Dawn Discovers Surprise 2nd Giant South Pole Impact Basin at Strikingly Dichotomous Vesta

Southern Hemisphere of Vesta; Rheasilvia and Older Basin. Colorized shaded-relief map showing identification of older 375-kilometer-wide impact basin beneath and overlapping with the more recent Rheasilvia impact structure at asteroid Vesta’s South Pole. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA


Scientists leading NASA’s Dawn mission have discovered a 2nd giant impact basin at the south pole of the giant asteroid Vesta, which has been unveiled as a surprisingly “dichotomous” and alien world. Furthermore, the cosmic collisions that produced these two basins shuddered through the interior and created vast Vestan troughs, a Dawn scientist told Universe Today.

The newly discovered impact basin, nicknamed ‘Older Basin’, is actually significantly older in age compared to the initially discovered South Pole basin feature named ‘Rheasilvia’ – perhaps by more than a billion years. And that is just one of the many unexplained mysteries yet to be reconciled by the team as they begin to sift through the millions of bits of new data streaming back daily to Earth.

Scientists speculate that ‘Older Basin’ is on the order of 3.8 Billion years old, whereas ‘Rheasilvia’ might be as young as about 2.5 Billion years, but those are just tentative estimates at this time and subject to change. Measurements so far indicate Rheasilvia is composed of basaltic material.

Shaded-relief topographic map of Vesta southern hemisphere showing two large impact basins - Rheasilvia and Older Basin.

“We found many surprising things at Vesta, which is quite unique and the results have exceeded our expectations”, said Dr. Carol Raymond, Dawn deputy principal investigator, of NASA’s Jet Propulsion Laboratory, Pasadena, Calif.

Researchers presented the latest findings from Dawn’s initial science mapping orbit at a news briefing at the annual meeting of the Geological Society of America in Minneapolis, Minn., on Oct. 13.

The team considers Vesta to be the smallest terrestrial planet.

Since achieving orbit in July, Dawn’s Framing Cameras (FC) have imaged most of Vesta at about 250 meter resolution and the Visible and Infrared mapping spectrometer(VIR) at about 700 meter resolution. The measurements were collected at the survey orbit altitude of 2700 km. Before Dawn, Vesta was just a fuzzy blob in humankind’s most powerful telescopes.

Vesta from Hubble (top) as a fuzzy blob and from Dawn in orbit (bottom) in crystal clear high resolution.

“There is a global dichotomy on Vesta and a fundamental difference between the northern and southern hemispheres”, said Raymond. “The northern hemisphere is older and heavily cratered in contrast to the brighter southern hemisphere where the texture is more smooth and there are lots of sets of grooves. There is a massive mountain at the South Pole. One of the more surprising aspects is the set of deep equatorial troughs.”

“There is also a tremendous and surprising diversity of surface color and morphology. The south is consistent with basaltic lithology and the north with impacts. We are trying to make sense of the data and will integrate that with the high resolution observations we are now collecting.”

Indeed Vesta’s completely unique and striking dichotomy can be directly traced back to the basins which were formed by ancient cataclysmic impacts resulting in shockwaves that fundamentally altered the surface and caused the formation of the long troughs that ring Vesta at numerous latitudes.

“The troughs extend across 240 degrees of longitude,” said Debra Buczkowski, Dawn participating scientist, of the Applied Physics Laboratory at Johns Hopkins University, Laurel, Md. “Their formation can be tied back to the two basins at the South Pole.”

Asteroid Vesta and Equatorial Grooves
This full view of the giant asteroid Vesta was taken by NASA’s Dawn spacecraft, as part of a rotation characterization sequence on July 24, 2011, at a distance of 3,200 miles (5,200 kilometers). A rotation characterization sequence helps the scientists and engineers by giving an initial overview of the character of the surface as Vesta rotated underneath the spacecraft. This view of Vesta shows impact craters of various sizes and grooves parallel to the equator. The resolution of this image is about 500 meters per pixel. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

In an exclusive follow up interview with Universe Today, Raymond said “We believe that the troughs formed as a direct result of the impacts,” said “The two sets of troughs are associated with the two large basins [Rheasilvia and Older Basin].”

