of Observations Kevin Reardon (Queen’s University Belfast / Osservatorio di Arcetri) Jay Pasachoff, Bryce Babcock (Williams College) Glenn Schneider (Steward Observatory, University of Arizona) Pascal Hedelt (Observatoire de Bordeaux) Serge Koutchmy (Institut d'Astrophysique de Paris) Mihalis Mathioudakis (Queen’s University Belfast) Paolo Tanga (Observatoire de la Cote d'Azur) Thomas Widemann (Observatoire de Paris, CNRS)

Introduction: This proposal describes an observing program designed to advance our understanding of planetary atmospheres, focusing on two key topics. These studies will take advantage of the visible from Sacramento Peak and Kitt Peak on 05 June 2012. This event will allow us to probe the dynamics and structure of the through measurements of the refracted, scattered, and transmitted solar radiation. Observations of the transit of a with a known atmospheric composition will allow us to develop better methods to characterize, predict, and explain the details of transits, providing direct support to a fundamental research topic in astrophysics in the coming decades.

The transit ofPROGRESS Venus, not to be visible from for 105 years, is aNATURE uniqueVol 450 29opportunity November 2007 j j to address these important questions. Venus, unlike , has a thick atmosphere and orbits withinTable 1 | Theour scientific solar payload system’s of habitable zone, yet is life-hostile. This makes it an Name(acronym) Description Measuredparameters important caseASPERA- study4 Detection to andexamine characterization ofas neutral we and charged seek particles ways to probe Electrons 1 eV –our20 keV; ionsneighborhood0.01–36 keV/q; neutral particles for0.1– 60signskeV MAG Dual sensor fluxgate magnetometer, one sensor on a 1-m-long boom B field 8 pT–262 nT at 128 Hz PFS Planetary Fourier Spectrometer (currently not operating) Wavelength 0.9–45 mm; spectral resolving power about 1,200 of life. The proposedSPICAV/SOIR Ultraviolet observations and infrared spectrometer make for stellar and use solar of NSO’sWavelengths comprehensive110–320 nm, 0.7–1.65 m mand and 2.2 –unique4.4 mm; spectral set of measurements and nadir observations resolving power up to 20,000 resources to VeRaacquire Radio high Science investigation quality for radio-occultation data of and both bi-static immediate measurements X- and and S-band Doppler historical shift, polarization value. and amplitude variations VIRTIS Ultraviolet–visible–infrared imaging spectrometer and high-resolution infrared Wavelength 0.25–5 mm for the imaging spectrometer and 2–5 mm for spectrometer the high-resolution channel; resolving power about 2,000 VMC Venus Monitoring Camera for wide-field imaging Four parallel channels at 365, 513, 965 and 1010 nm AtmosphericThese Studies instruments are expected of to producea Habitable more than 2 terabits of data Zone during the design Planet lifetime of four Venus: sidereal days (about 1,000 Earth days). Venus Express is operating in an elliptical polar orbit with a period of 24 h and an apocentre altitude of 66,000 km. The pericentre altitude is maintained between 250 and 400 km approximately over the north pole. q is elementary charge. While very similar in their size, composition, localized ‘weather’ phenomena, the overall organization of the atmo- a Sub-solar to and location sphericwithin circulation. the Threehabitable broad regimes zon are clearlye of present our in the Polar vortex anti-solar cell middle and lower atmosphere, with convective and wave-dominated Polar collar ,meteorology Venus in theand lower Earth latitudes are and an distinctly abrupt transition to Hadley cell smoother, banded flow at middle to high latitudes5. The latter ter- different in severalminates at about crucial 30u from theways. pole, where The the cold polar collar dis- Cold covered by earlier missions lies. This encloses a vast vortex-type atmosphere structureof Venus several thousand is extremely kilometres across dense with a complex (67 double 3 ‘eye’ that rotates every 2.5–2.8 Earth days. Simultaneous observations3 Warm Warm kg/m at the insurface, the ultraviolet compared and thermal infrared to spectral 1.2 ranges kg/m show corre- lated patterns, indicating that the contrasts at both wavelengths, on Earth) andalthough is predominantly representing different atmospheric composed levels, are drivenof by Cold CO (96.5%)the rather same circumpolar than dynamical nitrogen regime 5,6(78%). Spectroscopic as observa-on 2 tions indicate marked changes in the temperature and cloud struc- Earth. Giventure these in the vortex, divergent with the cloud conditions top in the polar collar on located the at an altitude of 70–72 km, about 5 km or one scale height higher than in two , theVenus eye. Night-side shows observations significant in the transparent spectral windows showed that the vortex structure and circulation exist at as least as b differences ingreat the a depth physical as the lower processes cloud deck at 50–55 at km, work although its Figure 1: The general circulation pattern 6 Recombination ‘dipole’ appearance seems to be confined to the cloud-top region . of O atoms of the atmosphere of Venus, showinginto O ( the) in structuringThe its edge atmosphere. of the polar collar at 50–60 Significantu latitude apparently marks the 2 ∆ poleward limit of the Hadley circulation, the planet-wide overturn- meridional flow driven by solar heating on progress hasing been of the atmosphere made in responsein identifying to the concentration these of solar heat- ing in the equatorial zones (Fig. 2a). Indirect evidence of such the sunward side. From Svedhem etNight-side al, mechanisms using terrestrial and spacecraft EUV flux airglow meridional circulation is provided by monitoring of the latitude 2007 distribution of minor constituents, especially carbon monoxide, as CO2 dynamical tracers in the lower atmosphere. Solar heating photodissociation The mesopause on Venus at 100–120 km altitude marks another transition between