Terrestrial Planet Optical Phase Curves. I. Direct Measurements of the Earth

NASA’s EPOXI mission used the Deep Impact spacecraft to observe the disk-integrated Earth as an analog to terrestial exoplanets’ appearance. The mission took five 24 hr observations in 2008–2009 at various phase angles (57.°7–86.°4) and ranges (0.11–0.34 au), of which three equatorial (E1, E4, E5) and two polar (P1, North and P2, South). The visible data taken by the HRIV instrument ranges from 0.3 to 1.0 μm, taken trough seven spectral filters that have spectral widths of about 100 nm, and which are centered about 100 nm apart, from 350 to 950 nm. The disk-integrated, 24 hr averaged signal is used in a phase angle analysis. A Lambertian-reflecting, spherical planet model is used to estimate geometric albedo for every observation and wavelength. The geometric albedos range from 0.143 (E1, 950 nm) to 0.353 (P2, 350 nm) and show wavelength dependence. The equatorial observations have similar values, while the polar observations have higher values due to the ice in view. Therefore, equatorial observations can be predicted for other phase angles, but (Earth-like) polar views (with ice) would be underestimated.

Optical and Infrared Phase Curves of the Lava Planet K2-141 b

<p>K2-141 b is a transiting, small (1.5 RE) Ultra-Short-Period (USP) planet orbiting its star every 6.7 hours discovered by the Kepler space telescope. The planet’s high surface temperature of more than 2000 K makes it an excellent target for atmospheric studies by the observation of its thermal emission. We present 65 hours of continuous photometric observations of K2-141 b collected with Spitzer’s IRAC Channel 2 at 4.5 microns spanning 10 full phases of the orbit. Our best fit model of the Spitzer data shows no significant offset of the thermal hotspot and is inconsistent with the observed offset of the well-studied USP planet 55 Cnc e at a 3.7 sigma level. We measure an eclipse depth of 142 +/- 40 ppm and an amplitude variation of 120 +/- 40 ppm in the infrared. The joint analysis of the observations collected in the two photometric bands favors a non-zero geometric albedo with Ag = 0.26 +/- 0.07 and a tentative temperature gradient. With a dayside temperature of 2141 -361 +352 K and a night-side temperature of 1077 -623 +473 K we also find no evidence of heat redistribution on the planet. We compare the observations to a 1D rock vapor model and a 1D circulation toy model and argue that the data are best explained by a thin rock vapor atmosphere with a thermal inversion.</p>

The Umov effect in cosmic dust analogue fluffy aggregates

ABSTRACT We investigate the effect of porosity in the Umov effect for the first time using the aggregate dust model. The Umov effect is an inverse correlation between the reflectivity (or geometric albedo) of an object and the degree of linear polarization of light scattered by it. Three different types of fractal aggregates: ballistic agglomeration (BA), ballistic agglomeration with one migration (BAM1), and ballistic agglomeration with two migrations (BAM2) having porosities 0.87, 0.74, and 0.64, respectively (which have the same characteristic radius ∼1 μm), are considered in our simulations. Using the multisphere T-matrix (mstm) code, maximum positive polarization (Pmax) and geometric albedo (A) are calculated for three different fractal aggregated structures considering amorphous silicate composition. Then Pmax and A are plotted against each other in logarithmic scale that shows a linear inverse correlation and a strong porosity dependence. This study shows that the porosity of the aggregates plays a crucial role in the Umov-law diagram. Further, we explore the effect of aggregate size parameter and the effect of composition in the Umov diagram for particles larger than the wavelength of incident radiation. A systematic study is presented in this paper.

