{"id":8397,"date":"2006-02-01T01:08:24","date_gmt":"2006-01-31T23:08:24","guid":{"rendered":"http:\/\/109.232.216.219\/~bilimvegelecek\/?p=8397"},"modified":"2017-07-17T11:03:08","modified_gmt":"2017-07-17T08:03:08","slug":"yok-olusun-kaynagi-dista-mi-icte-mi-sevgili-dinolarimizin-katili-kim","status":"publish","type":"post","link":"https:\/\/bilimvegelecek.com.tr\/index.php\/2006\/02\/01\/yok-olusun-kaynagi-dista-mi-icte-mi-sevgili-dinolarimizin-katili-kim","title":{"rendered":"Yok olu\u015fun kayna\u011f\u0131 d\u0131\u015fta m\u0131, i\u00e7te mi? Sevgili dinolar\u0131m\u0131z\u0131n \u2018katil\u2019i kim?"},"content":{"rendered":"

E\u011fer \u201cmaymundan geldi\u011fimiz sav\u0131\u201d sizi rahats\u0131z ediyorsa, bir d\u0131\u015f \u201cm\u00fcdahaleciye\u201d sar\u0131l\u0131n! Yok, \u201cEvrim s\u00fcrecini bask\u0131n olarak Yer\u2019deki etmenler belirler\u201d diyorsan\u0131z, siz di\u011fer sava sar\u0131lacaks\u0131n\u0131z! Her ikisinin birlikteli\u011fi olamaz m\u0131? Yine siz karar vereceksiniz. Ancak unutmayal\u0131m: Bilim hi\u00e7bir zaman \u201cson ger\u00e7ek\u201d tan\u0131maz! Richard Feynman\u2019\u0131n dedi\u011fi gibi, \u201cBilimsel kesinlik yok, bilimsel ilerleme var\u201d.<\/em><\/h4>\n

\u00d6nce Toutatis asteroidi, ard\u0131ndan Shoemaker-Levy 9 kuyruklu y\u0131ld\u0131z\u0131, \u015fimdilerde de Marduk \u201cDinozorlar\u0131n yazg\u0131s\u0131yla insan t\u00fcr\u00fcn\u00fcn yazg\u0131s\u0131 bir benzerlik g\u00f6sterecek mi?\u201d sorusunu s\u0131k s\u0131k g\u00fcndeme getirdi, getiriyor!<\/p>\n

\u201cYok olu\u015fun mant\u0131\u011f\u0131 ve Toutatis\u201d adl\u0131 yaz\u0131s\u0131nda Luis Walter Alvarez, g\u00f6kta\u015f\u0131 \u00e7arpmas\u0131yla dirimk\u00fcredeki baz\u0131 canl\u0131 t\u00fcrlerinin kitlesel yok olu\u015fu aras\u0131nda bir ili\u015fkinin varl\u0131\u011f\u0131na de\u011finiyor. Bu yaz\u0131da kitlesel yok olu\u015f tamamen Yer\u00f6tesi etmenlere ba\u011flan\u0131yor. Ancak dinozorlar\u0131n kitlesel yok olu\u015flar\u0131n\u0131 Yersel etmenlere ba\u011flayan bir a\u00e7\u0131klama da var! Bu a\u00e7\u0131klama, Yer\u2019deki etmenlerin evrim s\u00fcrecindeki \u00f6nemini ye\u011fler. Bu yaz\u0131da her iki g\u00f6r\u00fc\u015f\u00fcn savlar\u0131n\u0131 sunmaya \u00e7al\u0131\u015faca\u011f\u0131z. Yaz\u0131n\u0131n sonunda okuyucu belki kendi karar\u0131n\u0131 olu\u015fturup, \u201c\u0130\u015fte!…sevgili dinolar\u0131m\u0131z\u0131n ger\u00e7ek \u2018katili\u2019 bu etmendir!\u201d diyecek. Hemen \u015funu an\u0131msatal\u0131m: Bu kar\u015f\u0131la\u015ft\u0131rmal\u0131 yaz\u0131da ele al\u0131nan savlar, dinolar\u0131n katillerini belirlemenin \u00f6tesinde anlamlar ta\u015f\u0131r. Yer\u00f6tesi etmeni ye\u011fleyen sav, Darwin\u2019in evrim kuram\u0131nda kulland\u0131\u011f\u0131 \u201cs\u0131n\u0131rs\u0131z zaman<\/strong>\u201d ilkesini ve dolayl\u0131 olarak evrim kuram\u0131n\u0131 \u00e7\u00fcr\u00fctmeyi, \u201cba\u015f\u0131m\u0131za ya\u011fan ta\u015flar\u0131n\u201d evrim s\u00fcrecini h\u0131zland\u0131rd\u0131\u011f\u0131 sav\u0131n\u0131 g\u00fc\u00e7lendirmeyi ama\u00e7lamaktad\u0131r.<\/p>\n

Canl\u0131 evrimini \u201ch\u0131zland\u0131rma\u201d konusunda benzer bir \u00e7aba, \u00fcnl\u00fc \u0130ngiliz fizik\u00e7isi Lord Kelvin ve arkada\u015flar\u0131ndan gelmi\u015f ama ba\u015far\u0131s\u0131zl\u0131kla sonu\u00e7lanm\u0131\u015ft\u0131. E\u011fer \u201cmaymundan geldi\u011fimiz sav\u0131\u201d sizi rahats\u0131z ediyorsa, bir d\u0131\u015f \u201cm\u00fcdahaleciye\u201d sar\u0131l\u0131n! Yok, \u201cEvrim s\u00fcrecini bask\u0131n olarak Yer\u2019deki etmenler belirler\u201d diyorsan\u0131z, siz di\u011fer sava sar\u0131lacaks\u0131n\u0131z! Her ikisinin birlikteli\u011fi olamaz m\u0131? Yine siz karar vereceksiniz. Ancak unutmayal\u0131m: Bilim hi\u00e7bir zaman \u201cson ger\u00e7ek\u201d tan\u0131maz! Richard Feynman\u2019\u0131n dedi\u011fi gibi, \u201cBilimsel kesinlik yok, bilimsel ilerleme var\u201d.<\/p>\n

