Jupiter’s moon Europa continues to amaze and educate us about conditions in the “mid-range” region of the solar system. If you look at an image of Europa, you’ll notice right away that it appears to have jagged breaks in its icy crust, almost like a cracked egg. Yet, that surface is very smooth, which means it’s relatively young. There aren’t too many craters from impactors, and even the cracks look “iced over”.

We’ve long suspected that Europa has a significant fraction of water hiding in an ocean under its frozen crust. That’s led scientists to dub it an ‘ocean world’ or a ‘water world’.

Data from missions as far back as Voyager and the Galileo spacecraft helped us suss out Europa’s true nature. The Juno mission, currently in orbit around Jupiter, has also imaged Europa, mostly from a distance. Its data add to the collection of amazing observations we’ve been able to make. Europa’s appearance and the presence of a possible subsurface ocean inspire many questions: Why is it so smooth? What else is happening there? Could Europa support life? Does it HAVE life now? The answers lie in those ice-covered oceans.

Stretching Europa

We’ve long known that Europa is subject to tidal forces from Jupiter. It’s tidally locked to its parent planet. That is, Europa doesn’t spin on its axis the same way our planet does. That means the same side of Europa faces Jupiter as it orbits the planet. The tidal forces cause its surface to flex and stretch and similar flexing stretches and heats Europa’s rocky core. In addition, as Europa stretches, its oceans experience tides. Heat from the core and the action of the tides could affect the surface through warming and cracking. The cracks could be conduits for water to “leak out” to the surface and freeze.

Exploring Europa from Earth Orbit

In addition to spacecraft missions, scientists have also used HST over many years to study Europa. Their long-term observations also yielded intriguing facts about the place. For example, in 2013, Hubble Space Telescope found spectral evidence for plumes of water ice jetting out from Europa’s southern polar region. The most recent discovery from Hubble data shows a persistent water vapor “cloud” at Europa. Those data come from observations made between 1999 to 2015.

During that time, HST took more images and spectra of this moon, building up a significant database of information. A recent analysis of the images and data turned up an interesting fact. That water vapor “atmosphere” really only persists over the trailing hemisphere of Europa. Nobody’s quite sure why. That remains to be figured out. Now, the goal is to figure out where the vapor cloud originates. Is it primarily from the plumes? Or is something else going on, too?

Finding out More at Europa

Europa isn’t exactly a garden spot, despite its abundance of water. The surface is highly reflective, and it’s cold. The surface temperature at midday is around -260 F. You’d think it would be low enough to keep the surface relatively, well, frozen in place. Yet, it appears that the water ice on the surface is sublimating, that is, changing from an icy state to a vapor state. That could be contributing to the massing of water vapor over the trailing hemisphere, along with action from the plumes. We need more to pin this down, however.

The best way to directly find out the causes of Europa’s vapor atmosphere is to go there. It’s not likely that humans are going to travel to Europa and take surface and atmospheric measurements. It would be a lethal experience, due to the extremely high radiation associated with Jupiter’s magnetic field. It blasts Europa with a constant bath of high-energy particles. You could stand on the surface of this moon, but you wouldn’t last long. Even with radiation protection, in one day, the radiation you experience would be 1,800 times the radiation you’d get if you stayed here on Earth and lived at sea level. That’s enough to cause illness or death in a very short time.

Send in the Robotic Probe

The next best thing is to send spacecraft, which we’ve been doing. Another one is on the drawing boards, called the NASA Europa Clipper mission. It will do at least 45 flybys of Europa, using a variety of instruments to map and study the surface. It will also use gravitational measurements to confirm the existence and extent of that subsurface ocean. The main goal, however, is to determine whether or not Europa could, or does, support life. If all goes well, the mission could launch as early as 2024.

How did our galaxy form? The short answer is “galaxy collisions”. That keeps astronomers busy as they develop new tools to “look back” at the birth of our own galaxy. We know that the infant Milky Way began taking shape relatively early in the history of the universe. It probably existed for quite some time as a tiny dwarf galaxy. Then, it began to merge with others to “plump up” and grow.

Today, the Milky Way continues to dance the galaxy formation tango with other dwarf galaxies, such as the Sagittarius Dwarf. And, we know that it will eventually do an interactive dance with the Andromeda Galaxy. That series of galaxy collisions and interactions will happen several billion years from now. For now, it seems, that our galaxy is still forming through mergers. And, astronomers have some interesting tools to probe the details of the stars that got brought into the Milky Way.

The First Big Galaxy Collision

A couple of weeks ago, Ohio State University researchers released new information about the earliest stages of the Milky Way’s birth and growth. It seems the young Milky Way experienced a major merger with an orbiting satellite galaxy called Gaia-Enceladus. That happened about 10 billion years ago and influenced the distribution of stars in the galaxy.

Fiorenzo Vincenzo, who works at OSU’s Center for Cosmology and Astroparticle Physics, and a team of astronomers, wanted to know exactly what happened as a result of that merger. Specifically, what can the stars of our galaxy tell us about that ancient event?

They used several techniques to answer that, including asterochronology, asteroseismology, and a spectroscopic survey called APOGEE. Asterochronology studies the ages of stars to trace the basic “mergers and acquisitions” that helped form the Milky Way. Asteroseismology looks inside stars by studying their oscillations to get precise ages. The spectroscopic studies reveal the chemical compositions of stars. The combination of all three approaches gives amazingly good estimates for the ages and origins of stars in our galaxy.

“Our evidence suggests that when the merger occurred, the Milky Way had already formed a large population of its own stars,” said Vincenzo.

By calculating the ages of stars throughout our galaxy, the researchers determined something interesting. The stars captured from Gaia-Enceladus have similar or slightly younger ages than the majority of stars that already existed there.

During the merger, many of the infant Milky Way’s batch of “homemade” stars ended up in the “thick disk” that occupies the middle of the galaxy. The stars that were captured from Gaia-Enceladus during the merger exist mostly in the outer halo of our galaxy.

What Galaxy Collisions Change

Today, we know that most galaxies grow through mergers and acquisitions. So, it seems like a fairly normal occurrence in the universe. The merger of the baby Milky Way with Gaia-Enceladus was one of the most important in the Milky Way’s history. It shaped the galaxy we see today. It “shook things up”, by changing the orbits of stars already in the Milky Way.

Many stars were pushed into eccentric orbits. The stars from the long-disappeared Gaia-Enceladus, move in different ways than the “native” stars. Upon closer examination, stars that came from Gaia-Enceladus have different chemical compositions from those born inside the Milky Way.

Next Steps

Now that researchers peeked at the past collision and figured out what happened to the stars, what next? Their next step is to take the same approach and apply it to larger groups of Milky Way stars. The evolution of the Milky Way is, after all, an ongoing process. The end goal is to get a much clearer idea of that evolution and the changes mergers make to its stellar populations.