different global circulation regimes, this time in the vertical. The predominance of zonal super-rotation in the lower atmosphere below the mesopause is replaced by solar to antisolar flow in the thermosphere above, as revealed by non-LTE (non-local thermodynamic equilibrium) emission in the spectral band of O2 at 1.27 mm that originates from the recombination of oxygen atoms in descending flow on the night side (Fig. 2b). The observed emission Figure 2 | Schematic view of the general circulation of Venus’s atmosphere. patterns are highly variable, with the maximum at about the anti- a, The main feature is a convectively driven Hadley cell, which extends from the solar point and the peak altitude at about the mesopause7. A meso- equatorial region up to about 60u of latitude in each hemisphere. The trend is spheric temperature maximum is observed on the night side8, polewards at all levels that can be observed by tracking the winds (at about produced by adiabatic heating in the subsiding branch of the thermo- 50–65 km altitude above the surface), so the return branch of the cell must bein spheric solar to anti-solar circulation. the atmosphere belowthe clouds. A cold‘polar collar’ is found around each pole at about 70u latitude; the Hadley circulation evidently feeds a mid-latitude jet at Sequences of ultraviolet and infrared images have been used to its poleward extreme, inside which there is a circumpolar belt characterized by measure the wind speeds at different altitudes by tracking the remarkably low temperatures and dense, high clouds. Inside the collar a motions of contrast features in the clouds. Zonal winds at the cloud thinning of the upper cloud layer forms a complex and highly variable feature, tops (,70 km) derived from the ultraviolet imaging are in the range called the ‘polar dipole’ in earlier literature describing poorly resolved 100 6 10 m s21 at latitudes below 50u (ref. 5), in good agreement with observations, which appears bright in the thermal infrared6. Because in general the earlier observations9,10. The new data, which penetrate the bright terms thinner-than-average or lower-than-average cloud is often associated upper haze obscuring the main cloud at middle latitudes, find that with a descending air mass, and vice versa, the vortex may represent a second, the cloud-top winds quickly decline poleward of 50u. The infrared high-latitude circulation cell, resembling winter hemisphere behaviour on b observations6 sound the dynamics in the main cloud deck at ,50 km Earth. , Above about 100 km altitude the circulation regime on Venus changes completely to a sub-solar to anti-solar pattern. Oxygen airglow emission at altitude on the night side, finding strong vertical wind shear of about 21 21 1.27 mm reveals the recombination of oxygen atoms into molecular oxygen 3ms km below 50u, and no shear poleward of this latitude, when while descending to lower altitudes in the anti-solar region. Additional evidence compared with the higher-altitude ultraviolet-derived winds. The ofthiscirculationis givenbytheupper-atmospheretemperatureprofiles,which wind velocity profiles on Venus are found to be roughly, although show a pronounced temperature maximum on the night side that is due to not exactly, in agreement with those predicted by the cyclostrophic compressional heating in the downward branch of the circulation cell8. 630 © 2007 Nature Publishing Group 2 observations. Most recently, ESA’s Venus Express mission has provided new details on the atmospheric dynamics. In an interesting parallel to the global flows seen on the , the atmosphere of Venus shows a significant meridional flow, driven in this case by the solar heating from above in the equatorial zone (see overview in Svedhem et al., 2007). This drives an observed polarward flow in the troposphere (h< 60 km), with a return flow inferred to occur in the denser regions of the lower atmosphere. This poleward flow terminates at a latitude of 60-70° where a cold, jetlike circulation, called the “polar collar,” is found, encompassing polar vortices. Venus' middle atmosphere (60-120 km, also known as the mesosphere) is a transition region between this lower atmosphere circulation and the thermosphere (h > 120km), where diurnal heat and pressure gradients drive dynamic flows between the sunward and night–side of the planet. These cross-terminator1 flows occur both at the equator and over the poles (Drossart et al., 2007). Monitoring of thermal profiles and winds in the mesosphere has revealed significant time variability in the transition region where different types of circulation coexist. Questions remain, however, about the composition, vertical structure, and variability of the atmosphere. Aspects of these questions are powerfully addressed by transit observations. The transit geometry is unique because it provides for the simultaneous observation of the entire terminator of Venus with a constant solar zenith angle, compared with single-direction observations made from the Venus Express spacecraft. This simultaneous observation around the entire limb of the planet will allow determination of the latitudinal variation of the temperature or density structure of the mesosphere. This will help calibrate the Venus Express observations, in particular the lower mesosphere temperature structure using limb solar or radio occultations by the SOIR and VeRa experiments (Vandaele et al., 2008, Piccialli et al. 2011). The line-of-sight velocities around the full limb, caused by nightward flows or polar vortices, still have not been fully characterized. Because of the large background flux from the Sun, molecular absorption lines or refraction from the atmosphere of Venus can be measured with high spatial resolution and signal-to-noise in just a few seconds. The June transit will therefore provide a once-in-a-lifetime opportunity to apply the powerful tool of imaging spectroscopy to probe new dynamical timescales in the atmosphere.

