Date: October 14, 2011

Title: Hollows on Mercury

Podcaster: Bob Hirshon

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

Link: www.aaas.org

Description: Podcaster Bob Hirshon speaks with David Blewett, a planetary scientist from the Johns Hopkins University Applied Physics Laboratory and member of the MESSENGER science team, about a new landform discovered on the surface of 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:

Hollows on Mercury

Hirshon:

Welcome to the 365 Days of Astronomy podcast. I’m Bob Hirshon, host of the AAAS Science Update radio show and podcast. When scientists first planned the MESSENGER mission to the planet Mercury, there were several specific questions they sought to answer, including understanding the nature of Mercury’s magnetic field, and discovering whether or not the planet had water. But they also expected surprises, since no spacecraft had ever before gone into orbit around Mercury.

Well, members of the MESSENGER science team report in the journal Science that they’ve found at least one big surprise: a whole new landform on the planet, never before found in the solar system. David Blewett is a planetary scientist at the Johns Hopkins University Applied Physics Laboratory and a member of the MESSENGER science team. He says that some light-colored areas on Mercury’s surface, seen when Mariner 10 flew past the planet decades ago, had attracted their attention.

Blewett:

Even going back to Mariner 10, we knew that there were some craters on Mercury that had funny bright material in their floors. And MESSENGER made three flybys of Mercury before it went into orbit, and it found some more examples of this. We didn’t really know what this meant. They’re unusual because there are no craters like that on the moon. But they were sort of just a curiosity.

Hirshon:

Then MESSENGER went into orbit, and suddenly they got a better look.

Blewett:

It turns out that those craters we knew from the flybys that had this bright material, we can now see what that bright material looks like up close. And it turns out that it is composed of many small depressions, little pits in the ground, that have kind of flat floors, they’re sort of generally rounded, and they have bright interiors and sometimes bright halos around them. And this is, as I said, a big surprise, there’s nothing like this on the moon, and it was really unexpected. And it’s interesting because it’s telling us something about a geological process on Mercury that was totally unanticipated.

Hirshon:

He says a new feature required new nomenclature, since they couldn’t keep referring to them as “those new pit things.” And they wanted a name that would differentiate them from volcanic vents.

Blewett:

So we kicked around some ideas and decided to call these things “hollows,” because they’re a type of depression, sort of valley like feature.

Hirshon:

But what’s producing them? They’re clearly not volcanic in origin, nor were they produced by impacts.

Blewett:

And what we’re led back to every time is something involving volatile material. That is, there is a component in the rocks composing these central peaks and the floors of these craters, that is unstable at the surface of Mercury. And it appears that this stuff is, on rapidly rapid time scales for planetary geological processes, is undergoing some sort of sublimation or erosion of a kind that no one had expected.

Hirshon:

Intriguingly, Blewett says that on Mars, near the poles, there are pits that were formed from the sublimation of frozen carbon dioxide, boiling off into the atmosphere.

Blewett:

Now of course, Mercury is made of rocks, not carbon dioxide ice. But the similarity in the shapes of these features may be telling us that a similar process is taking place on Mercury.

Hirshon:

Blewett says that now that they are looking for hollows, they are finding them all over Mercury, wherever they have high-resolution images from MESSENGER.

Blewett:

And it’s a big mystery, really, what component in the rocks could be susceptible to some type of sublimation. Of course, the surface of Mercury is very hot so there could be a volatile-bearing phase that is essentially evaporating away and leading to collapse and formation of these depressions.

Hirshon:

Or, instead of sublimation, he says it’s possible that material is being eroded away.

Blewett:

Mercury is close to the sun so it is subjected to a high rate of bombardment by the solar wind and also bombardment by micro-meteorites—little tiny grains, dust sized and sand sized and pebble sized particles that blast the surface. And it’s possible that one of the minerals present in the rocks at the surface is susceptible to erosion by these processes.

Hirshon:

He says that figuring out what’s really producing the hollows will take more information on the composition of the planet’s surface, and the conditions there.

Blewett:

And as the mission proceeds, we’ll be getting more and more data on the distribution of these hollows, and the geochemical instruments will be telling us more and more about the abundance and the distribution of volatile elements in Mercury’s surface. So it’s quite exciting because it gives us an insight into a brand new geological process, a brand new landform on Mercury that we didn’t know about before, and it tells us about how Mercury formed in the solar nebula when all the other planets were forming. And of course, Mercury being closest to the sun is kind of the anchor at one end of the solar system. So if we really want to claim that we understand how the Earth formed, how Mars formed, how Venus formed, we really need to understand the full range of processes that was taking place in the early solar system. And Mercury is turning out to be a really important piece to that puzzle.

Hirshon:

And the discovery even has implications beyond our solar system, as scientists learn how planets formed around other stars.

Blewett:

Many of the other planets that have been discovered are very close to their host, to their parent star, as is Mercury to our Sun, so being able to understand how these other planets formed is informed by what we’re learning about Mercury in our own solar system.

Hirshon:

Well, that’s all for today. 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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