Science 13Mar2020

and/or differing properties of the salts present
on the cometary surface (such as concentra-
tion, mixing, and counter-ions). We investi-
gated other candidate compounds—fine water
ice grains, carboxylic acid, or hydrated minerals—
but these do not match the 3.2-mmfeature

(Fig. 1B). The laboratory spectra of five am-

monium salts we investigated are shown in

Fig. 2. Ammonium formate, ammonium sul-

fate, or ammonium citrate all reproduce the

3.1- and 3.3-mm absorption bands observed on

the comet. For ammonium carbamate and am-

monium chloride, the corresponding bands are

shifted to longer wavelengths or have different

spectral shapes (Fig. 2).

The similarity of band shapes and positions

leadsustoconcludethatNH 4 +in ammonium

salts is the main species responsible for the

Pochet al.,Science 367 , eaaw7462 (2020) 13 March 2020 2of6

Fig. 1. Comparison of NH 4 +HCOO–spectrum with
the average spectrum of comet 67P.(A) The
average reflectance spectrum of comet 67P in the
3.0-mm region [(a), black line; the vertical scale is
given in radiance factor I/F] and the spectrum
of a sublimate residue containing≲17 wt %
ammonium formate mixed with≳83 wt %
pyrrhotite grains at 170 to 200 K [(b), red line;
the vertical scale is given in reflectance factor
REFF]. The same spectra overlain on each other is
shown in fig. S5. Gaps in the comet 67P spectrum are
due to the instrument’s diffraction order sorting
filters. Both spectra have the same shape and
minima at 3.1 and 3.3mm (dashed gray vertical
lines). (B) Reflectance spectra (normalized at
2.5mm) of other compounds that do not
match the comet 67P spectrum: (c) a model
spectrum of 1-mm-diameter pure water ice grains
(solid line) [Hapke model ( 60 ), using optical
constants at 145 K ( 61 ), spectrum normalized,
scaled by a factor of 0.14, and offset by–0.08];
(d) a measured spectrum of a sublimate residue
containing≲17 wt % lactic acid mixed with pyrrhotite
grains at 170 to 200 K (dashed line, offset by–0.35) ( 12 ); and (e) spectrum of the primitive carbonaceous chondrite meteorite QUE 97990, which is rich in
hydrous silicates, measured under 400 to 475 K and high vacuum (dotted line, offset by–0.55) ( 62 ). Between 2.5 and 2.8mm, the spectra of sublimate residues
are affected by measurement artifacts because of the presence of water vapor in the optical path. Residual spectra, calculated by dividing the comet 67P spectrum
(a) by the experimental spectrum (b) or (d), are shown at the bottom of (A) and (B). Error bars indicate the ±1suncertainties.

0.4

0.6

0.8

1.0

1.2

(e)

QUE 97990

(d)

Lactic acid

Normalized Reflectance (offset)

1.5 2.0 2.5 3.0 3.5 4.0

1.54

1.61

1.68

Residual

Wavelength (μm)

2.5 3.0 3.5

1.19

1.26

1.33

Wavelength (μm)

Ratio (a)/(b) (a)/(d)

0.020

0.025

NH 4 +HCOO-

+ opaque grains

Reflectance (REFF)

(c)

H 2 Oice

(b)

0.025

0.030

0.035 B (a) Comet 67P

Reflectance (I/F)

Comet 67P

A

(a)

Fig. 2. Reflectance spectra of several ammonium salts.
Continuum-removed reflectance spectra of sublimate
residues made of ammonium salts (colored lines)
mixed with pyrrhotite grains (gray line) measured
in high vacuum at 170 to 200 K, compared with the
observed average spectrum of comet 67P (black line).
Ammonium formate, citrate, and sulfate are the closest
matches to the absorption features in comet 67P. We
attribute the other absorption features in the comet
spectrum at 3.35 to 3.6mmtoC−H stretching modes of
organic compounds ( 10 ). Between 2.6 and 2.8mm,
the laboratory spectra are affected by measurement
artifacts because of the presence of water vapor in the
optical path. The mass fractions of the salts mixed
with pyrrhotite are≲9 wt % for ammonium sulfate and
chloride,≲17 wt % for ammonium formate and
carbamate, and≲23 wt % for ammonium citrate
(the latter has been scaled by a factor of 0.6 for
display). These spectra are shown in fig. S6 with
uncertainties and before continuum removal.

Comet 67P

Ammonium formate

NH 4 +HCOO-+ Pyrr.

Ammonium citrate

+ Pyrrhotite

Ammonium sulfate

(NH 4 +) 2 SO 4 2-+Pyrr.

Ammonium carbamate

NH 4 +NH 2 CO 2 - +Pyrr.

Ammonium chloride

NH 4 +Cl-+ Pyrr.

Pyrrhotite

3.0 3.5 4.0

0.7

0.8

0.9

1.0

1.1

1.2

Reflectance (continuum removed, offset)

Wavelength (μm)

×0.6

+0.48

+0.13

+0.22

-0.06

-0.13

-0.29

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