Closed-formed solutions of geometric albedos and phase curves of exoplanets for any reflection law

The albedo of a celestial body is the frac-tion of light reflected by it. Studying the albe-dos of the planets and moons of the Solar Sys-tem dates back at least a century [1, 2, 3, 4, 5]. Of particular interest is the relationship between the albedo measured at superior conjunction (full phase), known as the “geometric albedo”, and the albedo considered over all phase angles, known as the “spherical albedo” [2, 6, 7]. Modern astronom-ical facilities enable the measurement of geomet-ric albedos from visible/optical secondary eclipses [8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20] and the inference of the Bond albedo (spherical albedo measured over all wavelengths) from in-frared phase curves [21, 22, 23, 24, 25] of transit-ing exoplanets. Determining the relationship be-tween the geometric and spherical or Bond albe-dos usually involves complex numerical calculations [26, 27, 28, 29, 30, 31, 32] and closed-form solu-tions are restricted to simple reflection laws [33, 34]. Here we report the discovery of closed-form solu-tions for the geometric albedo and integral phase function that apply to any law of reflection. The integral phase function is used to obtain the phase integral, which is the ratio of the spherical to the geometric albedos. The generality of the solu-tions stems from a judicious choice of the coor-dinate system in which to perform different parts of the derivation. The closed-formed solutions have profound implications for interpreting obser-vations. The shape of a reflected light phase curve and the secondary eclipse depth may now be self-consistently inverted to retrieve fundamental phys-ical parameters (single-scattering albedo, scatter-ing asymmetry factor, cloud cover). Fully-Bayesian phase curve inversions for reflectance maps and si-multaneous light curve detrending may now be per-formed, without the need for binning in time, due to the efficiency of computation. We demonstrate these innovations for the hot Jupiter Kepler-7b, inferring a revised geometric albedo of 0.12 ± 0.02, a Bond albedo of 0.18 ± 0.03 and a phase integral of 1.5 ± 0.1, which is consistent with isotropic scatter-ing. The scattering asymmetry factor is 0.04±0.15, implying that the aerosols are small compared to the wavelengths probed by the Kepler space tele-scope. In the near future, one may use the closed-form solutions discovered here to extract funda-mental parameters, across wavelength, from multi-wavelength phase curves of both gas-giant and ter-restrial exoplanets measured by the James Webb Space Telescope.

The 2017 May 20th stellar occultation by the elongated centaur (95626) 2002 GZ32

We predicted a stellar occultation of the bright star Gaia DR1 4332852996360346368 (UCAC4 385-75921) (mV= 14.0 mag) by the centaur 2002 GZ32 for 2017 May 20th. Our latest shadow path prediction was favourable to a large region in Europe. Observations were arranged in a broad region inside the nominal shadow path. Series of images were obtained with 29 telescopes throughout Europe and from six of them (five in Spain and one in Greece) we detected the occultation. This is the fourth centaur, besides Chariklo, Chiron and Bienor, for which a multi-chord stellar occultation is reported. By means of an elliptical fit to the occultation chords we obtained the limb of 2002 GZ32 during the occultation, resulting in an ellipse with axes of 305 ± 17 km × 146 ± 8 km. From this limb, thanks to a rotational light curve obtained shortly after the occultation, we derived the geometric albedo of 2002 GZ32 (pV = 0.043 ± 0.007) and a 3-D ellipsoidal shape with axes 366 km × 306 km × 120 km. This shape is not fully consistent with a homogeneous body in hydrostatic equilibrium for the known rotation period of 2002 GZ32. The size (albedo) obtained from the occultation is respectively smaller (greater) than that derived from the radiometric technique but compatible within error bars. No rings or debris around 2002 GZ32 were detected from the occultation, but narrow and thin rings cannot be discarded.

Component properties and mutual orbit of binary main-belt comet 288P/(300163) 2006 VW139

