Planetary Exploration

Forensic Sleuthing Ties Ring Ripples to Impacts

PASADENA, Calif. – Like forensic scientists examining fingerprints at a cosmic crime scene, scientists working with data from NASA’s Cassini, Galileo and New Horizons missions have traced telltale ripples in the rings of Saturn and Jupiter back to collisions with cometary fragments dating back more than 10 years ago.

The ripple-producing culprit, in the case of Jupiter, was comet Shoemaker-Levy 9, whose debris cloud hurtled through the thin Jupiter ring system during a kamikaze course into the planet in July 1994. Scientists attribute Saturn’s ripples to a similar object – likely another cloud of comet debris -- plunging through the inner rings in the second half of 1983. The findings are detailed in a pair of papers published online today in the journal Science.

“What’s cool is we’re finding evidence that a planet’s rings can be affected by specific, traceable events that happened in the last 30 years, rather than a hundred million years ago,” said Matthew Hedman, a Cassini imaging team associate, lead author of one of the papers, and a research associate at Cornell University, Ithaca, N.Y. “The solar system is a much more dynamic place than we gave it credit for.”

From Galileo’s visit to Jupiter, scientists have known since the late 1990s about patchy patterns in the Jovian ring. But the Galileo images were a little fuzzy, and scientists didn’t understand why such patterns would occur. The trail was cold until Cassini entered orbit around Saturn in 2004 and started sending back thousands of images. A 2007 paper by Hedman and colleagues first noted corrugations in Saturn’s innermost ring, dubbed the D ring.

A group including Hedman and Mark Showalter, a Cassini co-investigator based at the SETI Institute in Mountain View, Calif., then realized that the grooves in the D ring appeared to wind together more tightly over time. Playing the process backward, Hedman then demonstrated the pattern originated when something tilted the D ring off its axis by about 100 meters (300 feet) in late 1983. The scientists found the influence of Saturn’s gravity on the tilted area warped the ring into a tightening spiral.

Cassini imaging scientists got another clue when the sun shone directly along Saturn’s equator and lit the rings edge-on in August 2009. The unique lighting conditions highlighted ripples not previously seen in another part of the ring system. Whatever happened in 1983 was not a small, localized event; it was big. The collision had tilted a region more than 19,000 kilometers (12,000 miles) wide, covering part of the D ring and the next outermost ring, called the C ring. Unfortunately spacecraft were not visiting Saturn at that time, and the planet was on the far side of the sun, hidden from telescopes on or orbiting Earth, so whatever happened in 1983 passed unnoticed by astronomers.

Hedman and Showalter, the lead author on the second paper, began to wonder whether the long-forgotten pattern in Jupiter’s ring system might illuminate the mystery. Using Galileo images from 1996 and 2000, Showalter confirmed a similar winding spiral pattern. They applied the same math they had applied to Saturn – but now with Jupiter’s gravitational influence factored in. Unwinding the spiral pinpointed the date when Jupiter’s ring was tilted off its axis: between June and September 1994. Shoemaker-Levy plunged into the Jovian atmosphere during late July 1994. The estimated size of the nucleus was also consistent with the amount of material needed to disturb Jupiter’s ring.

The Galileo images also revealed a second spiral, which was calculated to have originated in 1990. Images taken by New Horizons in 2007, when the spacecraft flew by Jupiter on its way to Pluto, showed two newer ripple patterns, in addition to the fading echo of the Shoemaker-Levy impact.

“We now know that collisions into the rings are very common – a few times per decade for Jupiter and a few times per century for Saturn,” Showalter said. “Now scientists know that the rings record these impacts like grooves in a vinyl record, and we can play back their history later.”

The ripples also give scientists clues to the size of the clouds of cometary debris that hit the rings. In each of these cases, the nuclei of the comets – before they likely broke apart – were a few kilometers wide.

“Finding these fingerprints still in the rings is amazing and helps us better understand impact processes in our solar system,” said Linda Spilker, Cassini project scientist, based at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “Cassini’s long sojourn around Saturn has helped us tease out subtle clues that tell us about the history of our origins.”

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute in Boulder, Colo. JPL managed the Galileo mission for NASA, and designed and built the Galileo orbiter. The New Horizons mission is led by Principal Investigator Alan Stern of Southwest Research Institute, Boulder, Colo., and managed by the Johns Hopkins Applied Physics Laboratory, Laurel, Md., for NASA's Science Mission Directorate.