Yer\u00f6tesi etmen<\/em><\/strong><\/p>\n

Yerbilim, dirimbilim, ta\u015fbilim, g\u00f6kbilim ve fizik alan\u0131nda yap\u0131lan \u00e7al\u0131\u015fmalar Yer\u2019deki baz\u0131 canl\u0131 t\u00fcrlerinin kitlesel yok olu\u015fa u\u011frad\u0131\u011f\u0131na i\u015faret ediyor. Di\u011fer yandan Yer\u2019de b\u00fcy\u00fck \u00e7arpma kraterleriyle kar\u015f\u0131la\u015f\u0131yoruz. Yap\u0131lan \u00e7al\u0131\u015fmalar, her iki olay\u0131n da (kitlesel yok olu\u015f ve \u00e7arpma kraterleri) d\u00f6nemsel bir de\u011fi\u015fim sergiledi\u011fini g\u00f6steriyor. Daha ilginci, bu olaylar\u0131n d\u00f6nemi ayn\u0131 oldu\u011fu gibi evreleri de ayn\u0131d\u0131r! E\u011fer G\u00fcne\u015f\u2019in, b\u00fcy\u00fck eksen uzunlu\u011fu 2,8 \u0131\u015f\u0131k y\u0131l\u0131 olan ve olduk\u00e7a bas\u0131k bir y\u00f6r\u00fcngede dolanan bir k\u0131rm\u0131z\u0131 c\u00fcce<\/strong> yolda\u015f\u0131 oldu\u011funu varsayarsak, bu d\u00f6nemsel olaylar\u0131n ayn\u0131 evreli olu\u015funa bir a\u00e7\u0131klama getirebiliriz.<\/p>\n

Yolda\u015f Y\u0131ld\u0131z Modeli<\/strong>: R.A. Muller ba\u015fkanl\u0131\u011f\u0131ndaki Berkeley grubu, yukar\u0131da s\u00f6z\u00fcn\u00fc etti\u011fimiz iki olay aras\u0131ndaki s\u0131k\u0131 ili\u015fkiye a\u00e7\u0131klama getirebilmek amac\u0131yla, G\u00fcne\u015f\u2019in 26 milyon y\u0131ll\u0131k d\u00f6neme sahip bir yolda\u015f\u0131 oldu\u011funu varsay\u0131yor. Kepler yasalar\u0131 yolda\u015f y\u0131ld\u0131z\u0131n eliptik y\u00f6r\u00fcngesinin yar\u0131-b\u00fcy\u00fck eksenini (26 000 000) 2\/3<\/sup> = 88 000 AB olarak saptar. 1 AB = 150 000 000 km (Yer-G\u00fcne\u015f aras\u0131 uzakl\u0131kt\u0131r). Bu de\u011ferin birimini de\u011fi\u015ftirip sonucu 2 ile \u00e7arparsak y\u00f6r\u00fcngenin b\u00fcy\u00fck eksenini 2,8 \u0131\u015f\u0131k y\u0131l\u0131 olarak buluruz. I\u015f\u0131k y\u0131l\u0131 bir zaman \u00f6l\u00e7\u00fcs\u00fc de\u011fil, uzakl\u0131k \u00f6l\u00e7\u00fcs\u00fcd\u00fcr; \u0131\u015f\u0131\u011f\u0131n bir y\u0131lda kat etti\u011fi yol anlam\u0131na gelir. Yolda\u015f y\u0131ld\u0131z enberideyken (G\u00fcne\u015f\u2019e en yak\u0131n konumdayken) Oort kuyruklu y\u0131ld\u0131z bulutunun en yo\u011fun b\u00f6lgelerini tedirgin eder ve G\u00fcne\u015f dizgesinin i\u00e7 b\u00f6lgesine milyonlarca kuyruklu y\u0131ld\u0131z ya\u011fmas\u0131na neden olur. Ancak, J\u00fcpiter ve Sat\u00fcrn\u2019\u00fcn kuyruklu y\u0131ld\u0131zlara \u00e7ekimsel tedirginlik uygulamas\u0131 nedeniyle bu b\u00f6lge, kuyruklu y\u0131ld\u0131zlar\u0131n \u201c\u00f6ld\u00fcr\u00fcc\u00fc\u201d etkilerinden ba\u011f\u0131\u015f\u0131k kal\u0131r. T\u00fcm bu \u201cbody guard\u201dlar\u0131n koruyucu etkilerine kar\u015f\u0131n yine de 20-30 kuyruklu y\u0131ld\u0131z\u0131n Yer\u2019e \u00e7arpmas\u0131 beklenir. Dinolar bu t\u00fcr \u00e7arpmalar\u0131n bir b\u00f6l\u00fcm\u00fcnden kurtulmu\u015f olabilir. Ancak, Cretaceous<\/em> d\u00f6neminde \u00e7arpan b\u00fcy\u00fck bir kuyruklu y\u0131ld\u0131z bu dev s\u00fcr\u00fcngenlerin neslinin t\u00fckenmesine neden olmu\u015f olabilir (\u00c7izelge 1). \u00c7arpman\u0131n sonucu olarak b\u00fcy\u00fck niceliklerde toz atmosferin \u00fcst katmanlar\u0131na dek \u00e7\u0131kacak ve orada aylarca as\u0131l\u0131 kalacakt\u0131r. G\u00fcne\u015f \u0131\u015f\u0131\u011f\u0131ndan yoksun kalma fotosentezin durmas\u0131na, bitkilerin ge\u00e7ici olarak \u00f6lmelerine neden olacakt\u0131r. Ot yiyen dinolar\u0131n besinsiz kalmalar\u0131 ve s\u0131f\u0131r\u0131n alt\u0131ndaki s\u0131cakl\u0131klar bu neslin t\u00fckenmesi anlam\u0131na gelecektir.<\/p>\n

