Spitzer observations of interstellar object 1I/‘Oumuamua

# Spitzer observations of interstellar object 1I/‘Oumuamua

David E. Trilling    Michael Mommert    Joseph L. Hora    Davide Farnocchia    Paul Chodas    Jon Giorgini    Howard A. Smith    Sean Carey    Carey M. Lisse    Michael Werner    Andrew McNeill    Steven R. Chesley    Joshua P. Emery    Giovanni Fazio    Yanga R. Fernandez    Alan Harris    Massimo Marengo    Michael Mueller    Alissa Roegge    Nathan Smith    H. A. Weaver    Karen Meech    Marco Micheli
July 13, 2018
July 13, 2018
July 13, 2018
###### Abstract

1I/‘Oumuamua is the first confirmed interstellar body in our Solar System. Here we report on observations of ‘Oumuamua made with the Spitzer Space Telescope on 2017 November 21–22 (UT). We integrated for 30.2 hours at 4.5 m (IRAC channel 2). We did not detect the object and place an upper limit on the flux of 0.3 Jy (3). This implies an effective spherical diameter less than [98, 140, 440] meters and albedo greater than [0.2, 0.1, 0.01] under the assumption of low, middle, or high thermal beaming parameter , respectively. With an aspect ratio for ‘Oumuamua of 6:1, these results correspond to dimensions of [240:40, 341:57, 1080:180] meters, respectively. We place upper limits on the amount of dust, CO, and CO coming from this object that are lower than previous results; we are unable to constrain the production of other gas species. Both our size and outgassing limits are important because ‘Oumuamua’s trajectory shows non-gravitational accelerations that are sensitive to size and mass and presumably caused by gas emission. We suggest that ‘Oumuamua may have experienced low-level post-perihelion volatile emission that produced a fresh, bright, icy mantle. This model is consistent with the expected value and implied high albedo value for this solution, but, given our strict limits on CO and CO, requires another gas species — probably HO — to explain the observed non-gravitational acceleration. Our results extend the mystery of ‘Oumuamua’s origin and evolution.

comets: individual (1I/‘Oumuamua) — minor planets, asteroids: individual (1I/‘Oumuamua) — planetary systems
\move@AU\move@AF\@affiliation

Department of Physics and Astronomy
PO Box 6010
Northern Arizona University
Flagstaff, AZ 86011 \move@AU\move@AF\@affiliationLowell Observatory
Flagstaff, AZ 86001

Corresponding author: David E. Trilling\move@AU\move@AF\@affiliation

Department of Physics and Astronomy
PO Box 6010
Northern Arizona University
Flagstaff, AZ 86011 \move@AU\move@AF\@affiliationLowell Observatory
Flagstaff, AZ 86001

\move@AU\move@AF\@affiliation

Harvard-Smithsonian Center for Astrophysics
60 Garden St., MS-65
Cambridge, MA 02138

\move@AU\move@AF\@affiliation

Jet Propulsion Laboratory, California Institute of Technology
4800 Oak Grove Drive

\move@AU\move@AF\@affiliation

Jet Propulsion Laboratory, California Institute of Technology
4800 Oak Grove Drive

\move@AU\move@AF\@affiliation

Jet Propulsion Laboratory, California Institute of Technology
4800 Oak Grove Drive

\move@AU\move@AF\@affiliation

Harvard-Smithsonian Center for Astrophysics
60 Garden St., MS-65
Cambridge, MA 02138

\move@AU\move@AF\@affiliation

IPAC, California Institute of Technology
1200 E. California Boulevard

\move@AU\move@AF\@affiliation

The Johns Hopkins University Applied Physics Laboratory
Laurel, MD 20723-6099

\move@AU\move@AF\@affiliation

Jet Propulsion Laboratory, California Institute of Technology
4800 Oak Grove Drive

\move@AU\move@AF\@affiliation

Department of Physics and Astronomy
PO Box 6010
Northern Arizona University
Flagstaff, AZ 86011

\move@AU\move@AF\@affiliation

Jet Propulsion Laboratory, California Institute of Technology
4800 Oak Grove Drive

\move@AU\move@AF\@affiliation

Department of Earth & Planetary Science, University of Tennessee
306 EPS Building, 1412 Circle Drive
Knoxville, TN 37996, USA

