陨石撞击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.

外星陨石可以撞上地球，影响地球生态。地球上的岩石也可以飞向其他星球，送去生命。据英国广播公司12月11日报道，科学家研究发现，6600万年前导致恐龙灭绝的陨石撞击也可能把地球生命送上了其他星球。

***输出生命

6600万年前，一枚规模相当于一座小型城市的陨石撞上地球，人称希克苏鲁博撞击。有学者猜想这次撞击导致地球上火山大规模喷发，地表被烟雾覆盖，导致恐龙灭绝。与此同时，强烈的撞击也把700亿吨左右的地球岩石送入太空，有可能抵达其他星球，送去地球生命。这一发现已经在学术杂志《天体生物学》上发表。

第一作者、宾夕法尼亚州立大学的雷切尔•沃思说：“我们发现，地球和火星上面的岩石都有可能携带生命体抵达太阳系中的其他星球。我们在土卫六或者木星的卫星上寻找生命的时候必须记住，那些生命体可能独立产生，也有可能原本属于地球大家庭的一员。”

研究人员认为，从地球上飞出的岩石直径必须在3米以上才能有效保护所携带的生命体，保障其在最长可达1000万年的旅程中不会死于太阳辐射。这些岩石飞出地球后的命运各不相同。有的留在地球轨道中绕地飞行，有的慢慢掉落回地面，有的飞向太阳灰飞烟灭，还有的一头扎进浩瀚宇宙，永远离开地球周围。沃思说：“有的外星球可能适宜生命居住，有一部分地球岩石抵达那里，它们虽然数量较少，但已经值得我们注意。”

有证据表明，微小的生命体能够在严酷的太空环境下存活，细菌孢子休眠数百万年之后还能重新生长。不过，在多年的长途跋涉之后，它们还要面对着陆时撞击和高温的挑战，若目的地环境恶劣，也有可能就此死亡。

***木卫二

木卫二是木星的一个卫星，大约有6颗地球岩石飞抵那里。希克苏鲁博撞击送出的700亿吨地球岩石中，大约有20吨到达了木卫二。据估计，地球岩石带着生命体抵达木卫二的可能性也超过50%。木卫二上有液体海洋，被冰面覆盖。有人认为，冰面太厚，岩石无法穿透。沃思则表示，上冻的间隙，冰面有时候会破裂，露出液态水，给岩石创造机会进入冰面以下。沃思说：“那些卫星在并不遥远的过去还很温暖呢……即使现实一点，保守估计，这些生命体还是有可能在木卫二上的海洋里游来游去。”

***火星

另外，研究人员还估计，总共大约有36万颗地球岩石受陨石撞击影响飞上火星，也带去了生命。沃思说：“要是生命体没有活着到达火星，我才要吃惊呢。虽然这不在我们的研究范围内，但地球生命体很有可能在某个时候飞到那里。”而人类目前正努力在火星上寻找生命的迹象。

沃思认为，地球和火星之间的岩石来往相当频繁，如果微生物曾经在火星上存在过，就很有可能随着岩石来到地球。她说：“地球形成以来，已经飞过来了数十亿颗火星陨石。地球上的生命还有可能是火星上带过来的呢。”

***评论

除了沃思的团队之外，其他科研人员也在探究类似课题。普渡大学天体生物学家杰伊•梅洛什说：“一旦星球表面的岩石离开本体，就很有可能抵达其他星球。这项研究为此提供了强有力的证据。希克苏鲁博撞击就很有机会。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。我有时候会开玩笑说，咱们说不定能在月球上看见菊石的壳，它就是那次撞击的时候飞过去的。”

梅洛什补充说：“地球和太阳系其他星体之间有如此多样的途径交换物质，我们如果真的找到地外生命，说不定它们还是我们的远房亲戚呢。”