Sciency Words: Trojans

September 18, 2015 ~ J.S. Pailly ~ 2 Comments

Sciency Words is a special series here on Planet Pailly celebrating the rich and colorful world of science and science-related terminology. Today, we’re looking at the term:

TROJANS

Trojan asteroids are asteroids that share their orbits with a planet. This may not seem like a particularly safe arrangement for the asteroids (or the planet), but so long as the asteroids are positioned just right, their orbits will remain stable.

The asteroid must be located near something called a Lagrange point, specifically the L4 or L5 points. These are points in the orbital plane where the distance to the planet equals the distance to its host star. The combined gravitational pulls of the planet and star will cause the asteroid to circle round and round the Lagrange point in a bizarre, corkscrew-like orbital path.

The first known Trojans were discovered near Jupiter very early in the 20^th Century. The initial plan was to name them all after characters from the Trojan War, as told in Homer’s Iliad; however, it turned out that there were way, way more Trojan asteroids than named characters in that particular story.

We now know that Jupiter has over 6,000 Trojans, about 4,000 orbiting ahead of it and another 2,000 orbiting behind. Most of the other planets in the Solar System have Trojans too.

Neptune has a dozen confirmed Trojans, according to the IAU’s Minor Planet Center. Mars has four, which were probably captured from the asteroid belt. Earth and Uranus each have one. And Saturn… Saturn has none. No Trojans. Probably because Jupiter stole them all.

Aww, cheer up, Saturn! You have something way cooler than Trojan asteroids: Trojan moons!

Saturn is the only planet where moons are known to share orbits with each other. Tethys, Telesto, and Calypso orbit together, with Telesto near Tethys’s L4 point and Calypso near the L5 point. Dione, Helene, and Polydeuces make a similar set, with Helene and Polydeuces near Dione’s L4 and L5 points, respectively.

Trojan asteroids are interesting; Trojan moons moreso. But what would be really fascinating, should we ever discover them, are Trojan planets. Somewhere out there, could there be terrestrial worlds hovering near the L4 or L5 points of gas giants? Could these worlds support life? What sort of civilization might develop there, and what strange sights would they see in the night sky?

These are questions best answered by science fiction writers. At least for now.

Flying Through Saturn’s Rings

September 17, 2015 ~ J.S. Pailly ~ 4 Comments

Imagine you’re a hotshot space pilot on approach to Saturn. You know perfectly well you’re not supposed to fly through Saturn’s rings. The rings are protected by an intergalactic trust. You could get in a lot of trouble.

Okay, if we’re doing this, let’s do it the awesome way! The thickness and density of the rings varies somewhat, so let’s aim for one of the main rings (which astronomers call the A-ring, B-ring, and C-ring). In fact, aim for the middle band of the B-ring. That’s one of the thickest, densest parts.

Saturn’s rings are a mix of stuff, including rock and organic compounds, but most of the material is water ice. According to one source I found, the rings are composed of 90 to 95% ice. Most of this ice is in the form of teeny, tiny crystals, so basically we’re flying into a vast expanse of snowflakes.

Some of these snowflakes are floating free. Others have clumped together, forming larger objects. In 2005, the Cassini mission beamed radio waves at Saturn’s rings. By seeing which wavelengths bounced back, scientists got a rough estimate of the sizes of these tiny and sometimes not-so-tiny objects.

In the B-ring, most of the objects we’ll encounter will be at least 5 centimeters in diameter. Many will be larger than that. Perhaps much larger. Unfortunately, the ring particles in the middle of the B-ring are so close together that virtually all of Cassini’s radio waves bounced back. So it’s hard to say how large individual objects might be in there.

I guess we’re about to find out!

Don’t panic. It seems objects in Saturn’s rings are continuously coalescing and disintegrating due to collisions with each other and interactions with Saturn’s gravity. That suggests these objects are merely loose rubble piles of tiny ice crystals. Collide with one, and it should just burst apart like a lightly packed snowball.

Still, you should probably slow down a bit. At your current velocity, collisions with even the tiniest speck of dust could puncture your spaceship’s hull. Seriously. You need to slow down! Look out for the—!

Disclaimer: Today’s post is not intended as advice on whether or not you should attempt to fly through Saturn’s rings. If you decide to fly through Saturn’s rings, you do so at your own risk, and the author of this post cannot be held liable for any damages that may ensue.

Links

Cassini Radio Signals Decipher Saturn Ring Structure from NASA.