\move@AU\move@AF\@affiliation

Harvard-Smithsonian Center for Astrophysics
60 Garden St., MS-65
Cambridge, MA 02138

\move@AU\move@AF\@affiliation

Dept. of Physics & Florida Space Institute
University of Central Florida
4000 Central Florida Blvd.
Orlando, FL 32816-2385

\move@AU\move@AF\@affiliation

German Aerospace Center (DLR), Institute of Planetary Research
Rutherfordstrasse 2, 12489
Berlin, Germany

\move@AU\move@AF\@affiliation

Iowa State University, Department of Physics and Astronomy
A313E Zaffarano Hall
Ames, IA 50011, USA

\move@AU\move@AF\@affiliation

Kapteyn Astronomical Institute
Rijksuniversiteit Groningen
PO Box 800, 9700 AV
Groningen, The Netherlands \move@AU\move@AF\@affiliationSRON, Netherlands Institute for Space Research
PO Box 800, 9700AV
Groningen, The Netherlands

\move@AU\move@AF\@affiliation

Department of Physics and Astronomy
PO Box 6010
Northern Arizona University
Flagstaff, AZ 86011

\move@AU\move@AF\@affiliation

Department of Physics and Astronomy
PO Box 6010
Northern Arizona University
Flagstaff, AZ 86011

\move@AU\move@AF\@affiliation

The Johns Hopkins University Applied Physics Laboratory
Laurel, MD 20723-6099

\move@AU\move@AF\@affiliation

Institute for Astronomy
2680 Woodlawn Drive
Honolulu, HI 96822

\move@AU\move@AF\@affiliation

ESA SSA-NEO Coordination Centre
Largo Galileo Galilei, 1
00044 Frascati (RM), Italy

## 1 Introduction

‘Oumuamua (1I/2017 U1) was discovered on 2017 October 18. One week later it was announced that ‘Oumuamua’s orbit was unbound (Bacci et al., 2017) and that this was the first ever discovered interstellar body — an object that originated outside our Solar System.

It has long been thought that comets and asteroids exist in other planetary systems. Most current models of our own Solar System suggest that today’s small bodies are leftovers from the era of planet formation (e.g., Dones et al., 2015), implying that other planetary systems also produced comet and/or asteroid populations. Until now, it has been impossible to connect our own local small body populations to the large, but unresolved, groups of comets and asteroids found in exoplanetary circumstellar disks (e.g., Lisse et al., 2007, 2017).

‘Oumuamua was the subject of an intense, though brief, observing campaign (Jewitt et al., 2017; Ye et al., 2017; Knight et al., 2017; Bannister et al., 2017; Meech et al., 2017; Masiero, 2017; Bolin et al., 2018; Fitzsimmons et al., 2018; Belton et al., 2018; Fraser et al., 2018; Drahus et al., 2018; Micheli et al., 2018). In summary, ‘Oumuamua has a red, featureless visible/near infrared spectral slope; no directly-detected emission of gas or dust, though activity may be required to explain the presence of non-gravitational perturbations affecting its motion; a very elongated shape; and an excited rotation state. The color, spectral slope, density, and lack of apparent activity all suggest something like a D-type (primitive) asteroid, though the implied low-level activity points to a comet-like body. (The shape and rotation state do not particularly imply any specific analog in our Solar System.) Assuming the object to have asteroidal density, McNeill et al. (2018) showed that no significant cohesive strength is required for ‘Oumuamua to resist rotational fission, but even assuming a comet-like bulk density of 0.5 g/cm we find that a trivial cohesive strength of only 11 Pa is required..

The existence of ‘Oumuamua has implications for its formation and origin and on the small body populations in other planetary systems (Trilling et al., 2017; Ćuk, 2018; Feng & Jones, 2018; Do et al., 2018; Raymond et al., 2018a, b; Zwart et al., 2018; Gaidos, 2018; Jackson et al., 2018; Katz, 2018). Overall, these formation models generally prefer a comet-like body for interstellar interlopers.

As part of the observational campaign carried out before ‘Oumuamua became too faint, we observed this body with the Spitzer Space Telescope. Spitzer observations offered the best possibility to determine the diameter and albedo of this object by measuring its emitted thermal infrared radiation as our team has done for thousands of Near Earth Objects (NEOs) (Trilling et al., 2010, 2016)

Here we present the results of our Spitzer observations. We did not convincingly detect ‘Oumuamua and are left with an upper limit on its flux that corresponds to an upper limit on diameter and a lower limit on albedo. In Section 2 we present our observational approach and data reduction steps; details of the ephemeris and uncertainty calculations; and our observational results. In Section 3 we present our thermal modeling and the resulting limits on diameter and albedo, which strongly depend on choice of model parameters. We discuss our model results and search for activity in Section 4.