\"Jeolojik
\u00c7izelge 1. Jeolojik zaman \u00f6l\u00e7e\u011fi. Ya\u015f ve s\u00fcre milyon y\u0131l olarak verilmi\u015ftir. Paleozoic \u00e7a\u011f \u00e7izelgeye al\u0131nmam\u0131\u015ft\u0131r.<\/figcaption><\/figure>\n

Varsay\u0131lan yolda\u015f y\u0131ld\u0131z bug\u00fcne dek g\u00f6zlenemedi. Bu, y\u0131ld\u0131z\u0131n 7 kadirden daha s\u00f6n\u00fck oldu\u011fu anlam\u0131na gelir. Bu t\u00fcr y\u0131ld\u0131zlar Yale parlak y\u0131ld\u0131zlar dizininde yer almaz. E\u011fer yolda\u015f anakol y\u0131ld\u0131z\u0131ysa, k\u00fctlesi G\u00fcne\u015f k\u00fctlesinin (M\u009d<\/sub>) 0,3 denlisinden daha az, Oort kuyruklu y\u0131ld\u0131z bulutunu tedirgin edebilmesi i\u00e7in de 0,05 G\u00fcne\u015f k\u00fctlesinden daha fazla olmal\u0131d\u0131r. G\u00fcne\u015f\u2019e yeterince yakla\u015f\u0131p kuyruklu y\u0131ld\u0131z bulutunu tedirgin edebilmesi i\u00e7in y\u00f6r\u00fcnge bas\u0131kl\u0131\u011f\u0131 0,6-1,0 aral\u0131\u011f\u0131nda olmal\u0131d\u0131r. K\u00fctlesi, 0,05 M\u009d<\/sub> < M\u00ab<\/sub>< 0,3 M\u009d<\/sub> aral\u0131\u011f\u0131ndaysa, \u00f6ng\u00f6r\u00fclen y\u0131ld\u0131z\u0131n k\u0131rm\u0131z\u0131 c\u00fcce<\/strong> olmas\u0131 gerekir. K\u0131rm\u0131z\u0131 c\u00fcceler g\u00f6kadam\u0131zda bolca bulunan y\u0131ld\u0131z t\u00fcr\u00fcd\u00fcr. Bug\u00fcne dek yap\u0131lm\u0131\u015f olan yak\u0131n y\u0131ld\u0131z \u00e7al\u0131\u015fmalar\u0131nda yolda\u015f y\u0131ld\u0131z\u0131n g\u00f6zlenememi\u015f olmas\u0131n\u0131n nedeni, \u00f6zdevinimi ve dikine h\u0131z\u0131n\u0131n hemen hemen s\u0131f\u0131r olmas\u0131 olabilir. Berkeley grubu \u0131raks\u0131n\u0131m\u0131 (parallax) b\u00fcy\u00fck olan k\u0131rm\u0131z\u0131 y\u0131ld\u0131zlar\u0131 bu ama\u00e7la ara\u015ft\u0131r\u0131yor. Ancak hangi y\u00f6ne bakacaklar\u0131n\u0131 bilemiyor olmalar\u0131 b\u00fcy\u00fck bir engel! Varsay\u0131msal yolda\u015f y\u0131ld\u0131z\u0131n G\u00fcne\u015f \u00fczerine uygulad\u0131\u011f\u0131 \u00e7ekimsel tedirginlik, bize en yak\u0131n y\u0131ld\u0131z (4,3 \u0131\u015f\u0131k y\u0131l\u0131) olan Proxima Centauri<\/em>\u2019nin \u00e7ekimsel tedirginli\u011finden daha azd\u0131r. Bu t\u00fcr bir y\u0131ld\u0131z\u0131n G\u00fcne\u015f \u00e7evresindeki y\u00f6r\u00fcngesinin molek\u00fcl bulutlar\u0131 ve yak\u0131n kom\u015fulu\u011fumuzdan ge\u00e7en Samanyolu y\u0131ld\u0131zlar\u0131nca bozulmas\u0131 10 9<\/sup> y\u0131l s\u00fcrer. Bu denli y\u00fcksek y\u00f6r\u00fcnge kararl\u0131l\u0131\u011f\u0131 Yer\u2019deki kitlesel yok olu\u015flara a\u00e7\u0131klama getirebilir. Ancak, ayn\u0131 rakam yolda\u015f y\u0131ld\u0131z\u0131n Yer\u2019in olu\u015fumundan bu yana ayn\u0131 y\u00f6r\u00fcngede bulunamayaca\u011f\u0131na da i\u015faret eder! Berkeley grubu en olas\u0131 senaryoyu \u015f\u00f6yle kuruyor: Yolda\u015f y\u0131ld\u0131z bir zamanlar G\u00fcne\u015f\u2019e \u00e7ekimsel olarak s\u0131k\u0131 s\u0131k\u0131ya ba\u011fl\u0131yd\u0131. Yak\u0131n kom\u015fulu\u011fumuzdan ge\u00e7en y\u0131ld\u0131zlar\u0131n \u00e7ekimsel etkisiyle bu ba\u011f giderek zay\u0131flam\u0131\u015ft\u0131r. \u00c7arpma kraterleri \u00fczerine yap\u0131lan \u00e7al\u0131\u015fmalar bu spek\u00fclasyonu do\u011frulayabilir. \u00c7\u00fcnk\u00fc, krater \u00e7al\u0131\u015fmalar\u0131 kuyruklu y\u0131ld\u0131z \u201cya\u011fmurlar\u0131n\u0131n\u201d d\u00f6nemi ve \u015fiddetine ili\u015fkin bilgiler verir. \u00d6rne\u011fin, krater \u00e7al\u0131\u015fmalar\u0131 Ay \u00fczerine yap\u0131lan en b\u00fcy\u00fck kuyruklu y\u0131ld\u0131z bombard\u0131man\u0131n\u0131n 3,9 milyar y\u0131l \u00f6nce sona erdi\u011fine i\u015faret ediyor. Bu bombard\u0131man, varsay\u0131msal yolda\u015f y\u0131ld\u0131z\u0131n i\u00e7 y\u00f6r\u00fcngeden daha d\u0131\u015f bir y\u00f6r\u00fcngeye sa\u00e7\u0131lmas\u0131 nedeniyle durmu\u015f olabilir. Yer\u2019deki ilk ya\u015fam belirtileri \u00fczerine yap\u0131lan izotop \u00e7al\u0131\u015fmalar\u0131 ya\u015fam\u0131n bu bombard\u0131man\u0131n hemen ertesinde ba\u015flad\u0131\u011f\u0131na i\u015faret ediyor.<\/p>\n

