen would be cast off. Likewise, heavier elements like oxygen would tend to settle into the core of the star. So one would think we wouldn’t see much of either hydrogen or oxygen in the spectra of a white dwarf.

It turns out we do tend to see hydrogen, which could be an indication that some of that light element didn’t get thrown off during the red giant stage. But the presence of oxygen would seem to indicate this material was accreted by the star relatively recently on a cosmic scale. The authors found evidence of other elements such as silicon and iron, which are common in asteroids. One explanation for this is that the white dwarf accreted an asteroid, and its remnants are seen in the star’s spectra.

So the authors calculated the size of such an asteroid from the spectra, and found that it would have been about the size of Ceres. If that’s the case, then the strength of the oxygen and hydrogen spectra would imply that the asteroid was about 38% water when it was accreted. That’s a sizable amount. We know that asteroids in our solar system such as Ceres and Vesta contain water, but the amount is still under investigation. If this roughly 1/3 fraction is common, then there was plenty of water for a young Earth to gain by meteor impacts. The fact that this was seen around a white dwarf is also an indication that water-bearing asteroids may be common in planetary systems.

This isn’t conclusive proof that Earth’s water did in fact come from meteor impacts, but it adds to a growing pool of evidence supporting that model.

Paper: R. Raddi, et al. Likely detection of water-rich asteroid debris in a metal-polluted white dwarf. MNRAS 450 (2): 2083-2093 (2015)

The post Shores of Cosmic Oceans appeared first on One Universe at a Time.

Yesterday I wrote about how we know Enceledus has liquid water by its cryovolcanoes. There are other ways we can tell if a moon has a water interior, but one of the more interesting methods is to look at a moon’s aurora. This approach was recently used to show that Jupiter’s moon Ganymede has more liquid water than Earth.

You might have seen an aurora on Earth when there has been a rise in solar activity. On Earth, the aurora (or northern lights) are caused by charged particles emitted by solar flares that interact with Earth’s magnetic field. When a charge enters Earth’s magnetic field, the magnetic field causes the charge to spiral along the magnetic field. For this reason magnetic fields tend to trap charges along their field lines. The charges generally stay trapped unless they collide with each other (not likely in interstellar space) or interact with something else, such as our atmosphere. Where the particles strike our atmosphere depends upon the energy of the charged particles, and thus the activity level of the Sun. As the activity level of the Sun varies, the latitude at which aurora are most prominent can vary.

A Hubble image of aurora superimposed on an image of Ganymede. Credit: NASA/ESA

Aurora have been observed on other planets, as well as moons such as Ganymede. The difference is that the aurora of Ganymede are driven by the interaction with Jupiter’s magnetic field rather than the Sun. The process is much the same as aurora on Earth, but the latitude at which they are observed depends upon fluctuations of Jupiter’s magnetic activity. This has been known for a while, but in this new work the team demonstrated that the latitude variations of Ganymede’s aurora can be used to study the moon’s interior.

Since Ganymede’s aurora are driven by the activity of Jupiter, the team could calculate the amount of variation one would expect given the strength of Ganymede’s magnetic field, giving a variation of about 6 degrees. However, the observed variation is only about two degrees. It would seem that something is dampening latitudinal oscillation of the aurora. One mechanism for this kind of dampening is an interior ocean of saline water. Salt water is a good conductor of electricity, so as Jupiter’s magnetic field varies, it induces a magnetic field in addition to Ganymede’s regular magnetic field. As a result, there are less latitudinal fluctuations in aurora.

The team created a model of this interior ocean. They found that without an ocean there would be a fluctuation of about 6 degrees, but with an interior ocean the fluctuations are lessened. Given an observed fluctuation of about 2 degrees, the interior ocean would need to be about 100 kilometers thick, starting about 150 kilometers beneath the moon’s surface. That means Ganymede has about 70% more water than Earth.

Paper: Saur, J., et al. (2015), The search for a subsurface ocean in Ganymede with Hubble Space Telescope observations of its auroral ovals, J. Geophys. Res. Space Physics, 120, doi:10.1002/2014JA020778.

The post A Matter of Degree appeared first on One Universe at a Time.

We normally think of Earth as the “water world” of the solar system. While it’s true that Earth seems to be the only planet with liquid water on its surface, it isn’t the only world with water, nor is it the one with the most water. In the past we thought that Earth was the only planet with liquid water, but even that’s changing. As we’ve sent probes to the outer planets, we’ve found that many moons show evidence of liquid water in their interiors. It may be that liquid water is most common in the outer solar system. Take, for example, Saturn’s moon Enceladus.

