Solar System Spectroscopy with Millimetron: Comets and Asteroids

Darek Lis

Jet Propulsion LaboratoryCalifornia Institute of Technology

Millimetron Workshop, Sep 2019 1

NASA/Origins 2

Cosmic Inheritanceof Water

Images: NRAO/NASA

3

Observations of Cold Water

Putaud 2019 Coutens et al. 2014

• Key low-energy water l lines between 500 and 1700 GHz

• Atmosphere often completely opaque• Even SOFIA cannot observe cold water

4

Once upon a time the Earth

formed dry

• Water mass fraction increases with heliocentric distance

• “Textbook model”: temperature in the terrestrial planet zone was too high for water ice to exist

• Water and organics were most likely delivered by comets or asteroids

• Alternative: water could have survived, incorporated into olivine grains or through oxidation of an early H atmosphere by FeO in the magma ocean

Snow Line

may have distinct oxygen abundances because they sam-pled at different times and to differing extents regions ofthe solar nebula disk heterogeneous in oxygen (i.e., water)abundance is testable if the bulk oxygen abundances can bemeasured.

6.4.2. Uranus and Neptune

The carbon excesses measured from CH4 near-infraredspectroscopy give values of 30–50 for Uranus and Neptune,compared with 9 and 4 for Saturn and Jupiter (Encrenaz,2008). These values are consistent with the assumption ofan initial core of 10–15 Earth masses with heavy elementsin solar abundances. Unfortunately, nothing is known aboutthe water or oxygen content in Uranus and Neptune, sincewater condensation occurs at such deep levels that eventropospheric water vapor cannot be detected. Unexpect-edly, ISO detected water vapor in the upper atmospheresof both ice giants, with mixing ratios orders of magnitudehigher than the saturation level at the temperature inversion(Feuchtgruber et al., 1997). These observations can only beaccounted for by an external flux of water molecules, dueto interplanetary dust, or sputtering from rings or satellites.Similarly, Herschel maps of water of Jupiter demonstratethat even 15 years after the Shoemaker-Levy 9 impact, morethan 95% of the stratospheric Jovian water comes from theimpact (Cavalie et al., 2013). On the other hand, the lowD/H ratios in molecular hydrogen measured by Herschel inthe atmospheres of Uranus and Neptune may imply a lowerice mass fraction of their cores than previously thought (14–32%) (Feuchtgruber et al., 2013) .It is unlikely that information on the deep oxygen abun-

dance will be available for Uranus and Neptune anytimesoon, since reaching below the upper layers to determinethe bulk water abundance is very difficult. Thus, data com-plementary to that for Jupiter and Saturn will likely comefirst from observations of Neptune-like exoplanets.

7. WATER IN THE INNER SOLAR SYSTEM

Terrestrial planets were formed by accretion of rockyplanetesimals, and they have atmospheres that are only asmall fraction of their total masses. They are small andhave high densities compared with giant planets. Earth andVenus have very similar sizes and masses, whereas Marshas a mass of only 10% of that of Earth and a somewhatlower density (3.9 vs 5.2 gr cm−3). Terrestrial planets un-dergo different evolutionary processes compared with theirgas-rich counterparts. In particular, their atmospheres resultmostly from outgassing and from external bombardment.Internal differentiation of the solid material after formationleads to a structure in which the heavier elements sink tothe center, resulting in a molten core (consisting of metalslike iron and nickel), a mantle (consisting of a viscous hotdense layer of magnesium rich silicates) and a thin uppercrust (consisting of colder rocks).Water has very different appearances on Venus, Earth

and Mars, which is directly related to their distances from

–4

–3

–2

–1

0

Carbonaceous

log 1

0(w

ater

mas

s fr

acti

on

)

Distance from the Sun (AU)

20 50

–4

–3

–2

–1

0

Comets

TNOs

4.0

C P

1.50.3 2.0 2.5 3.0

Asteroids

Small

Large

MB Comets

Earth's water content

Ordinary

EnstatiteDry

Fig. 10.— Water content of various parent bodies at thetime of Earth’s growth as a function of radial distance fromthe Sun. Enstatite chondrites originating from asteroidsaround 1.8 AU in the inner disk are very dry. In contrast,carbonaceous chondrites originating from the outer asteroidbelt and beyond have a water content of 5–10%, and mainbelt comets, TNOs and regular comets even more. Figureby M. Persson, adapted fromMorbidelli et al. (2012).

the Sun at 0.7, 1.0 and 1.7 AU, respectively. On Venus, withsurface temperatures around 730 K, only gaseous water isfound, whereas the surface of Mars has seasonal variationsranging from 150–300 K resulting in water freezing andsublimation. Its current mean surface pressure of 6 mbaris too low for liquid water to exist, but there is ample ev-idence for liquid water on Mars in its early history. Mostof the water currently on Mars is likely subsurface in thecrust down to 2 km depth. Earth is unique in that its meansurface temperature of 288 K and pressure of 1 bar allowall three forms of water to be present: vapor, liquid and ice.In § 8, the origin of water on these three planets will befurther discussed. It is important to keep in mind that eventhough these planets have very different atmospheres today,they may well have started out with comparable initial watermass fractions and similar atmospheric compositions dom-inated by H2O, CO2, and N2 (Encrenaz, 2008).Earth has a current water content that is non-negligible.

