宇    宙    生    命    之    谜   

 多    年    来   , 人    们    一    直    在    探    索    宇    宙    生    命    问    题   。 我    们    常    常    有    这    样    的    疑    问   : 宇    宙    中   , 除    了    地    球    外   , 其    他    星    球    上    是    否    也    有    生    命    存    在   ? 为    了    解    决    这    个    疑    惑   , 有    位    同    学    找    到    了    这    篇    文    章   
 浏    览    了    这    一     段    后   , 发    现    它    对     了    解    地    球    之    外    是     否    有    生    命    没    有    帮     助   
 每    个    自    然    段     往    往    有    提    示    主    要     意    思    的    语    句   , 阅     读    时    我    要    注    意    找     出    来   。  古    时    候   , 科    学    不    发    达   , 人    们    一    直    向    往    着    天    上    的    世    界   。 于    是   , 有    了    许    许    多    多    的    故    事   : 嫦    娥    奔    月   , 仙    女    下    凡   , 蟠    桃    盛    会   …… 现    在   , 科    学    发    达    了   , 人    们    知    道    那    都    是    古    人    编    出    来    的   。 但    是   , 地    球    之    外    的    太    空    中    是    否    有    生    命    存    在   , 仍    然    是    一    个    吸    引    人    的    问    题   
 从    理    论    上    说   , 宇    宙    是    无    限    的   。 地    球    只    是    太    阳    系    中    的    一    颗    行    星   , 而    太    阳    系    只    是    银    河    系    中    一    个    极    小    的   
 哪    些    天    体    上    可    能    有    生    命    存    在    呢   ? 这    个    天    体    又    必    须    具    备    什    么    样    的    条    件    呢   ? 人    们    了    解    了    生    命    起    源    的    过    程    之    后   , 认    为    至    少    应    有    这    样    几    个    条    件   : 一    是    适    合    生    物    生    存    的    温    度   , 一    般    应    在   - 之    间   ; 二    是   
 本    文    作    者    张    申    碚   、 赵    晓    梅   ,  选    作    课    文    时    有    改    动   
 必    要    的    水    分   , 生    命    物    质    诸    如    蛋    白    质   、 核    酸    的    活    力    都    和    水    紧    密    相    关   , 没    有    水   , 也    就    没    有    生    命   ; 三    是    适    当    成    分    的    大    气   , 虽    然    已    发    现    少    数    厌    氧    菌    能    在    没    有    氧    气    的    条    件    下    生    存   , 但    氧    气    和    二    氧    化    碳    对    于    生    命    的    存    在    是    极    为    重    要    的   ; 四    是    要    有    足    够    的    光    和    热   , 为    生    命    系    统    提    供    能    源   
 这    一    段    对    解     决    问    题    很    重    要   ,  画    出    关    键    词    帮    我     理    清    了    生    命    存    在     的    条    件   
 根    据    这    些    条    件   , 科    学    家    首    先    对    太    阳    系    除    地    球    以    外    的    其    他    行    星    进    行    了    分    析   。 水    星    离    太    阳    最    近   , 朝    向    太    阳    的    一    面    表    面    温    度    达    到   , 不    可    能    存    在    生    命   。 金    星    是    一    颗    缺    氧   、 缺    水   , 有    着    浓    厚    云    层    的    行    星   , 太    阳    辐    射    和    云    层    造    成    的    温    室    效    应   , 使    得    金    星    表    面    温    度    极    高   , 不    可    能    有    生    命    存    在   。 木    星   、 土    星   、 天    王    星    和    海    王    星    离    太    阳    很    远   , 它    们    的    表    面    温    度   , 一    般    都    低    于   -, 因    此   , 也    不    可    能    有    生    命    存    在   
 这    段    的    段    首     提    到    了    地    球    外    的    其     他    行    星   。 通    过    提     取    关    键    信    息   , 我     知    道    这    些    星    球    上     是    不    存    在    生    命    的   
 太    阳    系    中    唯    一    还    可    能    存    在    生    命    的    行    星    是    火    星   。  火    星    与    地    球    有    不    少    相    似    之    处   : 地    球    自    转    一    圈    是     小    时    分    秒   , 火    星    自    转    一    圈    是    小    时    分   ; 地     与    公    转    轨    道    平    面    有    度    分    的    倾    角   , 而     火    星    的    倾    角    约    度    分   , 所    以    火    星    和    地    球    昼    夜    长     短    相    近   , 而    且    也    有    四    季    更    替   。 更    有    趣    的    是   ,  年   , 意    大    利    的    一    位    天    文    学    家    观    察    到    火    星    表    面    有    很     多    纵    横    的    黑    色    线    条   , 人    们    猜    测    这    是    火    星    人    开    挖    的     运    河   。 人    们    还    观    察    到    火    星    表    面    的    颜    色    随    着    季    节    而   