\u00c7arpma kraterlerinin d\u00f6nemi<\/strong>: Yer\u2019e \u00e7arpan bir kuyruklu y\u0131ld\u0131z\u0131n kraterinin bulunma olas\u0131l\u0131\u011f\u0131 y\u00fczde 10-25\u2019dir. Bu, d\u00fc\u015f\u00fck bir olas\u0131l\u0131kt\u0131r. \u00c7\u00fcnk\u00fc, bir\u00e7ok \u00e7arpma b\u00f6lgesi (\u00f6rne\u011fin, okyanus yataklar\u0131) \u00fczerine yap\u0131lan \u00e7al\u0131\u015fmalar hen\u00fcz tamamlanmam\u0131\u015ft\u0131r. Ancak, Yer\u00f6tesi etmen modeli her bir kuyrukluy\u0131ld\u0131z \u201cya\u011fmurunda\u201d birden fazla \u00e7arpman\u0131n olaca\u011f\u0131n\u0131 \u00f6ng\u00f6rd\u00fc\u011f\u00fcnden her bir \u201cya\u011fmura\u201d ili\u015fkin en az bir kraterin bulunma olas\u0131l\u0131\u011f\u0131 olduk\u00e7a y\u00fcksektir. \u00c7arpma kraterlerinin ya\u015flar\u0131 \u00fczerine yap\u0131lan \u00e7al\u0131\u015fmalar, 28 milyon y\u0131ll\u0131k bir d\u00f6nemin oldu\u011fu y\u00f6n\u00fcnde sonu\u00e7 veriyor. Bu bulgu, kitlesel yok olu\u015f d\u00f6nemi ve evresiyle \u00e7ak\u0131\u015fmaktad\u0131r. \u015eekil 1, \u00e7ap\u0131 10 km\u2019den daha b\u00fcy\u00fck olan \u00e7arpma kraterlerinin ya\u015f\u0131n\u0131 g\u00f6steriyor. Oklar, 28 milyon y\u0131ll\u0131k d\u00f6neme i\u015faret eder. D\u00f6nem \u00fczerine yap\u0131lan yan\u0131lg\u0131lar \u00b11 milyon y\u0131ld\u0131r.<\/p>\n

\"T\u00fcrlerin
\u015eekil 1. T\u00fcrlerin neslinin t\u00fckeni\u015f oran\u0131n\u0131 zaman g\u00f6re g\u00f6steren \u00e7izge. Oklar 26 milyon y\u0131ll\u0131k d\u00f6neme i\u015faret eder.<\/figcaption><\/figure>\n

\u00d6ng\u00f6r\u00fcler<\/strong>: T\u00fcrlerin kitlesel yok olu\u015funu a\u00e7\u0131klamaya \u00e7al\u0131\u015fan bu model G\u00fcne\u015f\u2019in bir yolda\u015f\u0131n\u0131n oldu\u011funu varsaymaktad\u0131r. E\u011fer bulunursa Berkeley grubu bu y\u0131ld\u0131za \u201cNemesis<\/strong>\u201d ad\u0131n\u0131n verilmesini \u00f6neriyor. Nemesis eski bir Yunan tanr\u0131\u00e7as\u0131d\u0131r; felaket ve \u00f6\u00e7 almay\u0131 simgeler! \u0130lgin\u00e7tir ki, Yunanca\u2019da felaket s\u00f6zc\u00fc\u011f\u00fc iki s\u00f6zc\u00fc\u011f\u00fcn bir araya gelmesiyle t\u00fcretilir: \u201cK\u00f6t\u00fc y\u0131ld\u0131z\u201d. Y\u0131ld\u0131z\u0131n sa\u011fduyulu bir ba\u015fka isim babas\u0131 da Texas \u00dcniversitesi\u2019nden Harlan Smith\u2019dir. Smith, yolda\u015f y\u0131ld\u0131za \u201cSiva<\/strong>\u201d denmesini \u00f6neriyor. Siva hem yok olu\u015fu hem de varolu\u015fu ayn\u0131 anda simgeleyen bir Hint tanr\u0131s\u0131d\u0131r. Ger\u00e7ekten de kitlesel yok olu\u015flar evrim s\u00fcrecinde ikili rol oynar: Dirimk\u00fcre sahnesinden bir canl\u0131 t\u00fcr\u00fc \u201c\u00e7ekilirken\u201d bir ba\u015fka t\u00fcr ortaya \u00e7\u0131kar.<\/p>\n

