A Trip to Palomar

Today, I visited the Palomar Observatory in the mountains north of San Diego. Palomar has an extensive history of astronomical discovery throughout the twentieth century, and continues to be in use today. The observatory is home to a massive 200 inch telescope built and operated by Caltech. The size of the telescope—200 inches—refers to the diameter of the primary mirror of the telescope, and is a good measure of a telescope’s light collecting power. A series of five other mirrors help to focus the light and direct it to various instruments, including a spectrometer, the housing of which I was allowed to climb inside! The entire assembly itself is housed in a massive dome with the same diameter of the ancient Roman Pantheon. Our tour guide stressed that the huge dome and extensive support structures were all designed to protect and align a thin layer of aluminum weighing only five grams in total.

Unlike the everyday mirrors in bathrooms which owe their reflectivity to silver surfaces, Palomar uses aluminum to create its mirrors. Silver mirrors use a simple chemical process to coat glass, called the Tollen's test. At Palomar, aluminum deposition onto its glass primary is carried out in a precisely controlled vacuum environment in order to ensure the mirror is devoid of imperfections. When making telescopes, minuscule imperfections can be a big problem. Any deviation from a perfectly parabolic surface will scatter or blur the valuable image the telescope aims to collect. Imperfections of sizes comparable to the wavelength of the observed light (in this case, visible light, which is several tenths of a micron in wavelength) can compromise the instrument. Much care is taken in order to hunt down these tiny flaws on a giant mirror for this reason. Every two to three years, the aluminum on the mirror is carefully stripped and recoated using the same high-precision process in order to repair the accumulation of dust and foreign material (read: bird droppings) that accumulate from nightly use. 

Below are some panoramas I took from various locations under the dome. The primary mirror is located under the big structure and is currently pointed straight up. The large cylindrical tank is the vacuum chamber where the mirror is repaired.

Here is a view from the south end of the telescope. The hole on the left is where I got to enter the telescope. The cage on the top of the telescope is the observing platform, and is separated from the rest of the telescope in order to isolate the vibrations of whoever was observing. Today, electronic instruments take data instead of astronomers' eyes. 

What Does Europa Taste Like?

Today, I attended a lecture by Mike Brown, a Caltech professor most famous for "killing Pluto" by discovering the Kuiper Belt object Eris. This time, he was talking about his current scientific project: learning about the chemistry (and taste!) of Europa. One of the great things about being a Caltech undergrad is that I get to attend talks like this from world-class scientists without travelling too far from home!

Europa is an icy moon of Jupiter that is about the size of our own moon. Often, we hear that Europa is exciting because it has a subsurface ocean. Brown's rationale for studying Europa is a bit more nuanced than that--Ganymede, another moon of Jupiter, also has a reserve of liquid water underneath its surface but is less interesting to Brown's research because it has more water than Europa. This is because Ganymede has a thick enough liquid water ocean that towards the bottom, the pressure is high enough for the water to freeze out into ice. If you could slice into Ganymede, you would discover an object with an icy outer shell, a liquid water ocean, an inner icy shell and a rocky core, or as Brown described it, "an icy water sandwich." In contrast, the smaller water ocean cannot produce ice at the bottom of the ocean. Water can come in contact with Europa's rocky core, creating a boundary or "interface" where water and rock can interact chemically.

A water-rock interface is important because here on Earth, the interaction of water and rock drives plate tectonics and creates hydrothermal vents inhabited by some of the planet's most exotic creatures.

Water and ice also interact with Europa's surface, producing linear features on its surface and jumbled terrains that resemble icebergs floating on a frozen ocean. These features are produced by upwelling of water from Europa's subsurface ocean. Brown is currently working on identifying the chemical composition of salts present in these features. By doing so, he can sneak a peek at the composition of the ocean beneath. Here on Earth, the main salt in our oceans is sodium chloride. Is this true of Europa, too?

Using spectroscopic measurements from the massive Keck Telescope on the Big Island of Hawai'i, Brown can find out. Europa has a quite varied landscape, with three main compositions roughly corresponding two hemispheres and their border. Europa is tidally locked to Jupiter like our moon is, so one side always faces the giant planet. On the side that leads the planet on its orbit, only pure water ice is detected. On the trailing side, Brown found traces of sulfuric acid, produced by the interaction of sulfur ions from another Jovian moon, Io, carried to Europa by Jupiter's massive magnetic field. Between these two extremes, the surface water is laced with magnesium, potassium, and sodium salts that likely originate from the interaction of the water ocean and rocky core of Europa.

What does this mean for the taste of Europa? Brown suggests a mixture of ice, salt, and grapefruit juice--the sour citric acid in grapefruit replacing the harsher sulfuric acid--for a drink that is out of this world.