Date: June 15, 2012
Title: Viewing Mercury with X-Ray Vision
Podcaster: Bob Hirshon
Organization: American Association for the Advancement of Science (AAAS)
Link: www.aaas.org
Description: While Superman can see through walls using X-ray vision, instruments aboard the MESSENGER spacecraft can use X-rays to do a lot more: they can determine the presence and abundance of a host of important elements in the planet’s surface. AAAS Science Update host Bob Hirshon spoke with geologist and MESSENGER post doctoral fellow Shoshana Weider about what the X-ray view is revealing about the planet.
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.
Everyone knows that Superman is faster than a speeding bullet, that he’s able to leap tall buildings in a single bound, and that he sees through solid walls with his X-ray vision. Fewer people know that the MESSENGER spacecraft, now in orbit around the planet Mercury, is also faster than a speeding bullet—in fact, it’s gone ten times faster—it too can leap tall buildings—not to mention entire planets—and it also has X-ray vision.
But it doesn’t use it to peer through walls. I spoke with geologist and MESSENGER post doctoral fellow Shoshana Weider, who studies data collected by MESSENGER’s X-Ray Spectrometer, or XRS, at the Carnegie Institution’s Department of Terrestrial Magnetism in Washington, DC.
She explains that MESSENGER’s XRS doesn’t produce x-rays—it relies on the Sun for that.
Weider:
When the sun is active, it has solar flares of different sizes. And if one of these solar flares is strong enough, the x rays which are emitted during that solar flare can hit the surface of another planet in the solar system, such as Mercury, such as the Moon. As long as those planets don’t have an atmosphere to absorb those incoming x rays, those x rays can excite the atoms in the surface of the planet. Electrons move around in those atoms, and an x ray equivalent to the change in energy of the electrons that have moved is emitted from the atom. And because energy levels within atoms are characteristic of each element, we can detect the energy of those x rays and work out from which atom they’ve come—so whether it’s come from a magnesium atom, or it’s come from a calcium atom—by the energy of that x ray. And so we use instruments on different missions such as MESSENGER, to measure these x rays and how many of them there are. And then we can start to quantify the amount of these elements within the surface.
Hirshon:
And how fine is it? Are you measuring what’s in a big swath of surface, or is it just a tiny little area?
Weider:
Well, a bit of both to be honest. So we only see down to a depth of a hundred microns. So that’s less than a millimeter. So we’re seeing a very thin layer of the surface. However we see wide areas on the surface. So we can see—with MESSENGER it varies, because we have this elliptical orbit around the planet. So our spatial resolution varies with where we are in the orbit. But we can see down to about a hundred km resolution with MESSENGER. In diameter. But they can go up to like half the planet, the footprints that we see.
Hirshon:
So you want solar activity; a lot of the science team members don’t want solar activity.
Weider:
Yes, we kind of run a fine line, in that we need the solar activity to produce our data, but anything too big, any of these huge x ray flares will knock out the instruments and destroy… well, destroy them for a little bit of time till they can recover. So, yeah, we run a fine line of wanting nice activity but not too much that the instruments are disabled.
Hirshon:
Over the past 15 months, as the MESSENGER spacecraft has been in orbit around Mercury, Dr. Weider says they’ve gotten a wealth of data about the planet’s surface composition, especially regarding the lower energy elements.
Weider:
So whenever we look at the planet, no matter what the solar activity is at the moment, we get data for magnesium, aluminium, ah, silicon, sulfur, ah, sorry, just the silicon. And then as the solar activity increases, we see data for elements such as sulfur and the calcium. And at the strongest levels of solar activity, we get data for titanium and iron. So we have very good coverage for the planet for the magnesium, aluminium and silicon; better for the sulfur and the calcium; and then, sorry, worse: NOT so great for the sulfur and the calcium; and then smaller patches for the higher energy elements such as the titanium and the iron.
Hirshon:
She says the results they got were surprising.
Weider
So very early on in the mission, we were seeing this strong signal for sulfur that was unexpected. The amount of sulfur on the surface of Mercury is about ten times higher than you get on any other terrestrial planet. That was a big surprise. And we’re seeing, we’re confirming the fact that there’s very little iron on the surface of Mercury.
Hirshon:
Many scientists had thought that Mercury’s large iron core could be explained by a catastrophic event early in the planet’s life that stripped off the outer layers of the crust, leaving a relatively larger core. But any such event would have also stripped away most of the sulfur and other volatile elements. The XRS results meant that Mercury could not have formed that way. Instead, Dr. Weider and her colleagues are trying to determine what sorts of chondritic meteor material could have formed the planet, and the conditions that would explain the planet’s odd composition.
More recently, XRS has been returning valuable data concerning vast volcanic plains on Mercury’s northern hemisphere.
Weider:
And from those data, we’re seeing that the northern plains, which is this large area of volcanic material, or what’s known as a flood basalt, is chemically distinct from the surrounding material on the planet. And we’re seeing that it’s poorer—it’s got less magnesium, calcium and sulfur and higher amounts of aluminium.
Hirshon:
And what does that tell the scientists?
Weider:
It tells us that the eruption style, the eruption temperatures, the age of this material is different, so that the volcanic plains are younger than the material that surrounds it; and from our data it seems that they erupted from cooler magmas and therefore maybe have come from different parts of the mantle.
Hirshon:
She says they’re now starting to compare the northern volcanic plains data with data from other volcanic plains on the planet.
Weider:
So there’s ones within and around the Caloris basin, which is the largest impact basin on Mercury. And we can start to see that the material within the Caloris basin is similar to the stuff which is in the northern volcanic plains. So we can start to draw conclusions if the volcanic plains all over the planet formed in similar ways and from similar material to the stuff that is the older material that surrounds them.
Hirshon:
Now while Dr. Weider refers to the material in these plains as “younger,” that’s a very relative term in this case. The oldest surfaces on the planet date back about four and a half billion years, while the younger plains are a mere three billion years old.
Weider:
But that’s what’s exciting about planets like the Moon and Mercury is that the surfaces are so old that we can look at what the Earth must have looked like four and half billion years ago, rather than the re-surfaced planet that we see today, covered in mortar, and that plate tectonics has destroyed a lot of the material on Earth.
Hirshon:
Studying this primordial material will help Dr. Weider and the rest of the science team better understand not only the formation of Mercury, but the conditions that existed in the early solar system, and provide clues to the origin of Earth as well.
Well, that’s it for today’s podcast. Please leave comments and let me know what you think, and 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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