Martian Clays Tell Story of Wet Past

Layers of clay-rich rock have been found in Mars’ Mawrth Vallis, a potential landing site for future rovers.  This work, published in the August 8 issue of Science, suggests that abundant water was once present on Mars and that hydrothermal activity may have occurred.

The Mawrth Vallis outflow channel is a feature in Mars’ northern highland region, a heavily cratered, ancient area of the Red Planet whose geology is a time capsule offering revelations to those who can read it. A team of researchers led by planetary scientist Janice Bishop of the SETI Institute has used the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) aboard the Mars Reconnaissance Orbiter (MRO) to examine infrared light reflected from clays situated in the many-kilometer wide channel. Mawrth Vallis resembles a dried-up, broad river valley through which water may have flowed.

The infrared spectra from CRISM show an extensive swath of phyllosilicate-bearing material.  This is a type of iron and magnesium-rich clay that forms in liquid water, and can be found on Earth in oceans and river beds.  It is familiar to anyone who’s nearly broken a shovel while trying to plant a tree. There is also evidence in the spectra for hydrated silica, which in its pure, clean form is known as opal.

The researchers have combined their data on the composition of soils in this region with topographic information collected by MOLA, the Mars Orbiter Laser Altimeter. They find that clay units in this region were emplaced in a layered fashion, with aluminum clays lying on top of hydrated silica and iron/magnesium clays.  These clays were likely formed when water came in contact with basalt – which is the dominant component of the Martian highlands, and probably was produced from volcanic ash, which once blanketed the planet.

“We were surprised by the variety of clay minerals in this region,” says Bishop.  “But what’s interesting is that we find the same ordering of the clay materials everywhere in Mawrth Vallis. It’s like a layer-cake of clays, one on top of another.  All these layers are topped with a ‘frosting’ of lava and dust.  We can see the clay layers where an impact crater has carved a hole through the surface or where erosion has exposed them.”

Since phyllosilicates have been found in a number of outcrops on Mars in CRISM images, these new data suggest that whatever mechanism formed clays at Mawrth Vallis has probably operated over much greater areas of the Red Planet.  Alteration by liquid water may have been widespread on early Mars.

These observations improve on others obtained by French scientists with the Mars Express/OMEGA instrument.  While those measurements were made with resolutions of hundreds of meters per pixel, the CRISM instrument can boast a sharper gaze: eighteen meters per pixel.  Additional scrutiny by MRO’s HIRISE camera, with a resolution of 26 cm per pixel, show distinct textures and layering in each of the clay units. 

Bishop is careful to note that this work is part of the long-term effort to establish just how widespread, and for what period of time, liquid water may have existed on Mars. 

“This is not evidence for life,” she notes.  “But it does suggest the long-term and common presence of liquid water – and concomitant active chemistry – on the Red Planet in the distant past.”

This CRISM image was taken on September 21, 2007, and shows a part of the Mawrth Vallis region centered near 24.7 degrees N latitude, 339.5 degrees E longitude. The image covers an area about 10 kilometers (6.2 miles) wide and is draped over MOLA terrain with 20X vertical exaggeration. Fe/Mg-phyllosilicate is shown in red, Al-phyllosilicate is shown in blue, hydrated silica and an Fe2+ phase are shown in yellow/green. 

NASA's Cassini Discovers Potential Liquid Water on Enceladus

NASA's Cassini spacecraft may have found evidence of liquid water reservoirs that erupt in Yellowstone-like geysers on Saturn's moon Enceladus. The rare occurrence of liquid water so near the surface raises many new questions about the mysterious moon.

"We realize that this is a radical conclusion - that we may have evidence for liquid water within a body so small and so cold," said Carolyn Porco, Cassini imaging team leader at the Space Science
Institute, Boulder, Colo. "However, if we are right, we have significantly broadened the diversity of solar system environments where we might possibly have conditions suitable for living organisms."

High-resolution Cassini images show icy jets and towering plumes ejecting large quantities of particles at high speed. Scientists examined several models to explain the process. They ruled out the
idea the particles are produced or blown off the moon's surface by vapor created when warm water ice converts to a gas. Instead, scientists have found evidence for a much more exciting possibility.
The jets might be erupting from near-surface pockets of liquid water above 0 degrees Celsius (32 degrees Fahrenheit), like cold versions of the Old Faithful geyser in Yellowstone.

"We previously knew of at most three places where active volcanism exists: Jupiter's moon Io, Earth, and possibly Neptune's moon Triton. Cassini changed all that, making Enceladus the latest member of this very exclusive club, and one of the most exciting places in the solar system," said John Spencer, Cassini scientist, Southwest Research Institute, Boulder.