Particle Sizes in Saturn’s Rings from Astronomy Picture of the Day.

Saturn’s Rings from NASA Jet Propulsion Laboratory (great chart to help you identify each of Saturn’s rings).

Saturn Rapidly Creates and Destroys Its Moonlets from Discovery News.

Sciency Words: Cryovolcano

September 11, 2015 ~ J.S. Pailly ~ 3 Comments

Sciency Words is a special series here on Planet Pailly celebrating the rich and colorful world of science and science-related terminology. Today, we’re looking at the term:

VOLCANO

Okay, I guess this is a scientific term. It’s certainly an important concept in geology, but how about we do something a little more exotic for today’s Sciency Words post:

CRYOVOLCANO

Much better! Instead of a volcano spewing fire and lava and ash, picture a volcano that erupts with a mix of icy cold fluids and/or vapors which scientists call “cryomagma.”

How Do Cryovolcanoes Work?

When cryovolcanoes erupt, their cryomagma tends to include lots of liquid water, but at temperatures well below zero degrees Celsius. We all know that salt lowers the freezing point of water. Other substances, including ammonia and methane, can have the same effect.

Cold as they are, cryovolcanoes still require a heat source. This heat can be generated in several ways, including tidal forces, radioactive decay, or perhaps even a subsurface greenhouse effect whereby translucent surface ice allows light energy from the Sun to be trapped as heat energy deep underground.

Where Can We Find Cryovolcanoes?

Cryovolcanoes were first discovered on Triton, one of Neptune’s moons, in 1989. In 2005, the Cassini spacecraft observed cryovolcanic activity on Enceladus, a moon of Saturn, leading to rampant speculation about Enceladus’s possible subsurface oceans and possible organisms swimming in those oceans.

Enceladus remains on the shortlist of places astrobiologists want to check for alien life. And since cryovolcanoes often vent materials into space, we could easily go collect a sample.

Much of what we now know about cryovolcanoes is thanks to our ongoing observation and study of Enceladus.

How Rare are Cryovolcanoes?

So which are more common: volcanoes or cryovolcanoes? Thus far, regular volcanoes have been found on Earth and Io (one of Jupiter’s moons), and strong evidence of volcanic eruptions was just recently observed on Venus. Meanwhile, cryovolcanoes have only been confirmed on two worlds: Triton and Enceladus.

Based on that, it might seem like regular volcanoes are ahead, but hints of cryovolcanism have been found on a long, long list of moons in the outer Solar System (also Pluto).

At the beginning of this post, I insinuated that cryovolcanoes are “exotic,” but I’d guess that Earth-like or Io-like volcanic activity is far less common. Small, icy objects with their weird (to us) cryovolcanoes are probably scattered all across the cosmos.

Links

Active Volcanoes in Our Solar System from Geology.com.

Learning about Volcanic Activity on Triton from Bright Hub.

Ocean on Saturn Moon Enceladus May Have Potential Energy Source to Support Life from Space.com.

How Saturn Got Its Rings (And How Earth Can Too)

September 7, 2015September 7, 2015 ~ J.S. Pailly ~ 1 Comment

All the gas giants in our Solar System have rings. None of the terrestrial planets do. Based on that, you could be forgiven for assuming that only gas giants can have rings, but astronomers have discovered at least one asteroid has rings, and there’s no real reason why a planet like Earth couldn’t have rings too.

If Earth wants rings of its own, it should seek advise from Saturn. Saturn has the most beautiful rings of them all, but how did Saturn get its rings? That is a matter for ongoing scientific debate.

Here are four possible explanations:

A Former Moon: One of Saturn’s moons wandered too close to the planet. Once its orbit crossed the Roche limit, this poor unfortunate moon was torn apart by Saturn’s gravity.
A Failed Moon: Alternatively, the rings may consist of material left over from the formation of Saturn and its moons. Because this material was within the Roche limit, it never coalesced into a moon in the first place.
Meteorite Debris: Debris from meteorite impacts on Saturn’s various moons may have gradually accumulated inside the Roche limit, preventing it from re-coalescing as a new, rubble pile moon.
Outgassing: Volatiles like water vapor are constantly spewing from certain Saturnian moons. Perhaps these outgassed materials gradually accumulated around Saturn, either alone or in combination with meteorite debris.