Exoplanet Analogues: Planets transiting across their host stars are now routinely observed throughout our galactic neighborhood. The signature of the geometric transit in the light curve has been used to identify more than a thousand extra–solar planet candidates with observations from the Kepler satellite (with the number still growing). In order to provide a “truth test” for these detections, the comparable measurement was performed during the 2004

1 The terminator is the boundary between the day and night side of the planet D. Ehrenreich et al.: Venus as a transiting exoplanet cross-sections are calculated from the HITRAN 2004 line3 list (Rothman et al. 2005)followingRothman et al. (1998, Sect. A.2.4) but using Voigt functions instead of Lorentzian line profiles. The UV cross-sections of CO2,O2,SO2,andO3 are taken from the AMOP2 and AMP3 data repositories. The pho- transittoabsorption of Venus cross using sections arethe plotted sun in-as Fig.-2aa.-star irradiance measurement of the ACRIMsat satelliteScattering (Schneider – Important et additionalal. 2006; absorption Pasachoff is caused et by al. 2011) to recover the “unknown” transit diffusion processes: Rayleigh scattering from atmospheric CO2 geometry.and, to a lesser extent, N2,CO,andH2O, emerging above the main cloud deck at 70 km, and Mie scattering caused by In theH2 SOeight4 droplets years inthe since upper that haze (70–90first transit, km). Rayleigh dif- fusion, which efficiency increases toward the blue following 4 however,λ− ,resultsfromthescatteringoflightbyparticleswithsizes the goal of detecting the exoplanet atmosphericr much smaller signature than the wavelength through (x =transmission2πr/λ 1), typi- cally, molecules composing the atmospheric gases." In the case spectroscopyof Venus, the dihasffusion now is mainly become caused by practical. CO2:thescattering cross section is 8/3x4(n2 1)2/(n2 +1)2 (see, e.g., Lecavelier des The Etangsstrong et al.molecular 2008a), where− bandsn is the refractive in planetary index of the gas. atmospheresFor CO2,weusetheformulaof produce broadSneep &absorption Ubachs (2005)tocalcu- late n as a function of wavelength. The upper haze is composed featuresby particles that with can sizes be larger detected, than the wavelength with sufficient (4 < x < 300). integration,Consequently, during the calculation the planetary of the extinction transits (scattering∼ .∼ + ab- sorption) cross section by the bimodal distribution of upper haze Theseparticles atmospheric is based on Mie signatures theory. The complex have refractive been index detectedof H2SO for4 droplets hot is taken from (e.g.,Hummel Charbonneau et al. (1988). Below 3 µm, the imaginary part of the refractive index is negligible et al.and 2002; the scattering Vidal dominates-Madjar the et Mie al. extinction. 2003, The 2011; absorption Snellencross et section, al. 2010) linked to.the Similar imaginary signatures part of the refractive are in- dex, becomes non-negligible above 3 µm, whereas the scatter- ∼ expecteding cross for section Earth drops.-sized The extinction planets, cross sectionthough is the sum FigureFig. 2. a) 2:Absorption (top) the crosscalculated sections transit of the consideredspectrum atmospheric for of the scattering and absorption. It is calculated for log-normal Venusgases, Rayleigh in transit scattering as seen cross from section Earth; of CO (2bottom)(dashed red the line), and with particlea reduced size distributions magnitude with the setbecause of light scattering of the routines Mie extinction cross sections for mode-1 (thick blue line) and mode-2 smalleravailable size at theof Universitythe planet of Oxford and Physics atmosphere Department4 and effective(thick violet height line) haze of absorption particles. b) Transit of different spectrum atmospheric of Venus transit- plotted as a function of wavelength for hazes with different par- componentsing in front of. the From Sun, asEhrenreich seen from Earth, et al., in relative2012. absorption and with ticlerespect size distributions to the inhost Fig. 2star.a. c) effective height of absorption. The different transit spectra shown are models with no upper haze (black), mode-1 upper haze (red), and