Sound dies quickly in the cold， thin air of Mars. Researchers have modeled a sound wave traveling through the Martian atmosphere and report that it doesn't go far——even a lawn mower's roar dies after a hundred meters or so. The model presents an unusually detailed¹ picture of how sound travels in an alien atmosphere and hints at what it would take to communicate on the Red Planet.

　　The shriek² of a baby， an ambulance's siren， or a violin sonata³ are all essentially⁴ the same thing： waves of pressure traveling through the air. Sound can also travel through water， or a solid like the ground， but because molecules⁵ must bump into each other to propagate the pressure wave， the denser⁶ the medium the better. Hoofbeats or footsteps travel farther through the ground than through the air， for example， because the molecules in air have to travel further to bump into one another than those in soil， thus losing energy more quickly.

　　The Martian atmosphere is mostly carbon dioxide and only 0.7% as dense⁷ as Earth's is， so sound should fade more quickly. But the details of how sound waves travel in the Martian atmosphere were unclear and could be important to future Mars missions.

　　Now， a computer model has given a molecule-by-molecule map of how sound moves on Mars. Graduate student Amanda Hanford and physicist⁸ Lyle Long of Pennsylvania State University in State College presented the model last week at a meeting of the Acoustical⁹ Society of America meeting in Providence¹⁰， Rhode Island. The model is unusual in its molecular¹¹ approach； most acoustical models of sound treat the medium it travels through as a continuous block with average properties. Such models are fine for dense atmospheres like Earth's， but treating the air like a loose bunch of freewheeling molecules is more realistic for Mars' rarefied atmosphere， say the researchers.

　　Hanford and Long first set up a virtual "box" filled with about 10 million carbon dioxide molecules floating about randomly¹²， at the same density¹³ as the Martian atmosphere. A sound wave then appeared on one side of the box， and the model calculated its progress across to the other side， computing¹⁴ nanosecond by nanosecond exactly how the carbon dioxide molecules bumped and moved. The results show that a noise that would travel several kilometers on Earth would die after a few tens of meters on Mars. Quieter sounds would travel far shorter distances， making eavesdropping¹⁵ on a quiet conversation nearly impossible.

　　Henry Bass¹⁶， a physicist at the University of Mississippi in Oxford¹⁷， notes that if people ever go to Mars and want to communicate audibly， they'll need to design devices that can work with the lower frequencies transmitted by the Martian atmosphere.