Personally, I prefer the idea that Saturn destroyed one of its own moons to make its rings. Firstly, that would be awesome. Secondly, the exceptional size and brightness of Saturn’s rings seems to suggest, at least in my mind, that they formed suddenly and relatively recently.

However, compelling cases can be made for all of these ideas, especially outgassing. At least some of Saturn’s many rings clearly originate from nearby outgassing moons (but I can’t imagine outgassing accounting for all the rings).

So if Earth really wants rings of its own, the best bet is probably to get a moon inside the Roche limit and let gravity tear it apart.

Sp03 Destroying the Moon — Maybe this is why the Moon is slowly and cautiously moving away from the Earth.

* * *

Today’s post is part of Saturn month for the 2015 Mission to the Solar System. Click here for more about this series.

Sciency Words: Roche Limit

September 4, 2015 ~ J.S. Pailly ~ 3 Comments

Sciency Words is a special series here on Planet Pailly celebrating the rich and colorful world of science and science-related terminology. Today, we’re looking at the term:

ROCHE LIMIT

Our yearlong voyage through the Solar System now brings us to Saturn, arguably the Solar System’s most beautiful planet.

Where did Saturn’s rings come from? That’s a subject of ongoing scientific debate, but whatever the explanation, it probably has something to do with the Roche limit.

First calculated in 1848 by French astronomer and mathematician Edouard Roche, the Roche limit describes the distance at which an object orbiting a larger object will be torn apart by the larger object’s gravity.

Gravity becomes exponentially weaker the farther you get from the source of that gravity. This is known as the inverse square law. What it means for a moon, especially a large moon, is that the gravitational pull on one side of the moon is stronger than on the other. Move that moon closer to its planet, and the discrepancy gets worse. Exponentially worse.

If our hypothetical moon strayed too close, the planet’s gravity could start pulling one side of the moon away from the other, causing the moon to crumble. The resulting debris field would tend to spread out, eventually creating planetary rings.

Calculating Roche limits can get complicated. The relative densities of the planet and moon matter a lot, since lower density objects (remember the rubble pile asteroids?) will fall apart much more easily. Molecular composition can also be a factor, as some molecular bonds are stronger than others and can do a better job holding a moon together.

But for a planet and moon of roughly the same density, the Roche limit equals about 2.5 times the planet’s radius (measured from the planet’s center). And it just so happens that Saturn’s rings extend to about that distance.

So thanks to the Roche limit, we can predict where planetary rings are likely to form, but not necessarily how. On Monday, we’ll look at four different ways that Saturn might have gotten its glorious rings.

Links

The Roche Limit from Teach Astronomy.

Roche Limit Visualization from YouTube.

Jupiter’s 63 Other Moons

August 31, 2015 ~ J.S. Pailly ~ 5 Comments

This month, we’ve met four of Jupiter’s moons. Four. Which means there are at least sixty-three other moons we haven’t met, and possibly more that have yet to be discovered.

It seems a little unfair to spend so much time on four moons and so little on all the rest, except those remaining sixty-three moons make up less than 1% of the total mass orbiting Jupiter.

Depending on who you ask (click here or here or here), the four Galilean moons pictured above constitute between 99.997 and 99.999% of the stuff in orbit around Jupiter. That includes all sixty-seven moons plus Jupiter’s rings.

Jupiter’s moons are divided into three groups. This might be hard to remember, but the four innermost moons are called the “inner moons.” Beyond the inner moons lie the four Galilean moons, and beyond them there’s a cloud of what astronomers call “irregular moons.”

Many of the irregular moons are in eccentric, inclined, or even retrograde orbit. Most if not all of them are either captured asteroids or debris from asteroid collisions. A few may only be temporary residents and might slip loose from Jupiter’s gravity sooner or later.

Compared to the Galilean moons, all these other moons look like pebbles. I don’t feel too bad about skipping them. Granted, the inner moons play an interesting role in shaping and maintaining Jupiter’s rings, but we’re going to be talking a lot about shaping and maintaining planetary rings very soon. I promise.

Sciency Words: Hot Jupiter

August 28, 2015 ~ J.S. Pailly ~ 6 Comments

$Sciency Words MATH$

Sciency Words is a special series here on Planet Pailly celebrating the rich and colorful world of science and science-related terminology. Today, we’re looking at the term:

HOT JUPITER

Hot Jupiters are defined as large gas giants, roughly Jupiter-sized, in orbits less than 0.5 AU from their host stars (half the distance between Earth and the Sun). Many hot Jupiters orbit much closer than that.

