Venus: Earth’s Evil Twin…

I have to say, I learn something new about space pretty much every day. A couple of weeks ago I was covering a lecture for an introductory level astronomy class here at UNH. The lecture topic was the formation of the solar system with a highlight on the details of the inner terrestrial planets. So, in preparation for the lecture (the night before), I looked through the textbook: The Cosmic Perspective. And while reading in there about Venus, my mind was blown. What exactly blew my mind? That Venus’s fate could easily have been Earth’s.

Venus, the second planet from the Sun, is the hottest place in the solar system. This radar image from the Magellan spacecraft shows the planet’s surface. Credit: JPL/NASA

Venus is almost the same size as Earth (95% in radius and 81.5% in mass) and made up of pretty much the same stuff. The similarity in size and density between Venus and Earth leads scientists to believe that they share a similar internal structure: a core, mantle, and crust. Both have significant volcanic activity, although Earth has active plate tectonics and we don’t see evidence of that on Venus. The big difference between the two planets comes in the stuff above the planet’s surface: the atmosphere. Earth’s atmosphere is made up primarily of nitrogen (78%) and oxygen (21%), with trace amounts  (~1% or less) of argon, carbon dioxide, water vapor, and other things. On Venus it’s an entirely different story. The Venusian atmosphere is almost entirely made up of carbon dioxide (97%), a very efficient greenhouse gas.

Now you’ve probably heard of greenhouse gases and their role in the greenhouse effect which is playing a role in concerns about global warming. A quick review of how the greenhouse effect works: a greenhouse gas, is a gas (most commonly a hydrocarbon) that allows visible light to pass through it but traps heat from infrared radiation. Okay, so who cares? Well the atmosphere is full of small amounts of naturally occurring greenhouse gases (such as carbon dioxide, methane, and water vapor) which allow the visible light from the Sun to pass through our atmosphere and reach the surface. That’s awesome, because if those gases were opaque to visible light– meaning they did not allow them to pass through– then all the plants on Earth would die and we’d all be sickly pale. It’s this next part that makes greenhouse gases significant. The Earth is warm, astronomically speaking compared to the cold vacuum of space, and emits its own radiation (or light) in the infrared. Back in the mid-19th century, scientists realized that anything that’s warm emits radiation– called blackbody radiation— most of it at wavelengths that we can’t see. The relation between the spectrum of light that an object radiates and its temperature is governed by Planck’s Law. A practical example are night-vision goggles which pick up on the thermal infrared radiation that’s given off by the warmth of a human’s body. Our Sun (10,340º F) is much, much hotter than Earth (~40º F) or a human (~98º F), so it emits a blackbody spectrum which peaks in the visible, whereas humans and the Earth emit mostly in the infrared. It’s the shifting of this peak which gives stars their different colors.

So Earth (and Venus because of its similar size) radiates most of its light in the infrared, which greenhouse gases do not allow to pass through the atmosphere. So instead of that newly radiated heat (IR light) being transmitted out through the atmosphere and into space, it gets trapped and increases the temperature for us on the surface. Now, normally for us on Earth that’s fine because our planet does a great job of regulating the surface temperature through a process known as the carbon dioxide (CO2) cycle. The CO2 cycle helps Earth to self-regulate its temperature. If it’s too hot, the excess carbon dioxide in the atmosphere dissolves in the water in the ocean, then settles and is stored in rocks of the ocean floor. If it’s too cold, then the carbon dioxide which returns to the interior of the planet (via subduction caused by plate tectonics) is released back into the atmosphere by volcanoes. So Earth can do a really good job at keeping track of its own thermostat, which has helped Earth maintain a stable surface temperature even though the temperature of the Sun has changed significantly over time. The problem with the greenhouse effect on Earth is that if we add too many new greenhouse gases (specifically carbon dioxide) that Earth can’t/isn’t ready to handle, then we can mess up the delicate balance that Earth is working so hard to maintain for us.