## 2 Observations and results

### 2.1 Observations and data reduction

Observations were obtained with Spitzer/IRAC (Fazio et al., 2004) as part of the DDT program 13249. Seven Astronomical Observing Requests (AORs) were used, six of 5 hour duration with 166100 second frames, and a final 2.9 hour (clock time) AOR with 94100 second frames, for a total of 1090 frames and 30.2 hours on-source frame time (acquired over 33 hours of clock time). The observations were divided in this way because of limits in the number of commands and data allowed in a single AOR. The data were taken with the “Moving Single” target mode with Full Array readouts, using a small cycling dither pattern. Two frames were taken at each dither position, to reduce the overhead of moving after each frame. Images were obtained in both arrays, but only the 4.5 m channel was nominally centered on the target position, since the object was expected to be brightest in that IRAC channel. With the information known at that time, we estimated that with this integration time we could achieve a 3 or better detection of the object if it was at its expected maximum brightness during the time of observation.

‘Oumuamua was discovered on 2017 October 19 (and identified as an interstellar body on 2017 October 25; Bacci et al. (2017)), but because of the constraints of the Spitzer observability zone, the earliest that the Spitzer observations could begin was late on November 20 (Figure 2.1). The ephemeris used to develop the original Spitzer observation sequence was based on ground-based astrometric data through the end of October and had a prediction uncertainty larger than the Spitzer FOV. On November 9 the Magdalena Ridge Observatory collected additional ground-based observations, which we used together with preliminary high-precision astrometry from Micheli et al. (2018) to refine the orbit of ‘Oumuamua. We found that the revised predicted positions could potentially put the object very close to or off the edge of the array for many frames in the AORs constructed with the original ephemeris. The Spitzer Science Center (SSC) was able to replan the observations with the latest orbit information. The first AOR began executing at 2017-11-21 10:13:26 UT, and the last AOR completed at 2017-11-22 18:52:06 UT; this is around 2.5 months after ‘Oumuamua’s perihelion passage. The average heliocentric distance of ‘Oumuamua during the observations was 2.0 au and the average Spitzer-centric distance was 1.8 au; the average phase angle was around 31 degrees (Figure 2.1). This geometry changed only very slightly during the 33 hours of clock time needed to carry out these observations. The rate of motion on-sky in these observations was around 68 arcsec/hour.

The data reduction method used was similar to that described in Mommert et al. (2014b). A mosaic of the field was constructed from the data set itself and then subtracted from the individual basic calibrated data (BCD) frames. After subtraction of the background mosaic, residual background sources and bright cosmic ray artifacts were masked in the individual BCDs before being mosaicked in the reference frame of the moving object.

### 2.2 Ephemeris and positional uncertainties

Micheli et al. (2018) later collected ground-based and Hubble Space Telescope astrometry of ‘Oumuamua, eventually extending the observational arc through 2018 January 2. Based on this longer sampling of the trajectory, they reported a 30 detection of a non-gravitational acceleration acting on the motion of the object, inferred from position measurements over time. This acceleration was not visible or expected when the Spitzer observation sequence was built in November and would have resulted in an ephemeris correction of about 100 arcseconds at the time of the Spitzer observations (Figure 2.2). This correction is along the Line of Variation (Milani et al., 2005), i.e., the direction corresponding to the semimajor axis of the uncertainty ellipse, which corresponds to a position angle (north to east) of 81.8 degrees. Though this correction is a statistically significant deviation (7.7) from the gravity-only ephemeris used to build the Spitzer observation sequence, the updated ephemeris still falls inside the Spitzer field of view, which is 5.2 arcmin on each side. The final mosaic presented below and our data analysis are based on the most recent solution for the position (i.e., Micheli et al. (2018)), so the only impact on our observations between the pre-HST solution (used for planning our observations) and the post-HST solution would be the error in the rate over the individual 100 second integrations. The difference between the two solutions (i.e., the degree of trailing introduced) over that length of time is completely negligible.

