Howdy Star Gazers and welcome to this episode
of Star Trails. My name is Drew and I'll be your
guide to the night sky for the week of March
the 22nd to the 28th. This week we're continuing
our exploration of our own cosmic neighborhood
by taking a look at some of the most unique moons
in the solar system. and which ones of those
might have the right conditions to develop life.
Later in the show, we'll take a look at this
week's sky and pick up the next two chapters
of our book club selection, Night Watch. Whether
you're tuning in from the backyard or the balcony,
I'm glad you're here. So grab a comfortable spot
under the night sky and let's get started. I
need to start this week's show by first thanking
our listeners. I received some interesting feedback
concerning last week's Pluto episode. Everything
from a wonderful poem about Pluto sent in by
a listener, to a handful of folks who mentioned
Pluto as being their favorite planet. Some folks
just enjoyed the story. At this point, I think
it's safe to say Pluto is as much of a cultural
object as it is an astronomical one. And we're
not done with Pluto yet. More on that later in
this show. And another listener who's currently
vacationing in New Zealand sent me a slew of
night sky images from Down Under, including one
of my favorite objects, the Large Magellanic
Cloud, plus a very bright image of the Milky
Way's core, and an upside down rendition of Orion.
It's kind of weird seeing Orion standing on his
head. As always, if you want to contact me, just
use the form over at StarTrails .Show. With that,
let's get into our main topic this week. When
most of us hear the word moon, we think of a
very specific place. A gray, cratered world hanging
quietly in our night sky. Airless, silent, unchanging.
And for most of human history, that was our template.
If you'd asked someone what a moon was, they
would have described something very much like
our own. And in many ways, our moon is extraordinary.
It likely formed when a Mars -sized world slammed
into early Earth, sending debris into orbit that
eventually coalesced into the moon. It's unusually
large for a rocky planet. so large that the Earth
-Moon system almost behaves like a binary pair.
Its gravity stabilizes Earth's tilt, helping
keep our climate relatively steady over long
periods of time. And, of course, it's the only
world beyond Earth where human beings have ever
set foot. But for all its importance, our Moon
also gave us the wrong idea. Because when we
finally began exploring the outer solar system,
We discovered that moons are not all like ours,
not even close. When the Voyager spacecraft flew
past Jupiter and Saturn and sent back images,
that changed everything. Suddenly, moons weren't
just dead rocks orbiting distant planets. They
were worlds, some of them violent, some of them
dynamic, and some of them are possibly habitable.
A moon is simply a natural satellite, anything
that orbits a planet or a dwarf planet, but maybe
that's a little too straightforward. Some moons
are bigger than planets. For example, Ganymede
and Titan are both larger than the planet Mercury.
Some systems behave like binary worlds, think
Pluto and Charon. Some moons may have formed
like planets, just in orbit around another world.
while others were wanderers captured by gravity.
Rings and moons are related. Inside a certain
distance, known as the Roche limit, material
can't clump together into a moon, so it forms
rings instead. In fact, if an object goes beyond
that limit, it's likely going to break apart
because of the planet's gravity. Saturn is the
king of the rings, of course, but Jupiter, Uranus,
and Neptune all have wispy ring systems. And
there are shepherd moons around these systems
that gravitationally keep the ring systems in
check. Outside the Roche limit, rings can eventually
coalesce into moons. So in a sense, rings are
moons that never formed, and nearby moons are
rings that finish the job. There's a great diversity
in moon types just in our solar system. Take
Io, one of Jupiter's moons. It's the most volcanically
active world in the entire solar system. Its
surface is constantly being reshaped by eruptions,
lava flows, sulfur plumes, and landscapes that
look more like a fever dream than a place in
space. And the reason is gravity. Jupiter's immense
pull stretches and squeezes Io over and over
again. generating heat inside the moon. That
process is called tidal heating. That same process
leads us to something even more remarkable. Because
if tidal forces can generate heat, they can melt
ice. And if they can melt ice, they can create
oceans. Hidden ones. Europa, another of Jupiter's
moons, looks like a frozen world. Its surface
is smooth, bright, and criss -crossed with long
fractures that resemble sea ice on Earth. But
beneath that icy shell, scientists believe there's
a global ocean, possibly deeper than all of Earth's
oceans combined. We haven't seen that ocean directly.