modes-1+2upperhaze(green).Thetopofthemainclouddeckissetat This field will see increasing attention and resources70 km. The in transit coming spectrum years. of the Earth Indeed calculated byit Kalteneggeris listed & as a 3.key Results priority and discussionin the latest Astronomy and AstrophysicsTraub (2009)isoverplotted(blue)andshiftedby decadal survey. Several+100 km for clarity.new missions under study –NASA’s FINESSE (Fast INfrared Exoplanet Survey Explorer) and The transmission spectrum of Venus is shown with a reso- be a factor 12.3 smaller because there is no effect – the ESA’slution EChO of 1 nm (Exoplanet in Fig. 2b, as Characterisation relative absorption ∆δ ( λObservatory) = distance to the) – exoplanet have equalscomplementary the distance to the transitedscience star. 2 goalsδ to(λ) detectδ (λmin ),atmospheres where δ (λ) = [R around+ h(λ)]/R rockyand♀ λplanets,min In termsthe ofcharacterization effective height of absorption of theirh(λ)(Fig. chemical2c), the is defined− as the wavelength where{ the transit depth$} is minimal; ♀ ♀ ♀ ♀ cytherean limb could be probed from 70 to 150 km, i.e., from composit is 1.67,ition, 2.50, and and the 2.65 identificationµmforthehaze-free,mode-1haze, of biomarkersthe in top the of the atmosphere. cloud deck up to aboveIn this the mesopause,context, in case Venusand is modes-1 our +closest2hazemodels,respectively.Thee model for telluricffective exoplanet height the targets upper haze. Obtaining is only composed its by transmission mode-1 particles. If larger of absorption h(λ)hasthesamemeaningasinKaltenegger & mode-2 particles are involved, it won’t be possible to probe the spectrumTraub (2009 during)andisplottedinFig. its transit 2acrossc. It can be the retrieved Sun on-line will servelimb below both80 as km, reacheda comparison at 2.65 µm. basis for (Table A.1). The amplitude of the spectrum reaches 25 ppm for ∼ transiting Earth-mass to be∼ observed inIn fact,the the future, lowest altitudeand a that proof it is possible of feasibility to reach with the prominent CO2 UV bands below 0.2 µm. From 0.2 µmand transmission spectroscopy is set by the dominant diffusion that suchabove, theobservations spectrum is dominated can by effectively Mie scattering probe (<2.7 µm) theregime, atmospheres Rayleigh or Mie. of Inexoplanets the case of Venus, in this the most mass re- rangeand. absorption (>2.7 µm) by upper haze particles, which can markable and extended spectral signature is that of Mie scat- significantly impact on the amplitude of the absorption from the tering by the upper haze. This situation contrasts with the Earth, other spectral features, such as the CO transitions around 2 µm A related experiment will be carried2 out during thewhose ToV transmission by a spectrumFrench is- dominatedled grou atp short with wavelengths which or the Hartley UV band of O3,dependingontheassumedpar- by Rayleigh scattering from N2 and the broad Chappuis band of ticle size distribution. The maximum amplitude of this effect we are collaborating. They will obtain observationsO3,asmodelledby of the moonEhrenreich by etthe al. ( 2006HST). This to firstdetect basic mod- (between UV and IR) is 6to8ppmformode-1andmodes- ∼ elling has been refined in the case of the Earth by Kaltenegger & these1+ spectra2hazemodels,respectively.Intheinfrared,themostno-l signatures in the reflected solar light.Traub (2009 Such), whose observations spectrum is reproduced are crucial in Fig. 2c. to ticeable feature is the ν3 vibrational band of CO2 at 4.3 µm In an atmosphere where Rayleigh scattering is the dom- help (test15 ppm). and For validate a real exoplanet the transit,sensitive these absorption detection∼ would techniques. A successful series of test ∼ inant diffusion process, it is fairly straightforward to assume observations of the lunar regions was made by that,HST eventhough in January there is 2012 no spectral. identification from spec- 2 http://amop.space.swri.edu/ tral lines, the main carrier of the Rayleigh scattering is the most 3 http://www.cfa.harvard.edu/amp/ampdata/ abundant atmospheric gas. That would be H2 for a giant ex- 4 http://www-atm.physics.ox.ac.uk/code/mie/index.html oplanet (Lecavelier des Etangs et al. 2008b)orN2 and CO2,