Since the 1990’s, astronomers have catalogued hundreds of hot Jupiters. Current models of planet formation indicate that gas giants cannot form so close to stars, so hot Jupiters must begin life father away and migrate inwards.

These planetary migrations can have dramatic effects on the rest of a star system.

The Creator of Worlds

As star systems coalesce, gas giants like Jupiter are among the first objects to appear. In some cases, a young gas giant might migrate inward while the other planets are still forming. The denser the protoplanetary disk, the more likely it is that a gas giant will migrate.

In computer simulations, researchers found that an inward migrating giant is actually good for a developing star system. Its passage stirs things up, encouraging planet formation.

Terrestrial planets that form in this way would benefit from the mixing of material from different regions of the protoplanetary disk. In the simulations, some ended up with way more water than Earth could ever dream of!

The Destroyer of Worlds

Of course if a giant planet migrates inward after the inner planets form, all bets are off. These smaller planets would either be gobbled up by the giant or hurled out of orbit by the giant’s gravity.

This scenario could happen if a Jupiter-sized planet were nudged by gravitational interactions with other large planets or by interactions with nearby stars. Gas giants in binary star systems would be at especially high risk.

The Destabilizer of Stars

Hot Jupiters are often found in high inclination (tilted) or retrograde (backward) orbits when compared to the orbits of their host stars. For a long time, astronomers wondered what happened to the orbits of these planets. A better question might be what happened to the rotations of these stars?

The presence of such a large object so close to a star could have a destabilizing effect on the star. New research suggests that hot Jupiters cause their stars to tilt sideways or tip upside down. This would explain the highly inclined and retrograde orbits we’ve observed.

Is This Normal?

Astronomers have discovered a whole lot of hot Jupiters, but that doesn’t mean they’re common. It’s just that with our current detection techniques, hot Jupiters are among the easiest planets to spot.

Rare or not, hot Jupiters would be worth closer inspection by futuristic space explorers. What sorts of adventures might these explorers have? Please share in the comments below.

Links

Why Doesn’t Our Solar System Have a Hot Jupiter? from Space Answers.

Build Your Own Orbit (Hot Jupiters) from Artifexian.

“Hot Jupiter” Systems May Harbor Earth-like Planets from Physics.org.

Mystery of “Hot Jupiter” Planets’ Crazy Orbits May Be Solved from Space.com.

What’s Jupiter Hiding?

August 27, 2015 ~ J.S. Pailly ~ 2 Comments

Jupiter’s hiding something. We can see the cloud tops. We can monitor the planet’s intense winds and observe its enormous cyclonic and anticyclonic storms like the Great Red Spot. But we don’t know what’s happening on the inside.

Maybe all the meteorological activity we see is only skin-deep. Maybe beneath the tumultuous “surface” lies a calm and tranquil atmosphere/ocean of gaseous/liquid hydrogen.

Or perhaps Jupiter’s interior is a violent and chaotic place. Perhaps storms like the Great Red Spot are driven by as yet unknown forces that extend deep into the planet’s innermost layers.

How can we settle the matter?

In July of 2016, NASA’s Juno spacecraft will enter a high eccentricity polar orbit around Jupiter. Jupiter’s upper atmosphere includes clouds of water (yes, you read that right… there’s water on Jupiter!). Using a microwave radiometer, Juno will attempt to figure out just how far down the water goes.

Also, as Juno skims near Jupiter, NASA will pay close attention to how Jupiter’s gravity affects the spacecraft. Subtle changes in Juno’s velocity will reveal variations in Jupiter’s gravity, indicating variations in the planet’s density. This technique, called gravity mapping, has been used to study the interiors of other planets, including Earth.

Juno also carries a magnetometer (in the illustration above, it’s that pointy thing connected to one of the solar panels). Since Jupiter’s magnetic field is generated by super pressurized metallic hydrogen and perhaps other metallic elements in the planet’s core, data from the magnetometer should give us a clearer understanding of conditions at the center of Jupiter.

Personally, I like the image of Jupiter’s chaotic surface activity concealing a deep, inner calm. It makes the planet sound really Zen. But we’ll have to wait until 2016 to find out if Jupiter is hiding a violent or tranquil interior.

P.S.: One of Juno’s instruments is named JEDI (short for Jovian Energetic particle Detector Instrument). Because NASA engineers can’t design a spacecraft without making at least one Star Wars reference.

