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NASA/Goddard/Arizona State University
NASA/Goddard/Arizona State University

How Are Moon Maps Made?

NASA/Goddard/Arizona State University
NASA/Goddard/Arizona State University

At the 46th annual Lunar and Planetary Science Conference in Houston last week, I met some of the planetary scientists who use the Lunar Reconnaissance Orbiter to craft maps of the moon—those spectacular, highly detailed images that appear in textbooks and online, and which are so ubiquitous that it rarely occurs to us to ask, “How was this done?” Planetary geologic mapping, as I learned, is not merely the result of clever computer algorithms painting photographs onto wireframe spheres (though computers have their place). Rather, human hands painstakingly refine such maps. Here is what a few scientists told me about mapping the moon and other celestial bodies.

The maps are huge.

The detail captured by the Lunar Reconnaissance Orbiter is extraordinary, capable even of focusing on the lunar footpaths left by the Apollo astronauts. When China put a rover on the moon in 2013, planetary scientists used the LRO to track the rover’s progress to counter the skepticism some had of information released by the Chinese government. Sure enough, the lander Chang’e 3 landed, and the rover Yutu roved.

Such level of detail comes at a price. The map files can be enormous. One-hundred-meter-per-pixel “global maps” clock in around 20 gigabytes for a single file. One map of the moon’s north pole—a mosaic comprised of thousands of separate images—came in at 3.3 terabytes (for a tiny slice of the moon). How big is this? If you printed the map out, it would cover a football field and then some. The map of the northern lunar pole was generated with the help of a program called the Integrated Software for Imagers and Spectrometers. It was tricky work, and planetary scientists had to deal with converging longitudes and lighting issues endemic to mapping poles. Consistency of lighting, especially, proved a challenge, but is essential for accuracy.

When you want precise work, you need a human brain.

Computers are great at stitching together maps from image sources, but the resulting product is not always usable. The reason is that computers don’t see images; they see only pixel values. Mapping issues that might stump the most powerful computer can sometimes be solved in seconds by the human brain, which has an uncanny ability to recognize when something is not quite right. The work of mapping a planet, asteroid, or moon is tedious work and requires meticulous, pixel-perfect effort on the part of scientists to get things usable for the wider field of planetary science.

Scanning celestial bodies isn’t like Star Trek.

The geological mapping of other worlds sometimes uses astronomical spectroscopy to measure electromagnetic radiation. Instruments on satellites and orbiters gather data from celestial bodies to map such things as minerals in rocks and soil. To actually interpret that data, however, scientists need laboratory measurements against which to compare. One problem: the laboratory measurements taken on Earth have a bias for this planet. To increase accuracy, geologists have to adjust conditions, and can use chambers able to manipulate pressures, temperature, and atmosphere to make things more like the body in question. They then create a database of their measurements to match with the data collected by instruments on such satellites as the Lunar Reconnaissance Orbiter.

Creating laboratory spectra is slow, meticulous work, and there’s a lot to it. It requires the characterization of thousands of different minerals calibrated to the data from orbiters. Furthermore, the viewing geometry of instruments—where the instrument is versus where the sun is versus where the surface of the moon is—can create differences, and planetary scientists must account for all such variables.

Planetary bodies change—a lot.

Planetary scientists use the measure of crater density—the number of craters of a given size range in a given area—to date in relative terms the age of the lunar surface. Older surfaces will have more craters than younger surfaces. In working out the relative age of the moon’s surface, however, not all craters are created equal. There are “primaries” and “secondaries.” Primaries are when bodies crash into the moon, as you’d expect. Secondaries are the result of the debris from the craters created by the primaries. (Think stones skipping on water.) Obvious secondaries have characteristic shapes and often overlap or result in a herringbone pattern, and must be removed from the crater count.

There are landslides on the moon.

Mapping the moon is made even more challenging because the face of the moon is always changing, which is why the Lunar Reconnaissance Orbiter has proven vital. Each LRO dataset essentially reveals a brand new moon. In the time between the start of the LRO mission and today, there have been over 10,000 surface changes recorded by planetary scientists. LRO data on surface changes allow scientists to constrain the impact flux on the moon, which is to say that ages derived from crater counts are becoming increasingly accurate as scientists learn the flux of impacting objects and the creation of craters in our current timescale. Because of LRO data, we now know that the surface of the moon is dynamic.

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Big Questions
Why Do They Build Oil Rigs in the Middle of the Ocean?
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Ryan Carlyle:

We put the rigs where the oil is!

There aren’t any rigs in the “middle” of the ocean, but it is fairly common to find major oilfields over 150 km off the coast. This happens because:

  • Shallow seas often had the correct conditions for oil formation millions of years ago. Specifically, something like an algae bloom has to die and sink into oxygen-free conditions on the sea floor, then that organic material gets buried and converted to rock over geologic time.
  • The continental shelf downstream of a major river delta is a great place for deposition of loose, sandy sediments that make good oil reservoir rocks.