L2, page 3 of 6 4

Figure 4: Map of worldwide visibility of 2012 transit Transit Circumstances : The transit of Venus as seen from Sacramento Peak will begin shortly after 16:00 local time (22:05 UT), with the Sun at an elevation of 47° (at an airmass of 1.35). Venus’s will be 57.8” and will be moving relative to the Sun at a rate of 0.0667 arcseconds/s (geocentric). First contact will occur on the northeast limb of the Sun. The entire ingress, the time between first and second contact, will last 18 minutes. Approximately one hour after second contact, the Sun’s elevation will have dropped to 30º. Greatest transit will occur at 01:27 UT, approximately three hours after 2nd contact, with the Sun at an elevation of seven degrees. Finally, the Sun will set at 02:08 UT (8 PM local time), four hours after the first contact. Local Circumstances

Sac Peak I = 22:05:44 II = 22:23:13 G = 01:25:35 Sunset = 02:08 Kitt Peak I = 22:06:09 II = 22:23:38 G = 01:25:31 Sunset = 02:29

Figure 3: The geocentric transit ephemeris prepared by Fred Espenak. Venus's elevation at different times during transit are marked in blue. 5

Transit Observations As the Venus moves onto the solar disk at the start of a transit, the portion of the planet protruding above the limb is encircled by a bright ring, an effect often reported as far back as the 1761 transit by Lomonsov2. After the first digital recordings of the aureole at the 2004 transit (Pasachoff, Schneider, and Widemann, 2011), Tanga et al. (2012) performed a quantitative examination of the spatial structuring of the aureole. They found that refraction is the predominant cause of aureole, with the background solar illumination bent through the mesosphere of Venus. The scale height and altitude are parameters that can be adjusted to reproduce the observed intensity profile of the arc. The transit observations thus serve as a useful probe of the upper atmosphere of Venus, allowing determination of the radial temperature profile as well as latitudinal and temporal variations of the atmosphere. However, the data obtained in 2004 are not sufficient to fully explore the presence and latitudinal distribution of aerosols in the upper atmosphere. At second contact, the disk of Venus is internally tangential to the solar limb and the “black-drop” effect is seen (Schneider et al. 2004, Pasachoff et al., 2005.). When the planet is on the disk it is fully illuminated from behind by the Sun. Below heights of approximately 60 km, the atmosphere of Venus is fully opaque at almost all wavelengths. However, as the solar radiation passes through the optically thin upper atmosphere of Venus it is selectively absorbed by the molecular species transitions. These absorption profiles encode information on the physical conditions in the atmosphere, including density, bulk flows, and unresolved “turbulent” broadening. The transmission spectra of Venus was observed in 2004 at the VTT on Tenerife using the Tenerife Infrared Polarimeter (TIP) in the infrared near 1600 nm, as discussed in Hedelt et al. (2011). They were able to isolate several CO2 lines above the disk of Venus. They used LTE radiative transfer calculations to generate model spectra, which were then compared to the observed spectra to find the best–fit parameters Figure 5: The aureole of for the temperature and radial Venus observed with the Swedish 1-m Solar distribution of CO2 in the Cytherean (SST) in atmosphere. While observations were 2004. The contrast of the obtained with the slit in a variety of region of the aureole has positions with respect to the planet, all been enhanced by a these data were summed in order to factor of nine. get sufficient signal to noise. This however results in the loss of all information about latitudinal and temporal variations in the atmosphere.

2Pasachoff and Sheehan (2012, in press) conclude that the effect was not, in fact, observed by M. Lomonosov at the St. Petersberg Observatory because his reports do not appear to match the timing of the 2004 observations. 6