### 2.3 Observational results

Our final mosaic is shown in Figure 2.3 along with the predicted location of ‘Oumuamua. There are no bright coherent sources in this image, so we conclude that we did not confidently detect the source. The 1 noise level in the final mosaic is 0.1 Jy per PSF. This noise level was determined by recovering synthetic sources of various brightnesses that were injected in the final mosaic. Sources as faint as 0.3 Jy could be reliably found and extracted with an error of 0.1 Jy. This noise floor is consistent with our expectations from Spitzer observations of other very faint moving objects (Mommert et al., 2014a, b).

There are several 2 “blobs” (essentially, single pixels) in the image, and a “source” that is around 1 that is located within the uncertainty ellipse. Since our final image is stacked in the moving frame of the target, the likelihood of any of these blobs corresponding to a true astrophysical object, which would have to be moving at the same rate as ‘Oumuamua over 30 hours, is vanishingly small. Nevertheless, the presence of these blobs in the image at the 1 or 2 level implies that there is correlated noise somewhere in our data stream. The final image has residuals from background stars not fully removed from the mosaics, which cause some streaking across the image. There are likely also fainter cosmic rays and other low-level array effects that survive our filtering and enter into the final image. These residuals get smeared out because of the offsets and mapping between the instrument pixels and the final image pixels on a smaller scale that will lead to correlated “noise.” The final image does not look like an image with only random pixel values in each pixel, but is consistent with what we would expect for the object of interest being too faint to detect. Alternately, this could be a tenuous detection of ‘Oumuamua at 1, or around 0.1 Jy. In the analysis below, we use a 3 upper limit for our calculations, which implies non-detection, or, at best, a weak detection.

There is no significant vignetting in the 4.5 micron IRAC channel near the edge of the field. However, there could be an impact on sensitivity from the object being off the edge of the detector for some frames due to the dithering. The 3-sigma position uncertainty ellipse shown in yellow in Figure 2.3 is fully covered by all exposures to within 2% of the median coverage of the central region (small variations are caused by rejection of bad pixels or pixels affected by background objects or cosmic rays during the exposures). To the right of the yellow ellipse, the coverage drops off roughly linearly along the red path until the end where it reaches a value of around 12% lower coverage than the central part of the image. Therefore, at that extreme end, the upper limit flux would then be 0.32 Jy for objects at this position in the mosaic. The coverage is similar along lines perpendicular to the major axes of the ellipses shown in Figure 2.3. Given the small area affected and the small (10%) difference we simply use the global 3 (0.3 Jy) upper limit for our analysis below.

## 3 Thermal modeling and interpretation of the non-detection

We rule out any detections of ‘Oumuamua at greater than 3 (0.3 Jy). Given the geometry of the observations, we have created a model spectral energy distribution that fits the available data: this 4.5 m upper limit and (the Solar System absolute magnitude in V band), which we take to be 22.4 (Meech et al., 2017) with an uncertainty of 0.09 (using the fractional uncertainty given in Meech et al. (2017)). At 4.5 microns and 1–2 au from the Sun the flux from this object is generally dominated by thermal emission (Trilling et al., 2016) (modulo some low-level gas emission, as described below), so a non-detection provides an upper limit on diameter and a lower limit on albedo.

We simulate the expected brightness of ‘Oumuamua in Spitzer IRAC Channel 2 in order to interpret our upper limit detection. Using the Near-Earth Asteroid Thermal Model (NEATM, Harris (1998)), we estimate the target’s brightness as a function of its absolute magnitude and a range in geometric albedo (). Since the physical properties of ‘Oumuamua are unknown, we consider a range of values for the thermal infrared beaming parameter: =[0.8, 1.1, 2.5]. The justification for these values, which span the range of plausible values for almost all NEOs (Trilling et al., 2016) and comets (Fernandez et al., 2013), is given in Section 4.3. We account for the target’s geometry at the time of our Spitzer observations and contributions from reflected solar light in IRAC Channel 2 (Mueller et al., 2011), assuming an infrared to optical reflectance ratio of 1.4 (Trilling et al., 2016). Furthermore, we account for color corrections of the thermal component of the target’s flux.

