Welcome back
to episode 2 of Season 3 and today we are going on another adventure but this
time it’s far beyond Moon to another Moon in our solar System. Today, I will
tell you about a very fascinating moon of Saturn called Iapetus.
Saturn is the
second biggest planet but with the largest number of Moons parked around its
orbit. As of March 2025, there are a total of 274 known Moons orbiting around
Saturn and there could be more in as more data is being processed which was
gathered from previously sent missions like Cassini. Iapetus is one of many
Moons of Saturn that is known to exist for more than 350 years to us. It was
discovered on October 25, 1671 by an Italian astronomer Giovanni Cassini. It
was discovered at the Paris Observatory and at the time of discovery was the
second known moon of Saturn after Titan which was discovered in 1655. It was
also the 6th known moon found in our solar system. Now moving
forward to the story of discovery, which is quite interesting and easy to
understand if you know the meaning of the term “tidally-locked”. Okay let me
help you recall. In Astronomy, when we say an object is tidally-locked, we mean
that it is locked in the orbit of the object it is revolving around. In other
words, a tidally locked object is something that only shows its one side
towards the object it is facing. The other side remains facing outwards in
opposite direction and we can’t see it from our point of view. Still confusing,
pardon me for my English but let me try to make it even easier for you with an
example. Our own Moon is tidally-locked with Earth because we only see its one
side facing us. The other side is dark and facing towards the outer space. The
same case is with Iapetus, which is tidally locked with Saturn. This has one of
the huge role in the discovery of this moon.
Back in
October 1671, when Giovanni Cassini pointed his refractive telescope at Saturn,
he discovered an object appearing to be hanging in the orbit around Saturn.
This was discovered in the western side of Saturn from our point of view. So
when it was revolving around Saturn, it was in the western side and very clear
from our point of view. To confirm his predictions, Cassini tried to look for
it again few months later but this time it was nowhere to be found on the eastern
side. Why was it happening? Did something swallow it? Obviously the concept of
black hole was unknown at that time but he tried it again but this time with a
much improved telescope. He pointed it again on the eastern side of Saturn and
there it was in orbit around Saturn. It had not gone anywhere, it was there the
whole time but the reason for it was very easy to understand. The side that was
facing towards us on the western side is icy and reflects back a lot of light
but on the other side, when it reaches on the eastern side of the planet, we
were met with a much darker side that does not reflects any light. However, you
must not forget that Iapetus is tidally-locked and only we can observe it’s
both sides from our point of view but to anybody standing on the Saturn (only
if it was possible), you can only see one side of the Moon.
Iapetus is
the third largest moon of Saturn and 11th largest in our Solar
System. Unlike other moons like Europa, it does not have any ocean beneath its
icy surface. This icy world is about 1469 km in diameter. A day here is about
79 Earth days long. Apart from these facts, there is one more feature about
this Moon which makes it standout against other natural satellites discovered
so far. Iapetus is a walnut shaped moon because of an equatorial ridge that
runs along the equator for about 1300 km long and it is about 20 km wide and
13km high, much higher than the Mt. Everest on Earth. Because of this height of
Ridge and its distance, this gives Iapetus a walnut like shape. Without any hypothesis,
this wouldn’t be actually part of Astronomy. So yes there is one that I like
the most which is that there used to have a ring system around Iapetus, which
later fell around its equator. Thus, the equatorial ridge was formed.
Another
question that rises here is have we ever visited this world before? And the
answer is both yes and no. There has never been any specific mission to this
Moon before but flyby have happened in the part as part of missions like
Cassini. The first attempt to study Saturn and its moon happened with
Pioneer-11 mission but it didn’t came that closer to it. The next series of
mission Voyager-1 and Voyager-2 and with this Voyager-1 made history as it
became the first ever spacecraft to ever send back a picture of Iapetus two
tone surface. That is the picture gave us more details about its dark and icy
sides surface. But the closer study of its surface wouldn’t happen till
December 31, 2004 when Cassini spacecraft made its first targeted flyby around
Iapetus. The closest and last flyby of Iapetus by Cassini happened in September
2007 and after that it never made any contact with the Moon. Data from Cassini
showed that its surface is heavily cratered and has a massive impact basin
about 580 km in diameter. This was from me for this month’s story. I will be
back next month with another episode. Thank you!
 |
| Episode Cover Art |