Instead at this transit we propose acquiring imaging spectroscopy data with IBIS that will allow us to (a) study atmospheric dynamics at small spatial and temporal scales (the 0.1” pixel scale of IBIS corresponds to 21 km on Venus); and (b) compare the characteristics of the equatorial and polar regions, valuable in interpreting the measurements made in situ by Venus Express. These data will be analyzed in the same manner as Heldelt et al. (2011) to infer information on the temperature, density, and bulk velocities as a function of latitude. The observation of the atmospheric profiles around the full circumference of the planet will also provide valuable in validating models of transiting planets, as discussed in Ehrenreich et al. (2012).

Observational Program – Sacramento Peak

Our primary goal is to obtain observations of Venus’s narrow atmospheric layer surrounding full planetary disk. We will isolate this region both spatially and spectrally. We intend to employ three different instruments multiplexed over separate spectral regimes in order to obtain as complete a dataset as possible. The three instruments are:

Instrument Spectral Range Image Scale Notes ROSA 3800, 3966, 4300, 4860 Å 0.08 arcsec/pixel FWHM 10–100 Å IBIS narrowband 5896, 6563, 7820, 8542 Å 0.1 arcsec/pixel R ~2–300,000 IBIS whitelight 6200 Å 0.1 arcsec/pixel FWHM 100 Å f/36 mode, 15000 – 16000 Å FIRS 0.3 arcsec/pixel 75” slit !" ~ 60 Å R ~200,000 The observations will be divided into two phases. The initial period will be that between first and second contact when the aureole of Venus is visible, going through to the brief period following second contact when the “black-drop” effect is visible. Once Venus is well inside the solar limb, observations dedicated to transmission spectroscopy can be obtained. Aureole In order to address the question of the presence of aerosols in the upper atmosphere of Venus, we propose to perform multi–wavelength observations with ROSA through four different filters ranging from the near-UV to green. These will be used to look for the wavelength dependence of the scattering (as compared to the solar spectrum) that would be the signature of Cytherian atmospheric aerosol. The surface brightness of the aureole increases as a greater fraction of the disk of Venus moves inside the solar limb (Tanga et al., 2012). Around first contact, this surface brightness can be anywhere from 10 to 100 times fainter than the intensity of the solar disk. Since the thickness of the refracting layer is close to our spatial resolution, the most important way to increase the detectability of the aureole arc is to maintain a high 7 spatial resolution, in particular through the use of short exposures and adaptive optics/tip-tilt correction3. ROSA will be run at a high frame rate in order to collect the multiple images needed for the application of post-facto image reconstruction. These techniques, in particular speckle reconstruction, have been shown to be photometrically correct and to work even on faint structures above the solar limb; they will be applied to these data in order to produce images with a maximum spatial resolution of 0.15 arcseconds, corresponding to 32 km at Venus. The pixel scale of the images (~0.075 arcsec/pixel) is constrained by the need to fit the entire 60 arcsecond diameter disk of Venus, framed by a sufficient portion of the solar surface, on the 1000 x 1000 pixel ROSA detectors. For this reason we are also looking into bringing an additional large-format (2k x 2k), rapid-readout (50 fps) camera to complement the ROSA system. The ROSA observations will continue through second contact to get a continuous series of observations for further studies of the black-drop effect (indicatively about 8 minutes past first contact, when Venus will be one planetary radius away from the limb). We will also obtain several sets of observations on the disk as well in order to look for any signature of the aureole enhancement around the disk during these later phases. Figure 5: Subfield of an image obtained at the Dutch Open Telescope during the 2004 transit (see Tanga et al. 2011). The width and altitude of the aureole (or Lomonosov arc) can be measured directly with an accuracy of 30 km or better.

