planetary science Archives - Page 3 of 38 - The Spacewriter's Ramblings

I don’t want to sound like one of those crazies standing in the public square holding a sign that says, “The End is Nigh”. However, in the case of the Cassini spacecraft, the end really is near. In fact, it’s five orbits away.

What Cassini mission scientists plan is a grand finale as the spacecraft makes its last orbits and plunges into the planet’s atmosphere.

The last of its trips around Saturn begins on Sunday (August 13), and these orbits will see it dip very close to the planet. Cassini’s final orbit will take it into the planet on September 15, 2017. Just before that, a quick gravity maneuver with Titan will slow the spacecraft down. That prepares it for the final dive into Saturn. On the way in, all its science instruments will be turned on. They’ll transmit data until it breaks up under the dense atmospheric pressure of the lower atmosphere. The idea is to get information about a region of Saturn hitherto unexplored and Cassini will be the first official probe of the planet’s atmosphere.

Leading Up to the Finale

Cassini grand finale — An artist’s concept of the Cassini mission as it begins its final five orbits of Saturn. Courtesy Cassini Mission/NASA.

In case you haven’t kept score on this long-running mission, the Cassini spacecraft left Earth on its one-way trip to Saturn twenty years ago, on October 14, 1997. I knew some of the folks on the team but was immersed in my own research into comets at the time. So, I followed it from afar for a while before getting hooked on its fantastic images a few years ago.

What a trip it’s been on! Cassini spent seven years getting to its target. It looped past Venus twice, and back by Earth before heading to Jupiter. Then, it slipped into orbit in the Saturnian system on July 1, 2004. By that time, it had already been imaging the planet and its moons and was ready to begin serious data-gathering.

Cassini’s Accomplishments In Review

pre grand finale aurorae sighted over southern pole — Aurorae over Saturn’s south pole. Courtesy NASA.

Over the years since then, the spacecraft has given us amazing looks at the moon Titan (both from orbit and via the Cassini-Huygens lander). It also revealed incredible details about the moon Enceladus, including in-depth looks (and fly-throughs) of the jets of ice particles spewing from that moon. Of course, the rings and the planet’s atmosphere were major targets. Who would have thought that the rings had propellers? And kinks? And waves? And aurorae in the atmosphere? And that the atmosphere had such fascinating structures? The Hexagon alone is worth another spacecraft visit, in my opinion! For 13 years, the Cassini spacecraft has delivered the best solar system gift a planetary scientist could ask for: an in-depth look at a distant gas giant world. It will be a long time before humans can get out there and study it in person.

The Grand Finale Orbits

Now that it’s down to the last five orbits, what will Cassini be doing? As it gets closer to the planet, the spacecraft’s instruments will continue to measure Saturn’s immense gravitational field and its magnetic field. Such observations give valuable clues to the interior structure of the planet. It may actually reveal just how fast Saturn’s interior is rotating (which is where its magnetic field is generated).

Not surprisingly, with the spacecraft drawing ever-closer to the planet, the cameras are going to get a good look at the structure of the upper atmosphere as well as the rings. While it passes through ring-plane crossing, specialized instruments called “particle detectors” will study the tiny bits of ice that exist in the rings. In addition, Cassini scientists should get a very good feel for just how dense those rings really are. Finally, as the spacecraft plunges into the atmosphere, the mass spectrometer on board will sample the “air” and reveal more about the mix of gases that float high above the planet.

Why Send it Into Saturn?

It seems rather odd to be sending a working spacecraft into the planet at the end of its mission. In truth, the propellants are running low. In addition, Cassini’s instruments will eventually stop working. But, why not leave the spacecraft in a final orbit as a testament to the fantastic mission? The answer lies on the Saturn moons: pristine ice surfaces. Enceladus and Titan, in particular, could be targets if the spacecraft’s orbit should decay and cause it to stray through their orbits. That would bring Earth-based contaminants to these worlds. It’s not something anybody wants, particularly if there’s a chance of life existing on either of those moons. Or, what’s worse, if they’re home to compounds that could form life in the future. In either case, sending an Earth-based spacecraft their way would mess up the delicate balance that leads to or supports life. So, the safest thing is to send Cassini into the Saturnian atmosphere. There, it will burn up and any bio-contaminants would presumably be destroyed.

There’s a bit over a month left to enjoy the fruits of Cassini’s final orbits. Follow the news on the mission Web page and watch for announcements of new findings and images. It’s going to be a tremendous grand finale!

Earth as a Planetary Science Topic

When I travel, I often pay close attention to the geological landmarks in the regions I visit. I do this because I’ve always been fascinated by rock formations, mountain ranges, and how features form. If you’ve never studied geology, it’s a great science to know. With a little practice, you can look at a landform and know the history of that region pretty quickly. It’s not so easy with water, particularly when I’m traveling via the ocean. However, this important substance plays a big role on our planet (and on other worlds, too). It’s a complex topic, but worth taking a brief look at here and then getting into some more details in future posts.