Enceledus is a small moon, only about 500 km in diameter. We’ve known for a while that it is geologically active, since we’ve observed active cryovolcanoes in the southern hemisphere. Given its size, this activity is likely driven by tidal effects from Saturn. But new evidence shows that not only is there liquid water within Enceledus, it is most likely warm. This comes from observations of the moon’s cryovolcanic plumes gathered by the Cassini probe. It seems that the plumes not only contain saline water, but also silica. The origin of this silica is not entirely clear, but its likely that it dissolved in the subsurface water before it was ejected. If that’s the case, the water would only only need to be in contact with rock, but also at a temperature of about 160 degrees Celsius.

There’s a scenario that could produce warm water in contact with rock, and that’s a geothermal vent, such as has been observed on Earth. This has raised speculation about whether Enceledus could harbor life, since thermal vents on Earth are known to be rich with life. One should be cautious in speculating too far in that direction. For one, though the thermal vent model is a likely cause, it isn’t a confirmed cause. We’ll need more observational data to confirm that. For another, the fact that Earth vents are rich with life doesn’t mean that extraterrestrial vents will also have life. It might be possible for life to survive there, but not for abiogenesis to occur.

Still, evidence of warm liquid water on outer moons is growing, and that means there are areas in the solar system beyond Earth where life as we know it could survive and thrive. It may just happen that the best place to look for life on another world might not be Mars, but rather the icy moons of Saturn and Jupiter.

Paper: Hsiang-Wen Hsu, et al. Ongoing hydrothermal activities within Enceladus. Nature 519, 207–210 (2015)

The post Water, Water, Every Where appeared first on One Universe at a Time.

We’ve known for a while that Mars was a wet planet about 4 billion years ago, but less clear was where that water went. Did it freeze into the surface, or evaporate into the planet’s thin atmosphere? Now a new paper in Science answers a bit of that question, and shows that Mars once had even more water than we’ve thought.

The work looks at water evaporating from the polar ice caps of Mars. In particular, the authors measured the level of deuterium water in the atmosphere compared to regular water. Deuterium is an isotope of hydrogen that has a proton and neutron in its nucleus, rather than just a proton. Since deuterium is almost twice as heavy as regular hydrogen, deuterium water (HDO) is a bit heavier than regular water (H₂O). This means that when water evaporates, HDO is a bit more likely to be left behind. In Earth’s oceans deuterium isn’t very common compared to hydrogen, and exists at about 26 parts per million.

What they found was that the deuterium levels in Martian water is about seven times greater than that of Earth. Since the water of Earth and Mars would likely have had similar origins, the higher deuterium level on Mars means that much of the planet’s water must have evaporated into the atmosphere, and eventually into space. From the deuterium levels it was calculated that Mars must have had about 20 million cubic kilometers of water on its surface. Given the terrain of Mars, that would be an ocean covering most of the planet’s northern plains.

Of course that amount of water is calculated just from the evaporation of water. Mars could have had even more water that froze below the planet’s surface, which wouldn’t have changed the ratio of deuterium to hydrogen. So the level calculated is a lower bound on the amount of water Mars once had. It’s clear then that Mars, like Earth, was once a blue ocean world.

Paper: G. L. Villanueva, et al. Strong water isotopic anomalies in the martian atmosphere: Probing current and ancient reservoirs. Science DOI: 10.1126/science.aaa3630 (2015)

The post Blue Mars appeared first on One Universe at a Time.

Results are starting to come in from the Rosetta mission, including a new article in Science on the composition of water on the comet 67P/C-G. The results support the idea that Earth’s water didn’t come from cometary bombardments.

The origin of Earth’s water has been a matter of some debate. The traditional view is that early Earth was too hot to retain its primordial water, so our planet was later wetted by cometary impacts during the late heavy bombardment period. It seems reasonable if you imagine comets as dusty snowballs. But as we now know, comets are more like snowy dustballs. And while asteroids are often thought of as dry rocks, they actually contain a great deal of water embedded in their minerals. So Earth could have gotten water from either meteor or comet impacts (or both).

We can actually determine the origin of Earth’s water by looking at trace isotopes within it. Typical water consists of two parts hydrogen to one part oxygen, hence H₂O. But there are other variations such as D₂O, which is two parts deuterium instead. The ratio of these two varieties of water (known as the D/H ratio) can tell us about the water’s origin. The D/H ratio found in water-rich meteorites is fairly consistent, and it is similar to the ratio of Earth’s water. That would indicate that our water came from asteroids, not comets.

In this new work, the authors measured the D/H ratio of water vented from 67P, and found it was nearly four times higher than Earth levels. This means that our water most definitely did not come from this kind of comet. Comet 67P is part of the Jupiter family of comets. Observations of cometary tails from other Jupiter family comets have given similar results, but this new result is much more accurate.

While this excludes one family of comet, it is possible that other comets might have contributed to Earth’s water. But since periodic comets such as 67P once originated from the same Oort cloud as other comets, that doesn’t seem likely.