So what, Venus has more carbon dioxide in its atmosphere, why does that matter to Earth? Well, because it could very easily have happened to us. The only reason that Venus is a horrible, deadly world, the hottest in the solar system, with temperatures over 700º F is because it’s closer to the Sun than Earth is. The average distance of Venus’s from the Sun (called the semi-major axis) is ~72% of that of Earth’s; that means it’s roughly 66,928,200 miles away from the Sun (that’s a mere 26,027,600 miles– or 109 times the distance to the Moon– closer than Earth is). Astronomically speaking, that’s a very small distance. If primordial Earth’s orbit was altered enough to move it that roughly 26 million miles closer to the Sun– by, say an asteroid impact similar to the one that we believe created the Moon— then Earth could have ended up the same as Venus. If Earth were to move that mere 30% closer to the Sun, then the liquid water oceans which we have on Earth that dissolve the excess carbon dioxide, removing it from our atmosphere, would have evaporated. Water vapor is actually one of the best greenhouse gases, so the evaporation of the water vapor into the air combined with the failure to remove carbon dioxide from the atmosphere, would result in a runaway heating effect that the planet would have no way to stop.

So it’s by a very small distance, a distance that’s only 30 times larger than the diameter of the Sun, that Earth escaped being a hellish, fiery deathtrap and became the one oasis of life that we know of in the universe.

– Cheers,
Ian Cohen
Manager, UNH Observatory

Modified version of original blog post from The Sky’s the Limit.

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A spotlight on Saturn’s rings

Saturn is without a doubt my favorite object to look at in the night time sky. The first time I viewed it through my first telescope I was instantly mesmerized by the rings. It’s really a celestial treasure right in our own backyard. If you’ve never looked at Saturn through a telescope, do it. I promise, you won’t regret it. Now you might be wondering, what will Saturn look like through a telescope? Well you won’t see this:

This natural color image of Saturn was created from a series of images that were taken by the Cassini spacecraft during its encounter with the planet in October 2004. Credit: NASA

…but you probably will be able to see something like this:

Here’s a glimpse of Saturn taken through the UNH Observatory‘s C14 by UNH Observatory volunteer and SkyGuy John S. Gianforte on February 21, 2007.

So you see why it’s something that sticks with you. So yes, you should definitely look at Saturn through a telescope. Now let’s learn a little bit more about the 2nd largest planet in our solar system!

Of the five planets visible to the naked eye from Earth, Saturn is the furthest and slowest moving across the sky. That’s actually how Saturn got its name; the ancient Greeks named the planet after Chronos (Saturn to the Romans), the father of Zeus (Jupiter) and the God of time (a reason why chronos is a root of time-related words, like chronology). Galileo was the first to resolve and discover the rings of Saturn when he looked at it through his telescope in 1610. It’s incorrect status as the only ringed planet in the solar system survived until well into the latter half of the 20th century. In 1977, Saturn was joined in the ringed planet club by Uranus, after scientists observed a star passing behind the planet, a phenomenon called an occultation. Most unexpectedly, the star’s light blinked on and off nine times before disappearing behind the disc of the planet; this proved that although the rings were too dim to be seen from Earth, the material was present. Only a few years later, Voyager 1 discovered the rings of Jupiter and it wasn’t until the mid-1980’s that another stellar occultation proved the existence of Neptune’s ring system. So it’s true, all of the gaseous outer planets, or “gas giants”, have rings, but Saturn’s are by far the most visually impressive and the only ones visible from Earth. So that leads us to a very interesting and mysterious question…why? What makes Saturn’s rings special?

As telescopes on Earth have become more and more advanced and we continue our extensive exploration of the solar system, we have pieced together a much more comprehensive understanding of Saturn’s rings than Galileo had 400 years ago. Although the rings look solid and sheet-like from Earth, we now know that the rings of Saturn are actually comprised of billions of particles of rock, ice, and dust ranging in size from microscopic to meters-wide. The brighter, more dense regions of the rings have more material to reflect light while the dark regions or “gaps” are much more scarcely populated. The debris that makes up the planet’s rings are in a very well-defined plane, only a few tens of meters thick, but extending almost 130,000 km (80,778 mi) above the planet’s surface. The gallery below shows the evolution of our understanding and imaging of Saturn’s rings.