“The key piece of evidence presented was that the set of troughs in the northern hemisphere, that look older (more degraded) are circumferential to the older impact basin,” Raymond told me.

“The equatorial set are circumferential to Rheasilvia. That Rheasilvia’s age appears in places to be much younger is at odds with the age of the equatorial troughs. An explanation for that could be resurfacing by younger mass wasting features (landslides, slumps). We will be working on clarifying all these relationships in the coming months with the higher resolution HAMO (High Altitude Mapping Orbit) data.”

Dawn has gradually spiraled down closer to Vesta using her exotic ion thrusters and began the HAMO mapping campaign on Sept. 29.

Surface features are dated by crater counting methodology.

“Preliminary crater counting age dates for the equatorial trough region yields a very old age (3.8 Billion years). So there is a discrepancy between the apparent younger age for the Rheasilvia basin and the old age for the troughs. These could be reconciled if Rheasilvia is also 3.8 Billion years old but the surface has been modified by slumping or other processes,” Raymond elaborated.

Time will tell as further data is analyzed.

Dawn launch on September 27, 2007 by a Delta II Heavy rocket from Cape Canaveral Air Force Station, Florida. Credit: Ken Kremer

“Vesta is full of surprises, no more so than at the South Pole,” said Paul Schenk at the GSA briefing. Schenk is a Dawn participating scientist of the Lunar and Planetary Institute, Houston, Texas.

The ‘Rheasilvia’ basin was initially discovered in images of Vesta taken a decade ago by the Hubble Space Telescope which revealed it as a gaping hole in the southern hemisphere. But it wasn’t until Dawn entered orbit on July 16, 2011 after a nearly four year interplanetary journey that Earthlings got their first close up look at the mysterious polar feature and can now scrutinize it in detail to elucidate its true nature.

“The South Pole [Rheasilvia] basin is a roughly circular, impact structure and a deep depression dominated by a large central mound,” said Schenk. “It shows sharp scarps, smooth areas, landslide deposits, debris flows. It’s about 475 km in diameter and one of the deepest (ca. 20 -25 km) impact craters in the solar system.”

The central peak is an enormous mountain, about 22 km high and 180 km across- one of the biggest in the solar system. “It’s comparable in some ways to Olympus Mons on Mars,” Schenk stated.

“We were quite surprised to see a second basin in the mapping data outside of Rheasilvia. This was unexpected. It’s called ‘Older Basin’ for now.”

‘Older Basin’ is about 375 km in diameter. They overlap at the place where Rheasilvia has a missing rim.

“These basins are interesting because we believe Vesta is the source of a large number of meteorites, the HED meteorites that have a spread of ages,” Schenk explained.

Images showing key components of Rheasilvia impact basin on Vesta in high resolution ,referred to Shaded-relief topographic map. Credit: NASA/JPL-Caltech/ UCLA/MPS/DLR/IDA

Multiple large impacts over time may explain the source of the HED (Howardite, Eucrite and Diogenite) meteorites.

“We did expect large impacts on Vesta, likely associated with the late heavy bombardment recognized in the lunar impact record,” Raymond told Universe Today. “The surprising element is that the two apparently largest impacts – keeping in mind that other larger impact basins may be lurking under the regolith – are overlapping.”

Dawn’s VIR spectrometer has detected pyroxene bands covering Vesta’s surface, which is indicative of typical basaltic material, said Federico Tosi, a VIR team member of the Italian Space Agency, Rome. “Vesta has diverse rock types on its surface.”

“VIR measured surface temperatures from 220K to 270 K at the 5 micron wavelength. The illuminated areas are warmer.”

So far there is no clear indication of olivine which would be a marker for seeing Vesta’s mantle, Tossi elaborated.

The VIR spectrometer combines images, spectral information and temperature that will allow researchers to evaluate the nature, composition and evolutionary forces that shaped Vesta’s surface.

The team is absolutely thrilled to see a complicated geologic record that’s been preserved for study with lots of apparent surface layering and surprisingly strong and complex structural features with a large range of color and brightness.

Stay tuned for a year of Vestan delights !