Earth and Its Water

Planet Earth is a water world. During a recent talk to a general public audience, I explained how Earth’s oceans are thought to have formed, and how oceans appear to exist (or have existed) on other worlds in the solar system. I ended by talking a little bit about how Earth’s atmosphere, oceans, and land all interact with each other. These are topics familiar to planetary scientists, and of course, to climate and atmospheric sciences.

Earth’s systems interact in ways that we don’t always think about as we go about our day-to-day lives. For example, if it rains or snows, we all know that the water ends up in ponds, lakes, streams, and rivers. It makes its way to the oceans and exists in snow drifts and glaciers (in the case of snow). It also seeps underground and it can stay there for a long time. If you drink well water, you know that what you use comes from deep underground, basically from rocks. For many city dwellers, the idea that water can come from rocks is a complete surprise. I had a lengthy conversation with a woman on board the ship I just lectured aboard about this. After a talk about Mars, where I talked about the possibility of water underground on the Red Planet, she came up afterwards and asked about how it could exist in rocks. We had a great conversation about that. In the process, we also talked about how everyone from farmers to technology firms, transport companies, and many other entities have a vested interest in how we use our water on the planet.

The Role of Water for Life

Water is pretty important to life, so much so that it is one of the three main “requirements” that astrobiologists look for when they talk about life existing on other worlds, the others being warmth (energy), and organic material (food for the life to consume). All forms of life need to be wet in some way. We tend to focus on human needs for water, but every organism has some need of it to live and evolve and reproduce. That’s why so many people recognize the need for clean water to sustain life. It’s part of a healthy environment on any planet, but particularly on Earth.

Where Did the Oceans Come From?

On one of my most recent trips, I was crossing the Atlantic. On nearly every cruise trip where I am a science speaker, someone asks me where the water came from. This time, I did a talk specifically about that. The truth is, however, scientists are still figuring out all the ways that Earth got its water. We used to say it came from comets. It’s possible that some of Earth’s water did come from specific families of comets, but it’s still an open question about how much of it did. Some comets did NOT contribute their water to early Earth, as we found out from the Rosetta mission when it studied Comet 67P/Churyumov-Gerasimenko. Comets of its type existed when Earth and the other planets were formed and being bombarded by solar system leftovers, but they didn’t contribute a lot to Earth’s water supply. Other families of comets may well have been suppliers, however.

Water and the Newborn Solar System

Infant Earth likely formed from water-rich planetesimals, and as this young world solidified and cooled, the water vapor from those protoplanetary objects condensed to form the first pools, rivers, lakes, and oceans. We also can’t ignore the contribution that water-rich asteroids and meteoroids made to Earth’s water budget. Early in Earth’s history, these objects slammed into the infant planet Earth (and possibly Venus and Mars), and contributed their water to the growing worlds.

If that delivery mechanism worked across the planets (and it’s pretty clear it did, due to the number of worlds (including moons and dwarf planets) that have (or had) water) then water is a ubiquitous part of the solar system. It holds the clues to a lot of things we still need to understand about the early solar system, and is an important part of the chemistry of the worlds where it exists.

Protecting Our Water

All that being said, water on Earth — particularly the clean variety — is a precious resource. It fuels us, but it also keeps the planet’s ecosystems in balance. Swamps and wetlands provide ways to filter water, bring oxygen to the atmosphere, and sustain a huge variety of organisms that we depend on (such as fish) for food and other products. Fresh water in rivers and aquifers provide drinking water for millions of people, and our bodies evolved to use that fresh water. Polluting it pollutes our bodies, and the life around us.

The oceans teem with life, with many forms not even yet cataloged (and some only recently being discovered). As we swept along the ocean, I kept thinking about all the life below us in that aquatic realm, and what it has meant to the planet. We know so little about that “world beneath the waves” even though we’ve learned a lot in recent years. We owe it to ourselves to protect and understand that world because, for all we know (and we know a lot), the world above the waves depends in many ways on the “aquasphere” of planet Earth.

Understanding our Planet

It’s important to study our planet (just as we do the other worlds of the solar system) as if we were exploring a new world (as we do in planetary science). Understanding its systems and features helps us understand other worlds. And, in a most interesting way, understanding THEM gives us new insights into the planet we call home. Our lives depend on what we learn. Just ask a farmer, a sailor, a fishing boat worker, or any of the many hundreds of people in professions who depend on sources of water, and the people whose lives are affected by access to clean water. This goes way beyond politics and opinion; the fact is, we are still learning about this place we live on and that’s an important part of humanity’s education.

The Spacewriter's Ramblings

Category Archives: planetary science

Cassini’s Grand Finale Begins