"Other moons in the solar system have liquid-water oceans covered by kilometers of icy crust," said Andrew Ingersoll, imaging team member and atmospheric scientist at the California Institute of Technology, Pasadena, Calif. "What's different here is that pockets of liquid water may be no more than tens of meters below the surface."

"As Cassini approached Saturn, we discovered the Saturnian system is filled with oxygen atoms. At the time we had no idea where the oxygen was coming from," said Candy Hansen, Cassini scientist at NASA's Jet Propulsion Laboratory (JPL) in Pasadena. "Now we know Enceladus is spewing out water molecules, which break down into oxygen and hydrogen."

Scientists still have many questions. Why is Enceladus so active? Are other sites on Enceladus active? Might this activity have been continuous enough over the moon's history for life to have had a chance to take hold in the moon's interior?

In the spring of 2008, scientists will get another chance to look at Enceladus when Cassini flies within 350 kilometers (approximately 220 miles), but much work remains after the spacecraft's four-year prime mission is over.

"There's no question, along with the moon Titan, Enceladus should be a very high priority for us. Saturn has given us two exciting worlds to explore," said Jonathan Lunine, Cassini interdisciplinary scientist, University of Arizona, Tucson, Ariz.

Mission scientists report these and other Enceladus findings in this week's issue of Science. The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency.

JPL, a division of the California Institute of Technology, manages the Cassini-Huygens mission for NASA's Science Mission Directorate. The Cassini orbiter was designed, developed and assembled at JPL.

NASA'S HUBBLE DISCOVERS ANOTHER MOON AROUND PLUTO

WASHINGTON -- Astronomers using the Hubble Space Telescope discovered a fourth moon orbiting the icy dwarf planet Pluto. The tiny, new satellite, temporarily designated P4, was uncovered in a Hubble survey searching for rings around the dwarf planet.

The new moon is the smallest discovered around Pluto. It has an estimated diameter of 8 to 21 miles (13 to 34 km). By comparison, Charon, Pluto's largest moon, is 648 miles (1,043 km) across, and the other moons, Nix and Hydra, are in the range of 20 to 70 miles in diameter (32 to 113 km).

"I find it remarkable that Hubble's cameras enabled us to see such a tiny object so clearly from a distance of more than 3 billion miles (5 billion km)," said Mark Showalter of the SETI Institute in Mountain View, Calif., who led this observing program with Hubble.

The finding is a result of ongoing work to support NASA's New Horizons mission, scheduled to fly through the Pluto system in 2015. The mission is designed to provide new insights about worlds at the edge of our solar system. Hubble's mapping of Pluto's surface and discovery of its satellites have been invaluable to planning for New Horizons' close encounter.

"This is a fantastic discovery," said New Horizons' principal investigator Alan Stern of the Southwest Research Institute in Boulder, Colo. "Now that we know there's another moon in the Pluto system, we can plan close-up observations of it during our flyby."

The new moon is located between the orbits of Nix and Hydra, which Hubble discovered in 2005. Charon was discovered in 1978 at the U.S. Naval Observatory and first resolved using Hubble in 1990 as a separate body from Pluto.

The dwarf planet's entire moon system is believed to have formed by a collision between Pluto and another planet-sized body early in the history of the solar system. The smashup flung material that coalesced into the family of satellites observed around Pluto.

Lunar rocks returned to Earth from the Apollo missions led to the theory that our moon was the result of a similar collision between Earth and a Mars-sized body 4.4 billion years ago. Scientists believe material blasted off Pluto's moons by micrometeoroid impacts may form rings around the dwarf planet, but the Hubble photographs have not detected any so far.

"This surprising observation is a powerful reminder of Hubble's ability as a general purpose astronomical observatory to make astounding, unintended discoveries," said Jon Morse, astrophysics division director at NASA Headquarters in Washington.

P4 was first seen in a photo taken with Hubble's Wide Field Camera 3 on June 28. It was confirmed in subsequent Hubble pictures taken on July 3 and July 18. The moon was not seen in earlier Hubble images because the exposure times were shorter. There is a chance it appeared as a very faint smudge in 2006 images, but was overlooked because it was obscured.

Hubble is a project of international cooperation between NASA and the European Space Agency. NASA's Goddard Space Flight Center in Greenbelt, Md., manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy Inc. in Washington.

 

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