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Acknowledgments: We thank the NIH (R37GM-55382)
for support of this work and Chevron for a fellowship to P.S.F.
A provisional patent application has been filed on this reaction.
Supporting Online Material
Materials and Methods
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25 July 2013; accepted 22 October 2013
10.1126/science.1243759
orbit of Venus based on photometry from an ex-
terior viewpoint.
Imaging of a Circumsolar Dust Ring
Near the Orbit of Venus
M. H. Jones,1* D. Bewsher,2 D. S. Brown2
The STEREO mission (16), launched in
October 2006, uses two nearly identical space-
craft, A and B, to provide synoptic observations
of the Sun and the heliosphere interior to 1 AU.
Each spacecraft carries a Heliospheric Imager
instrument (17), HI-2, both of which continuous-
ly monitor the inner zodiacal cloud. These HI-2A
and HI-2B instruments have a field of view of
about 70° centered on ecliptic latitude b ≈ 0° at
helioecliptic longitude |l′| = 53.7°. In searching
for a dust ring at the orbit of Venus, the lines of
sight that are of interest are close to the ecliptic
plane and have 40° < |l′| < 50° (depending on the
location of the spacecraft) (Fig. 1).
The gravitational interaction of dust in the zodiacal cloud with individual planets is expected to
give rise to ringlike features: Such a circumsolar ring has been observed associated with Earth,
but such resonance rings have not been confirmed to exist for other planets. Here, we report on
sensitive photometric observations, based on imaging from the STEREO mission, that confirm
the existence of a dust ring at the orbit of Venus. The maximum overdensity of dust in this ring,
compared to the zodiacal cloud, is ~10%. The radial density profile of this ring differs from the
model used to describe Earth’s ring in that it has two distinct steplike components, with one
step being interior and the other exterior to the orbit of Venus.
In normal science operations, the HI-2 instru-
circumsolar dust ring is known to exist
Searches for resonance structures around ment generates a 1024 pixel–by–1024 pixel im-
around the orbit of Earth (1, 2). It arises the orbits of Mars and Jupiter have not yet been age with ≈4–arc min resolution every 2 hours.
from the trapping of interplanetary dust successful (10). The situation at Venus is more Further processing, including positional calibra-
A
grains, primarily of asteroidal and cometary ori- complex. A detection of enhanced scattering tion (18), results in the level-1 data used here.
gin (3–5), into orbits that are resonant with Earth’s was reported (11) from Venera 9 and 10 but is The instrument is sensitive to wavelengths of 400
orbit. Dust grains of sizes from 1 to 100 mm are attributed to circumplanetary rather than circum- to 1000 nm, and the dominant diffuse source in
subject to Poynting-Robertson (P-R) drag and to solar dust. On the basis of a reanalysis of pho- HI-2 images is solar radiation scattered by dust
a lesser extent solar wind drag, which results in a tometry from the Helios mission, data consistent grains [with radii of 10 to 100 mm (19)] in the
gradual decay of their orbits [about 104 years for with a dust ring just outside the orbit of Venus zodiacal cloud. The typical surface brightness at
a 10-mm particle at 1 astronomical unit (AU)] (6). have been presented (12), but the existence of (l′ ≈ 45°, b ≈ 0°) is ≈8 DN s−1 pixel−1 [DN is data
As a dust grain slowly spirals inward toward the such a ring could not be confirmed beyond number, 1 DN ≅ 15 photoelectrons is the default
orbit of the Earth, it successively passes into lo- doubt.
unit used by the instrument team (17)]. The HI-2
cations in which it may be trapped temporarily A circumsolar dust ring at Venus would pro- instrument was designed (17) to monitor coronal
(for time scales of 104 to 105 years) into a par- vide observational data, which should lead to mass ejections having a surface brightness of about
ticular orbital resonance (1, 7–9). Dust in in- improved understanding of the factors affect- 1% of that of the zodiacal light. By combining
dividual resonances will cluster into periodic ing the formation of resonance rings, such as ~100 HI-2 level-1 images, photometry on the
patterns around the orbit, and, when multiple res- (P-R and solar wind) drag forces, an elliptical zodiacal cloud to an accuracy of order 0.1% can
onances combine to form a real dust ring, these planetary orbit, and gravitational perturbation be extracted. A major limitation is the presence of
underlying structures result in the azimuthal dis- by an exterior planet (1, 7, 8). Furthermore, un- systematic errors that compromise this photom-
tribution of the ring being nonuniform (1).
derstanding of circumstellar dust rings is im- etry. These arise from various sources: saturation
portant in the context of exoplanetary systems stripes, ghosting caused by bright objects, and the
(13). Not only are they an important considera- presence of the galactic plane within 30° of the re-
tion in proposed space-based interferometric im- gion of interest. To avoid contamination, data con-
aging of exoplanets (14), large scale rings have taining these features were not used in our analysis.
1Department of Physical Sciences, The Open University, Walton
Hall, Milton Keynes, Buckinghamshire MK7 6AA, UK. 2Jeremiah
Horrocks Institute, University of Central Lancashire, Preston,
Lancashire PR1 2HE, UK.
been imaged directly, as in the case of Fomalhaut
Photometry of HI-2 data involved combining
*Corresponding author. E-mail: m.h.jones@open.ac.uk
(15). Here, we describe a search for a ring at the level-1 images from 10 consecutive days (i.e., up to
960