Asteroid Vesta from Dawn
South Pole Rheasilvia basin is at lower right. NASA's Dawn spacecraft obtained this image of the giant asteroid Vesta with its framing camera on July 24, 2011 from a distance of about 3,200 miles (5,200 kilometers). Dawn entered orbit around Vesta on July 16, and will spend a year orbiting the body.

Read Ken’s continuing features about Dawn and Vesta starting here:
Amazing New View of the Mt. Everest of Vesta
Dramatic 3 D Imagery Showcases Vesta’s Pockmarked, Mountainous and Groovy Terrain
Rheasilvia – Super Mysterious South Pole Basin at Vesta
Space Spectacular — Rotation Movies of Vesta
3 D Alien Snowman Graces Vesta
NASA Unveils Thrilling First Full Frame Images of Vesta from Dawn
Dawn Spirals Down Closer to Vesta’s South Pole Impact Basin
First Ever Vesta Vistas from Orbit – in 2D and 3D
Dawn Exceeds Wildest Expectations as First Ever Spacecraft to Orbit a Protoplanet – Vesta

Rheasilvia – Super Mysterious South Pole Basin at Vesta is Named after Romulus and Remus Roman Mother

A False-Color Topography of Vesta's South Pole. This false-color map of the giant asteroid Vesta was created from stereo images obtained by the framing camera aboard NASA’s Dawn spacecraft. The image shows the elevation of surface structures with a horizontal resolution of about 750 meters per pixel. The terrain model of Vesta's southern hemisphere shows a big circular structure with a diameter of about 300 miles (500 kilometers), its rim rising above the interior of the structure for more than 9 miles (15 kilometers.) Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Video caption: Rheasilvia Impact Basin and Vesta shape model. This false-color shape model video of the giant asteroid Vesta was created from images taken by the framing camera aboard NASA’s Dawn spacecraft. Rheasilvia – South Pole Impact Basin – shown at bottom (left) and head on (at right). Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

‘Rheasilvia’ – that’s the brand new name given to the humongous and ever more mysterious South Pole basin feature being scrutinized in detail by Dawn, according to the missions top scientists in a Universe Today exclusive. Dawn is NASA’s newly arrived science orbiter unveiling the giant asteroid Vesta – a marvelously intriguing body unlike any other in our Solar System.

What is Rheasilvia? An impact basin? A crater remnant? Tectonic action? A leftover from internal processes? Or something completely different? That’s the hotly debated central question consuming loads of attention and sparking significant speculation amongst Dawn’s happily puzzled international science team. There is nothing closely analogous to Vesta and Rhea Silvia – and thats a planetary scientists dream come true.

“Rheasilvia – One thing that we all agree on is that the large crater should be named ‘Rheasilvia’ after the mother of Romulus and Remus, the mythical mother of the Vestals,” said Prof. Chris Russell, Dawns lead scientist, in an exclusive interview with Universe Today. Russell, from UCLA, is the scientific Principal Investigator for Dawn.

“Since we have never seen any crater just like this one it is difficult for us to decide exactly what did happen,” Russell told me. “The name ‘Rheasilvia’ has been approved by the IAU and the science team is using it.”

Craters on Vesta are being named after the Vestal Virgins—the priestesses of the Roman goddess Vesta. Other features will be named for festivals and towns of that era. Romulus and Remus were the mythical founders of Rome.

‘Rheasilvia’ has the science team in a quandary, rather puzzled and reevaluating and debating long held theories as they collect reams of new data from Dawn’s three science instruments – provided by the US, Germany and Italy. That’s the scientific method in progress and it will take time to reach a consensus.

Prior to Dawn’s orbital insertion in July 2011, the best views of Vesta were captured by the Hubble Space Telescope and clearly showed it wasn’t round. Scientists interpreted the data as showing that Vesta’s southern hemisphere lacked a South Pole! And, that it had been blasted away eons ago by a gargantuan cosmic collision that excavated huge amounts of material that nearly utterly destroyed the asteroid.

