Poster’s Note: One of the many under-appreciated aspects of NASA is the extent to which it publishes quality science content for children and Ph.D.’s alike. NASA Space Place has been providing general audience articles for quite some time that are freely available for download and republishing. Your tax dollars help promote science! The following article was provided for reprinting in April, 2014.
By Dr. Ethan Siegel
Put more mass beneath your feet and feel the downward acceleration due to gravity increase. Newton’s law of universal gravitation may have been superseded by Einstein’s, but it still describes the gravitational force and acceleration here on Earth to remarkable precision. The acceleration you experience is directly proportional to the amount of mass you “see,” but inversely proportional to the distance from you to that mass squared.
The denser the mass beneath your feet, the stronger the gravitational force, and when you are closer to such a mass, the force is even greater. At higher elevations or even higher altitudes, you’d expect your gravitational force to drop as you move farther from Earth’s center. You’d probably also expect that downward acceleration to be greater if you stood atop a large mountain than if you flew tens of thousands of feet above a flat ocean, with nothing but ultra-light air and liquid water beneath you for all those miles. In fact this is true, but not just due to the mountain’s extra mass!
Earth is built like a layer-cake, with the less dense atmosphere, ocean, and crust floating atop the denser mantle, which in turn floats atop the outer and inner cores of our planet. An iceberg’s buoyancy is enough to lift only about one tenth of it above the sea, with the other nine tenths below the surface. Similarly, each and every mountain range has a corresponding “invisible mountain” that dips deep into the mantle. Beneath the ocean floor, Earth’s crust might be only three to six miles thick, but it can exceed 40 miles in thickness around major mountain ranges like the Himalayas and the Andes. It’s where one of Earth’s tectonic plates subducts beneath another that we see the largest gravitational anomalies: another confirmation of the theory of continental drift.
A combination of instruments aboard NASA’s Gravity Recovery and Climate Experiment (GRACE) satellites, including the SuperSTAR accelerometer, the K-band ranging system and the onboard GPS receiver, have enabled the construction of the most accurate map of Earth’s gravitational field ever: to accelerations of nanometers per second squared. While the mountaintops may be farther from Earth’s center than any other point, the extra mass of the mountains and their roots – minus the mass of the displaced mantle – accounts for the true gravitational accelerations we actually see. It’s only by the grace of these satellites that we can measure this to such accuracy and confirm what was first conjectured in the 1800s: that the full layer-cake structure of Earth must be accounted for to explain the gravity we experience on our world!
Image credit: NASA / GRACE mission / Christoph Reigber, et al. (2005): An Earth gravity field model complete to degree and order 150 from GRACE: EIGEN-GRACE02S, Journal of Geodynamics 39(1),1–10. Reds indicate greater gravitational anomalies; blues are smaller ones.
About NASA Space Place
The goal of the NASA Space Place is “to inform, inspire, and involve children in the excitement of science, technology, and space exploration.” More information is available at their website: http://spaceplace.nasa.gov/
The NASA News service provides up-to-date announcements of NASA policy, news events, and space science. A recent selection of space science articles are provided below, including direct links to the full announcements. Those interested in receiving these news announcements directly from NASA can subscribe to their service by sending an email to:
NASA laboratory experiments suggest the dark material coating some geological features of Jupiter’s moon Europa is likely sea salt from a subsurface ocean, discolored by exposure to radiation. The presence of sea salt on Europa’s surface suggests the ocean is interacting with its rocky seafloor — an important consideration in determining whether the icy moon could support life.
The study is accepted for publication in the journal Geophysical Research Letters and is available online.
“We have many questions about Europa, the most important and most difficult to answer being is there life? Research like this is important because it focuses on questions we can definitively answer, like whether or not Europa is inhabitable,” said Curt Niebur, Outer Planets Program scientist at NASA Headquarters in Washington. “Once we have those answers, we can tackle the bigger question about life in the ocean beneath Europa’s ice shell.”
A new NASA study finds the last remaining section of Antarctica’s Larsen B Ice Shelf, which partially collapsed in 2002, is quickly weakening and likely to disintegrate completely before the end of the decade.
A team led by Ala Khazendar of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, found the remnant of the Larsen B Ice Shelf is flowing faster, becoming increasingly fragmented and developing large cracks. Two of its tributary glaciers also are flowing faster and thinning rapidly.
