Scientists who search for life beyond Earth would love to analyze samples from distant planets and moons in the lab. They can’t do that yet — so they’re using light to hunt for life. Europa is one of Jupiter’s largest moons. Its surface is solid ice.
But below the icy crust of Europa, there might be a warmer liquid ocean. And scientists wonder if life might exist in that ocean. If it does, some evidence for life might have been pushed up through Europa’s surface ice. That’s why scientists can analyze light reflected from Europa’s surface — to search for life in the oceans below. The Galileo spacecraft collected light reflected from Europa. Brad Dalton at NASA’s Ames Research Center in California tried to make sense of the data.
Brad Dalton: I was asking myself, if there were evidence of life on Europa, what would that look like?
In 2002, Dalton chilled bacteria in a laboratory to Europa’s surface temperatures. In December he announced that the light signatures reflected by these bacteria were similar to light signatures collected when the Galileo spacecraft studied Europa. But charged particles from Jupiter interfered with readings from Galileo’s sensors.
Brad Dalton: These data are tantalizing, but are not in themselves proof of extraterrestrial life. What we really need to do is to go back to Europa with better instruments with higher spectral precision and higher resolution, and higher signal range.
The National Research Council released a report in 2002 saying that a mission to Europa is one of the top priorities for space scientists in the coming decade. NASA is looking into such a mission, but it probably won’t fly for another decade.
According to Guy Webster at NASA’s Jet Propulsion Laboratory, NASA is working on a mission called “Jupiter Icy Moons Orbiter” that probably won’t fly any sooner than 2011. It would carefully study the moons Callisto, Ganymede and Europa. It would use new technologies — a nuclear fission reactor and ion propulsion.
1. Noise in the data
“The problem with the Galileo data is that Europa is deep within the magnetosphere of Jupitar and is subjected to continuous bombardment by charged particles striking the surface at very high speeds and energies. This can destroy some molecules on the surface of Europa. Also, if you have a spacecraft in orbit around Europa, or near Europa, its also being struck by these high energy particles. And they will strike the detectors of the sensors (in the instruments) and provide false positives,” Dalton said.
“So we have a fair amount of instrumental noise in the data due to the high radiation environment. It’s just like the static you pick up when you are trying to listen to the radio. And trying to see our signal with this much noise around it can be difficult at times.”
Many scientists use spectroscopy — studying the light reflected and absorbed by a material to figure out what it’s made of. Dalton describes it this way:
“When I talk to my friends, I usually say, if you’ve got a glass of orange juice sitting across the room, you can’t actually drink it to determine whether its orange juice, but you can be pretty sure looking at it that it’s orange juice because you recognize that color of orange. What we do in spectroscopy is we look at colors your eye doesn’t see and we compare them to colors we’ve looked at very closely in the lab.
Everything has color because of the way it interacts with light, and what spectroscopists do, we look at light to see what the interactions were, and from that we can determine things like the chemical bonds; the chemical structure; and what molecules are present.”
Ordinary sunlight is made up of light of many different energies. Our eyes process some of these energies — we can tell red light from blue light, for example. But even though we can’t see infrared or ultraviolet light, materials reflect these types of lights too. Spectrometers are instruments that collect and record how much light — of many different energies — is reflected by an object. The number of light energies the spectrometer measures puts a limit on how detailed the signature will be. The more energies measured, the better the portrait. This is where things went awry on the recent Galileo mission. When an antenna malfunctioned on the spacecraft, it meant that less information could be sent back to Earth. The scientists decided to program the spectrometer to collect fewer light energies. In a way, its almost like asking the spectrometer to collect only blue light and green light but not turquoise light, leaving gaps in the final spectral signature. This has made it difficult to compare Europa’s spectral signature with the much more detailed signatures researchers can obtain in the lab.
Unfortunately, spectrometers need to be close to their target — you couldn’t use a spectrometer on Earth to measure light reflected from Europa. So we will have to wait for another trip to the icy moon before the moon’s signatures can be sharpened.
Says Dalton, ” We’re using data from the Galileo space craft, which was able to get up close, which telescopes on Earth can’t do. When you look at Europa through a telescope and you get a little dot, and the spectra of all places on the planet are averaged together. From the space craft, you can take spectra from small localized areas where you may have concentrations of materials. That’s why we’re using the Galileo spacecraft data to look at linnea and chaos regions. “
According to Dalton, ” We’ve known [Europa] has got a water-ice crust for a long time. But since the early eighties, we’ve been aware that the the spectral shape of the water spectrum is not typical of water. It looks a little different. The band positions, the absorption, are in the right position, but the shapes are not what you’d see with water, water ice, or water frost.
What it does relate to is water bound into some other material. We have a number of hydrated minerals — these are minerals that incorporate water into their structure. A common mineral is gypsum, it incorporates water — as a there are two water molecules for every molecule of CaSO4, that’s sheet rock, your walls are made of it. It’s usually mined from deposits from alkaline lakes. The lake dries up and leaves behind gypsum.
We see evidence of some hydrated material, on Europa. Which material it is, is not totally clear, because they tend to look similar. They tend to have the same set of absorption features which based on the water features, just distorted from what water would look like.
At this point, the front runner for the surface of MgSO4 — one of its hydrates is known as Epsom salt, you can buy it in the drugstore. There are other hydrated forms of MgSO4. These were based on measurements in the lab, done with comparison to alkaline lakes on Earth. They were compared to Galileo data, and it was a reasonably good match. But when I did the experiment of measuring the salts hydrated salts at the temperature of Europa, which is far, far colder than terrestrial measurements, I found that the spectrum changed. This is not an uncommon. Often things changed with temperature.
And so I got to thinking, what other hydrates haven’t we measure yet that might match? THere’s actually a lot that haven’t been measured yet. And I also got to thinking, we’re all looking for life on Europa, life contains water in a bound state, what would that look like? So I got some samples and did some measurements and found that it actually matches very well. Using these spectra, and comparing to the Europa data, I’ve found something that’s rather intriguing, although it’s not proof of life of on Europa by any means. ”
4. Liquid Water
Q: If Europa is so cold, how could a liquid ocean exist? How come it isn’t frozen like the surface?
If there is an ocean on Europa — and scientists aren’t sure if there is — it exists because of a gravity tug-of-war between Jupiter and the larger moons. Tidal forces — that is, the gravitational pull of nearby moons and planet — squish and stretch Europa, and could generate a lot of heat in the planet’s interior. This same gravity tug-of-war also drives the volcanoes on Jupiter’s moon Io.