Modelin bir \u00f6ng\u00f6r\u00fcs\u00fc \u015fudur: \u00c7al\u0131\u015fmalar kuyruklu y\u0131ld\u0131z \u00e7arpma kraterlerinin kitlesel yok olu\u015fla ili\u015fkisini kurdu\u011funa g\u00f6re, krater yak\u0131nlar\u0131nda iridyum katmanlar\u0131 olu\u015fmu\u015f olmal\u0131d\u0131r. Ger\u00e7ekten de Permian\/Triassic<\/em> d\u00f6neminde olu\u015fmu\u015f olan bir krater yak\u0131nlar\u0131nda iridyum tabakas\u0131 bulunmu\u015ftur. \u0130ridyum Yer\u2019de do\u011fal olarak bulunmayan bir kimyasal elementtir. Bu elementin varl\u0131\u011f\u0131, ve ek olarak \u00e7arpma b\u00f6lgesi s\u0131n\u0131rlar\u0131ndaki maddelerin \u015fok dalgalar\u0131nca etkilenmi\u015f oldu\u011funun belirlenmesiyle \u00e7arpma modeli g\u00fc\u00e7lenecektir.<\/p>\n

Model, t\u00fcm de\u011filse bile baz\u0131 kitlesel yok olu\u015flarda birden fazla \u00e7arpman\u0131n\u00a0 ger\u00e7ekle\u015fmi\u015f olmas\u0131n\u0131 \u00f6ng\u00f6r\u00fcyor. Bu da ara\u015ft\u0131rmac\u0131lar\u0131 birden fazla iridyum katman\u0131n\u0131n varl\u0131\u011f\u0131n\u0131 ara\u015ft\u0131rmaya itiyor. Bug\u00fcne dek yap\u0131lan \u00e7al\u0131\u015fmalarda birden fazla iridyum katman\u0131 bulundu. Di\u011fer bir deyi\u015fle, \u00e7oklu katman\u0131n olmad\u0131\u011f\u0131na ili\u015fkin kesin bir kan\u0131t yok! Alvarez\u2019in \u00e7al\u0131\u015fma grubu \u015fimdi yeni bir iridyum algac\u0131yla \u00e7al\u0131\u015f\u0131yor. Bu alga\u00e7, \u0130ridyum-192 radyoaktif elementinin bozunmas\u0131yla a\u00e7\u0131\u011fa \u00e7\u0131kan gamma \u0131\u015f\u0131nlar\u0131n\u0131 317 keV ve 468 keV erke d\u00fczeylerinde alg\u0131lamaya \u00e7al\u0131\u015facak. Bu alga\u00e7 yard\u0131m\u0131yla ad\u0131 ge\u00e7en modelin \u00f6ng\u00f6r\u00fcleri s\u0131nanabilecektir.<\/p>\n

Yok olu\u015f s\u00fcresi<\/strong>: Kuyruklu y\u0131ld\u0131z ya\u011fmurunun s\u00fcresi yolda\u015f y\u0131ld\u0131z\u0131n y\u00f6r\u00fcnge d\u0131\u015f merkezlili\u011fiyle orant\u0131l\u0131d\u0131r. Yolda\u015f\u0131n Oort kuyruklu y\u0131ld\u0131z bulutuna uygulayaca\u011f\u0131 \u00e7ekimsel tedirginli\u011fin ye\u011finli\u011fi, yolda\u015f y\u0131ld\u0131z\u0131n buluta olan uzakl\u0131\u011f\u0131n\u0131n k\u00fcb\u00fcyle ters orant\u0131l\u0131d\u0131r (1\/r 3<\/sup><\/em>). E\u011fer yolda\u015f y\u0131ld\u0131z\u0131n y\u00f6r\u00fcnge d\u0131\u015f merkezlili\u011fini 0,6-0,9 aral\u0131\u011f\u0131nda d\u00fc\u015f\u00fcn\u00fcrsek, kuyruklu y\u0131ld\u0131z ya\u011fmur s\u00fcresi 100 000 – 2 000 000 y\u0131l olacakt\u0131r. Ya\u011fmurun k\u0131sa s\u00fcrmesi i\u00e7in y\u00f6r\u00fcnge d\u0131\u015f merkezlili\u011finin \u00e7ok b\u00fcy\u00fck ve karars\u0131z olmas\u0131 gerekir. Yukar\u0131daki de\u011ferlerle hesaplayacak olursak, tipik bir kuyruklu y\u0131ld\u0131z ya\u011fmurunda Yer\u2019e 1 milyon y\u0131ll\u0131k bir s\u00fcre i\u00e7inde, her bir \u00e7arpma aras\u0131 50 000 y\u0131l olmak \u00fczere 10 \u00e7arpma ger\u00e7ekle\u015fecektir.<\/p>\n

