A false-color image of Ceres' Occator Crater shows differences in surface composition. On Ceres, blue is generally associated with bright material, found in more than 130 locations, and seems to be consistent with salts, such as sulfates. It is likely that silicate materials are also present. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
In 2007, a Delta II rocket carrying a NASA spacecraft called Dawn launched from Cape Canaveral, Florida. In 2011, it arrived at the asteroid Vesta, where it spent 14 months in orbit studying that object. It then left, powered by ion engines. Two and half years later Dawn entered the orbit of Ceres, a mysterious dwarf planet characterized by a complex geomorphology, that is located in the asteroid belt between Mars and Jupiter.
The spacecraft recently completed the low-altitude, global mapping of every inch of the dwarf planet's surface. It did this over four months through a series of orbits, scanning one strip of land at a time at a resolution of 35 meters/pixel. Thanks to this intensive scrutiny, we now have some new insights into some of Ceres's features. And yet other aspects remain mysterious—and confounding.
“This is a body different than any we’ve examined thus far," Chris Russell said last week at the Lunar and Planetary Science Conference in The Woodlands, Texas. Russell is the principal investigator of the Dawn mission. "It fills a gap in our understanding of planet evolution. It may still be evolving.”
WHAT ARE THOSE BRIGHT SPOTS?
The bright spots of Ceres' Occator crater as captured by NASA's Dawn spacecraft on June 25, 2015. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
If you've heard of Ceres, it's likely because of those mysterious white spots that scientists tell us are definitely not alien cities. They exist in Occator Crater, a complex feature that is 92 kilometers in diameter, 3 kilometers deep, and 70 to 80 million years old. The crater's rim is irregular and scalloped. Its floor is bluish and its walls are red hued. The different coloring suggests compositional and physical differences in the composite materials. The famed central cluster of bright spots is 10 kilometers across and very bright at the center, with a mysterious dome.
“The Occator dome continues to fascinate us," Russell said. In high-resolution view, this "ever intriguing" planetary splotch bears an uncanny resemblance to “a dropped bag of flour,” Russell said, adding, "That bright spot has continued to beguile us.”
Notably, there are bright spots elsewhere on Ceres, and to the right of the bright cluster and dome are subsidiary spots, which are dimmer by a factor of two. Whatever is at work in Occator is not a wholly unique process, but we don't know what that process is yet.
HOW TO SCAN AN ALIEN WORLD
Dawn is loaded with delicate instruments to help decipher the dwarf planet's secrets. The Gamma Ray and Neutron Detector (GRaND) maps elements on the asteroid so that scientists can make sense of the surface and processes at work. The instrument works like this. Galactic cosmic rays smack into the regolith (the loose surface layer; on Earth, think: dirt), and interactions with the surface lead to emissions of neutrons and gamma rays. GRaND detects these emissions as they bounce into space. Neutrons at different energy levels correspond to different surface elements.
During the regolith interaction, when the cosmic rays hit the nucleus of an atom, the nucleus explodes, sending neutrons and protons in all directions. Some neutrons escape the regolith, some smash into other nuclei. Here's where it gets interesting. If a neutron hits the nucleus of a hydrogen atom, it loses energy in the interaction, similar to the way a cue ball stops when it hits another ball in a game of pool. When GRaND is counting neutrons, therefore, lower numbers suggest more hydrogen.
That's what is shown on the above map, which is color-coded for the presence of hydrogen. (Blue is more; red is less.) The area in blue is the north pole of Ceres, and as the map reveals, it's teeming with hydrogen, relatively speaking. This indicates the presence of water ice—H2O—near the dwarf planet’s surface. This is the first time such ice has been detected, and the finding is consistent with longstanding scientific predictions. Planetary scientists will continue analyzing the data collected by GRaND and other instruments in order to better understand the origin and evolution of Ceres.
"We're sampling about a meter below the surface," said Thomas Prettyman of the Planetary Science Institute, who leads the GRaND team. "So we're below the optical surface, which makes it complementary to the optical spectrometers." Because cosmic rays strike from every direction in space, these measurements can even be made in total darkness, further supplementing the optical instrument data.
They're also planning to take some color data on the surface, Russell said. “We’ll take color of selected regions—we can’t do the whole body—and we’ll be doing topography over the next few months.”
SOLVING THE MYSTERY
There is some discussion as to whether Ceres, now classified as a dwarf planet, should still be considered an asteroid. Such taxonomic debates speak to the exciting forward march of planetary science. The more scientists learn, the blurrier the lines get, and there is time yet for the lines to get blurrier still.
“Our stay at Ceres will likely be through early 2017," said Carol Raymond, the deputy principal investigator of Dawn. "We still have quite a way to go to fill in blanks at lowest altitude. We expect we’ll get lots of interesting data for a long time to come.”
That data will come in many forms. "We’ve been focusing on the camera data," Raymond told mental_floss, noting that in addition to the Framing Camera and GRaND, the Visible & Infrared Spectrometer (VIR) instrument is trying methodically to capture data to fill in gaps, though that's a slower process. "We can’t always predict ahead while we’re sequencing; it's a game of cat-and-mouse to capture high-value areas."
Space has taken its toll on Dawn over these nine years, and the spacecraft's reaction wheels are no longer fully reliable. The wheels, which act as a sort of gyroscope, help point and position the spacecraft. This won't end the mission, though it doesn't make the job any easier. In the months and years to come, data sets will continue being built, analyses conducted, and models run.
Take a closer look at Ceres in the video below.