venerdì 23 aprile 2010

I crateri di Titano raccontano la sua storia

Gli ultimi studi del numero di crateri da impatto presenti su Titano, potrebbero svelare nuovi segreti sulle possibiltà che la luna ospiti la vita.


Dal 2004 al 2007, Cassini ha mappato il 22% della superficie di Titano . Gli scienziati hanno analizzato le immagini riprese dalla sonda ad alta risoluzione Radar Mapper, trovando 49 crateri da impatto. "I crateri da impatto si formano sulla superficie di ogni pianeta a causa dell'impatto degli asteroidi, comete e altri detriti", ha detto Charles Wood, scienziato presso l'Istituto di Scienze Planetarie a Tucson, Arizona, e autore principale dello studio. "L'analisi dei crateri da impatto è una tecnica utilizzata per conoscere la storia di un pianeta".Titano è stato descritto dalla NASA come uno dei maggiori mondi simili alla Terra studiato fino ad oggi, eppure è anche uno dei più ostili per la nascita e lo sviluppo della vita. [Nella foto: crateri da impatto nella regione Xanadu. Crediti: JPL NASA]

Uno dei principali ostacoli è la temperatura di circa 290 gradi F (-179 C). A queste temperature l'acqua sulla superficie diventa dura come la roccia, privando la vita della molecola principale necessaria alle reazioni chimiche basilari. Nonostante tutto, però il fascino di Titano non è mai venuto meno e anzi è aumentato, in quanto assomiglierebbe molto alla Terra primordiale, secondo l'opinione di Wood. Titano è l'unico corpo planetario del Sistema Solare ad eccezione della Terra e di Venere, ad avere una superficie solida e una densa atmosfera, ricca di azoto e metano. Il metano è tuttavia presente in forma liquida, gassosa e solida e gli scienziati ritengono che potrebbe svolgere nei processi biologici il ruolo che sulla Terra svolge l'acqua. "Alcuni scienziati infatti, sostengono che Titano è la versione fredda della Terra primordiale", e se fosse stato più caldo avrebbe sicuramenre ospitato la vita", ha detto Wood.
Comprendere quindi i processi chimici su Titano potrebbe aiutare a capire come ebbe inizio la vita sulla Terra, miliardi di anni fa.

I crateri da impatto recenti sono facili da identificare a causa della loro forma quasi circolare e i pavimenti interni piatti. Ma anche i vulcani possono creare depressioni circolari che assomigliano molto ai crateri da impatto. Gli scienziati hanno numerosi metodi per distinguere se i crateri siano stati creati dai vulcani o da un impatto. Anche se non si sa molto sull'attività vulcanica di Titano, gli scienziati pensano che i vulcani di Titano non espellano pietra lavica incandescente, come sulla Terra, ma acqua o un liquido mescolato con l'ammoniaca o altre sostanze chimiche che poi congelano in superficie. I vulcani potrebbero anche essere una fonte di metano per l'atmosfera di Titano.

Wood dice che c'è una forte evidenza che la maggior parte dei crateri osservati siano stati creati da impatti.
"Sul 22% di Titano, cinque sembravano essere crateri da impatto", ha detto Wood. In questa analisi, gli scienziati hanno identificato infatti il KSA, il Sinlap, il Menrva, l'Afekan e i crateri Selk. I restanti 44 crateri sono stati classificati di classe 2 (impatto quasi certo) e Classe 3 (crateri da impatto probabile). "Penso che abbiano fatto un buon lavoro, data la natura dei dati," ha detto Alfred McEwen, professore di scienze planetarie e direttore del Planetary Image Research Lab presso l'Università di Arizona.

Finora, le immagini di Cassini hanno rivelato solo circa un quinto della superficie di Titano, insufficiente per trarre delle conclusioni definitive circa il potenziale per qualsiasi tipo di forma di vita esistente su Titano. Wood dice che si scoprisse che la superficie di Titano è "molto antica," significherebbe che ci sono possibilità di trovare la vita. Se un corpo planetario ha molti crateri, come la luna di Giove Callisto, è perché la superficie non è stata alterata da attività vulcanica o intemperie atmosferiche. L'impatto con un asteroide o una cometa su un mondo così inattivo lascia traccie dei crateri per miliardi di anni. Ma per un pianeta attivo come la Terra, i crateri da impatto tendono a scomparire nel corso del tempo rimodellati dalzl'attività atmosferica, geologica o biologica. Titano ha pochissimi crateri da impatto, indicando che potrebbe essere una luna attiva con una superficie molto fresca.

In un altro recente studio, un team di scienziati ha esaminato i crateri da impatto su una superficie di Titano conosciuta come Xanadu, ciò che essi descrivono come "la regione più insolita su Titano". Xanadu, è grande all'incirca come l'Australia e sembra essere molto luminosa forse perchè fatta di ghiaccio d'acqua. Xanadu ha più crateri da impatto di ogni altra regione esaminata fino ad ora da Cassini, ma la distribuzione di questi crateri sembra essere molto irregolare. Ciò significa che l'età di Xanadu varia a seconda della posizione. Al momento, gli scienziati non hanno dati sufficienti per fare conclusioni finali circa l'età o l'attività della superficie di Titano, ed è per questo motivo che si rende necessario un esame completo della superficie.

"Nei prossimi sette anni, la proroga della missione Cassini finanziata dalla Nasa consentirà allo strumento radar di rivelare meglio la superficie di Titano", ha detto Wood. "Speriamo che entro la fine della missione, sia possibile ottenere i dati del 50% della superficie di Titano, riuscendo quindi a trovare più indizi sulla storia Titano"<\i>. McEwen spera in una scelta ancora migliore. "Una nuova missione su Titano sarebbe naturalmente ancora meglio. Ci piacerebbe avere dati più completi e ora che siamo in grado di capire Titano, possiamo progettare strumenti migliori per capire di più sulla sua superficie."


Adattamento a cura di Arthur McPaul

Link: http://www.space.com/scienceastronomy/titan-craters-moon-history-100422.html

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