Kuyruklu y\u0131ld\u0131z ya\u011fmur modelinden \u00e7\u0131kan ilgin\u00e7 bir sonu\u00e7 \u015fudur: Ya\u011fmur s\u0131ras\u0131nda t\u00fcm t\u00fcrlerin nesli ayn\u0131 zamanda t\u00fckenmez. Baz\u0131 t\u00fcrler \u00e7arpman\u0131n erken a\u015famalar\u0131nda yok olur. Baz\u0131lar\u0131 bir\u00e7ok \u00e7arpmadan yara almadan kurtulurken son ve en b\u00fcy\u00fck \u00e7arpmaya yenik d\u00fc\u015fer. Baz\u0131 ta\u015fbilimciler t\u00fcrlerin kitlesel yok olu\u015funun ans\u0131z\u0131n de\u011fil, 1 milyon y\u0131l veya daha uzun zaman dilimlerinde ger\u00e7ekle\u015fti\u011fini savunur.<\/p>\n

\"\"<\/p>\n

Evrim<\/em><\/strong><\/p>\n

E\u011fer kuyruklu y\u0131ld\u0131z ya\u011fmur modelinin \u201cdo\u011frulu\u011fu\u201d g\u00f6sterilebilirse, Yer\u2019in dirimk\u00fcresinde s\u00fcregelen evrim s\u00fcre\u00e7leri bug\u00fcne dek ileri s\u00fcr\u00fclen savlardan olduk\u00e7a ayr\u0131 bir karaktere b\u00fcr\u00fcnecektir. Darwin\u2019in evrim kuram\u0131n\u0131n varsay\u0131m\u0131, evrimi belirleyen temel etmenin t\u00fcrler aras\u0131ndaki rekabet oldu\u011fu<\/strong> y\u00f6n\u00fcndeydi. \u00d6nerilen model, b\u00f6ylesi bir evrim s\u00fcrecinin ancak iki kuyruklu y\u0131ld\u0131z ya\u011fmuru aras\u0131nda ge\u00e7en, g\u00f6reli olarak sakin, 26 milyon y\u0131ll\u0131k d\u00f6nemde olabilece\u011fini savlar. Evrim s\u00fcrecinde, her 26 milyon y\u0131lda bir yeni bir a\u015fama geli\u015fir. Yer, t\u00fcm gezegeni etkileyen bir \u00e7arpma \u201cfelaketine\u201d u\u011frar. Bu t\u00fcr bir felaket olmasayd\u0131 memeliler dinozorlar\u0131 \u201calt ederek\u201d tarih sahnesine \u00e7\u0131kamazlard\u0131. Kuyruklu y\u0131ld\u0131z ya\u011fmuruyla geli\u015fen s\u00fcrecin bizim bildi\u011fimiz evrim s\u00fcrecine olan etkisi nedir bilemiyoruz; ancak, \u00f6nemli bir etkide bulunma olas\u0131l\u0131\u011f\u0131 vard\u0131r. Bu s\u00fcre\u00e7, \u201cbask\u0131n\u201d t\u00fcrlerin neslini t\u00fcketirken \u201c\u00e7ekinik\u201d t\u00fcrlerin geli\u015fmesine yol a\u00e7abilir.<\/p>\n

Evet, evrim s\u00fcrecini d\u0131\u015f etmene ba\u011flayan g\u00f6r\u00fc\u015f b\u00f6yle savlar ileri s\u00fcr\u00fcyor. \u015eimdi de hem dinolar\u0131n yazg\u0131s\u0131n\u0131 hem de evrim s\u00fcrecini Yersel etmenlere ba\u011flayan g\u00f6r\u00fc\u015f\u00fc inceleyelim.<\/p>\n

Yersel etmen<\/em><\/strong><\/p>\n

Yer\u00f6tesi etmeni ye\u011fleyen model ot yiyen dinolar\u0131n besinsizlikten yok olduklar\u0131n\u0131 savundu\u011funa g\u00f6re, Yersel etmeni ye\u011fleyen modelin o d\u00f6nemin bitki \u00f6rt\u00fcs\u00fc \u00fczerine olan g\u00f6r\u00fc\u015flerini incelemede yarar var!<\/p>\n

Yersel etmen modeline g\u00f6re, ya\u015fam\u0131n Yer \u00fczerinde yapt\u0131\u011f\u0131 en \u00f6nemli etkilerden biri, bitkilerin Yer iklimi \u00fczerine yapt\u0131\u011f\u0131 de\u011fi\u015fikliklerdir. Bitkiler G\u00fcne\u015f erkesini kullanarak suyu topraktan yapraklar\u0131na pompalar. Yapraklarda depolanm\u0131\u015f olan su daha sonra buhar olur. Bu s\u00fcre\u00e7 ya\u011fmur \u00e7evriminin s\u00fcreklili\u011fini sa\u011flar. Ya\u011fmuru olu\u015fturan su damlac\u0131klar\u0131 G\u00fcne\u015f\u2019in s\u00fcrekli etkisi alt\u0131nda kal\u0131r ve G\u00fcne\u015f erkesini so\u011fururlar. Buhar yo\u011funla\u015f\u0131p ya\u011fmur olarak d\u00fc\u015ferken sal\u0131nan erke Yer\u2019in r\u00fczgar dizgesini olu\u015fturur. Nemli havay\u0131 okyanuslardan karalara ve da\u011flara ta\u015f\u0131yan bu r\u00fczgarlar daha fazla ya\u011fmur ya\u011fmas\u0131na ve daha fazla erke sal\u0131nmas\u0131na neden olur. Bu s\u00fcre\u00e7, Yer y\u00fczeyini so\u011futur, bitki ya\u015fam\u0131 ve t\u00fcm Yer konu\u015flu organizmalar\u0131n kulland\u0131\u011f\u0131 su niceli\u011fini dev boyutlarda artt\u0131r\u0131r. Karalardan denizlere s\u0131zan besinler oradaki ya\u015fam\u0131n da artmas\u0131na neden olur. En yo\u011fun bi\u00e7imiyle u\u00e7laklarda (kutuplarda) g\u00f6zlenen so\u011fuma daha fazla r\u00fczgar ve ya\u011fmur uyartarak erke ak\u0131s\u0131ndaki art\u0131\u015fa katk\u0131da bulunur.<\/p>\n

