Venus nightside: the dark hemisphere at 1 micron and visible wavelengths with amateur equipment

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1 Venus nightside: the dark hemisphere at 1 micron and visible wavelengths with amateur equipment D. Gasparri ¹ ¹ Università di Bologna, Dipartimento di Astronomia, Via Ranzani 1, Bologna, Italy Abstract I present an imaging work on the Venus nightside, focused at the wavelength of 1 micron. Using an amateur equipment and a science grade (amateur) 16 bit CCD camera, I followed, at resolution of 2,6, the nightside of the planet for one week. The images were processed and analysed to show the correlation between the visible features and the radar data taken from the Magellan spacecraft. The rotational period, contrast and surface magnitude have been analysed, as well as the photometric change on the brightness due to the presence and motion of different optical depth lower clouds layers. An image at visible wavelengths is presented and could be a strong evidence for the Ashen light. 1. Introduction The surface of Venus is hidden by a thick atmosphere which preclude any observation at optical wavelengths, revealed only with radar images taken by spacecrafts (Pioneer, Magellan, Venus Express). On the nightside of the planet, the high and uniform temperature ( 750 K) lets the surface to emit near infrared radiation, peaked at λ Peak 3. 9 microns, with a Planck blackbody trend. The synthetic spectra of the nightside (Lecacheux & al. 1993) and the measured radiance from the Cassini spacecraft (Baines & al. 2000) quantified a strong transparency peak at 1,01 micron. This transparency window on the CO 2 atmosphere allows the thermal radiation coming from the surface to escape without dramatic contamination caused by the thick cloud layer (Carlson 1991; Lecacheux & al. 1993; Meadows & Crisp 1996; Baines & al. 2000; Pellier 2004; Shiga 2004). Lecacheux & al. (1993), at a Pic du midi telescope, showed a good correlation between the features visible and the altimetric map of Venus surface taken by the Magellan. Thanks to a steep temperature gradient of nearly 8 /km (Carlson & al. 1993), and the slope of the blackbody Planck function on the Rayleigh range, this window is particularly sensitive to temperature variation. Comparing two or more images taken in consecutive days, it is possible to show the photometric evolution of the lower clouds layers, which are not completely transparent to the thermal radiation coming from the surface. The emission of the nightside at visible wavelengths is well known as a faint airglow produced by the recombination of the oxygen atoms (Slanger & al ), while the brighter Ashen light is more elusive and controversial (Russel & Philips, 1990). I imaged the planet s dark hemisphere for a week, from March 12 th to March 18 th 2009 from my backyard observatory near Perugia, Italy (lat = , long = ); here are some results of this observative campaign at 1 micron wavelength. The observation at visible light is presented later and separately in this paper. 2. Equipment The observations of the nighside thermal emission were taken with a completely amateur equipment: a SC 235mm f10 telescope, a CCD microlens camera SBIG ST-7XME, without antiblooming gate, and a Schott RG 1000 band pass filter. Using a CCD camera cooled, with linear response and a 16 bit dynamic (FWC 10 5 e ) is fundamental to have enough S/N and dynamical range to show the features, which have an apparent contrast less than 1% at the imaging conditions (background luminosity, dayside brightness and sampling). This could be the main reason for the very low S/N features imaged with bit amateur devices (Pellier 2004). The convolution of the spectral sensitivity of the CCD with the filter gave a nearly Gaussian instrumental transmission (Fig.1) with FWHM 120 nm, peaked at λ Peak 965nm. Pellier (2004) was the first amateur to show, with similar instrumental transmission, the thermal radiation of the nightside and some low contrast, not precisely identified, features. 1

