隕石撞擊或把地球生命送上火星

The asteroid that wiped out the dinosaurs may have catapulted life to Mars and the moons of Jupiter, US researchers say.

They calculated how many Earth rocks big enough to shelter life were ejected by asteroids in the last 3.5bn years.

The Chicxulub impact was strong enough to fire chunks of debris all the way to Europa, they write in Astrobiology.

Thousands of potentially life-bearing rocks also made it to Mars, which may once have been habitable, they add.

"We find that rock capable of carrying life has likely transferred from both Earth and Mars to all of the terrestrial planets in the solar system and Jupiter," says lead author Rachel Worth, of Penn State University.

"Any missions to search for life on Titan or the moons of Jupiter will have to consider whether biological material is of independent origin, or another branch in Earth's family tree."

Panspermia - the idea that organisms can "hitchhike" around the solar system on comets and debris from meteor strikes - has long fascinated astronomers.

But thanks to advances in computing, they are now able to simulate these journeys - and follow potential stowaways as they hitch around the Solar System.

In this new study, researchers first estimated the number of rocks bigger than 3m ejected from Earth by major impacts.

Three metres is the minimum they think necessary to shield microbes from the Sun's radiation over a journey lasting up to 10 million years.

They then mapped the likely fate of these voyagers. Many simply hung around in Earth orbit, or were slowly drawn back down.

Others were pulled into the Sun, or sling-shotted out of the Solar System entirely.

Yet a small but significant number made it all the way to alien worlds which might welcome life. "Enough that it matters," Ms Worth told BBC News.

About six rocks even made it as far as Europa, a satellite of Jupiter with a liquid ocean covered in an icy crust.

"Even using conservative, realistic estimates... it's still possible that organisms could be swimming around out there in the oceans of Europa," she said.

Travel to Mars was much more common. About 360,000 large rocks took a ride to the Red Planet, courtesy of historical asteroid impacts.

Big bang theory

Perhaps the most famous of these impacts was at Chicxulub in Mexico about 66 million years ago - when an object the size of a small city collided with Earth.

The impact has been blamed for the mass extinction of the dinosaurs, triggering volcanic eruptions and wildfires which choked the planet with smoke and dust.

It also launched about 70 billion kg of rock into space - 20,000kg of which could have reached Europa. And the chances that a rock big enough to harbour life arrived are "better than 50/50", researchers estimate.

But could living organisms actually survive these epic trips?

"I'd be surprised if life hasn't gotten to Mars," Ms Worth told BBC News.

"It's beyond the scope of our study. But it seems reasonable that at some point some Earth organisms have made it over there."

It has been shown that tiny creatures can withstand the harsh environment of space. And bacterial spores can be revived after hundreds of millions of years in a dormant state.

But even if a hardy microbe did stow away for all those millennia, it might simply burn up on arrival, or land in inhospitable terrain.

The most habitable places in range of Earth are Europa, Mars and Titan - but while all three have likely held water, it may not have been on offer to visitors.

Europa's oceans are capped by a crust of ice that may be impenetrably thick.

"But it appears regions of the ice sheet sometimes break into large chunks separated by liquid water, which later refreezes," Ms Worth said.

"Any meteorites lying on top of the ice sheet in a region when this occurs would stand a chance of falling through.

"Additionally, the moons are thought to have been significantly warmer in the not-too-distant past."

Moon fossils

On Mars, there is little evidence of flowing water during the last 3.5bn years - the likeliest window for Earth life to arrive.

But what if the reverse trip took place?

The early Martian atmosphere appears to have been warm and wet - prime conditions for the development of life.

And if Martian microbes ever did exist, transfer to Earth is "highly probable" due to the heavy traffic of meteorites between our planets, Ms Worth told BBC News.

"Billions have fallen on Earth from Mars since the dawn of our planetary system. It is even possible that life on Earth originated on Mars."

While her team are not the first to calculate that panspermia is possible, their 10-million-year simulation is the most extended yet, said astrobiologist Prof Jay Melosh, of Purdue University.

"The study strongly reinforces the conclusion that, once large impacts eject material from the surface of a planet such as the Earth or Mars, the ejected debris easily finds its way from one planet to another," he told BBC News.

"The Chicxulub impact itself might not have been a good candidate because it occurred in the ocean (50 to 500m deep water) and, while it might have ejected a few sea-surface creatures, like ammonites, into space, it would not likely have ejected solid rocks.

