This is planetary radio we'll talk with Adriana Ocampo.

Born in South America, educated in the U.S. and now a senior research scientist with the European Space Agency.

Adriana will tell us about her work as a planetary

geologist, particularly on the earth-shattering effects of past asteroid impacts.

Bruce Betts will be here helping us to start the year with a look at what's up in tonight's

sky and what to expect in 2003.

First though, let's learn why the big planets get all the pretty rings on our new Q&A segment.

I'll be back with Adriano Ocampo in just a minute. Hi, I'm Emily Lockwala with questions and answers. A member from Halifax, Nova Scotia,

asked, why do only the Gas giants Jupiter, Saturn, Uranus, and Neptune have rings,

while the rocky planets Mercury, Venus, Earth, Mars, and Pluto have none.

We asked Larry Esposito, an expert on planetary rings and atmospheres at the University of Colorado, to answer this excellent question.

Dr Esposito explained that although planetary rings appear to be a solid disk, they are actually made of myriad small particles in orbit around the planet.

These dusty rings are only found close to their parent body, where the planet's gravity is strong enough to overcome the particles' tendency to stick together and grow into a small moon.

Planetary rings are transient.

Ring particles are constantly being ground down by collisions and swept away by frictional drag.

Rings must be replenished by fresh supplies. collisions and swept away by frictional drag.

Rings must be replenished by fresh supplies of dust knocked from small satellites.

The balance between these acts of destruction and creation yields the rings we see today.

Earth has no rings now, but it might have in its past.

I'll explain about Earth's rings when I return in a few minutes.

Now, back to planetary radio.

Adriano Capo is on the phone with us now. Radio.

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