2 Fig.1 The red line is the transmission of the RG1000 filter, the yellow the QE of the Kaf-0402 CCD and the blue line is the effective instrumental transmission of my setup 3. Data acquisition The purpose of this imaging campaign was to acquire good quality (high resolution and high S/N) images of the Venus nightside, at 1 micron, during some days, to show the nature and properties of the details visible and the amateur equipment potential. Each session started 10 minutes after the sunset and finished when the planet lowered under 5 of altitude. The CCD device was set to a temperature of 5.0 ± 0. 2 C ; dark frames and flat field were acquired in every session. The focal length (2350mm) and the pixels size of the sensor ( 9 µ m ) gave a theoretical sampling of 0,79 /pixel; the measured sampling on the images was ( 0.86 ± 0.01)arc sec/ pixel. The theoretical resolution, according to Rayleigh, is about 1 (250 Km at current Venus distance from Earth). Exposure time was set and regulated according to the background brightness; the maximum exposure was 10 seconds, perfect compromise between the signal of the nightside and the increasing luminosity of the thin crescent, that caused a large amount of blooming. For any day I took 150 to 300 single frames, calibrated and averaged at last into 2 groups, to create two raw images (redundancy) and compare them after the processing step, to avoid any type of processing artifact. Despite a very low peak effective QE 0,003 (Fig.1), the thermal emission was obvious with just 2 seconds exposure. Actually, the main problem is the glow of the background, which set a limit to the effective window at 40 minutes after the sunset (if the inclination of the ecliptic is propitious). The background emission is too high and hide completely the thermal emission when the Sun is above the horizon, at any elongation. 4. Data processing Every single frame was calibrated with master dark frame (median of 15 single darks) and master flat field (average of 30 frames). The raw images obtained by averaging at least 60 frames of each observative session were processed with different software and filters. The best combination was achieved with a two step processing: 1) Unsharp mask (UM) with large radius (about 4 pixels) 2) Deconvolution (Lucy-Richardson, synthetic PSF with radius of pixels, 10 to 50 iterations). The same features are present regardless the processing step and the software used. Once the verification of the details gave a perfect matching between the sets taken in the same day, I averaged them to built the final processed image. The smallest spots visible have size of about 2,6, that is a real resolution of 650 Km, against a theoretical value of 1 (250 Km) and a sampling value of 1,7. The atmospheric turbulence at these wavelengths does not affect the image quality as much as at visible light, despite an average exposure of 8 seconds and an altitude smaller than 20. 2

3 5. Data interpretation In the (Fig.2) there are all the final images of the campaign. On the left the raws, at the centre the images processed with UMs and at right after a deconvolution. The orientation, due to the presence of the blooming, was adjusted in the acquisition stage to have the line of the cusps nearly perpendicular to the major side of the CCD, and changes as Venus approaches the Sun, but it is easy to define by knowing the orientation of the thin crescent and the path of the planet during the time. March 12 March 13 March 14 March 16 3

4 March 17 March 18 Fig.2: the final images of the imaging campaign. On the left, the raw, unprocessed, at the centre after an UM processing, at right after a deconvolution. The maximum brightness contrast of the nightside against the scattered light background is 9% In any image we can notice two types of features: 1) The dark spots at low scale seem to do not change significantly with time and have sizes and shapes well defined. They belong easily to the surface. 2) Those at larger scale are fainter and look to change totally from one day to another and sometimes modify the shape and contrast of the lower scale features. Not always these details can be easily noticed, as shown in the images taken on March 16 and 17. According to Carlson 1991; Lecacheux & al. 1993; Meadows & Crisp 1996; Baines & al. 2000; Pellier 2004; Shiga 2004, the features visible on the nightside at those wavelengths are a mix of topography and lower clouds contribution. I tried to measure the contrast between bright and dark features. An evaluation can be done only after the correction for the background and the scattered light of the dayside. From the photometrical profile (Fig.3), it is easy to see that the trend of the scattered light along the nightside have a nearly exponential shape, decreasing rapidly near the limb. The isophotes are also nearly parallel to the line of the cusps. The limb darkening seems to be negligible, as confirmed by Lecacheux & al Accounting for these observations, we can simplify the correction step. For the measurement, I selected two features near the limb: a dark spot located at latitude +29.0, longitude 285.0, identified as the Beta Regio, and the bright spot located between Beta Regio itself and Phoebe Regio, at lat , long. 270 (System I). I measured the mean background value just outside the nightside limb and subtract it to the image. With this analysis we have an easy correction and relatively small errors introduced. The measurement of the specific brightness of these regions, on the images taken on March , gave a contrast: 0.20 < C < 0. 30, in agreement with Meadows & al