Meet a Moon: Callisto

August 24, 2015 ~ J.S. Pailly ~ 3 Comments

Our ongoing journey through the Solar System now brings us to Callisto: the least interesting of Jupiter’s Galilean moons. Callisto doesn’t participate in the Laplace resonance. It exhibits no geological activity, past or present. It doesn’t have a magnetic field, and its thin atmosphere is the generic CO₂ atmosphere that almost all rocky planetoids in the Solar System have.

Callisto’s surface consists of a mix of rock and water ice, and there may be a small ocean of liquid water deep underground. That’s pretty nifty, I guess, but it’s kind of old news after Europa and Ganymede. And without geological activity to feed nutrients into that ocean, it seems unlikely that life could have developed on Callisto.

Yet NASA has taken a special interest in Callisto. If all goes according to plan, astronauts could set foot on this not-so-small moon as early as 2040.

No, Callisto may not be as exciting as its neighbors. It lacks Io’s sulfur volcanos, Europa’s potentially habitable oceans, or Ganymede’s protective magnetic field. But according to a 2003 concept study called HOPE (Human Outer Planet Exploration), Callisto may be one of the safest locations in the outer Solar System to build a scientific research base. Why? Precisely because it’s so boring.

No geological activity means we don’t have to worry about volcanoes or earthquakes (Callisto-quakes?).
Since the odds of Callisto supporting native life are negligible, we don’t have to worry much about contaminating the Callistonian ecosystem… or about having the Callistonian ecosystem contaminate us.
Even though Callisto doesn’t have a magnetic field like Ganymede’s to shield us from solar or cosmic rays, Callisto orbits outside Jupiter’s radiation belt. Radiation levels on Callisto are actually lower than on Ganymede.

Once we’ve established an outpost on Callisto, astronauts could use it as a base of operations for further exploration of Jupiter and its other moons. Callisto’s water can also be converted into rocket fuel (liquid hydrogen and liquid oxygen), so the outpost could also serve as a fuel depot for missions beyond Jupiter.

I can’t remember any references to Callisto in science fiction (though the mythical Callisto appeared in a few episodes of Xena). But if this seemingly boring moon has attracted so much attention from NASA, maybe it’s worth exploring as a setting for Sci-Fi stories as well.

P.S.: A mission to Callisto in the 2040’s would follow close on the heels of NASA’s planned mission to Mars in the 2030’s. While the HOPE study made some excellent points about the viability of a Callisto outpost, I won’t comment on how realistic the mission timetable that might be.

Links

Concept for a Human Mission to Callisto in the 2040s from Beyond Earthly Skies.

Is There Life on Callisto? from Mysterious Universe.

Jupiter’s Moons in Fiction: Callisto from Wikipedia.

Sciency Words: Chaos Terrain

August 21, 2015 ~ J.S. Pailly ~ 2 Comments

Sciency Words is a special series here on Planet Pailly celebrating the rich and colorful world of science and science-related terminology. Today, we’re looking at the term:

CHAOS TERRAIN

Chaos terrain is a weird concept, so I’ve decided to let a master of chaos terrain formation explain.

First, you’ll need an ocean of liquid water with a layer of water ice on top. For best results, I recommend using pure or nearly pure ice and really salty ocean water, so that they’ll have dramatically different melting/freezing points.

Next, set up some volcanoes or hydrothermal vents on your ocean floor. A little volcanic activity will cause the sporadic melting and refreezing of your ice, allowing ice water and saltwater to mix. If you do this right, you’ll end up with a salty “lake” trapped between layers of ice.

As we all know, liquid water is denser than water ice, so your lake will cause the ice above to sag and eventually cave in.

Cracks and fissures will form. Chuncks of ice will break apart, and that salty liquid water will get the chance to seep into the gaps, causing more melting and refreezing mayhem.

Finally, when your lake refreezes, it will expand (remember: ice is less dense than water) pushing all that cracked and broken material upward.

The resulting terrain will look truly bizarre—chaotic, you might say!—with huge ice blocks jutting up above an otherwise perfectly smooth landscape.

So, fellow planets and moons, what else can we do to confuse the humans? Share your ideas in the comments below!

Links

Active Formation of “Chaos Terrain” over Shallow Subsurface Water on Europa from Nature (beware of paywall).

Europa’s Chaos Terrains from NASA Visualization Explorer.