Instead, we've detected it through a kind of
cosmic detective work. The surface geology suggests
movement. Magnetic measurements from the Galileo
spacecraft indicate a conductive, likely salty,
layer beneath the ice. Models of tidal heating
show there's enough energy to keep water liquid,
so all signs point to an ocean in the dark. And
Europa isn't alone. At Saturn, a tiny moon called
Enceladus shocked scientists when the Cassini
spacecraft discovered geysers erupting from its
south pole, jets of water vapor and ice blasting
into space. Cassini flew through those plumes
and detected salts, organic molecules, even hydrogen,
evidence that this water has been in contact
with rock deep below the surface. In other words,
Enceladus has an ocean, and it's leaking into
space. Then there's Titan. Titan is something
else entirely. It's larger than the planet Mercury
and has a thick nitrogen atmosphere, denser than
Earth's in some ways, and a surface shaped by
weather, clouds, rain, rivers, and lakes. But
the liquid isn't water. It's methane and ethane.
Titan is a world where it rains hydrocarbons
and rivers carve through landscapes of ice as
hard as rock. And we've been there. In 2005,
the Huygens probe parachuted through Titan's
orange sky and landed on its surface, the most
distant landing humanity has ever achieved. It
sent back images of an eerie landscape, rounded
stones of water ice scattered across what looked
like a dried riverbed. And beneath Titan's crust,
there may be yet another hidden ocean. Back at
Jupiter, Ganymede is the biggest moon in the
solar system. If it and Titan were orbiting the
Sun instead of gas giants, we'd be calling them
planets. Ganymede has its own magnetic field,
and it's the only moon we know of that does.
Beneath its surface may lie multiple layers of
ocean, stacked between sheets of ice like a planetary
layer cake. Callisto, also a Jovian moon, is
heavily cratered and ancient. It too may also
hide a deep ocean beneath its battered exterior.
And farther out orbiting Neptune is Triton, a
strange icy world that likely didn't form around
Neptune at all. It was probably captured from
the Kuiper Belt. It orbits backward against the
rotation of the planet. And Voyager 2 observed
nitrogen geysers erupting from its surface. Even
at the edge of the solar system, these moons
are active. And then there are the quieter, lesser
-known worlds. Mimas, a small moon of Saturn,
looks like the Death Star from Star Wars. It's
dominated by a massive impact crater. But recent
evidence suggests that it may have a, guess what,
a hidden ocean beneath its icy shell. Iepidus
is split dramatically in two, one hemisphere
bright as snow, the other dark as coal, creating
one of the starkest contrasts in the solar system.
Miranda, a moon of Uranus, looks like it was
shattered and reassembled with cliffs and terrain
so bizarre that it defies easy explanation. And
out at Pluto, Charon, or is it pronounced Cairon,
is so large relative to Pluto that the two orbit
a point in space between them. The New Horizons
flyby in 2015 revealed vast canyons and signs
of ancient geological activity. In addition to
Charon, Pluto has four irregular tiny moons,
making it a complex six -body system. They don't
behave like our moon or even like most of the
large moons of Jupiter and Saturn. Instead of
being tidally locked, some of them tumble unpredictably.
Their rotations are influenced by the constantly
shifting gravitational pull of both Pluto and
Charon. It's a dynamically messy system. We've
talked about some of the most well -known moons
in this episode, but here are a couple of really
weird ones you might not know about. Hyperion
orbits Saturn and resembles a sponge or a piece
of coral drifting through space. Its surface
is covered in deep, sharp -edged craters giving
it a porous appearance. But the truly strange
part is its motion, and this is a trait it shares
with some of Pluto's moons. Hyperion isn't tidally
locked, and it tumbles around Saturn in an unpredictable
way. If you were standing on Hyperion, which
would be complicated, Saturn wouldn't rise and
set in a regular way. It would drift unpredictably
across the sky. Another excellent oddball is
Phoebe, also a moon of Saturn. I'm beginning
to think Saturn is probably the most fun place
to be in the solar system. It has everything.
Rings, weird moons, and more. Anyway, Phoebe
orbits Saturn backward, much like Triton does
around Neptune. So it didn't form with Saturn,
and like Triton, it's possibly a relic from the
Kuiper Belt. It's dark, irregular, and ancient.
A fossil from the early solar system just hanging
out in Saturn's orbit. Before we drift away from
Saturn, I also have to mention Janus and Epimetheus.
These two moons share nearly the same orbit,
and every few years when they get near each other,
they effectively swap places. These are called
co -orbital moons. And what I mean by swapping
places is this. One moon orbits closer to the
planet than the other. When they approach one
another, a gravitational dance takes place between
the two, and one assumes the closer inner orbit,
trading places with the other, now pushed to
the outer track. And they do this repeatedly.
