40亿年前的火星曾什么样？
<p><span word="A">A</span> <span word="new">new</span> <span word="animation">animation</span> <span word="by">by</span> <span word="NASA">NASA</span> <span word="scientists">scientists</span> <span word="illustrates">illustrates</span> <span word="what">what</span> <span word="Mars">Mars</span> – <span word="the">the</span> <span word="fourth">fourth</span> <span word="planet">planet</span> <span word="from">from</span> <span word="the">the</span> <span word="Sun">Sun</span> <span word="and">and</span> <span word="the">the</span> <span word="second">second</span> <span word="smallest">smallest</span> <span word="planet">planet</span> <span word="in">in</span> <span word="the">the</span> <span word="Solar">Solar</span> <span word="System">System</span> – <span word="may">may</span> <span word="have">have</span> <span word="looked">looked</span> <span word="like">like</span> <span word="billions">billions</span> <span word="of">of</span> <span word="years">years</span> <span word="ago">ago</span>.</p><p><span word="Today">Today</span>, <span word="Mars">Mars</span> <span word="is">is</span> <span word="a">a</span> <span word="cold">cold</span>, <span word="desert">desert</span> <span word="world">world</span>. <span word="Liquid">Liquid</span> <span word="water">water</span> <span word="cannot">cannot</span> <span word="exist">exist</span> <span word="pervasively">pervasively</span> <span word="on">on</span> <span word="its">its</span> <span word="surface">surface</span> <span word="due">due</span> <span word="to">to</span> <span word="the">the</span> <span word="low">low</span> <span word="atmospheric">atmospheric</span> <span word="pressure">pressure</span> <span word="and">and</span> <span word="surface">surface</span> <span word="temperature">temperature</span>, <span word="although">although</span> <span word="there">there</span> <span word="is">is</span> <span word="evidence">evidence</span> <span word="for">for</span> <span word="spurts">spurts</span> <span word="of">of</span> <span word="liquid">liquid</span> <span word="flow">flow</span> <span word="that">that</span> <span word="perhaps">perhaps</span> <span word="consist">consist</span> <span word="of">of</span> <span word="a">a</span> <span word="briny">briny</span> <span word="solution">solution</span> <span word="with">with</span> <span word="reduced">reduced</span> <span word="freezing">freezing</span> <span word="temperature">temperature</span>. <span word="Water">Water</span> <span word="under">under</span> <span word="current">current</span> <span word="conditions">conditions</span> <span word="can">can</span> <span word="be">be</span> <span word="ice">ice</span> <span word="or">or</span> <span word="sublimate">sublimate</span> <span word="directly">directly</span> <span word="into">into</span> <span word="vapor">vapor</span> <span word="without">without</span> <span word="staying">staying</span> <span word="in">in</span> <span word="a">a</span> <span word="liquid">liquid</span> <span word="phase">phase</span>.</p><p><span word="Around">Around</span> 4 <span word="billions">billions</span> <span word="of">of</span> <span word="years">years</span> <span word="ago">ago</span> <span word="when">when</span> <span word="the">the</span> <span word="planet">planet</span> <span word="was">was</span> <span word="young">young</span>, <span word="it">it</span> <span word="appears">appears</span> <span word="to">to</span> <span word="have">have</span> <span word="had">had</span> <span word="a">a</span> <span word="thick">thick</span> <span word="atmosphere">atmosphere</span> <span word="that">that</span> <span word="was">was</span> <span word="warm">warm</span> <span word="enough">enough</span> <span word="to">to</span> <span word="support">support</span> <span word="oceans">oceans</span> <span word="of">of</span> <span word="liquid">liquid</span> <span word="water">water</span> – <span word="a">a</span> <span word="critical">critical</span> <span word="ingredient">ingredient</span> <span word="for">for</span> <span word="life">life</span>.</p><p><span word="The">The</span> <span word="animation">animation</span> <span word="shows">shows</span> <span word="how">how</span> <span word="the">the</span> <span word="surface">surface</span> <span word="of">of</span> <span word="the">the</span> <span word="Red">Red</span> <span word="Planet">Planet</span> <span word="might">might</span> <span word="have">have</span> <span word="appeared">appeared</span> <span word="during">during</span> <span word="this">this</span> <span word="ancient">ancient</span> <span word="clement">clement</span> <span word="period">period</span>, <span word="beginning">beginning</span> <span word="with">with</span> <span word="a">a</span> <span word="flyover">flyover</span> <span word="of">of</span> <span word="a">a</span> <span word="lake">lake</span>.</p>
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