5 Fig.3: above on the left the brightness profile along the latitude of Beta Regio. On the right the profile of the comparison region for the contrast estimation. The average slope is nearly exponential, and for values near the limb the trend is the same. No evidence for significant limb darkening can be seen. On the left a 3D brightness profile. In March 12 image, in the same field of view of Venus, there is a star (HD3884 m v = 7. 70, G0V), useful to estimate the surface brightness of the nightside. Considering the non photometric conditions (scattered light from the dayside and the instrumental transmission at 1 micron that do not match with any standard photometric filter) we can just have a rough approximation. The magnitude difference between the star and the specific pixel emission of the nighside give a value of mag IR = 3.8 ± The surface magnitude of the nightside can be obtained by correcting for the area difference of times between the sampling ( 0.86 ± 0.01)"/ pixel and the unitary surface (1 ) then by estimating the magnitude of the star with this instrumental transmission. The estimated color index is an average value between the V-I and V-J colors; the value found is just an approximation and introduces some large errors. Considering an approximated color index CI Star = 0.80 ± 0. 20, and a magnitude correction of 2 mag Area = 0.32 ± 0.02, the surface brigthness is found to be: 12.0 ± 0.3mag / arc sec, 10 magnitudes lower than the dayside, in agreement with Lecacheux & al In March 13 image (Fig. 4, right), the region at lat +11.0, long 270 appear darker, and the brightness difference with the Beta Regio decreases to nearly zero. This dark, filiform feature is due to a layer of clouds at greater optical depth than the rest of the disk, moving much faster than the solid body. Similar filiform dark features also appear in the march 12 image (Fig. 4, left), at greater longitude, but data are not enough to understand if one of them is the same cloud imaged the following day. 5

6 Fig.4 Features difference between the 12 March image (left) and 13 March (right) To show and erase the clouds contribution, professional astronomers usually take images at longer wavelengths, to show only the contribution of the lower clouds, and subtract them to the 1 micron images. Unfortunately, the silicon based devices do not allow to take images at wavelengths longer than 1 micron. The amateurs do not have the necessary equipment to take images at longer wavelengths, so it is not possible to apply this correction. The technique adopted is the following. The clouds lower layers and the surface have rotational periods difference of at least one magnitude, then in two or three consecutive images we expect to see a very slow rotation of the features coming from the surface and a fast rotation of the clouds. These are just the two groups of features we have identified above, based on shape and size. By observing carefully the images taken on March , the best of the whole campaign, we can also see nearly the same small scale features, contaminated just a little by some changes in contrast and brightness (around Beta Regio, for instance). Making the median of these three images, we almost delete the variable contribution of the clouds and we have a final picture made nearly of surface details, with a better S/N. An harder processing with deconvolution and UMs gave the final topography image (Fig. 5, right). The same procedure was adopted also to the March images to have a comparison between the features visible (Fig.5, left). A comparison with the Magellan spacecraft radar altimetric image made a nearly perfect matching: the dark features visible are identified with higher regions on the Venus surface, which have lower temperature, then emit less thermal radiation. The surface visible from this campaign has coordinates: 30 < latitude < 60 and 250 < longitude < 330 with C.M. at 329 (average value). To have the final evidence that the main features belong to the surface, we can estimate the rotational period (sidereal). Taking the images of March 12 and 17, and measuring the longitude (System I) of 10 features, two times each, we have an average longitude shift of S = 6.02 ± Taking into account for the difference due to the elongation between March 12 and 17: C = , we have a rotational period of P = 240 ± 6 days. Although this value is just a rough approximation, this is a definitive evidence that the dark spots visible in the nightside belong to the surface of the planet, which has a sidereal period of 243 days. A brief list with latitude and longitude of the main features identified in (Fig. 5) is visible in table 1. 6