While high-resolution images were obtained from the solar in the Canaries and from TRACE in 2004, no spectroscopy of the aureole has yet been performed. The imaging spectroscopy capabilities of IBIS are well suited to rapidly recording the scattered spectral profiles in the arc. The modification of the profiles after passing through atmosphere of Venus could provide additional information higher levels of the atmosphere than accessible with transmission spectroscopy later in the transit. We propose to use IBIS to perform high-spatial-resolution imaging spectroscopy of the full aureole arc in several different spectral lines. These will include the solar Na D1 5896, Hα 6563, and Fe I 7090 Å lines, as well as the CO2 7822 Å lines formed directly in the Cytherean atmosphere (see below). This latter may reveal interesting features due the integration over a long refracted path length through the atmosphere. We will

3 The ability to use the adaptive optics system during the ingress phase needs to be further evaluated. More likely, only a tip tilt correction, using the on-disk edge of Venus or the limb of the Sun, will be possible. 8

Figure 6: The calculated transmission spectrum of the atmosphere Venus in the 782.2 nm CO2 band during transit (purple). The spectrum is the combination of the solar, Cytherean, and terrestrial spectrum convolved with the 35 mÅ FWHM bandpass of IBIS. The solar atlas spectrum (red) and terrestrial absorption spectrum (green) show almost no features in this spectral range. The yellow shows the theoretical passband of the 782.3 nm prefilter. evaluate whether the IBIS observations of the off–limb arc should be acquired in the single Fabry-Perot (FPI) mode to increase throughput and allow short (5-10 msec) exposures. Once Venus is mostly or fully on the disk, the narrow FPI could be quickly reinserted into the beam and the observations continue.

Transmission Spectroscopy: Once Venus is well inside the limb (approximately 15 minutes after 2nd contact), we will perform spectroscopy of the atmosphere seen in absorption. Here we will use several CO2 lines inside one of the molecular bands present in the transmission spectrum. The use of the molecular lines has the significant advantage that all the CO2 absorption can be ascribed to the atmosphere of Venus, with no confusion from corresponding solar lines (as in the , when the sodium absorption from the exosphere lies in the wing of the much stronger solar sodium absorption).

We have identified a sequence of CO2 lines at 7822 Å in a clean region of the solar spectrum with few solar or telluric lines to conflate the transit signal. A dedicated filter is being ordered from Andover Corporation for this spectral region. We will perform repeated spectral scans of several of the CO2 lines in this range. We expect to be able to achieve a velocity resolution on the order 0.1 km/sec at all azimuthal angles around the planet’s back-lit limb. We will perform two different sets of observations, one optimized to high temporal resolution, with repeated scans through a single CO2 line with only a single image at each wavelength postion. Another set will be acquired with approximately 10 images per wavelength position. With this latter set, image 9 reconstruction techniques can be applied to the narrowband data to assure the highest spatial resolution (at the expense of the temporal cadence).