\u00d6rne\u011fin, 20 milyon y\u0131l \u00f6nce ilk kez geli\u015ftiklerinde otlar k\u0131talar\u0131n i\u00e7 b\u00f6lgesinde o zaman \u00e7\u00f6l olan b\u00f6lgeleri kaplam\u0131\u015ft\u0131. Bu otlar, suyu toprakta tuzaklayarak ve onu havaya ta\u015f\u0131yarak s\u0131cak olan k\u0131ta i\u00e7i b\u00f6lgelerin so\u011fumas\u0131na neden oldular. B\u00f6ylece h\u0131zl\u0131 so\u011fuma e\u011filimi ba\u015flad\u0131. \u0130lk kez Antarktika\u2019da buz b\u00f6lgesi olu\u015ftu. Yer\u2019in ilk buzulu 200 milyon y\u0131lda olu\u015ftu. So\u011fuma daha nemli hava \u00fcretti: Bitki \u00f6rt\u00fcs\u00fc artt\u0131, buzul \u00e7a\u011f\u0131nda doruk noktas\u0131na ula\u015ft\u0131 ve so\u011fuma son 200 milyon y\u0131l boyunca, buzul \u00e7a\u011flar\u0131 aras\u0131 d\u00f6nemde s\u00fcrd\u00fc. Bug\u00fcn, tropik b\u00f6lgelerdeki ya\u011fmur ormanlar\u0131n\u0131n kesilmesi sonucunda bu etkinin belki de tersine tan\u0131k oluyoruz. Ya\u011fmur ormanlar\u0131 d\u00fcnyan\u0131n di\u011fer b\u00f6lgelerine k\u0131yasla iki veya \u00fc\u00e7 kez daha fazla ya\u011fmuru \u00e7evrime sokuyor ve r\u00fczgar olu\u015fumu i\u00e7in gerekli erkeyi \u00fcretiyor. \u00c7evrimi ortadan kald\u0131rd\u0131\u011f\u0131m\u0131z zaman so\u011fuma etkisini hemen durdurmu\u015f oluyoruz. Atmosferdeki erke ak\u0131s\u0131 ve t\u00fcm D\u00fcnya\u2019daki r\u00fczgarlar azal\u0131yor. Bunlarla birlikte, nemli havan\u0131n deviniminden kaynaklanan ve k\u0131talar\u0131n i\u00e7 b\u00f6lgelerine dek s\u0131zabilen ya\u011fmurlar da azal\u0131yor. \u0130nsanlar\u0131n tar\u0131ma y\u00f6nelik orman \u201ckatliamlar\u0131\u201d b\u00fcy\u00fck bir olas\u0131l\u0131kla t\u00fcm D\u00fcnya\u2019da \u0131s\u0131nmaya neden oluyor. Bu etki, sera etkisine neden olan karbondioksit gibi gazlar\u0131n etkisinden daha fazlad\u0131r. Bu y\u00fczy\u0131l\u0131n ba\u015fl\u0131ca \u0131s\u0131nma e\u011filimi g\u00f6steren d\u00f6nemi, 1920-1940 ve 1980\u2019li y\u0131llard\u0131r. Bu d\u00f6nemlerin hi\u00e7biri sanayi geli\u015fiminin h\u0131zla ilerledi\u011fi d\u00f6nemleri i\u00e7ermiyor. Ancak her iki d\u00f6nemde de dev boyutlarda orman \u201ckatliamlar\u0131\u201d ve 1930\u2019lu y\u0131llarda tar\u0131m alanlar\u0131n\u0131n \u201ckatliam\u0131\u201d olmu\u015ftur.<\/p>\n

Dirimsel s\u00fcre\u00e7ler jeolojik s\u00fcre\u00e7ler yard\u0131m\u0131yla da y\u00fcksek erke ak\u0131s\u0131na katk\u0131da bulunmu\u015f olabilir. \u00d6rne\u011fin y\u00fcz milyon y\u0131l \u00f6nce, k\u00f6kleri daha derinlere inebilen ve daha yayg\u0131n olan \u00e7i\u00e7ekli bitkiler dinozorlar\u0131n nesli t\u00fckendikten 35 milyon y\u0131l sonra k\u0131ta co\u011frafyas\u0131nda g\u00f6zlenen b\u00fcy\u00fck de\u011fi\u015fikliklere katk\u0131da bulunmu\u015ftur. Dinozorlar\u0131n \u00e7a\u011f\u0131 olan Mesozoic<\/em> d\u00f6nemin tamam\u0131nda dinozorlar k\u0131talar\u0131n i\u00e7 b\u00f6lgelerindeki al\u00e7ak denizlerin \u00e7evresinde bulunan batakl\u0131k y\u00f6relerde ya\u015f\u0131yorlard\u0131. K\u0131y\u0131 \u015feridinin dalga devinimleriyle a\u015f\u0131nmaya u\u011framas\u0131 ve bitki yakalayan sedimentler yoluyla olu\u015fan deltalar aras\u0131ndaki dengenin \u00e7ok az da olsa bozulmas\u0131, bu denizlerin h\u0131zla ortadan kalkmas\u0131na neden olacakt\u0131 ki bu durum ger\u00e7ekten de 65 milyon y\u0131l \u00f6nce ger\u00e7ekle\u015fti. Denge, k\u0131y\u0131 bitkilerinde daha etkin k\u00f6klerin geli\u015fmesi ve sedimantasyonun artmas\u0131yla bozulmu\u015f olabilir. K\u0131ta i\u00e7 b\u00f6lgelerinin \u00e7o\u011funu kaplayan al\u00e7ak denizlerin ortadan kalkmas\u0131 daha de\u011fi\u015fken bir iklim yaratm\u0131\u015ft\u0131r. K\u0131talar\u0131n i\u00e7 b\u00f6lgeleri k\u0131\u015f\u0131n daha so\u011fuk ve yaz\u0131n daha s\u0131cak olmaya ba\u015flad\u0131. Bu s\u0131cakl\u0131k farkl\u0131l\u0131\u011f\u0131 r\u00fczgar ve ya\u011fmurlar\u0131 artt\u0131rd\u0131 ve ya\u015fam i\u00e7in kullan\u0131mda olan erkenin artmas\u0131na katk\u0131da bulundu.<\/p>\n