Context. The binary asteroid 288P/(300163) is unusual both for its combination of wide-separation and high mass ratio and for its comet-like activity. It is not currently known whether there is a causal connection between the activity and the unusual orbit or if instead the activity helped to overcome a strong detection bias against such sub-arcsecond systems. Aims. We aim to find observational constraints discriminating between possible formation scenarios and to characterise the physical properties of the system components. Methods. We measured the component separation and brightness using point spread function fitting to high-resolution Hubble Space Telescope/Wide Field Camera 3 images from 25 epochs between 2011 and 2020. We constrained component sizes and shapes from the photometry, and we fitted a Keplerian orbit to the separation as a function of time. Results. Approximating the components A and B as prolate spheroids with semi-axis lengths a < b and assuming a geometric albedo of 0.07, we find aA ≤ 0.6 km, bA ≥ 1.4 km, aB ≤ 0.5 km, and bB ≥ 0.8 km. We find indications that the dust production may have concentrated around B and that the mutual orbital period may have changed by 1–2 days during the 2016 perihelion passage. Orbit solutions have semi-major axes in the range of (105–109) km, eccentricities between 0.41 and 0.51, and periods of (117.3–117.5) days pre-perihelion and (118.5–119.5) days post-perihelion, corresponding to system masses in the range of (6.67–7.23) × 1012 kg. The mutual and heliocentric orbit planes are roughly aligned. Conclusions. Based on the orbit alignment, we infer that spin-up of the precursor by the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect led to the formation of the binary system. We disfavour (but cannot exclude) a scenario of very recent formation where activity was directly triggered by the break-up, because our data support a scenario with a single active component.

A multi-chord stellar occultation by the large trans-Neptunian object (174567) Varda

Context. We present results from the first recorded stellar occultation by the large trans-Neptunian object (174567) Varda that was observed on September 10, 2018. Varda belongs to the high-inclination dynamically excited population, and has a satellite, Ilmarë, which is half the size of Varda. Aims. We determine the size and albedo of Varda and constrain its 3D shape and density. Methods. Thirteen different sites in the USA monitored the event, five of which detected an occultation by the main body. A best-fitting ellipse to the occultation chords provides the instantaneous limb of the body, from which the geometric albedo is computed. The size and shape of Varda are evaluated, and its bulk density is constrained using Varda’s mass as is known from previous works. Results. The best-fitting elliptical limb has semi-major (equatorial) axis of (383 ± 3) km and an apparent oblateness of 0.066 ± 0.047, corresponding to an apparent area-equivalent radius R′equiv = (370±7) km and geometric albedo pv = 0.099 ± 0.002 assuming a visual absolute magnitude HV = 3.81 ± 0.01. Using three possible rotational periods for the body (4.76, 5.91, and 7.87 h), we derive corresponding MacLaurin solutions. Furthermore, given the low-amplitude (0.06 ± 0.01) mag of the single-peaked rotational light-curve for the aforementioned periods, we consider the double periods. For the 5.91 h period (the most probable) and its double (11.82 h), we find bulk densities and true oblateness of ρ = (1.78 ± 0.06) g cm−3, ɛ = 0.235 ± 0.050, and ρ = (1.23 ± 0.04) g cm−3, ɛ = 0.080 ± 0.049. However, it must be noted that the other solutions cannot be excluded just yet.

138175 (2000 EE104) and the Source of Interplanetary Field Enhancements

&lt;p&gt;We present the first optical observations taken to characterize the near-Earth object 138175 (2000 EE104). &amp;#160;This body is associated with Interplanetary Field Enhancements (IFEs), thought to be caused by interactions between the solar wind magnetic field and solid material trailing in the orbit of the parent body. &amp;#160;Based on optical photometry, the radius (in meters) and mass (in kilograms) of an equal-area sphere are found to be&amp;#160; 250(0.1/p)^{1/2} and&amp;#160; 1e11(0.1/p)^{3/2}, respectively, where p is the red geometric albedo and density 1500 kg/m3 is assumed. &amp;#160;The measured colors are intermediate between those of C-type (primitive) and S-type (metamorphosed) asteroids but, with correction for the likely effects of phase-reddening, are more consistent with a C-type classification than with S-type. No evidence for co-moving companions larger than 40(0.1/p) meter in radius is found, and no dust particle trail is detected, setting a limit to the trail optical depth &lt; 2e-9. &amp;#160;Consideration of the size distribution &amp;#160;produced by impact pulverization &amp;#160;makes it difficult to generate the &amp;#160;mass of nanodust (minimum 1e5 kg to 1e6 kg) required to account for IFEs, unless the size distribution is unusually steep. &amp;#160;While the new optical data do not definitively refute the hypothesis that boulder pulverization is the source of IFEs, neither do they provide any support for it.&lt;/p&gt; &lt;p&gt;Journal: Planetary Science Journal, submitted&lt;/p&gt;