“These are warning signs that the remnant is disintegrating,” Khazendar said. “Although it’s fascinating scientifically to have a front-row seat to watch the ice shelf becoming unstable and breaking up, it’s bad news for our planet. This ice shelf has existed for at least 10,000 years, and soon it will be gone.”
…
The research team included scientists from JPL, the University of California, Irvine, and the University Centre in Svalbard, Norway. The paper is online at: go.nasa.gov/1bbpfsC
NASA uses the vantage point of space to increase our understanding of our home planet, improve lives and safeguard our future. NASA develops new ways to observe and study Earth’s interconnected natural systems with long-term data records. The agency freely shares this unique knowledge and works with institutions around the world to gain new insights into how our planet is changing.
For more information about NASA’s Earth science activities, visit: www.nasa.gov/earth
NASA’s WISE Spacecraft Discovers Most Luminous Galaxy in Universe
A remote galaxy shining with the light of more than 300 trillion suns has been discovered using data from NASA’s Wide-field Infrared Survey Explorer (WISE). The galaxy is the most luminous galaxy found to date and belongs to a new class of objects recently discovered by WISE — extremely luminous infrared galaxies, or ELIRGs.
“We are looking at a very intense phase of galaxy evolution,” said Chao-Wei Tsai of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, lead author of a new report appearing in the May 22 issue of The Astrophysical Journal. “This dazzling light may be from the main growth spurt of the galaxy’s black hole.”
The brilliant galaxy, known as WISE J224607.57-052635.0, may have a behemoth black hole at its belly, gorging itself on gas. Supermassive black holes draw gas and matter into a disk around them, heating the disk to roaring temperatures of millions of degrees and blasting out high-energy, visible, ultraviolet, and X-ray light. The light is blocked by surrounding cocoons of dust. As the dust heats up, it radiates infrared light.
On June 3, 1965, NASA astronaut Ed White became the first American to walk in space. NASA is celebrating the 50th anniversary of the United States’ first extravehicular activity (EVA) — better known as a spacewalk — through a number of commemorative features on NASA Television and NASA.gov.
On Monday, June 1, NASA will premiere a documentary narrated by actor Jon Cryer on the history and future of humans working on a tether in space. The film, Suit Up, marks five decades of spacewalks and will air on NASA Television starting at 10 a.m. EDT, and be available on the agency’s website and YouTube account.
The documentary features interviews with NASA Administrator and astronaut, Charles Bolden, NASA Deputy Administrator and spacesuit designer, Dava Newman, as well as other astronauts, engineers, technicians, managers and luminaries of spacewalk history. They share their personal stories that cover the full EVA experience– from spacesuit manufacturing to spacewalk maneuvering — all brought to life through historical and HD footage.
…
For NASA TV streaming video, schedules and downlink information, visit: www.nasa.gov/nasatv
NASA’s Hubble Finds Pluto’s Moons Tumbling in Absolute Chaos
If you lived on one of Pluto’s moons, you might have a hard time determining when, or from which direction, the sun will rise each day. Comprehensive analysis of data from NASA’s Hubble Space Telescope shows that two of Pluto’s moons, Nix and Hydra, wobble unpredictably.
“Hubble has provided a new view of Pluto and its moons revealing a cosmic dance with a chaotic rhythm,” said John Grunsfeld, associate administrator of NASA’s Science Mission Directorate in Washington. “When the New Horizons spacecraft flies through the Pluto system in July we’ll get a chance to see what these moons look like up close and personal.”
The moons wobble because they’re embedded in a gravitational field that shifts constantly. This shift is created by the double planet system of Pluto and Charon as they whirl about each other. Pluto and Charon are called a double planet because they share a common center of gravity located in the space between the bodies. Their variable gravitational field sends the smaller moons tumbling erratically. The effect is strengthened by the football-like, rather than spherical, shape of the moons. Scientists believe it’s likely Pluto’s other two moons, Kerberos and Styx, are in a similar situation.
Put more mass beneath your feet and feel the downward acceleration due to gravity increase. Newton’s law of universal gravitation may have been superseded by Einstein’s, but it still describes the gravitational force and acceleration here on Earth to remarkable precision. The acceleration you experience is directly proportional to the amount of mass you “see,” but inversely proportional to the distance from you to that mass squared.