 前    面    排    除    了     那    些    不    具    备    生    命     存    在    条    件    的    行    星   ,  这    一    段    提    示    火    星     上    有    可    能    存    在    生     命   , 关    于    火    星    的     内    容    很    重    要   。  变    化   , 有    人    认    为    这    是    火    星    表    面    植    物    随    着    季    节    的    变    化    而    改    变    了    颜    色   
 为    了    揭    开    火    星    神    秘    的    面    纱   , 科    学    家    们    决    定    利    用    探    测    器    对    火    星    作    近    距    离    的    观    测   。 年   , 美    国    发    射    的    水    手    号    探    测    器    进    入    了    环    绕    火    星    飞    行    的    轨    道   , 给    火    星    拍    摄    了    大    量    的    照    片   。 这    些    照    片    表    明   , 意    大    利    天    文    学    家    观    察    到    的    所    谓    运    河   , 原    来    是    一    连    串    的    暗    环    形    山    和    暗    的    斑    点   。 通    过    近    距    离    观    测    还    发    现   , 以    前    观    察    到    的    火    星    表    面    上    所    谓    颜    色    的    四    季    变    化   , 并    不    是    植    物    的    生    长    和    枯    萎    造    成    的   , 而    是    风    把    火    星    表    面    上    的    尘    土    吹    来    吹    去    引    起    的    颜    色    明    暗    变    化   
 科    学    家    们    还    发    现   : 火    星    是    一    个    非    常    干    燥    的    星    球   , 在    它    的    大    气    中    虽    然    找    到    了    水    汽   , 但    含    量    极    少   , 只    有    地    球    上    沙    漠    地    区    的   ; 火    星    的    大    气    层    非    常    稀    薄   , 是    二    氧    化    碳   , 氧    气    含    量    极    少   ; 火    星    表    面    温    度    很    低   ; 火    星    上    没    有    磁    场   , 它    的    大    气    层    中    又    没    有    臭    氧    层   , 因    而    不    能    抵    御    紫    外    线    和    各    种    宇    宙    线    的    照    射   。 所    有    这    些    因    素    都    说    明   , 在    火    星    上    生    命    难    以    存    在   
 近    年    来    科    学     家    在    对    火    星    的    研     究    中    可    能    有    了    新     的    发    现   。  为    了    对    火    星    作    进    一    步    的    考    察   , 年   , 美    国    发    射    了    两    个    名    叫    海    盗    号    的    探    测    器   。 这    两    个    探    测    器    在    火    星    着    陆   , 进    行    了    一    系    列    的    分    析    和    测    试   , 得    到    两    个    重    要    结    果   : 一    是    在    火    星    的    土    壤    中    未    检    测   
 到    有    机    分    子   ; 二    是    在    对    火    星    表    面    取    样    的    培    养    中   , 未    发    现    微    生    物    的    存    在   。 这    证    明   , 在    探    测    器    着    陆    的    地    区   , 火    星    表    面    没    有    生    命    存    在   。 科    学    家    又    提    出   , 生    命    物    质    是    否    会    存    在    于    火    星    的    岩    层    之    中    呢   ? 这    有    待    进    一    步    研    究   
 人    们    至    今    尚    未    在    地    球    以    外    的    太    空    中    找    到    生    命   ,  但    仍    然    相    信    遥    远    的    太    空    存    在    着    生    命   。 近    年    来   , 科     学    家    对    落    在    地    球    上    的    一    些    陨    石    进    行    分    析   , 发    现    陨     石    上    存    在    有    机    分    子   , 说    明    太    空    可    能    存    在    生    命   
 地    球    之    外    是    否    有    生    命    存    在   , 是    人    类    一    直    探    索    的    宇    宙    之    谜   
 与    问    题    相    关    的    内    容    我    会     仔    细    读   , 必    要    时    会    多    读    几    遍   。  有    的    段    落    和    我    想    要    了    解    的    问     题    关    系    不    大   , 就    不    需    要    细    读   。  有    的    信    息    可    能    是     不    准    确    的   , 需    要    再    查     查    相    关    资    料    加    以    判    断   

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