We will also use FIRS to observe the CO2 lines in the infrared region. This observation is comparable to the observations performed at the VTT during the 2004 transit and analyzed by Hedelt et al. (2011). Since the multiple slits of FIRS (with a separation of ~43” separation) will not efficiently sample the disk of Venus, we will instead remove the DWDM4 filter and operate in the single slit mode. This will allow us to achieve a larger spectral coverage, approximately 50 Å with a 0.05 Å sampling, which is important in detecting as many CO2 lines to model as possible. In the spatial direction the slit will cover 75 arcseconds with a spatial sampling of 0.08 arcsec/pixel.

Coordinated Observations: The proposed observations are already being directly coordinated with the observations from several other solar telescopes, including:

McMath–Pierce / NAC SOLIS / ISS SOLIS / VSM SOLIS / FDP Mees Solar Observatory Hinode / SOT Hinode / XRT SDO / AIA SDO / HMI Yunnan Observatory, China

This broad set of complementary observations will be assembled into a cohesive dataset for analysis many different groups. We making the DST data freely available for other researchers.

This work is funded by a National Geographic Society, Committee for Research and Exploration Grant, a NASA/AAS Small Research Grant (for purchase of the CO2 filter, and a NSF grant for the analysis of the acquired data at Williams College.

4 Dense wavelength division multiplexing 10

References

Drossart, P.; Piccioni, G.; Gérard, J. C.; Lopez-Valverde, M. A, 2007, “A dynamic upper atmosphere of Venus as revealed by VIRTIS on Venus Express,” Nature, 450, 7170, 641-645.

Ehrenreich, D.; Vidal-Madjar, A.; Widemann, T.; Gronoff, G.; Tanga, P.; Barthélemy, M.; Lilensten, J.; Lecavelier Des Etangs, A.; Arnold, L., 2012, “Transmission spectrum of Venus as a transiting exoplanet”, Astron. & Astrophys., 537, id.L2

Hedelt, P.; Alonso, R.; Brown, T.; Collados Vera, M.; Rauer, H.; Schleicher, H.; Schmidt, W.; Schreier, F.; Titz, R., 2011, “Venus transit 2004: Illustrating the capability of exoplanet transmission spectroscopy“, Astron. & Astrophys., 533, id.A136.

Pasachoff, Jay M., Glenn Schneider, and Leon Golub, 2005, "The black-drop effect explained," in Transits of Venus: New Views of the Solar System and Galaxy, IAU Colloquium No. 196 (U.K., 2004), D. W. Kurtz and G. E. Bromage, eds., 242-253.

Pasachoff, Jay M., Glenn Schneider, and Thomas Widemann, 2011, "High-resolution Satellite Imaging of the and Asymmetries in the Cytherean Atmosphere," Astron. J., 141, #3, 112-120 (March). doi: 10.1088/0004- 6256/141/4/112. Cover article and image: http://ej.iop.org/pdf/aj/covers/141/AJ_141_4_cover.pdf

Schneider, G., J. M. Pasachoff, and L. Golub 2004, "TRACE Observations of the 15 November 1999 Transit of Mercury and the : Considerations for the 2004 Transit of Venus," Icarus 168, 249-256.

Schneider, G., J. M. Pasachoff, and Richard C. Willson, 2006, "The Effect of the Transit of Venus on ACRIM's Total Solar Irradiance Measurements: Implications for Transit Studies of Extrasolar Planets," Astrophys. J. 641, 565-571.

Svedhem, H.; Titov, D. V.; Taylor, F. W.; Witasse, O., 2007, “Venus as a more Earth-like planet,“ Nature, 450, 7170, 629-632.

Tanga, P., T. Widemann, B. Sicardy, J. M. Pasachoff, J. Arnaud, L. Comolli, A. Rondi, S. Rondi, and P. Suetterlin, 2012, "Sunlight refraction in the mesosphere of Venus during the transit on June 8th, 2004," Icarus 218, 207-219, March. http://dx.doi.org/10.1016/j.icarus.2011.12.004; http://arxiv.org/abs/1112.3136 http://www.sciencedirect.com/science/article/pii/S0019103511004696