The mass of the water contained in the Earth’s crust (in-cluding the oceans and the atmosphere) is 2.8× 10−4 Earthmasses, denoted as ‘one Earth ocean’ because almost all ofthis is in the surface waters of the Earth. The mass of thewater in the present-day mantle is uncertain. Lecuyer et al.(1998) estimate it to be in the range of (0.8–8)×10−4 Earthmasses, equivalent to 0.3–3 Earth oceans. More recently,Marty (2012) provides arguments in favor of a mantle wa-ter content as high as ∼7 Earth oceans. However, an evenlarger quantity of water may have resided in the primitiveEarth and been subsequently lost during differentiation andimpacts. Thus, the current Earth has a water content ofroughly 0.1% by mass, larger than that of enstatite chon-drites, and it is possible that the primitive Earth had a wa-ter content comparable with or larger than that of ordinary

16

5

“Textbook” D/H in Water in the Solar Nebula

• Variations in the D/H ratio: progressive isotopic exchange reactions between HDO and H2

• Water was initially synthesized by interstellar chemistry with a high D/H ratio (>7.2×10-4; highest value measured in clay minerals)

• The D/H ratio in the solar nebula then gradually decreased with time

• Turbulent mixing of grains condensed at different epochs and locations in the solar nebula ➟ D/H gradient

Horner et al. 2007

Jupiter Neptune

6

• A coupled dynamical and chemical model

• D/H may decrease in the outer regions

• Water thermally processed in the inner disk transported outward

Other D/H Models

Yang et al. (2013)

7

Isotopic Ratio Measurements

• Sample return or in-situ — detailed studies of individual objects — H, O, N, noble gases

8

OSIRIS-REx

Rosetta

Herschel

• Remote sensing — statistical studies of objects that have atmospheres

Deep Impact/EPOXI

Comets

• Comets are among the most primitive bodies formed before planets and asteroids

• Jupiter Family comets originate in the Kuiper Belt, or associated scattered disc, beyond the orbit of Neptune

• Long-period comets come from the Oort cloud, but formed in the Jupiter-Neptune region

• Sent toward the Sun by gravitational perturbations from the outer planets or nearby stars, or due to collisions

9

D/H Observations

• Comets: variations between one and three times terrestrial value• No trends with physical or dynamical parameters

10

D/H vs. Active Fraction

12

• Hyperactive comets typically have terrestrial D/H ratios• Large reservoir of ocean-like water in the outer Solar System

Lis et al. 2019

Oxygen Isotopic Ratios

• Expect mass dependent fractionation: fractionation of 17O/16O half of that of 18O/16O• Mass independent fractionation observed — why?

McKeegan 2011

13

Nitrogen Isotopic Ratios• Comets enriched in

15N?• Cometary

measurements mostly in CN and HCN

• Rosetta: ammonia 4.7 times more abundant than HCN

• Ammonia may not be isotopically equilibrated with CN and HCN

Sarafian et al. 2014

14

• Fundamental NH3 line at 572 GHz detected with Herschel, but not 15NH3

Tempel 2Biver et al. 2012

Millimetron Sensitivity

15

u Herschel/HIFI observations of comet 103P/Hartley — heterodyne spectroscopy

u Figure of Merit (FOM) =1028s-1/0.21au=4.8×1028

u 10! detection of the HDO 509 GHz in 340+82 min (7 h)

u Expected improvement for Millimetron: × 16 u Rx sensitivity: 2 (Trx 50 K DSB at 500 GHz demonstrated)

u Telescope diameter: 10/3.5=2.9

u Two pixels:

u 4 × longer integration (up to ~30 h per source): 2

u Sensitivity limit: FOM = 1.5×1027 (for a 5! detection of HDO; SOFIA~1.5×1029)

2

u Frequency settings:

• D/H and O — Ch 1: HDO 509.3 GHz, H218O 547.7 GHz / H217O 552.0 GHz + Ch 2: H216O 1097.4 (or 1153.1, 1162.9, 1228.8 GHz — modeling required)

• N — Ch 1: NH3 572.5 GHz / 15NH3 572.1 GHz (with H216O 556.9 GHz) + Ch 2: HDO 894 GHz

How Many Comets?

NASA, Origins 16

~85 comets in 5 years

Up to 2500 h observing program, 500 h per year~20 days plus overheads~8% of the mission

Main Belt Comets/Active Asteroids

176P/LINEAR — de Val-Borro et al. 2012

• Intrinsically icy bodies on asteroidal orbits in the Main Belt

• Observed to eject dust — satisfy the observational definition of a comet — no water outgassing detected

• Formed and remained at their current location — not captured

• Herschel: Q(H2O)<4×1025 s-1 (3", 5h)• DGR=1 expect: Q(H2O)<2.3×1024 s-1 (highly uncertain)• Millimetron would detect water emission, but is unlikely

to measure isotopic ratios

P/2013 R3 breaking apart

Summary

• Vision and Voyages explicitly identified “determining the deuterium/hydrogen and other crucial isotopic ratios in multiple comets” as key measurement for understanding Solar System beginnings (D/H, but also O and N isotopic ratios)

• Observations of water emissions in active asteroids provide information about water content of the asteroid belt and are of key importance for future in situ missions under consideration

• Important for understanding habitability of other planetary system 20

© 2019 California Institute of Technology. Government sponsorship acknowledged.

21