Figure 3 shows the distribution of predicted IRAC Channel 2 flux densities for the three different beaming parameters . Black lines indicate levels that are equal to integer multiples of the flux density noise level measured from our data. For = [0.8, 1.1, 2.5], the 3 lower limit on the target’s geometric albedo is [0.2, 0.1, 0.01], respectively (Figure 3). (Technically, any albedo is allowed for the case; we set here the minimum value to be 0.01 to allow for finite diameters.) Correspondingly, we find a diameter upper limit of [98, 140, 440] meters, respectively (Figure 3).

## 4 Discussion

### 4.1 Search for activity

Based on the discovery of non-gravitational accelerations acting upon the orbit of ‘Oumuamua (Micheli et al., 2018), we investigate the possibility of dust and gas activity in this object during our observations. Our non-detection enables the placement of upper limits on the production rates of dust, as well as CO and CO gas; we are unable to constrain the production of other gas species. We use the same formalism that we used in detecting cometary behavior in the NEO Don Quixote (Mommert et al., 2014c)Bauer et al. (2015) used similar approaches with WISE data — and our measured 3 4.5 micron flux density limit (0.3 Jy). Within a 6 pixel (5.2 arcsec) radius aperture (our standard size) we derive Af cm following the definition of A’Hearn et al. (1984), where A is albedo, f is the filling factor, and is the linear radius of the emission (here, an upper limit). Assuming a dust particle radius of 10 m, a dust bulk density of 1 g cm, an albedo of 0.03 that is comparable with cometary dust and compatible with the range of possible albedos that we derived for ‘Oumuamua, and a dust ejection velocity equal to the expansion velocity of gas at this distance from the Sun (Ootsubo et al., 2012), we find a 3 upper limit on the dust production rate of 9 kg s. Similarly, we calculate the 3 upper limit on the CO gas production rate as 910 molecules s. This can be scaled into a 3 upper limit for the production of CO (910 molecules s) based on the ratio of the CO and CO fluorescence efficiencies (Crovisier & Encrenaz, 1983).

This CO upper limit is much lower than the Micheli et al. (2018) value of 4.5 molecules/s (the most sensitive search in the literature) and implies that the outgassing from ‘Oumuamua cannot have CO (or, presumably, CO) as a significant component, though the Micheli et al. (2018) CO production rate assumes a relative large body and albedo of 4%. If ‘Oumuamua’s size were 10–20 times smaller than the Micheli et al. (2018) diameter of 220 meters then the amount of CO outgassing at the upper limit would produce sufficient acceleration. However, an effective spherical diameter of 10–20 meters would require an unacceptably high albedo and unacceptably low , as described below, so this argument is rejected. Overall, we find these upper limit production rates and our upper limit of Af to be very low compared to the ensemble of comets (A’Hearn et al., 1995; Ootsubo et al., 2012), supporting the inactivity of ‘Oumuamua during our observations.

### 4.2 Uncertainties

Our analysis of ‘Oumuamua’s physical properties is based on a measured flux density upper limit and thermal modeling performed with the NEATM. This model has been specifically designed for use on near-Earth asteroid observations and has been shown to be reasonably accurate over a wide range of cases (Harris et al., 2011; Mommert et al., 2018). It is applicable to thermal emission from any airless body and has been used extensively for comet nuclei as well (Lisse et al., 2005, 2009; Fernandez et al., 2013). A more sophisticated thermophysical model (Mommert et al., 2014a, b, 2018) is not appropriate here due to the lack of information on the target (e.g., spin pole orientation and complex rotation state; shape is somewhat known but not uniquely so) and the upper-limit nature of the flux density measurement.

‘Oumuamua is known to have a high-amplitude lightcurve (e.g., Jewitt et al., 2017; Knight et al., 2017; Meech et al., 2017; Bolin et al., 2018; Micheli et al., 2018) with, most likely, a period of 6–8 hours. Since our observations spanned 33 hours (clock time) any lightcurve effects are smoothed out and we observe only the average flux. Furthermore, even though the Spitzer viewing geometry of ‘Oumuamua is very different from that seen by observatories on and near the Earth, because ‘Oumuamua is in an excited rotation state (Fraser et al., 2018; Belton et al., 2018; Drahus et al., 2018), we likely observed the same time-averaged projected surface area that would have been seen from Earth. The lightcurves presented in Belton et al. (2018) are not sinusoidal but rather have broad maxima and narrow minima, so our 33 hour integration is likely not corrupted by faint epochs in the lightcurve. Even in the case of a 55 hour period, one possible solution suggested by Belton et al. (2018), our observations span a significant fraction of the entire rotation and therefore included something close to the time-averaged cross-sectional area, except in the case of a pathological orientation.

