In part two of this interview with Astrobiology Magazine, Grady discusses the varieties of dust and meteorites that have fallen to Earth, and explains what they tell us about the history of the solar system and its potential for life. Read part one of this interview here.

Astrobiology Magazine (AM): What is the importance of studying different meteorites?

Monica Grady (MG): Looking at meteorites that come from different asteroids, we can see that some meteorites have been altered and some haven't. Some have been melted and differentiated, while others are still primitive and full of carbon. Why? Because there's a spectrum of compositions within the asteroid belt, and a spectrum of different parent bodies that have accumulated and broken apart in the asteroid belt. The more meteorites we look at, the more parent bodies we think there were. It gives you a more complete picture of the solar system and the heterogeneity of the material that we came from. The turbulent cloud of gas and dust that was the solar nebula managed to produce some things that are full of carbon, and others that aren't full of carbon.

One of the projects I'm working on is Stardust, which now has primitive dust that was collected in a comet. The dust hasn't evolved in the last 4.5 billion years. The amazing thing about the particles collected by the Stardust mission is that some of the grains seem to be rich in calcium and aluminum, which are refractory elements in grains that formed close to the sun. But we expected comets to only have grains that formed far away from the sun. So Stardust puts a different spin on our knowledge of how the solar system formed.

I've been studying dust for a long time, looking at infrared spectroscopy observations made by astronomers on planet formation, star formation, and the presence of disks around stars, and linking that with what we see on Earth -- in terms of interplanetary and interstellar dust which coagulated to form asteroids and meteorites. I'm interested in the process from planet formation right through to meteorites arriving on the Earth.

Infrared spectroscopy is a way you can follow that, because you can make measurements on Earth and with telescopes. I've also shifted wavelengths and gone from infrared to the visible and ultraviolet regions to try and make sense of the spectra and look at the spectral signatures of the materials that are coming in on dust grains, meteorites, and star dust particles to see what that can tell us about the materials. Ultraviolet visible spectroscopy is sensitive to mineral composition and also the presence or absence of organics.

AM:Are you looking for the organics that may have played a role in the origin of life?

MG: Just the precursors that might be within cometary particles. But I am looking for the conditions that we think could have led to the origin of life on Mars. My work with martian meteorites has led to my involvement in a project to develop a water sensor for Mars. Not just to look for signs that water's been there in terms of channels or minerals, but actually to look for liquid water, to see if it's there as a film on minerals. I'm working with teams from Norway and Portugal to build a sensor. We're hoping to persuade the European Space Agency that this small sensor can be integrated with its other density, porosity and the permittivity sensors, fitting them all on ExoMars.

AM: So are comets and meteorites our friends or our foes?

MG: They're both! They're friend and foe. They can be foe if they are too big. Something on the 1 to 10 kilometer scale would be catastrophic. But certainly they were friendly in the past, bringing water and the organic building blocks of life. And they can be our friends today, because as long as a meteorite doesn't kill anybody or destroy a house when it falls, then it stimulates the public's interest in meteorites.

AM: Plus, they're our friends because of the changes they've helped to bring about in evolution?

MG: The K-T boundary is what people always point to as being caused by a 10 kilometer-sized meteorite. The evolutionary consequences of that were enormous, setting in motion a whole chain of environmental changes -- wildly swinging fluctuations in temperature of the atmosphere, the opacity of the atmosphere, the acidity of the surface waters, and changes which led eventually to the extinction of many species. But it also allowed evolutionary niches to be inhabited and expanded by other species. Change in the balance from vertebrates to mammals occurred at the K-T boundary.

AM:How often do incoming meteorites arrive on the Earth today?

MG: Meteorites come all the time, and tiny cosmic dust arrives by the ton every year. I think it's somewhere between 40 to 60 thousand tons, but estimates vary. So each year we receive a huge amount of material: several thousand tons of dust, about a thousand football-sized meteorites, and maybe one washing machine-sized meteorite.

AM:And yet we're not seeing these things raining down on us through the atmosphere constantly. Are they getting burned up?

MG: Well, you don't notice it because it's mostly dust. We can pick it up in the stratosphere, we can collect it in Antarctica, or we can dredge it up from the ocean floor.

AM: How often is that done? How many people are studying and collecting this cosmic dust?

MG: The Antarctic Ice Program has been in place for 20 or so years now. I don't know that they are collecting any stratospheric dust at the moment, but NASA has made such samples available internationally.

AM:Are you referring to the Don Brownlee experiments where he essentially waved fly paper around in the stratosphere to collect dust?

MG: Yes.

AM:What about Chandra Wickramasinghe's sterile collection from a balloon and his interpretation of the biogenic materials as originating from cosmic dust?

MG: I think the experiments he's been doing with the balloons are fascinating. If he has managed to collect some cosmic dust, it would be great. I have asked for some to analyze, but I don't know whether I'll get some or not.

But I have no great belief that this is biological material that has come from beyond Earth. It could well have come from beyond the stratosphere; it could be bits and pieces from shuttle astronauts or satellites. But I don't believe it's biological material that has traveled from many light years away.

AM: If Panspermia -- life being delivered within or beyond our solar system -- happened in the past, why should it not continue to happen now?

MG: Yes, but Chandra and Max Wallace are specifically talking about bacteria. I'm not a believer that Panspermia happened in the past, in the sense of bringing bacteria to the Earth. I'm happy with the idea that carbon dioxide and water were brought to the Earth in the past. I believe in Panspermia to that extent, but not the extent of bringing actual living organisms.

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