Blame the Sun for the mysterious dark blemishes on Saturn鈥檚 moon Iapetus. New photos from the Cassini spacecraft reveal the splotches are mainly found on the sunward-facing slopes of craters and mountains, suggesting a runaway heating process is tainting portions of the moon.
The 1470-kilometre-wide Iapetus has a yin-yang appearance, with a black face and a bright back. The dark, organic-rich material on the front probably comes from one of Saturn鈥檚 other moons and simply splattered on Iapetus鈥檚 leading hemisphere like insects on a car鈥檚 windshield.
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Since dark material absorbs more sunlight, some scientists suggested that the moon鈥檚 face got slightly warmer than its back. Over time, this led to the evaporation of thin layers of ice, exposing even more dark material that was already mixed in with the ice. The net result: a feedback loop that produced the satellite鈥檚 distinctive look.
That theory has received a strong boost from the latest Cassini images, acquired during the spacecraft鈥檚 close flyby of Iapetus on 10 September (see Bizarre Saturn moon mottled like a Dalmatian). The results were presented today at a planetary sciences meeting in Orlando, Florida, US.
鈥楻unaway process鈥�
鈥淲e got a very much better look at the relation between the black material and the ice,鈥� says Carolyn Porco, leader of Cassini鈥檚 imaging team.
Team members found that there are sharp-edged dark spots all over the surface, even on the moon鈥檚 bright trailing hemisphere. Careful analysis revealed that these isolated spots are preferentially located on the sunward facing slopes of craters.
That suggests that as the slopes get slightly warmer, ice there starts to evaporate. That exposes more dark stuff in the ice, which is then ready to retain even more solar heat. 鈥淚t鈥檚 a runaway process,鈥� Porco told 快猫短视频.
Unfortunately, knowing that so-called thermal segregation is at play doesn鈥檛 reveal anything about the exact nature of the dark stuff. Part of it has apparently been mixed in with the ice for a very long time, while another part was swept up by Iapetus more recently.
According to Porco, the presence of dark material on some crater floors suggests that some dark material may have slid down the slopes of the craters. 鈥淲e certainly don鈥檛 know all the details yet,鈥� she says.
Cassini: Mission to Saturn 鈥� learn more in our continually updated .