But many questions still remain about why Saturn’s rings are so much brighter than the other gassy planets. The answer scientists think, lies with the sixth-largest of Saturns 60+ moons: a small, icy world called Enceladus(shown below).

Saturn’s 6th largest moon, Enceladus, as imaged by NASA’s Cassini spacecraft. The blue fissures in the surface seen in the southern hemisphere are known as tiger stripes. Credit: NASA

Enceladus is roughly 500 km in diameter (14% of the Moon) and 1.1 × 10²º kg in mass (0.2% of the Moon), but what it lacks in stature it makes up for in output. Literally. Discovered by William Herschel in 1879 (only 2 years before his discovery of the planet Uranus), Enceladus first came into the spotlight for scientists in early 1980. As John Spencer recalls in a recent article in Physics Today, that was

“…when scientists using Earth-based telescopes acquired new images of a faint outer ring of Saturn—the E ring—which had been discovered in the 1960’s. Those images revealed that the E ring’s brightness peaked at the orbit of Enceladus. They also showed that unlike Saturn’s other rings, the E ring scattered sunlight more efficiently at shorter wavelengths, which indicated that the ring was dominated by particles not much larger than the wavelength of light. Sputtering by charged particles in Saturn’s magnetosphere would erode away such micron-sized particles on time scales of decades to hundreds of years, so something had to be replenishing the ring on comparable time scales. The peak in ring density at Enceladus pointed to that moon as the likely source.”

Since then interest in the small ice-world increased exponentially and as a result Enceladus became a primary target of investigation for the joint NASA/ESA mission of the Cassini spacecraft. After only slightly whetting their appetite with two flybys of the small moon in early 2005, researchers decided to make a third flyby at a much closer range, 170 km (105 mi) instead of the planned 1000 km (621 mi). The dramatic results from this third Enceladus flyby in July 2005 were released in a special March 2006 issue of the journal Science. It was this flyby that got the high-resolution images seen above and first discovered the “four prominent parallel fractures, dubbed tiger stripes, surrounded by an intensely tectonically disrupted landscape” that the image depicts. In an even more interesting find, Cassini caught evidence of multiple plume jets erupting from the four tiger stripe fractures seen near Enceladus’s south pole.

Multiple plume jets erupting from the four tiger-stripe fractures near Enceladus’s south pole are visible in this Cassini image. The jets appear not only on the edge of Enceladus’s disk but also where they rise up into sunlight from sources on the night side of the moon. Credit: NASA/JPL/SSI; Mosaic: Emily Lakdawalla

These plumes (shown above), currently ejecting mass at an astounding 200 kg/s, have two observable components: micron-sized ice grains and gas (99% water vapor). It is speculated that the water vapor and ice crystals that are being deposited into Saturn’s ring system by Enceladus are what have kept the planet’s rings so bright and reflective for so long.

In spite of all of this other extremely intriguing science, the most interesting thing for scientists is Enceladus’ potential for life. As one of the few places in the solar system where we know water exists, Enceladus has become a key target in the search for extraterrestrial life. Scientists speculate that liquid water might occur in several places on the tiny moon: as a global ocean between the silicate core and the ice crust, as a more local south polar sea beneath the ice shell), or as localized bodies of water in the ice shell itself.

Here is Saturn’s tiny icy moon Enceladus, imaged by NASA’s Cassini spacecraft. Just above the smaller moon we can see the planet’s rings and Saturn’s largest moon, Titan, looming in the background. Credit: NASA/JPL-Caltech/Space Science Institute

So there you have it, Enceladus is our celestial ring-bearer, if you will. Hope you learned something! Until next time, friends!

– Cheers,
Ian Cohen
Manager, UNH Observatory

References:

Modified version of original blog post from The Sky’s the Limit.