Evrim \u00e7evrenin de\u011fi\u015fmesine neden olurken, \u00e7evre de\u011fi\u015fikli\u011fi de yan\u0131t olarak evrimi \u201ch\u0131zland\u0131r\u0131yordu\u201d. Daha sert k\u0131ta iklimleri, memeliler gibi \u00e7abuk de\u011fi\u015fen iklim ko\u015fullar\u0131na uyabilecek organizmalara gereksinim duyuyordu. Bu organizmalar hemen hemen iki y\u00fcz milyon y\u0131ll\u0131k dinozor \u00fcst\u00fcnl\u00fc\u011f\u00fcn\u00fc sona erdiriyor, sevgili dinolar\u0131n neslinin t\u00fckenmesine neden olan tek etmen olmasa da bu t\u00fckeni\u015fe b\u00fcy\u00fck katk\u0131da bulunuyorlard\u0131.<\/p>\n

\u0130\u015fte Yersel etmeni ye\u011fleyen modelin sav\u0131 da b\u00f6yle! Yersel etmen savunucular\u0131n\u0131n Yer\u00f6tesi etmenin savunucular\u0131na y\u00f6neltti\u011fi soru: \u201cSon y\u0131llarda dinozor neslinin t\u00fckeni\u015fi, Yer-kuyruklu y\u0131ld\u0131z \u00e7arp\u0131\u015fmas\u0131 gibi tamamen geli\u015fig\u00fczel ve bir d\u0131\u015f etmene ba\u011flan\u0131yor. Bu g\u00f6r\u00fc\u015f\u00fcn savunucular\u0131n\u0131n \u015fu soruyu yan\u0131tlamas\u0131 gerekiyor: kuyruklu y\u0131ld\u0131z modeli k\u0131ta i\u00e7i denizlerin kurumas\u0131n\u0131 nas\u0131l a\u00e7\u0131kl\u0131yor?<\/strong> Bu denizlerin kurumas\u0131 Mesozoic<\/em> \u00e7a\u011f\u0131n sonu ve dinozor neslinin t\u00fckenmesiyle e\u015fzamanl\u0131d\u0131r.\u201d<\/p>\n

Orman ve tar\u0131m alanlar\u0131 katliamlar\u0131yla, kloroflorokarbonlarla, n\u00fckleer s\u0131z\u0131nt\u0131larla kendi neslimizi t\u00fcketmeden \u00f6nce bu sorunun yan\u0131t\u0131n\u0131 bulaca\u011f\u0131m\u0131z\u0131 umuyorum!<\/p>\n

KAYNAKLAR<\/strong><\/p>\n

1) E. Lerner, The Big Bang Never Happened, Times Books, Random House, Inc., NY, 1991.<\/p>\n

2) R. A. Muller, Evidence for a solar companion star, The Search for Extraterrestrial Life: Recent Developments, ed. M.D. Papagiannis, IAU Symposium No: 112.<\/p>\n

 <\/p>\n","protected":false},"excerpt":{"rendered":"

E\u011fer \u201cmaymundan geldi\u011fimiz sav\u0131\u201d sizi rahats\u0131z ediyorsa, bir d\u0131\u015f \u201cm\u00fcdahaleciye\u201d sar\u0131l\u0131n! Yok, \u201cEvrim s\u00fcrecini bask\u0131n olarak Yer\u2019deki etmenler belirler\u201d diyorsan\u0131z, siz di\u011fer sava sar\u0131lacaks\u0131n\u0131z! Her ikisinin birlikteli\u011fi olamaz m\u0131? Yine siz karar vereceksiniz. Ancak unutmayal\u0131m: Bilim hi\u00e7bir zaman \u201cson ger\u00e7ek\u201d tan\u0131maz! Richard Feynman\u2019\u0131n dedi\u011fi gibi, \u201cBilimsel kesinlik yok, bilimsel ilerleme var\u201d. \u00d6nce Toutatis asteroidi, ard\u0131ndan […]<\/p>\n","protected":false},"author":475,"featured_media":8424,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[62,211,21,32,28,232],"tags":[421,200,420,239,230,354,422],"class_list":["post-8397","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-24-sayi","category-antropoloji","category-biyoloji","category-evrim","category-sosyal-bilimler","category-yerbilimleri","tag-dinozorlar","tag-evrim","tag-feynman","tag-fosiller","tag-insanlik-tarihi","tag-paleontoloji-fosilbilim","tag-yerin-evrimi"],"acf":[],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t