The ancient collision left behind a colossal 300 mile (500 km) diameter and circular gaping hole in the southern hemisphere – nearly as wide as the entire asteroid (530 km) and leaving behind an as yet unexplained and enormous central mountain peak, measuring some 9 miles (15 km) high and over 125 miles (200 km) in diameter. The mountain has one of the highest elevations in the entire solar system.

“We are trying to understand the high scarps that we see and the scarps that should be there and aren’t,” Russell explained. “We are trying to understand the landslides we think we see and why the land slid. We see grooves in the floor of the basin and want to interpret them.

“And the hill in the center of the crater remains as mysterious today as when we first arrived.”

Viewing the South Pole of Vesta and Rheasilvia Impact Basin
This image obtained by Dawns framing camera and shows the south pole of the giant asteroid Vesta. Scientists are discussing whether the Rheasilvia circular structure that covers most of this image originated by a collision with another asteroid, or by internal processes early in the asteroid's history. Images in higher resolution from Dawn's lowered orbit might help answer that question. The image was recorded from a distance of about 1,700 miles (2,700 kilometers). The image resolution is about 260 meters per pixel. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Another top Dawn scientist described Rheasilvia in this way:

“I would say that the floor of the impact feature contains chaotic terrain with multiple sets of intersecting grooves, sometimes fairly straight and often curvy, said Carol Raymond to Universe Today. Raymond is Dawn’s Deputy Principal Investigator from NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

“The crater rim is not well-expressed”, Raymond told me. “We see strong color variations across Vesta, and the south pole impact basin appears to have a distinct spectral signature.

“The analysis is still ongoing,” Russell said.

“The south is distinctly different than the north. The north has a varied spectrum and the south has a distinct spectral feature but it has little variation.” Time will tell as additional high resolution measurements are collected from the forthcoming science campaign at lower orbits.

Russell further informed that the team is rushing to pull all the currently available data together in time for a science conference and public briefing in mid-October.

“We have set ourselves a target to gather everything we know about the south pole impact feature and expect to have a press release from what ever we conclude at the GSA (Geological Society of America) meeting on October 12. “We will tell the public what the options are.”

“We do not have a good analog to Vesta anywhere else in the Solar System and we’ll be studying it very intently.”

Impressive South Pole MountainTop at Rheasilvia Crater on Vesta
This mountain, which measures about 125 miles (200 kilometers) in diameter at its base, is one of the highest elevations on all known bodies with solid surfaces in the solar system. The image has been recorded with the framing camera aboard NASA's Dawn spacecraft from a distance of about 1,700 miles (2,700 kilometers). The image resolution is about 260 meters per pixel. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Right now Dawn is using its ion propulsion system to spiral down four times closer to Vesta, as it descends from the initlal survey orbit(about 2700 km, 1700 mi) to the new science orbit, elegantly named HAMO – or High Altitude Mapping Orbit (about 685 km.)

“Our current plan is to begin HAMO on Sept. 29, but we will not finalize that plan until next week,” Dr. Marc Rayman told Universe Today. Rayman, of NASA’s JPL, is Dawn’s Chief Engineer.

“Dawn’s mean altitude today (Sept. 20) is around 680 km (420 miles),” said Rayman .

“Dawn successfully completed the majority of the planned ion thrusting needed to reach its new science orbit and navigators are now measuring its orbital parameters precisely so they can design a final maneuver to ensure the spacecraft is in just the orbit needed to begin its intensive mapping observations next week.”

Watch for lots more stories upcoming on Vesta and the Dawn mission

Read Ken’s continuing features about Dawn
Space Spectacular — Rotation Movies of Vesta
3 D Alien Snowman Graces Vesta
NASA Unveils Thrilling First Full Frame Images of Vesta from Dawn
Dawn Spirals Down Closer to Vesta’s South Pole Impact Basin
First Ever Vesta Vistas from Orbit – in 2D and 3D
Dawn Exceeds Wildest Expectations as First Ever Spacecraft to Orbit a Protoplanet – Vesta
Dawn Closing in on Asteroid Vesta as Views Exceed Hubble
Dawn Begins Approach to Asteroid Vesta and Snaps First Images
Revolutionary Dawn Closing in on Asteroid Vesta with Opened Eyes