I didn't forget about Mars. It has Phobos and
Deimos, two little rocky potatoes that were likely
captured by the Martian gravity. In other words,
they didn't form alongside Mars the way our moon
formed alongside Earth. Our moon is the only
large moon of the inner solar system, which makes
us quite an outlier. The outer planets are moon
machines. Jupiter now has more than 90 known
moons, Saturn more than 140, and Uranus and Neptune
both have dozens. Astronomers think the difference
in numbers between the outer and inner planets
is similar to why the outer planets are so much
larger. The inner solar system was hotter when
it formed. Close to the sun, lighter materials
like water, ice, and gases couldn't condense
easily. that left mostly rock and metal. Out
beyond the frost line, ices could form, and ice
is abundant. That gave the outer planets far
more raw material to build not just planets,
but entire systems of moons. Also, since the
outer planets are all enormous, their gravity
is strong enough to capture passing objects and
hold on to large numbers of moons. None of that
explains why Pluto, smaller than even our moon,
somehow has five moons of its own. Well, the
moons of Pluto and Earth may actually share an
origin story. Earth's moon is likely the result
of a giant impact, which is a rare and chaotic
event. Pluto's moons may have also formed from
such an impact. A collision early in Pluto's
history would have thrown debris into orbit,
which later coalesced into Charon and the smaller
moons. Mercury and Venus may have once had moons,
but over billions of years, gravitational interactions
with the sun could have destabilized those systems,
causing moons to either crash into the planet
or escape. So the outer solar system ends up
looking like kind of a miniature solar system
factory. with planets surrounded by their own
collections of worlds, while the inner solar
system is comparatively sparse. I remember reading
in Carl Sagan's Cosmos years ago that there could
be multiple places to sustain life in our solar
system, not just Earth and Mars. Of course, Sagan
was including many of the moons in the outer
solar system that had been recently discovered.
The possibility that some of these moons might
be habitable, not on the surface or in sunlight,
but beneath the ice, is a fascinating one. On
Earth, we've discovered ecosystems thriving in
complete darkness, deep in the ocean, powered
not by sunlight, but by chemical energy from
hydrothermal vents. If similar environments exist
on Europa or Enceladus, liquid water, energy,
chemistry, then the basic ingredients for life
may be present. And here's the part that changes
how we think about the universe. When we search
for habitable worlds, we tend to look for planets
like Earth, rocky, sitting at just the right
distance from their star. But our own solar system
is telling us something surprising. Moons warmed
by gravity instead of sunlight, oceans hidden
beneath ice, small distant worlds orbiting giant
planets, their oceans circulating quietly in
the dark. These moons are intriguing areas for
future exploration. Remember, many of these moons
have only been seen once during the Voyager flybys
of the 1980s. There's a lot more science to learn.
Future missions such as Europa Clipper, which
was launched in 2024, is designed to study the
subsurface ocean of Europa. And Dragonfly is
expected to launch in 2028 on a mission to Titan.
It's a robotic helicopter designed to fly on
Titan where it will study habitability and chemistry.
And, of course, NASA is making plans to return
to our moon with the Artemis program, with a
landing scheduled for 2027. We have one more
episode about our solar system to close out March.
Next week, we journey into the Kuiper Belt and
beyond. After a quick break, we'll be back to
cover this week's night sky. Stay with us. Welcome
back. Let's step outside now and take a look
at this week's sky. We're coming off of a new
moon, which means this is one of the best weeks
of the month for deep sky observing. As the week
begins, a delicate, waxing crescent moon hangs
low in the western sky just after sunset, growing
a little fuller each night. By mid -week, the
moon reaches first quarter, appearing half -lit,
and on the evening of March 26th, it forms a
nice pairing with Jupiter in the constellation
Gemini. This is a great time to notice something
subtle, earth -shine. That faint glow illuminating
the dark portion of the crescent moon is actually
sunlight reflected off of Earth. Now let's talk
planets. The standout this week is Venus, blazing
in the western sky just after sunset. It's reemerged
as the evening star, shining brighter than anything
else in the night sky except the moon. You'll
find it low in the west, lingering for more than
an hour after sunset. If you catch it with a
small telescope you may even notice it showing
a crescent phase, much like a tiny version of
the moon. I've been seeing Venus on my evening
walks lately and its brightness is so startling
I have a hard time looking away. It's really
one of my favorite sights in the night sky. Higher
up dominating the evening is Jupiter. It's bright,
steady, and unmistakable. Look at it in a telescope
and you'll see it flanked by the moons we've
been discussing, Io, Europa, Ganymede, and Callisto.