Paper: K. Altwegg, et al. 67P/Churyumov-Gerasimenko, a Jupiter family comet with a high D/H ratio. Science DOI: 10.1126/science.1261952 (2014)

The post A Comet’s Tale appeared first on One Universe at a Time.

For an inner planet, Earth is bountiful with water. The origin of that water has been a matter of some debate. One idea is that a combination of Earth’s strong magnetic field and distance from the Sun allowed Earth to retain much of the water emitted from rocks as the planet cooled. Another is that water came to Earth through cometary or asteroid bombardment. But now it seems the origin of Earth’s water is more complex and more interesting that we’ve thought.

Last month an article in Science showed that much of Earth’s water existed before the formation of the solar system. The authors demonstrated this by looking a levels of deuterium in terrestrial water. Deuterium is an isotope of hydrogen that has a proton and neutron in its nucleus, rather than just a proton. As a result, it’s almost twice as heavy as regular hydrogen, and this means the way it chemically reacts is slightly different from regular hydrogen.

Deuterium isn’t very common compared to hydrogen, and exists at about 26 parts per million. When the team measured levels of deuterium in the water of Earth and other solar system bodies, they found the water contained deuterium at about 150 parts per million. This is interesting, because deuterium water is more likely to form in interstellar space. Water formed in the heat of a young solar system isn’t likely to produce much deuterium water. Given measured deuterium levels, the authors calculate that about half of Earth’s water was produced in the depths of space, before the solar system was formed.

This month another paper in Science found that water arrived on Earth earlier than expected. In this paper the team compared chondrite minerals on Earth with chondrite asteroids, specifically ones that likely originated from Vesta. Chondrite asteroids have a high quantity of water chemically bound to them, and one idea is that they could have been the source of Earth’s water. When they looked at the chemical makeup of terrestrial chondrites, they found them to be remarkably similar. This likely means terrestrial chondrites were themselves the source of Earth’s water. If that’s the case, then Earth was likely a water world a hundred million years earlier than the bombardment model predicts.

So it seems that Earth’s seas are more ancient both in origin and composition than we once thought.

Paper: Cleeves, L. I., et al. The ancient heritage of water ice in the solar system. Science, 345 (6204), p. 1590 – 1593 (2014)

Paper: Sarafian et al. Early accretion of water in the inner solar system from a carbonaceous chondrite–like source. Science, 346 (6209) p. 623-626 (2014)

The post Ancient Seas appeared first on One Universe at a Time.

Yesterday I talked about how water can form on the Moon. It might seem a bit surprising that water exists on the Moon, but it doesn’t sound like a crazy idea. What does sound crazy is the idea that there is water on the Sun’s surface, and yet we know that there is.

The surface of the Sun (specifically the photosphere) has a temperature of about 6000 K. It is so hot that hydrogen atoms are ionized, and molecules can be ripped apart. It’s pretty inhospitable for a molecule such as water. Despite this, the potential for water is there. Oxygen is produced in stars through the CNO fusion cycle, and we have observed quantities of oxygen in the Sun’s spectra. Hydrogen is the most abundant element in the universe, and most of the Sun’s mass is hydrogen. All that’s needed is a cool enough temperature for the hydrogen and oxygen to come together to form water.

It turns out there is just such a place in sunspots. We normally think of sunspots as dark regions on the Sun. They aren’t actually dark, but they are cooler and dimmer than the rest of the Sun, which is why they appear dark in solar images. Within a large sunspot, the temperature can be as cool as 3500 K, which is cool enough for water to form. Naturally it only exists as water vapor, but there really is water on the Sun’s surface.

Spectra for hot water. The red line is the theoretical spectra for water at 3000K. The blue line is the observed curve in sunspots. Credit: Polyansky, et al.

The presence of water on the Sun has long been suspected, but proving it has been a real challenge. That’s because water has a complex spectra with millions of absorption lines. These lines also vary with temperature, making it even more challenging. Experimentally measuring the line spectra of water vapor at 3500 K isn’t feasible, so you need to calculate the expected spectra using computer simulations.

In 1997, a team did just that. They were able to calculate more than 6 million absorption lines for very hot water, and then compared the results to observed spectra deep within sunspots. They found a clear match, showing that water does indeed form within “cool” sunspots.

So yes, there really is water on the Sun’s surface. It’s a fact worth remembering if you ever want to win a bar bet.

 Paper: Polyansky et al. Water on the Sun: Line Assignments Based on Variational Calculations. Science 277 (5324): 346-348 (2014)

The post Hot Water appeared first on One Universe at a Time.

The Moon is a dry, airless rock. At least that is how we imagine it. At basic level, that’s a pretty accurate description. It is drier than any desert on Earth, and its surface would be considered a hard vacuum. But at a more subtle level, that isn’t quite true. The Moon does have the faintest trace of atmosphere, consisting of elements such as argon, helium and hydrogen. The Moon also has traces of water on its surface, mostly locked up within minerals.

