[苏教版五下火星——地球的“孪生兄弟”教学反思]课文围绕了：为什么说火星和地球是一对 孪生兄弟 ？火星上的水是从哪儿来的？为什么地球上有水并且能够孕育生命，而火星上不能？三个问题展开了讨论。抓住了排比句让学生明确排比句的...+阅读

Sound dies quickly in the cold， thin air of Mars. Researchers he modeled a sound we treling 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 detailed picture of how sound trels in an alien atmosphere and hints at what it would take to municate on the Red Pla.

The shriek of a baby， an ambulance's siren， or a violin sonata are all essentially the same thing： wes of pressure treling through the air. Sound can also trel through water， or a solid like the ground， but because molecules must bump into each other to propagate the pressure we， the denser the medium the better. Hoofbeats or footsteps trel farther through the ground than through the air， for example， because the molecules in air he to trel 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 dense as Earth's is， so sound should fade more quickly. But the details of how sound wes trel in the Martian atmosphere were unclear and could be important to future Mars missions.

Now， a puter model has given a molecule-by-molecule map of how sound moves on Mars. Graduate student Amanda Hanford and physicist Lyle Long of Pennsylvania State University in State

College presented the model last week at a meeting of the Acoustical Society of America meeting in Providence， Rhode Island. The model is unusual in its molecular approach； most acoustical models of sound treat the medium it trels through as a continuous block with erage 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 randomly， at the same density as the Martian atmosphere. A sound we then appeared on one side of the box， and the model calculated its progress across to the other side， puting nanosecond by nanosecond exactly how the carbon dioxide molecules bumped and moved. The results show that a noise that would trel several kilometers on Earth would die after a few tens of meters on Mars. Quieter sounds would trel far shorter distances， making eesdropping on a quiet conversation nearly impossible.

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

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