NAI Ask an Astrobiologist

It seems that members of the animal kingdom can live a plant like existence (e.g. sponges), but you would not expect members of the plant or fungus family to evolve into something that could grow legs and walk around. So fundementally what is the difference between plants and animals, such that animals can move around independently?

Hi Glyn! To answer your question, let’s consider this: moving behavior is not solely attributed to animals. We know not all plants are sedentary - consider, for example, slime molds, or choanoflagellates, which exhibit similar morphological properties to sponges.

Therefore we cannot list moving as a fundamental distinctive property distinguishing plants from animals. To understand why some organisms are sedentary and some are not, we need to take the environment into consideration. Remember: environment selects!

Think about the lifestyle of a plant, for instance, and what it needs to survive: nutrients, sufficient soil, air and light. Therefore it would make sense for a plant not to move in order to survive and to satisfy its basic needs. A plant does not have to actively pursue its food like animals must do.

We often see some closely related species exhibiting different properties. For those cases, we need to keep in mind that the divergence between these species may have occurred millions of years ago. Thus the organisms had a really long time to selectively adapt to their local environment.

I hope this answers your question, feel free to send in any follow questions you may have.

Dr. Betul Kacar, Blue Marble Space Institute of Science

I have read multiple articles on Europa due to a project I have. In multiple articles there are conflicting information about liquid water under the ice. Is it liquid water or another form of liquid?

A lot of what we know about Europa comes from looking at the surface, but some of the data we have also tells us about its interior. Even from telescopes on Earth, we could tell that Europa's surface was extremely reflective, like that of ice or snow, and its infrared spectrum, the pattern of infrared light it reflects, tells us the surface is made of water ice.

We know from gravity measurements that Europa is mostly rocky inside, with an outer layer about 150 km thick with a rocky mantle and iron core just like the Earth. When the Voyager spacecraft flew by Europa in 1979, the first images were returned showing a complex icy surface, with different geology from any place we'd seen in the solar system. Scientists then debated whether the ice was solid all the way through to Europa's rocky interior, or was there a water layer in between the ice and rock? It was the Galileo spacecraft that answered this question, not with images but with a magnetometer that measured Europa's magnetic properties.

The magnetometer detected an induced magnetic field, meaning that Jupiter's magnetic field was inducing currents in a layer right below Europa's icy surface. The strength of that field tells us that a conductive layer of salty water exists within about 50km of Europa's surface, proving that there is an ocean, much like the Earth's, just below the ice. There's lots of evidence for other pockets and small amounts of water within the ice shell as well, but most of the water is down in the ocean. We know it's water because of its density, the contact with the ice shell, and how the layer conducts, other liquids wouldn't behave the same way or be expected there! Hopefully the next mission to Europa will confirm the ocean, and tell us even more about shallow water in the shell.

Dr. Britney Schmidt, University of Texas at Austin

It is said that dying red giant stars could act like a defibrillator and bring icy planets back from the dead & that this rebirth could also lead to new breeding grounds for life. My question is that how can new life survive without light?

While most life on Earth is powered by photosynthesis, it's not the only way for an organism to survive. Organisms can get their energy from chemical reactions as well as sunlight, and these organisms are known as chemotrophs There are actually a surprising number of ecosystems in places that never see the sun. The most famous of these, perhaps, are the communities found around hydrothermal vents, which are volcanic fissures in the ocean floor. Being on the bottom of the sea, they get essentially zero sunlight, but nonetheless, they host a thriving ecosystem, which is based on bacteria that "eat" the sulfur emitted by the vents, rather than on plants. If you want to learn more about these fascinatingly strange communities, Searching For Life Where the Don't Shine is a terrific five-part series of articles examining how things live without sunlight, and our efforts to learn more about them.

Dr. Mason Fisher

What would the Earth look like to scientists if it were observed by the Kepler Observatory orbiting the planet Kepler 62e? What would they presume about the Earth's capacity to develop life?

If Kepler were in orbit around Kepler-62e, Earth would look a lot like what Kepler-62e looks like from Kepler as it currently orbits Earth. There are some important differences: Kepler-62e is about 40% bigger than Earth, and it gets more energy from its star compared to what Earth gets from the Sun. And it might be a little too hot for life as we know it. However, its neighbor-planet, Kepler-62f, gets less energy, and none of its known properties are "surface-life killers." So based on what we know, Kepler-62f could potentially harbor Earth-like life. We have no evidence of life on Kepler-62f, the mere possibility is tantalizing.

The other point to note is that we aren't really "seeing" Kepler-62e, Kepler-62f, or any other exoplanets of a size similar to Earth. For a planet detected by Kepler, all we can usually "see" is the amount of light the planet blocks from its star when the planet passes in front of it. Most of these planets are too far away for us to take pictures of them with a telescope. Someday we exoplanet searchers hope to be able to do that for similar planets that are closer to us. We don't yet know of any exoplanets that would be suitable to this type of study, but the existence of Kepler-62e, Kepler-62f, and other planets suggests we'll find some soon.

Dr. Shawn Domagal-Goldman, Blue Marble Space Institute of Science

How could Earths atmosphere be transformed to Titan like environment?

This is a good question with an unfortunate answer: it could not.

The temperature of the Earth's surface is set by the thickness and composition of the atmosphere and by the amount of incoming light from the Sun. Titan's atmosphere is a little denser than that of the Earth. It is predominately nitrogen and methane, and is sufficiently thick to give the surface a temperature of 94 K / -179 C. All other things being equal, an object with a similar atmosphere at the Earth's distance from the Sun would be heated to an average surface temperature of 290 K / 18 C, a few degrees hotter than the Earth is right now. But not all other things are equal: Titan's atmosphere has the composition that it does because Titan is so cold. Warming it up to the temperature of Earth would melt Titan's current icy surface, putting a huge amount of water vapor into the atmosphere in the process. That would warm the place further, and since Titan's surface is the outer surface of a thick shell of water ice, you would end up with a very hot and humid atmosphere which blended seamlessly into a very deep global ocean.

Nor would moving your Titan-Earth out to the distance of Saturn let you give it a Titan-like atmosphere. The height of an object's atmosphere is determined by its surface gravity and the temperature: at the same temperature, less gravity gives a taller atmosphere. Titan's surface gravity is 14% that of Earth, giving a very tall atmosphere. Something the mass of Earth at the same temperature would have a much shorter atmosphere.

Finally, the composition of Earth makes an atmosphere like Titan's impossible. Both objects have nitrogen-rich atmospheres. But Earth's overall composition is dominated by silicate oxides (rocks), with only a little bit of water mixed in and floating on top. Titan has a rocky core, but that is covered by thick layers of high-pressure ice, a global subsurface ocean, and the outer ice crust. The methane in the atmosphere, lakes, and near-surface is steadily, albeit slowly, destroyed by the chemical reactions that produce the haze. It is replenished only by slow leaking of a primordial supply of methane from Titan's deep interior. The Earth has no such large reservoir of methane.

In other words, to make the atmosphere of the Earth like that of Titan, you would need to: move the object out to the distance Titan is from the Sun, change its mass to be similar to that of Titan, and change its bulk composition to match that of Titan. At that point, you don't have the Earth anymore.

Dr. Michael Busch, Blue Marble Space Institute of Science