We do not include uncertainties on the ratio of infrared to optical reflectances, as the impact of this ratio barely affects the overall results of this study, especially in the light of the large uncertainties on the beaming parameter and hence the geometric albedo .

We investigate the applicability of NEATM for this study given the high aspect ratio of ‘Oumuamua (Meech et al., 2017; McNeill et al., 2018) and the assumption of sphericity in NEATM. For this purpose, we use an asteroid thermophysical model (Mommert et al., 2014b, a, 2018) to derive the thermal and reflected solar flux density of the body, assuming both a highly elongated shape and a highly oblate shape (following Belton et al., 2018). Based on McNeill et al. (2018), we assume a triaxial ellipsoidal shape with semi major axes 6:1:1 for the highly elongated shape and 1:: for the highly oblate shape, both in arbitrary units. We furthermore use the geometry during our Spitzer observations, the period (7.34 hr) derived by Meech et al. (2017), and assume a geometric albedo of 0.03 (in agreement with our NEATM-derived lower limit) and typical small-body values for thermal inertia and surface roughness. We simulate the flux density observed at Spitzer over one quarter of the target’s rotation and derive the average flux density, which is the quantity measured in our observations by combining all available data. Finally, we form the ratio of the average flux density derived for a spherical body (NEATM assumption) to the average flux density derived from the elongated shape and oblate shape models. Deriving this ratio minimizes the effects of the choice of the geometric albedo, surface roughness, and thermal inertia used in the simulation.

We find that this flux density ratio varies as a function of the target’s spin axis latitude (as a proxy for the aspect angle of our observations). In the case of the elongated shape, a spin axis latitude of 90(equator-on view), the ratio is 1, and rotational effects are averaged out during our observations. The ratio decreases to 0.5 for spin axis latitudes approaching 0 (pole-on view), which represents an extreme case. In the case of the oblate shape, we find ratios of around [0.5, 1, 2] for =[0, 32.7, 90] degrees, respectively. As no information on the spin axis orientation of ‘Oumuamua is available, we use the average333To compute this average, we assume a uniform distribution of spin poles on the sphere of the body. This is obtained by uniform sampling in longitude and uniform sampling in the sine of the latitude. Now, we compute the average of latitude knowing that sine of latitude is uniform. The average from -1 to 1 is zero (since this distribution is symmetric), but if we take only the northern hemisphere (for example) then we calculate which is 32.7. We note that even under other assumptions of the average value the deviation from our nominal solutions are insignificant in all cases. latitude of 32.7, leading to a flux density ratio of 0.5 for the elongated shape model and 0.7 for the prolate shape.. This mismatch between the flux densities of the different shapes is insignificant compared to the uncertainties introduced by the lack of knowledge of the surface properties () of ‘Oumuamua. We therefore conclude that our use of NEATM is acceptable. We also note that the uncertainties in the results from the uncertainties are small compared to the uncertainties from our lack of constraints on .

### 4.3 Discussion of possible solutions

#### 4.3.1 Low albedo solution

Since ‘Oumuamua is in an excited rotation state, absorption of solar energy could be significantly more uniform around the surface than for rotation around a single axis. This implies the temperature distribution would be smoother than a single axis rotator, requiring a higher than would otherwise be appropriate (Myhrvold, 2016). The exact influence of the excited rotation state on the thermal emission of ‘Oumuamua is difficult to model given our ignorance on its exact rotation state and overall shape. The extreme of the high case would be represented by the Fast Rotator thermal model (FRM) (Mommert et al., 2018). The FRM for this case produces virtually the same result as (Figure 3). While the FRM is technically not suitable for complex rotation, it should be a reasonable approximation (especially since the rotation period is not very short).

Under the conservative assumption of (the high solution) any albedo is allowed (Figure 3 and Figure 3). This includes arbitrarily low values. A comet-like value of 0.04 (Lamy et al., 2004), as was assumed in Micheli et al. (2018), implies a diameter of 220 meters, and D-type asteroids have similarly low albedos (Thomas et al., 2011). This relatively large body can still experience non-gravitational accelerations but requires relatively large impulses and, consequently, relatively high activity rates that are not commensurate with our CO/CO outgassing limits presented above.

