在火星可以大聲(At Mars can be loud)

　　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 detailed picture of how sound travels in an alien atmosphere and hints at what it would take to communicate on the red planet.

　　the shriek of a baby, an ambulance's siren, or a violin sonata are all essentially the same thing: waves of pressure traveling through the air. sound can also travel through water, or a solid like the ground, but because molecules must bump into each other to propagate the pressure wave, the denser 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 dense 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 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 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 randomly, at the same density 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, computing 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 eavesdropping 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 communicate audibly, they'll need to design devices that can work with the lower frequencies transmitted by the martian atmosphere.

　　在火星上，嗓門可得大一點!

　　研究顯示火星大氣的吸音效果特好，是地球大氣的100倍。

　　聲音依靠介質粒子碰撞傳遞能量，聲波的傳播速率與衰減決定于介質的狀態(tài)與種類。火星地表的大氣壓僅有地球的0.7%，大氣成分95%是二氧化碳、再來是2.7%的氮氣、1.6%的氬氣，而平均溫度是零下63℃�？梢灶A期，聲音在火星上可能衰減的很快，但是細節(jié)卻不甚清楚。多數模擬聲音傳播的數值模式，把介質視為連續(xù)體來處理：不去考慮分子個別行為的影響，只以整體的平均性質來做計算。這樣的觀點，適用于地球表面的厚重大氣，但不合用于稀薄的火星大氣。賓州州立大學的研究生amanda hanford與物理學家lyle long則建立一個分子模式來模擬聲音在火星的傳播：他們模擬被局限在虛擬盒子內一千萬個二氧化碳分子的運動：當聲波出現在盒子的一側后，以每奈秒為時間間隔計算二氧化碳分子的碰撞與移動，來看聲波如何傳遞到盒子另一側。模式結果已發(fā)表在六月初的聲學會議(the acoustical society of america)上。

　　模式結果顯示，火星大氣的吸音效果特好，是地球大氣的100倍，這表示原本在地球上會吵到數公里遠的噪音，在火星上在數百公尺的距離就已經聽不見。所以，未來在火星上，我們若想要”呼”朋引伴，除非有其他裝置的協助，不然嗓門可得大一點。而怕吵的人，火星會是個好地方，或者月球也行!