Date: July 20, 2012
Title: A Soggier Mars
Podcaster: Bob Hirshon
Organization: American Association for the Advancement of Science (AAAS)
Link: www.aaas.org
Description: The Martian landscape appears absolutely arid. But deep below, in the Martian mantle, there could be an ocean’s worth of H2O. AAAS Science Update host Bob Hirshon spoke with staff scientist Erik Hauri, at the Department of Terrestrial Magnetism at the Carnegie Institution of Washington. He and his colleagues analyzed meteorites from Mars and conclude that the planet may hold 30 times more water than was thought.
Bio: Bob Hirshon is Senior Project Director at the American Association for the Advancement of Science (AAAS) and host of the daily radio show and podcast Science Update. Now in its 24th year, Science Update is heard on over 300 commercial stations nationwide. Hirshon also heads up Kinetic City, including the Peabody Award winning children’s radio drama, McGraw-Hill book series and Codie Award winning website and education program. He oversees the Science NetLinks project for K-12 science teachers, part of the Verizon Foundation Thinkfinity partnership. Hirshon is a Computerworld/ Smithsonian Hero for a New Millennium laureate.
Today’s Sponsor: This episode of 365 Days of Astronomy is sponsored by The Education and Outreach team for the MESSENGER mission to planet Mercury. Follow the mission as the spacecraft helps to unlock the secrets of the inner solar system at www.messenger-education.org
Additional sponsorship for this episode of 365 days of astronomy was provided by iTelescope.net – Expanding your horizons in astronomy today. The premier on-demand telescope network, at dark sky sites in Spain, New Mexico and Siding Spring, Australia.
Transcript:
Hirshon:
Welcome to the 365 Days of Astronomy Podcast. I’m Bob Hirshon, host of the AAAS radio show and podcast Science Update.
In everyday experience, water is the thing that escapes. Leave it out and it evaporates. Fail to tighten the cap on your water bottle enough, and you’re in for a thirsty hike. Without constant replenishment, everything from your body to a lake inexorably dries up.
But from a cosmic perspective, water is remarkably persistent. It arcs through the vacuum of space for millions of years in the form of enormous frozen comets. It encrusts asteroids orbiting between Mars and Jupiter. It makes up most of the mass of Saturn’s moon Rhea, and, on Enceladus, another moon of Saturn, it spurts from geysers.
In fact, water is even found in the boiling hot, high pressure caldron of Earth’s mantle. This according to staff scientist Erik Hauri, at the Carnegie Institution of Washington’s Department of Terrestrial Magnetism.
Hauri:
So the water content, the present day water content of the earth’s interior is high enough that you can stuff at least one ocean’s worth of water into the earth’s interior. If you completely melted the earth’s interior right now and released all its water, it would be at least one ocean full of water and probably several.
Hirshon:
Where does all that water come from? Hauri says it’s mostly pulled into Earth’s interior as mid-ocean tectonic plates slide down into the mantle.
Hauri:
You have subduction zones where one plate is basically plunging into interior of the earth beneath another. And because these subduction zones are all deep trenches in the oceans, when the plate goes deep into the mantle, it carries sea water with it.
Hirshon:
On the other hand, he says volcanoes pump that water back up to the surface and release it into the atmosphere. Earth is constantly cycling water back and forth, down into the mantle at midocean ridges and then back up to the surface through volcanic vents.
So what about Mars? There’s no more active plate tectonics there to pull water down, but also no more volcanic activity to spew it out. There is substantial water ice in patches on the surface. Is it possible that primordial water also exists far deeper, sealed in its mantle?
To find out, Hauri and his colleagues examined bits of Mars that fell to Earth as meteorites. Of the 50,000 or so meteorites discovered on Earth, about 100 have been found to be of Martian origin. They’re hunks of rock blown into space by large impacts on Mars a few million years ago, some of which eventually landed on Earth’s surface. Some of them contain a mineral called apatite, which forms in a planet’s mantle, and binds tightly to water ions there.
Hauri:
So when we measure the water content of an apatite mineral, we actually know what the relationship is between the water content of the mineral and the water content of the magma that the mineral formed in. And so from that relationship, which is called a partition coefficient, these can be experimentally determined. From this partition coefficient, we can take the water content of the mineral and calculate how much water was in the magma.