"I sometimes joke that we might find ammonite shells on the Moon from that event.

"But other large impacts on the Earth may indeed have ejected rocks into interplanetary space."

Another independent expert on panspermia, Mauricio Reyes-Ruiz of the National Autonomous University of Mexico, said the new findings were "very significant".

"The fact such different pathways exist for the interchange of material between Earth and bodies in the Solar System suggests that if life is ever found, it may very well turn out to be our very, very distant relatives," he said.外星隕石可以撞上地球，影響地球生態。地球上的岩石也可以飛向其他星球，送去生命。據英國廣播公司12月11日報道，科學家研究發現，6600萬年前導致恐龍滅絕的隕石撞擊也可能把地球生命送上了其他星球。

***輸出生命

6600萬年前，一枚規模相當於一座小型城市的隕石撞上地球，人稱希克蘇魯博撞擊。有學者猜想這次撞擊導致地球上火山大規模噴發，地表被煙霧覆蓋，導致恐龍滅絕。與此同時，強烈的撞擊也把700億噸左右的地球岩石送入太空，有可能抵達其他星球，送去地球生命。這一發現已經在學術雜誌《天體生物學》上發表。

第一作者、賓夕法尼亞州立大學的雷切爾•沃思說：“我們發現，地球和火星上面的岩石都有可能攜帶生命體抵達太陽系中的其他星球。我們在土衛六或者木星的衛星上尋找生命的時候必須記住，那些生命體可能獨立產生，也有可能原本屬於地球大家庭的一員。”

研究人員認爲，從地球上飛出的岩石直徑必須在3米以上纔能有效保護所攜帶的生命體，保障其在最長可達1000萬年的旅程中不會死於太陽輻射。這些岩石飛出地球后的命運各不相同。有的留在地球軌道中繞地飛行，有的慢慢掉落回地面，有的飛向太陽灰飛煙滅，還有的一頭扎進浩瀚宇宙，永遠離開地球周圍。沃思說：“有的外星球可能適宜生命居住，有一部分地球岩石抵達那裏，它們雖然數量較少，但已經值得我們注意。”

有證據表明，微小的生命體能夠在嚴酷的太空環境下存活，細菌孢子休眠數百萬年之後還能重新生長。不過，在多年的長途跋涉之後，它們還要面對着陸時撞擊和高溫的挑戰，若目的地環境惡劣，也有可能就此死亡。

***木衛二

木衛二是木星的一個衛星，大約有6顆地球岩石飛抵那裏。希克蘇魯博撞擊送出的700億噸地球岩石中，大約有20噸到達了木衛二。據估計，地球岩石帶着生命體抵達木衛二的可能性也超過50%。木衛二上有液體海洋，被冰面覆蓋。有人認爲，冰面太厚，岩石無法穿透。沃思則表示，上凍的間隙，冰面有時候會破裂，露出液態水，給岩石創造機會進入冰面以下。沃思說：“那些衛星在並不遙遠的過去還很溫暖呢……即使現實一點，保守估計，這些生命體還是有可能在木衛二上的海洋裏游來游去。”

***火星

另外，研究人員還估計，總共大約有36萬顆地球岩石受隕石撞擊影響飛上火星，也帶去了生命。沃思說：“要是生命體沒有活着到達火星，我纔要吃驚呢。雖然這不在我們的研究範圍內，但地球生命體很有可能在某個時候飛到那裏。”而人類目前正努力在火星上尋找生命的跡象。

沃思認爲，地球和火星之間的岩石來往相當頻繁，如果微生物曾經在火星上存在過，就很有可能隨着岩石來到地球。她說：“地球形成以來，已經飛過來了數十億顆火星隕石。地球上的生命還有可能是火星上帶過來的呢。”

***評論

除了沃思的團隊之外，其他科研人員也在探究類似課題。普渡大學天體生物學家傑伊•梅洛什說：“一旦星球表面的岩石離開本體，就很有可能抵達其他星球。這項研究爲此提供了強有力的證據。希克蘇魯博撞擊就很有機會。它撞在50到500米深的海洋上，有可能把菊石等海洋生物送上太空。我有時候會開玩笑說，咱們說不定能在月球上看見菊石的殼，它就是那次撞擊的時候飛過去的。”

梅洛什補充說：“地球和太陽系其他星體之間有如此多樣的途徑交換物質，我們如果真的找到地外生命，說不定它們還是我們的遠房親戚呢。”