#### 4.3.2 Mid-range albedo solution

The default approach used in our Spitzer NEO program is to derive from phase angle; Trilling et al. (2016) present in some detail the correlation and dispersion in the correlation between those two parameters. In this case, the phase angle of 31 degrees implies around 1.1. This value yields and diameter less than 140 meters. These values are intermediate in the range of acceptable solutions for ‘Oumuamua (Figure 3). However, even this moderate albedo is generally inconsistent with cometary albedos.

#### 4.3.3 High albedo solution

Finally, a lower value appears to be more appropriate for comets (Fernandez et al., 2013). As our bounding case we take . This implies diameter less than 98 meters and albedo greater than 0.2 (Figure 3). This small size is preferred from an activity and non-gravitational acceleration perspective, but the high albedo is unexpected since radiolysis of the surface during its interstellar passage would presumably have darkened the surface (and reddened it; a red color is indeed observed). One possible explanation is that ‘Oumuamua’s recent passage by the sun was sufficient to emplace a thin layer of bright, fresh ice on the surface, as discussed below.

If ‘Oumuamua has a high albedo then its inferred size (98 meter diameter) is substantially smaller than the 220 meter diameter that was assumed by Micheli et al. (2018), and its mass is smaller by the cubed ratio of these solutions (). With a smaller mass, greater acceleration is produced for a given force (i.e., outgassing). However, force is proportional to the production rate, and the CO production rate derived here is  times less than that used by Micheli et al. (2018) to explain the measured astrometry. This rules out the the possibility that CO or CO outgassing was responsible for the non-gravitational acceleration that Micheli et al. (2018) detected. However, we can not put constraints on outgassing of water ice, which is the other main volatile ice found in comets, using our data (see below).

### 4.4 Summary of results and a possible interpretation

There are several possible interpretations of our results, as follows. We note that in all cases, given our upper limit on CO and CO production rates, some other gas species (e.g., water) must also have been emitted to explain the non-gravitational acceleration observed by Micheli et al. (2018). We can place no constraint on these other gas species.

(1) ‘Oumuamua could have a high , which would not be unusual for asteroids but would be very unusual for comets — although a body in an excited rotation state might have a higher than expected value. In the high case, the albedo is low, which means the diameter is large. However, a large body implies a large outgassing rate, which we do not see for CO and CO and for dust. Conversely, (2) ‘Oumuamua could have a low , in accordance with expectations for cometary bodies. However, this requires an albedo that is much higher than that expected for comets. This high albedo corresponds to a small diameter, which is favored, considering our upper limits on gas and dust production. (3) Intermediate values of , albedo, and diameter are also possible, though these are not really consistent with any expectations.

In conclusion, there is no simple asteroidal or cometary physical model that agrees with expectations and previous work (including non-gravitational acceleration) and our results for all of , albedo, and diameter. One plausible explanation is that ‘Oumuamua was a dormant comet nucleus reactivated, after millions of years in interstellar space, by heating during its close passage by the Sun. This reactivation either destroyed the thin dark mantle expected to be created by cosmic rays and galactic ultraviolet radiation (e.g., Lisse et al., 1998, 2004) and/or coated the surface with an optically thick layer of new, fresh ice. In the latter case, the bright coating could plausibly have come from CO, CO, or water, as follows.

We assume that ‘Oumuamua is outgassing  molecules of CO per second (see above). In the high albedo case, the effective spherical diameter of ‘Oumuamua is around 98 meters, and the surface area is therefore around  m (taking 49 meters as the radius of the equivalent sphere). If we require a uniform surface layer that is 10 microns thick — so that the surface appears bright with CO ice for observations made at 4.5 microns — then the volume of this surface layer is around 0.3 m.

The density of CO ice is approximately 1.5 g/cm. The mass required to create a surface layer of 0.3 m is therefore  g. We calculate the number of CO molecules required as

 4.5×105 g44 g/mole×6.02×1023 molecules/mole

which is around  molecules of CO. At  molecules/sec that corresponds to around 67,000 seconds, or around 0.8 days — far less than the few weeks of ‘Oumuamua’s perihelion passage time. Thus, even if the efficiency of this process is small, it is still quite plausible that a low level of activity — induced by solar heating of a near-subsurface CO reservoir — could produce enough material to coat the surface with bright, fresh CO and increase the albedo to the relatively high value required in our high-albedo case.