You'll see them change position from night to
night, and sometimes they render shadows on the
Jovian surface. If you're up early, there's a
more subtle challenge waiting for you. Late in
the week, Mercury begins to peek into the pre
-dawn sky, very low on the horizon before sunrise.
It's not an easy target, but if you have a clear
eastern horizon and a bit of patience, you might
catch it briefly before the sun washes it away.
Now this is the time of year astronomers quietly
get excited about. Spring brings what's known
as galaxy season. As the Milky Way dips below
the horizon in the evening, we're able to look
past the plane of our galaxy into deep space.
And that means galaxies and lots of them. In
the constellation Leo, you can find the Leo Triplet,
three galaxies interacting with each other across
millions of light years. In Ursa Major, near
the Big Dipper, look for M81 and M82, a pair
of galaxies, one graceful and spiral, the other
ragged and explosive. And if you sweep your telescope
toward Virgo and Coma Berenices, you'll begin
to encounter entire fields of galaxies. Faint
smudges of light, each one a vast island universe.
For something a little more low -key, try tracking
down the Coma star cluster in Coma Berenices,
a loose, delicate scattering of stars that's
best seen with binoculars under dark skies. Or,
if you're feeling ambitious, hunt down the Ghost
of Jupiter Nebula in Hydra, a faint planetary
nebula that resembles a pale ghostly disk. And
finally, take a moment this week to simply look
up without a plan. Find Orion setting in the
west, slowly leaving our evening skies. Look
overhead to Gemini, with Jupiter shining among
the twin stars Castor and Pollux. And rising
in the east, you'll begin to see the constellations
of Spring, Leo, Virgo, and Boötes taking their
place. We're slowly creeping towards the end
of our book club selection, Night Watch, and
there are just a few chapters left. Chapters
8 and 9 bring us back to something familiar.
our own moon and its relationship with the Sun.
These are two very short chapters, and I'm not
sure if they offer much to experienced observers,
but since our topic this week was all about the
moons of our solar system, it seemed appropriate
to talk about them in this episode. Chapter 8
is all about observing the Moon and Sun. These
two familiar objects are easy to take for granted,
but endlessly rewarding if you spend a little
time with them at the eyepiece. The Moon, especially,
is one of the best targets for backyard astronomy.
Even a small telescope will reveal an incredible
landscape of craters, mountain ranges, and vast
lava plains known as maria. One of the best times
to observe is along the line between light and
dark, called the Terminator, where shadows stretch
across the surface and give those features depth
and texture. This chapter also includes some
nice maps of the moon's surface. Chapter 8 also
encourages something we don't talk about often,
observing the sun. Now, this comes with an important
caveat. You must use proper solar filters. But
when you do, even a modest telescope can reveal
sunspots, those dark shifting regions tied to
the sun's magnetic activity. With specialized
equipment, you can even see solar prominences,
massive arcs of plasma rising from the sun's
surface. Then in chapter 9, we move into eclipses.
These are the moments when the sun, earth, and
moon align just right to create something extraordinary.
A solar eclipse happens when the Moon passes
in front of the Sun, casting its shadow on Earth.
A lunar eclipse occurs when Earth moves between
the Sun and the Moon, and our shadow darkens
the lunar surface. Total solar eclipses, in particular,
have captivated people throughout history. In
fact, the book describes so -called eclipse cults.
These are people who travel across the globe
to stand in the narrow path of totality where
day briefly turns to night. Lunar eclipses are
more common and a bit more forgiving. You don't
need special equipment to enjoy them and they
often turn the moon a deep coppery red as Earth's
atmosphere filters sunlight into the shadow.
You can refer to the tables in this chapter to
discover when the next eclipses are happening.
We'll wrap up our discussion on the remaining
chapters of Night Watch in two weeks. We're almost
done. If you're a newer observer and you've made
it this far in the book, then your knowledge
of the night sky and how to view it has increased
exponentially since we began this journey. That's
going to do it for this week. If you found this
episode interesting, please share it with a friend
who might enjoy it. The easiest way to do that
is by sending folks to our website, StarTrails
.Show. And if you want to support the show, use
the link on the site to buy me a coffee. That
really helps. Be sure to follow Star Trails on
Blue Sky and YouTube. Links are in the show notes.
Until we meet again beneath the stars, clear
skies everyone.