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14 Maret 2015

Bulan Terbesar di Tata Surya Menyimpan Samudra di Bawah Permukaannya — Samudra yang tersembunyi di bawah bulan terbesar di tata surya, Ganymede, terungkap. Publikasi di Journal of Geophysical Research mengungkap bahwa samudra pada bulan milik Jupiter itu punya kedalaman 30 kilometer.

Alih-alih mengamati samudra tersembunyi itu secara langsung, para astronom menemukan lautan luas di Ganymede dengan cara mengamati aurora, cahaya alami di kutub yang muncul akibat adanya interaksi dengan medan magnet.

Ganymede lain dengan bulan-bulan di tata surya karena seolah-olah memiliki dinamo internal yang bisa membangkitkan medan magnet sendiri. Keberadaan medan magnet itulah yang membuat bulan yang ditemukan pada 7 Januari 1610 itu punya aurora.

Uniknya, karena juga dipengaruhi oleh medan magnet Jupiter, aurora di Ganymede tak cuma diam lalu hilang. Aurora itu memanjang dan memendek, berputar dengan kecepatan tertentu.

Astronom memprediksi, dengan mempertimbangkan tarik-menarik medan magnet Ganymede-Jupiter, aurora di Ganymede mestinya bergerak dengan spin 6 derajat. Namun, hasil pengamatan ternyata tak demikian.

"Aurora dalam kasus ini hanya bergerak cuma (dengan spin) sekitar 2 derajat," kata Joachim Saur, astronom dari University of Cologne, yang melakukan studi seperti dikutip National Geographic, Kamis (12/3/2015). 

Dengan pemodelan komputer, Saur mengungkap bahwa sebab aurora hanya bergerak dengan spin 2 derajat adalah lautan di Ganymede. Menurut dia, air asin di Ganymede memengaruhi medan magnet bulan itu, mencoba menyeimbangkan pengaruh Jupiter.

Keberadaan samudra di bulan berdiameter 5.200 kilometer itu sebenarnya tak begitu mengejutkan. Wahana Galileo milik Badan Penerbangan dan Antariksa Amerika Serikat (NASA) yang mengorbit Jupiter dari tahun 1995-2003 pernah memberikan indikasinya.

Namun, indikasi yang diberikan Galileo berupa pembacaan data magnetik belum kuat. Efek yang sama bisa diberikan oleh faktor medan magnetik internal bulan itu sendiri. "Waktu itu masih fifty-fifty," kata Saur. 

Saur, seperti dikutip BBC, Kamis, mengatakan, "Lautan yang ada di Ganymede tak mungkin lebih dalam dari 330 kilometer. Jika lebih dalam, itu tak akan menerangkan data yang diperoleh."

"Data konsisten dengan adanya lautan yang punya kedalaman 100 kilometer dan memiliki konsentrasi garam 5 gram per 1 liter air. Namun, bisa juga yang ada adalah lautan dengan kedalaman 10 kilometer dengan garam 10 kali lebih besar," kata Saur.

Penemuan samudra di Ganymede ini menarik perhatian ilmuwan karena di mana pun ada air, di sana ada potensi kehidupan. Walaupun demikian, masih banyak yang harus dipelajari hingga ilmuwan bisa menyatakan bahwa kehidupan memang mungkin ada di sana.

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