Alternately, heating of water ice into gas could present a plausible scenario. Cometary dust:gas ratios are typically around 5:1, so our dust emission upper limit of 9 kg/sec corresponds to 1.8 kg/sec as an upper limit for gas emission. If we assume that all of this gas emission is in HO, then we find

 1.8×103 g/sec18 g/mole×6.02×1023 molecules/mole

which is around  molecules/sec, enough to produce the non-gravitational accelerations reported by Micheli et al. (2018).

CO+CO ice in typical Solar System comets is around 15% of the water abundance. Here our limits imply around 0.15% for this ratio — a factor of 100 times smaller. This could imply that ‘Oumuamua was heated to 100 K prior to our observations — either by our Sun, or before entering our Solar System. ‘Oumuamua, if propelled by water ice sublimation at  molecules/sec while producing only  molecules/sec of CO+CO, must have been previously devolatilized of these more volatile ices.

### 4.5 Possible analogies

Unfortunately, we do not have pre-perihelion observations to compare to these post-perihelion observations to test the hypothesis that ‘Oumuamua brightened during its perihelion passage. Further modeling of ‘Oumuamua’s outgassing — whether CO, CO, or some other species — would be very beneficial.

A plausible analogy for such activity-produced resurfacing is the well-studied comet 67P, the target of the Rosetta mission. Keller et al. (2017) and Liao et al. (2018) showed that the nucleus of 67P was partially resurfaced through re-condensation of volatiles released from the nucleus; Liao et al. (2018) found that the deposition rate of water ice could be up to several microns in an hour near perihelion. While this does not correspond directly to low levels of activity and an albedo increase, as proposed here for ‘Oumuamua, it is nevertheless evidence that activity can resurface small body surfaces after perihelion passage, at the order of magnitude required for ‘Oumuamua (1–10 microns deposited in days or weeks). Bolin et al. (2018) saw no color changes as a function of rotation, which could imply a relatively uniform resurfacing process. This suggested emission must be too small to create a measurable change in velocity after the first ‘Oumuamua observations (i.e., the beginning of the observational arc), or else occurred after perihelion but before the discovery observation of ‘Oumuamua (see Figure 2.1), in order to be consistent with the results reported in Micheli et al. (2018).

Another possible analogy is the well-studied comet Shoemaker-Levy 9. Sekanina (1995) shows that after the breakup of this body from tidal forces exerted by Jupiter many fragments appeared intrinsically brighter – as if fresh ice had just been revealed or deposited onto their surfaces. It is possible that the shape and/or rotation state of ‘Oumuamua were affected by its passage near the Sun, and interior volatiles may also have been liberated onto the surface at the same time.

## 5 Conclusions

We observed interstellar body ‘Oumuamua for 30 hours of integration time at 4.5 microns with the Spitzer Space Telescope. We did not convincingly detect the object and place upper limits on its flux during our observations. Depending on the assumptions used in our thermal model, we find low-, medium-, and high-albedo solutions (and corresponding limits on the effective spherical diameter). We do not detect any activity from ‘Oumuamua and place upper limits for CO and CO emission that are far lower than were derived by Micheli et al. (2018) under the assumption of a body with 4% albedo; we can place no constraints on emission of other gas species (e.g., water ice). The nature of the gas emission and the origin of the non-gravitational accelerations are still unknown.

It is not clear what type of body in our Solar System is the most similar to ‘Oumuamua, as there are significant failures with both comets and primitive (D-type) asteroids as end-member analogs. One possible scenario that appears to explain many of the observed properties of ‘Oumuamua, including our observations, is exposure or creation, from outgassing, of a fresh, icy, bright surface due to thermal reactivation during ‘Oumuamua’s close perihelion passage in September, 2017. However, due to the geometry of ‘Oumuamua’s passage through the Solar System, there will be no more observations of this object, so it is likely that we will never know the true nature of this interstellar interloper.

This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA.

We thank the SSC Director for approving these DDT observations and the SSC staff for rapidly implementing these observations with their usual technical excellence. Part of this research was conducted at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. KM acknowledges support from NSF awards AST1413736 and AST1617015.

Facilities: Spitzer(IRAC)

Software: MOPEX (Makovoz et al., 2006), IRACproc (Schuster et al., 2006)

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