SCIENCE LAB SPECIAL : MERCURY FALLING

WHY our smallest planet just keeps getting smaller : Mercury is, by far the smallest planet in our solar system - about one-29th the width of Jupiter and just one-third the width of Earth.  

One day it might be even smaller.

When Mercury formed about 4.5 billion years ago in the raucous collision of the early solar system, it was a much hotter planet. Over many millenniums, however, its metallic core has cooled and contracted, causing the planet's surface to warp and wrinkle.

Previous estimates suggested that Mercury had shrunk by just a few miles since its formation. Now, scientists say it is shrinking up to 30 percent faster than expected. It might have even shrunk by as much as 14.5 miles [23.3 kilometers], which is nearly 1 percent of its current radius of 1,516 miles.

This new estimate, published in Geophysical Research Letters, gives astronomers a new understanding of how the planet - and others like it formed and evolved.

'' Many bodies, like the moon and Mars, are shrinking nowadays,'' said Gaku Nishiyama, a planetary scientist at the German Aerospace Center Institute of Space Research who led the study.

'' But in the case of Mercury, it can shrink more because of a large metal core.''

Dr. Nishiyama's team used images from the 2004 flyby of NASA Messenger spacecraft and more recent digital models to analyze the planet's ridges and valleys - the surface ''wrinkles'' that form as the planet shrinks.

They also looked at features that might obscure them, like lava flows and impact debris, that could make it hard for researchers 97 million miles away to see the full picture.

The reason why the estimation remains difficult is that we have to rely on topographies that are visible on the surface of the planet,'' Dr. Nishiyama said.

More data and imagery of the planet's surface will be needed to nail down how much Mercury has shrunk over the past 4.5 billion years, but the study's range of 10 to 30 percent faster than previous estimates unlocks another piece of the puzzle of planet evolution, experts say.

!WOW! thanks K.R. Callaway.