That doesn’t mean these minerals are wet by any means. Initial studies of lunar rocks gathered during the Apollo missions found no evidence of water. Only during later, more sophisticated studies was a trace of water discovered. With modern satellites we can detect such traces of water across the lunar surface, such as seen in the image above.

It’s generally been thought that lunar water originated on the Moon in much the same way as it originated on Earth, through water-rich meteorites (chondrites) and comets. But that doesn’t seem to be the case. While some of the Moon’s water clearly did come from impacts, the majority of lunar water is due to a rather surprising source: the Sun.

The discovery was published recently in PNAS, and it looks at isotopes in lunar water. Typical water consists of two parts hydrogen to one part oxygen, hence H₂O. But there are other variations such as D₂O, which is two parts deuterium instead. The ratio of these two varieties of water (known as the D/H ratio) can tell us about the water’s origin. The D/H ratio found in water-rich meteorites is fairly consistent, and it is one of the ways we know meteorites contributed more water to Earth than comets. The D/H ratio found in lunar water doesn’t match that of meteorites. The authors estimate that less than 15% of lunar water could have come from chondrites.

The rest of the water seems to have come from the solar wind. The solar wind consists of protons and electrons that stream away from the Sun. On Earth, these charged particles are caught by our planets magnetic field, causing them to strike the upper atmosphere near the poles, which creates aurora. The Moon lacks a strong magnetic field, so these particles can strike the lunar surface. When protons from the solar wind strike the Moon, they can bond with elements on the surface, such as oxygen. This can lead to the formation of water. Of course, solar-wind produced water also has a distinctive D/H ratio, and the authors were able to show that lunar water was a good match.

So it turns out water can appear on a dry, airless rock. All you need is a bit of solar wind.

Paper: Alice Stephant and François Robert. The negligible chondritic contribution in the lunar soils water.  PNAS, DOI:10.1073/pnas.1408118111 (2014)

The post Water from the Sun appeared first on One Universe at a Time.

The transit method of exoplanet observation looks at the light from a star to observe dips in brightness. When a planet passes in front of a star (transits), it blocks some of the starlight, making the star appear slightly dimmer. Normally when we are looking for exoplanets we simply look for an overall dip in brightness, from which we can determine things such as the size of the planet relative to the star.

But once an exoplanet is known, you can begin to make more subtle observations, such as observing the dip at different wavelengths. This was done by a team recently to study the atmosphere of an exoplanet known as Gliese 1214 b. Their results were recently published in the Astrophysical Journal.

This particular planet is a “super earth”, with a mass about 6.5 earths, and a diameter about 2.5 times larger than Earth. It orbits a small red dwarf star. Because of the planet’s size and mass, its density is only about twice that of water, which has led some to speculate that it is a watery world with a relatively thick atmosphere. Now in this latest paper there is some evidence to support that idea.

If we could observe the planet directly, it would look like a disk, where the outer edge of the disk is the planets atmosphere. If you’ve seen a picture of Earth from space, you get the idea. As the planet passes in front of the star, the atmosphere starts to block the star before the disk of the planet does. Likewise, as the planet finishes transiting, the disk of the planet unblocks the star before the atmosphere does. So the beginning and ending portions of the transit are more strongly due to the atmosphere of the planet.

The team made observations of planetary transits at different wavelengths, and compared the different dips at those wavelengths to find the amount of light absorbed or scattered by the planet’s atmosphere. They then compared their results to three atmospheric models: one mostly of hydrogen; a water-rich atmosphere (similar to Earth’s); and a highly cloudy atmosphere (such as Venus or Titan). These three different types of atmospheres interact with light in different ways, as seen in the figure below.

Credit: NAOJ

In a hydrogen dominated atmosphere, Rayleigh scattering scatters most of the blue light, leaving red wavelengths to dominate. A similar effect occurs in our atmosphere, but to a less extent. You might remember from a previous post that the amount of Rayleigh scattering depends upon the size of atmospheric molecules. Since hydrogen is much much smaller than the nitrogen of our atmosphere, so we would expect more scattering. A water rich atmosphere would have less depth than a hydrogen one, so the absorption and scattering of light would be less severe and more uniform. Finally, a cloudy atmosphere would block much of the light at various wavelengths.

The model that best fit observation was one of a water rich atmosphere similar to our own. This seems to support the idea that Gliese 1214 b is a watery world. Because of this, news of the results have hit the popular press. But the authors note that hydrogen-rich atmosphere with heavy clouds is also a possibility. So while it seems likely that this is a watery world, more observations are needed.

It’s pretty amazing that we are now able to start analyzing the atmospheres of planets around other stars, but it’s even more amazing when you realize these observations were made with a ground based telescope, the Subaru telescope at Mauna Kea.

The post Water in the Sky appeared first on One Universe at a Time.