These two types of rock—organic-rich source rock and permeable reservoir rock—must be deposited in the correct order in the same place for there to be economically viable oil reservoirs. Sometimes, we find ancient shallow seas (or lakes) on dry land. Sometimes, we find them underneath modern seas. In that latter case, you get underwater oil and offshore oil rigs.

In the “middle” of the ocean, the seafloor is primarily basaltic crust generated by volcanic activity at the mid-ocean ridge. There’s no source of sufficient organic material for oil source rock or high-permeability sandstone for reservoir rock. So there is no oil. Which is fine, because the water is too deep to be very practical to drill on the sea floor anyway. (Possible, but not practical.)

This post originally appeared on Quora. Click here to view.

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Big Questions
Why is Friday the 13th Considered Unlucky?
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Today, people around the globe will feel uneasy about getting out of bed, leaving their homes, or going about their normal daily routines, all because of a superstition. These unfortunate folks suffer from paraskavedekatriaphobia, a common neurosis familiar to us all: the fear of Friday the 13th. But just where did this superstitious association come from, and how did it catch on?

The truth is that no one is absolutely sure where the idea that Friday the 13th is unlucky originated. Donald Dossey, the founder of the Stress Management Center and Phobia Institute in Asheville, North Carolina, suspects the fear can be traced back to a Norse myth about 12 gods who had a dinner at Valhalla—the fabled hall where legendary Norse heroes feasted for eternity after they died—that was interrupted by a 13th guest, the evil and mischievous god Loki.

According to legend, Loki tricked Höðr (the blind god of winter and son of Odin, the supreme god in Norse mythology) into shooting his brother Baldr (the benevolent god of summer who was also a son of Odin) with a magical spear tipped with mistletoe—the only substance that could defeat him. Thus the number 13 was branded as unlucky because of the ominous period of mourning following the loss of such powerful gods by this unwanted 13th guest.

For whatever reason, among many cultures, the number 12 emerged throughout history as a "complete" number: There are 12 months in a year, 12 signs of the zodiac, 12 Gods of Olympus, 12 sons of Odin, 12 labors of Hercules, 12 Jyotirlingas or Hindu shrines where Shiva is worshipped, 12 successors of Muhammad in Shia Islam, and 12 tribes of Israel. In Christianity, Jesus was betrayed by one of his 12 Apostles—Judas—who was the 13th guest to arrive for the Last Supper. Surpassing the number 12 ostensibly unbalances the ideal nature of things; because it is seen as irregular and disrespectful of a sense of perfection, the number 13 bears the stigma of misfortune and bad luck we know today.

WHY FRIDAY?

Friday joins in the mix mostly because all of the early accounts of Jesus’s crucifixion agree that it took place on Friday—the standard day for crucifixions in Rome. As Chaucer noted in The Canterbury Tales, "And on a Friday fell all this mischance." Yet perpetuating Friday as an unlucky day in America came from the late 19th-century American tradition of holding all executions on Fridays; Friday the 13th became the unluckiest of days simply because it combined two distinct superstitions into one. According to the Oxford University Press Dictionary of Superstitions, the first reference to Friday the 13th itself wasn’t until 1913. (So despite actually occurring on Friday, October 13, 1307, the popular notion that the Friday the 13th stigma comes from the date on which the famed order of the Knights Templar were wiped out by King Philip of France is just a coincidence.)

The repercussions of these phobias reverberated through American culture, particularly in the 20th century. Most skyscrapers and hotels lack a 13th floor, which specifically comes from the tendency in the early 1900s for buildings in New York City to omit the unlucky number (though the Empire State Building has a 13th floor). Some street addresses also skip from 12 to 14, while airports may skip the 13th gate. Allegedly, the popular Friday the 13th films were so-named just to cash in on this menacing date recognition, not because the filmmakers actually believed the date to be unlucky.

So, is Friday the 13th actually unlucky? Despite centuries of superstitious behavior, it largely seems like psychological mumbo jumbo. One 1993 study seemed to reveal that, statistically speaking, Friday the 13th is unlucky, but the study's authors told LiveScience that though the data was accurate, "the paper was just a bit of fun and not to be taken seriously." Other studies have shown no correlation between things like increased accidents or injuries and Friday the 13th.

And Friday the 13th isn't a big deal in other cultures, which have their own unlucky days: Greeks and Spanish-speaking countries consider Tuesday the 13th to be the unluckiest day, while Italians steer clear of Friday the 17th. So today, try to rest a little easy—Friday the 13th may not be so unlucky after all.

Additional Source: 13: The Story of the World’s Most Popular Superstition.

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