Universe Today
In 2011, NASA's
Dawn spacecraft
established orbit around the large asteroid (aka. planetoid) known as Vesta. Over the course of the next 14 months, the probe conducted detailed studies of Vesta's surface with its suite of scientific instruments. These findings revealed much about the planetoid's history, its surface features, and its structure - which is believed to be differentiated, like the rocky planets.
In addition, the probe collected vital information on Vesta's ice content. After spending the past three years sifting through the probe's data, a team of scientists has produced a
new study
that indicates the possibility of subsurface ice. These findings could have implications when it comes to our understanding of how Solar bodies formed and how water was historically transported throughout the Solar System.
Their study, titled
"Orbital Bistatic Radar Observations of Asteroid Vesta by the Dawn Mission
", was recently published in the scientific journal
Nature Communications.
Led by Elizabeth Palmer, a graduate student from Western Michigan University, the team relied on data obtained by the communications antenna aboard the Dawn spacecraft to conduct the first orbital bistatic radar (BSR) observation of Vesta.
[caption id="attachment_117707" align="aligncenter" width="580"]
Artist rendition of Dawn spacecraft orbiting Vesta. Credit: NASA/JPL-Caltech
[/caption]
This antenna - the High-Gain telecommunications Antenna (HGA) - transmitted X-band radio waves during its orbit of Vesta to the
Deep Space Network
(DSN) antenna on Earth. During the majority of the mission, Dawn's orbit was designed to ensure that the HGA was in the line of sight with ground stations on Earth. However, during occultations - when the probe passed behind Vesta for 5 to 33 minutes at a time - the probe was out of this line of sight.
Nevertheless, the antenna was continuously transmitting telemetry data, which caused the HGA-transmitted radar waves to be reflected off of Vesta's surface. This technique, known as bistatic radar (BSR) observations has been used in the past to study the surfaces of terrestrial bodies like Mercury, Venus, the Moon, Mars, Saturn's moon Titan, and the comet 67P/CG.
But as Palmer explained, using this technique to study a body like Vesta was a first for astronomers:
[caption id="attachment_116446" align="aligncenter" width="580"]
This high-res geological map of Vesta is derived from Dawn spacecraft data. Brown colors represent the oldest, most heavily cratered surface. Credit: NASA/JPL-Caltech/ASU
[/caption]
By studying the reflected BSR waves, Palmer and her team were able to gain valuable information from Vesta's surface. From this, they observed significant differences in surface radar reflectivity. But unlike the Moon, these variations in surface roughness could not be explained by cratering alone and was likely due to the existence of ground-ice. As Palmer explained:
In the end, Palmer and her colleagues concluded that the presence of buried ice (past and/or present) on Vesta was responsible for parts of the surface being smoother than others. Basically, whenever an impact happened on the surface, it transferred a great deal of energy to the subsurface. If buried ice was present there, it would be melted by the impact event, flow to the surface along impact-generated fractures, and then freeze in place.
Much in the same way that moon's like Europa, Ganymede and Titania experience surface renewal because of the way cryovolcanism causes liquid water to reach the surface (where it refreezes), the presence of subsurface ice would cause parts of Vesta' surface to be smoothed out over time. This would ultimately lead to the kinds of uneven terrain that Palmer and her colleagues witnessed.
[caption id="attachment_122694" align="aligncenter" width="580"]
The planetoid Vesta, which was studied by the Dawn probe between July 2011 and September 2012. Credit: NASA
[/caption]
This theory is supported by the large concentrations of hydrogen that were detected over smoother terrains that measure hundreds of square kilometers. It is also consistent with geomorphological evidence obtained from the
Dawn Framing Camera
images, which showed signs of of transient water flow over Vesta's surface. This study also contradicted some previously-held assumptions about Vesta.
As Palmer noted, this could also have implications as far as our understanding of the history and evolution of the Solar System is concerned:
This work was sponsored by NASA's
Planetary Geology and Geophysics
program, a JPL-based effort that focuses on fostering the research of terrestrial-like planets and major satellites in the Solar System. The work was also conducted with the assistance of the USC's
Viterbi School of Engineering
as part of an ongoing effort to improve radar and microwave imaging to locate subsurface sources of water on planets and other bodies.
Further Reading: USC, Nature Communications