Hirshon:
Earlier attempts concluded there was very little water inside Mars—just 3 ppm or so. But those samples were of minerals that lost much of their water as they erupted from the Mars interior hundreds of millions of years ago. Others had lost their water as a result of the impact that jarred them loose from the Martian surface and sent them hurtling into space.
Hauri:
But if the meteorite comes from far enough away from the impact crater, then it is subject to lower and lower peak shock temperatures and peak shock pressures, so that helps to keep the water inside the constituent minerals of the rock that you study.
Hirshon:
In other words, they had to find just the right meteorite samples to study: ones that would have retained their water through the eruption process, and that had avoided severe shocks that could fracture them and caused their water to escape later. Fortunately, two of the nearly 100 Martian meteorites fit the bill, allowing Hauri and his team to estimate water concentrations within magma in the Martian mantle.
Hauri:
And when we do that, we find the water content of the magma that’s actually very similar to the kinds of water contents you see in terrestrial mid-ocean ridge basalts from the mid-Atlantic ridge or East Pacific Rise. These mid ocean ridge basalts on the earth are the most productive types of magmas that are generated even though they’re below the sea floor and people can’t really see them, it’s really the most voluminous magmas that erupt on the earth and so they’re very common, they’re very well studied in terms of their water content, and the water contents match up pretty closely. And so from that we know roughly how much melting takes place in the mantle to produce those magmas. And so we can back calculate the water content of the mantle source, deep in the interior of Mars. And when we do that, again, we find that water content for the Martian mantle—at least this particular part of the Martian mantle that gave rise to the shurgertites—that water content for the Martian mantle is very similar to the water content of earth mantle.
Hirshon:
That came to 100 to 300 ppm—about 30 to a hundred times more water that scientists had thought.
As remarkable as that finding is, Hauri says that in some ways the discovery of water inside Mars and Earth isn’t all that surprising, considering the water content of smaller bodies, like chondritic meteors, that came together to form the planets in the first place.
Hauri:
Some of the most water rich meteorites contain ten percent water. That’s a, that’s a ton of water. But you look at the earth and the average water content of the upper mantle of the earth is somewhere between 150 and 200 parts per million. So that’s a big difference. And you’ve lost many more orders of magnitude of water than you retain during that planetary process. And that’s the situation for the Earth and it appears to be the situation for Mars as well.
Hirshon:
Hauri says that depending on a planet’s tectonic history, its size, its distance from the sun, and other factors, it may either conserve much of its original water, or degas it into space. Hauri suspects that Venus is an example of the latter.
Hauri:
Venus, essentially, its interior overturns almost completely every 500 million years, and so there’s a lot of opportunity to degas the planetary interior of water. And we know from the isotopic composition of hydrogen in the marti—in the venusian atmosphere that the planet has lost a lot of water from its atmosphere. So the present day situation on Venus could be very very dry.
Hirshon:
Mercury, on the other hand, even though it’s the closest planet to the sun, could conceivably have much more water inside. Hauri says volcanic rock spewed from its mantle has been spotted by the spacecraft MESSENGER, now in orbit around the tiny planet. Hauri says he would love to get a bit of that rock into his laboratory, to see what it reveals about Mercury’s interior.
Hauri:
On Mercury, … It’s been essentially a one-plate planet almost from the time that it was formed. At the same time, you have abundant evidence for volcanism on the surface of Mercury. And a lot of these volcanic deposits look very similar to the type of volcanic deposits you see on the moon. And so if I were to design a spacecraft mission that would retrieve volcanic samples from any of the terrestrial planets, my first target would be Mercury. We know where those deposits are, we know what they look like, we know where to go, all you need is technology and the motivation to go there, get the samples and bring them back.
Hirshon:
Unfortunately, merely getting into orbit around Mercury is a monumental task, because of its small size and proximity to the Sun. That feat was only just accomplished by the MESSENGER spacecraft. Conducting a return sample mission to the tiny planet won’t be accomplished for a long time.
But the latest studies by Hauri and his colleagues are already shedding light on the role water plays in the evolution of rocky planets, on the conditions that allow water to persist, and on its abundance both within our solar system, and beyond, a finding that also has implications on the chances of finding signs of life on other worlds.
Well, that’s all for today’s podcast. Please leave comments and let me know what you think, and don’t forget to suggest ideas for future podcasts.
Thanks for listening. For the 365 Days of Astronomy podcast, I’m Bob Hirshon.
End of podcast:
365 Days of Astronomy
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