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Date: November 11, 2011

Title: MESSENGER Overturns Origin of Mercury

Podcaster: Bob Hirshon

Organization: American Association for the Advancement of Science (AAAS)

Description: Science Update host Bob Hirshon speaks to Research Scientist Patrick Peplowski at the Johns Hopkins University Applied Physics Laboratory about new findings from the MESSENGER spacecraft that contradict the most popular theories on the origins of the planet Mercury.

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.

Sponsor: This episode of “365 Days of Astronomy” has been 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.

Transcript:

IYA Podcast for 11/11/11
MESSENGER Overturns Origin of Mercury
Bob Hirshon, AAAS

Welcome to the 365 Days of Astronomy podcast and happy eleven-eleven-eleven! I’m Bob Hirshon, senior project director at the American Association for the Advancement of Science and host of the Science Update radio show and podcast.

The origin of the planet Mercury has been a mystery ever since the Mariner 10 spacecraft flew by the planet in the 1970s. This according to research scientist Patrick Peplowski, at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland.

Peplowski:

One of the things we learned about Mercury in those flybys was that it is the most dense of all the terrestrial planets. What that means is that there is more metal per unit volume in Mercury than in Venus, Earth and Mars. And this is a bit of a puzzling challenge, because you have to ask yourself “what formation process could have made Mercury have such a different composition than the other terrestrial planets?”

Hirshon:

The most popular theories argued that Mercury started out about the same as the other rocky planets, with a small iron core compared to the rest of the planet, but then something happened to strip away the planet’s outer layers. One possibility was that it was smacked by large meteors and the impacts ripped off much of the outer part of the planet, leaving a much smaller planet behind, with a relatively large metal core. Another possibility was that billions of years ago, Mercury was either closer to the sun or the sun was much hotter, and the sun’s energy just burned off the planet’s outer layers.

But there was no way to test these theories, because since the Mariner 10 flybys in the 1970s, there have been no return trips to Mercury. In the late 1990s, NASA scientists began planning the MESSENGER mission, in which a spacecraft would, for the first time, go into orbit around Mercury.

Peplowski:

So one of the things that scientists were thinking about when they went into MESSENGER was “how can we answer this question about Mercury’s formation process?” As a result, an instrument suite that does remote chemical measuring of the surface was added. One of the instruments is the gamma ray spectrometer.

Hirshon:
As you might guess, this is an instrument that measures gamma rays coming from the surface of Mercury. And one of the sources of the gamma rays is the decay of radioactive isotopes on the surface of the planet. By measuring the amount of these isotopes in the space around Mercury, scientists can calculate the abundance of the elements that produce them. These include potassium, thorium and uranium. In March of 2011, MESSENGER went into a highly elliptical orbit around Mercury. At its most distant point from the planet, the spacecraft is over 15,000 kilometers away, but at its closest point, it’s just over 200 km from the surface. This Fall, in the Journal Science, Peplowski and his colleagues described what they see each time the spacecraft approaches the planet.

Peplowski:

As we get closer, we start to see a gamma ray peak from potassium. By summing all the low altitude data, and measuring the total flux of gamma rays that we counted, we can actually determine the abundance of this element on the surface. In this case, we figured there was a little bit more than a thousand parts per million potassium.

Hirshon:

These levels of potassium were far higher than expected. And another instrument, the X-Ray Spectrometer, detected high levels of sulfur on the planet. So what do the abundances of these elements have to do with how Mercury formed?

Peplowski:

We were actually able to rule out a number of the formation theories for Mercury. Several of these required removing the outer layers of the planet. For instance, one theory said that a number of giant impacts stripped off the outer layers. Another theory says that Mercury used to be either closer to the sun, or the sun used to be a lot hotter, and that the outer layers were actually burned off, leaving a smaller planet that has a larger metal core as a result. Both of these would give the larger iron abundance that we measured, and that we inferred from the Mariner 10 data, but it turns out that those measurements would have resulted in removal of potassium from the surface, as well as sulfur.

Hirshon:

In a matter of weeks, MESSENGER overturned two of the most satisfying explanations for the origin of the planet. So where does that leave planetary geologists?

Peplowski:

It actually looks like, based on the measured abundances, that the planet probably formed from material that looks a lot like chondritic meteorites. We have samples of these meteorites on earth, so we know their composition. We also know that they’re primitive bodies and that they’re left over from the early days of the solar system. And as we get more and more data from MESSENGER, we’re finding that the composition of the planet looks a lot like these chondritic meteorites. So we think something like that probably resulted in the formation of Mercury.

Hirshon:

In other words, Mercury is an oddball, less like Venus, Earth and Mars than was thought.

Peplowski:

It’s still a little puzzling as to why the composition of Mercury is different from the other planets, but this is something that we’re investigating as we get more and more data.

Hirshon:

So, not all that surprisingly, MESSENGER has added immensely to our understanding of this tiny planet, and also added a brand new mystery. But with many months of data collection left in the MESSENGER mission, perhaps that, too, will become clear before the spacecraft and the science team are through. We’ll keep you posted in future podcasts.

But that is all we have for you today. For the 365 Days of Astronomy podcast, I’m Bob Hirshon.

End of podcast:

365 Days of Astronomy
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