7 Fig.5: averaging the (above, left) and (above, right) images I built two final images with better S/N. An harder processing shows all the features, identified with common names. The Magellan altimetric map is shown for comparison. The north pole of the planet is up. According to Hashimoto & al. (2001), some possible short period lava flows can be imaged at relatively low 2 scale (about 25Km ) but in the images there are no visible time-changing hot spots. It is impossible to discriminate the contribution of the clouds on the single image, but we can try to emphasize the photometric change on the nightside between two days, as follows. We take two raw images shot in consecutives days, with similar S/N, so that the rotation of the surface can be neglected at the sampling resolution of 1,7. Normalizing and dividing them, we have a new image that will show only photometric changes, due to the different optical depth of the clouds. Applying this technique to March images, we have the situation below (Fig.6): Venus 17 March divided 16 March 18 March divided 17 March 18 March divided 16 March Fig.6: photometric changes in the nightside during the days. Large structure clouds can be easily seen All the topography details disappeared, as we expected. The images show lager scale brightness gradients, due to the change in the clouds opacity, according to Russo & al. (2006) The filiform patterns seen and described earlier (Fig. 4) are still present (for instance, the thin cloud around Beta Regio in March pictures), but we can also notice greater and fainter brigthness gradients, hidden in the images by the higher contrast of surface features. 7

8 The visible light question From the 12 March observations I have also some frames taken at visible wavelengths with 1 second exposure, with same telescope and CCD camera than 1 micron observations. The exposures were just a test to study the instrumental response to high level of illumination and to manage the damage caused by the blooming to the image. A Schott BG38, with transmission 300 nm < λ < 750nm peaked at λ Peak 550nm, has been used. A later careful examination of 20 frames revealed a faint glow on the nightside of the planet, confirmed visually with the same telescope. The night glow seems to be very sensitive to the background transparency and to the seeing; it also seems to change brightness at time scale of few minutes. The presence of this visible light nightside is not CCD orientation dependent (Fig.7); the shape is symmetrical and matches perfectly the orientation and size of the planet, then it cannot be an image artifact. The instrumental transmission at 700nm is just 0.14 and fall to 0.02 at 800nm, against a peak of 0.50 at 530 nm, out of the thermal emission from the surface, starting at 850nm (Baines & al. 2000): the emission imaged belongs to the visible domain. Fig.7 Three 1 second exposure frames. The visible light glow is visible in every frame and does not depend on the position and orientation of the CCD. Averaging 12 best frames we have the final image (Fig.8, left). The features visible at large scale seems to be real, at least the dark spot located at equatorial latitude and long 280. This detail is present in every frame and do not depend neither on the orientation of the camera nor on the position of Venus along the CCD. A comparison with the image taken at 1 micron (Fig.8) seems to show some similarities with the Phoebe and Beta Regio, but shapes and sizes of the features do not match perfectly the 1 micron, quality reduced, image. It is possible that there is not a direct connection among them; in any case, it would be interesting to know the physical explanation for the spot. At these wavelengths the airglow of the upper atmosphere can be detected (Lawrence & al. 1977; Slanger & Black 1978; Bougher & Borucki 1994; Crisp 2001; Slanger & al ), with no uniform brightness, but it should be much fainter than imaged. Fig.8: the nightside emission at visible wavelengths (left) compared with IR 1 micron image reduced in quality (right) 8

9 Thanks to the star in the field of view (HD3884 m v = ) I tried to estimate the surface brightness in the same way for the 1 micron image. The evaluation is affected by the uncertainty due to the filter used. The magnitude difference, after background correction, is mag V = 4.4 ± Assuming a V magnitude for HD3884 and the same area-correction coefficient of IR image, we have a value 2 of 11.8 ± 0.2mag / arcsec. Could it be the elusive and controversial Ashen light? (Russell and Philips, 1990) Actually, this brightness value could be easily visually seen, but there are some problems for an immediate detection: 1) We need a very dark sky, small crescent, and a good ecliptic geometry, that allow dark sky observation at altitudes greater than 10 2) The altitude of the planet will be never as high as 20, with some considerable extinction, in order of one magnitude. 3) The brightness of the day crescent is very high, times (in flux) the surface brightness of the dayside. We need some high magnification and to put the crescent out of the field of view to have a good chance to see it visually These points could explain the visual observation difficulties and the controversial reports (regardless the nature and behaviour of the radiation incoming). The problems for the imaging are somewhat greater. The very low contrast against the diffuse light of the planet 6%, make the nightside detectable only with high dynamical range, thermal stabilized, CCDs. The image sample is fundamental: an image scale greater than 1 /pixel returns a too small disk and the scattered light hide completely the night glow. An image scale smaller than 0,30 /pixel returns a too big disk, with exposure time increasing, contrast decreasing and grater turbulence damage. Recent observation made by the Venus Express confirmed the presence of lightings (Russell & al. 2006), which could be associated to the Ashen light (Russell and Philips, 1990), but further investigations are needed to discover the nature and properties of this emission. Conclusion The study of Venus at 1 micron, with amateur equipment, for a relatively long time, gives direct information of the surface features, which can be compared directly with radar altimetric images taken by the spacecrafts. This is the unique method to investigate directly the surface of the planet at optical wavelengths. Beyond these well known features, it is possible to show the changes on the clouds absorption. Monitoring the nightside, in phases smaller than 0.35, let the amateur astronomers to map the clouds structure, the surface details, and spot possible volcanic eruptions (Hashimoto & al. 2001). This last point seems to be very interesting, since the discussion about the volcanic activity is still open. The thermal print of a relatively small volcanic eruption, assuming a lava temperature similar to Earth s (1300 K), could generate a thermal specific emission more than 3000 times the average specific emission of the surface. Taking into account the dynamic range of my setup, the real resolution of 2,6, and the imaging 2 conditions, this means that it is possible to spot volcanic eruption as large as 25Km. The ground-based amateur observations are useful to the Venus Express mission, as a part of the ESA Venus observing amateur project, and will provide a global and time lasting coverage of the nightside, a work impossible to carry out by the professional community. The registered emission in the visible light represents an important quantitative observation of the visible glow, or the Ashen light, often reported visually by many observers, but never imaged. Feature Approximate central Latitude Approximate central Longitude Beta regio Asteria regio Phoebe regio Navka planitia Devana chasma Table 1: latitude and longitude of the main features identified in the Venus nigthside (planetocentric, System I), detected at 1 micron wavelength 9

10 References Baines & al. 2000: Detection of Sub-Micron Radiation from the Surface of Venus by Cassini/VMS, Icarus 148 [2000] thermal emission at 850 and 900 nm from the surface. Carlson & al. 1991: Galileo Infrared Imaging Spectroscopy Measurements at Venus, Science 253 [1991 Sep. 27] Hashimoto & al. 2001: Elucidating the Rate of Volcanism on Venus: Detection of Lava Eruptions Using Near-Infrared Observations Icarus 154, Issue 2, December 2001, Pages Lecacheux & al. 1993: Detection of the surface of Venus at 1.0 µm from ground-based observations, Planet. Space Sci. 41 # 7 [1993] Meadows & al. 1992: Groundbased near-ir observations of the surface of Venus, In Lunar and Planetary Inst., Papers Presented to the International Colloquium on Venus p (SEE N ) Meadows & Crisp 1996: Ground-based near-infrared observations of the Venus nightside: The thermal structure and water abundance near the surface, Journal of Geophysical Research 101 E2 [1996] Russell al. 2006: Lightning detection on the Venus Express mission, Planetary and Space Science, Volume 54, Issues 13-14, November 2006, Pages Russell & Philips 1990: the Ashen light: Adv. Space Res., Vol. 10, No. 5, pp. (5)137-(5)141, 1990 Pellier 2004: Thermal Emission on the Venusian Nightside, - website with his own observations from May 2004 Russo & al. 2006: Imaging of the Venus night side with the Venus Monitoring Camera onboard Venus Express, European Planetary Science Congress Berlin, Germany, September 2006., p.428 Shiga 2004: Amateur Images Venus' Surface, skyandtelescope.com/news/article_1266_1.asp (2004 June 2) Slanger & al. 2001: Discovery of the atomic oxygen green line in the Venus night airglow, Science, 2001, vol. 291, no5503, pp Slanger & al. 2005: The Venus nightglow: Ground-based observations and chemical mechanisms: Icarus, Volume 182, Issue 1, p