Roadmap for Ocean Worlds Ceres and Small Bodies

J. Castillo-Rogez J. Scully, M. Neveu, S. Hosseini, B. Schmidt, M. Eubanks, M. Poston

A. Hendrix and T. Hurford – Chairs

OPAG Charge to ROW • OPAG chartered ROW; we are coordinating with SBAG since some

“SBAG-owned” bodies could be ocean worlds • Identify and prioritize science objectives for Ocean Worlds

– tied to the Decadal Survey • Design roadmap to explore these worlds to address science

objectives – Mission sequences, sustained exploration effort

• Assess where each Ocean World fits into the overall roadmap • Summarize broad mission concepts

– Considering mission dependences & international cooperation • Recommend technology development and detailed mission studies

in support of the next decadal survey • Place exploration of Ocean Worlds into the larger context of Solar

System exploration

Ocean World Themes

• Four themes – Identify ocean worlds – Characterize Oceans – Assess Habitability – Search for Life

• Theme groups came up with an initial set of science questions for each theme – Ranging from high level to very detailed

Ceres/Small Bodies WG

• Focused on Ceres, Large Asteroids, Trojan Asteroids • Looked at observational constraints and theoretical

predictions • The geophysical prospect for the long-term

preservation of liquid is unfavorable for most asteroids – Ceres is the exception – but it’s a dwarf planet!

Search for

Life

Identify Ocean Worlds

Characterize

Oceans

Assess Habitability

Energy Sources

Ocean Signatures

Solvents Rock/Ocean Interface

Energy for Life

Physico-chemical Conditions for Life

Biomarkers

Oce

an W

orld

s

Enceladus

Europa

Titan

Ganymede

Callisto

Poss

ible

Oce

an W

orld

s

Ceres

Pluto/Charon/ KBOs

Triton

Other Saturnian Icy Satellites *

Other Uranian Icy Satellites **

* Mimas, Tethys, Dione, Rhea, Iapetus ** Miranda, Ariel, Umbriel, Titania, Oberon

Primary Mission Contributor

New Horizons: 2006-Present

Voyager: 1977-Present

Galileo: 1996-2003

Cassini: 2004-2017

Dawn: 2007-Present

Solid Foundation

Key Information

• 482 x 482 x 446 km • mean radius 470 km • Rotation period 9.074 hr • Ceres’ surface reflects <10% of

incident sunlight • Surface temperature 110-155K • Density 2.162 kg m-3

• Rock mass fraction ~73 wt.% • Ceres as a whole is ~50 vol.% water

NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Ceres is Large – Geophysically Speaking

Most previous models predicted the long-term preservation of a deep ocean

EUROPA Surface temp: 102K Energy source: tidal

CERES Surface temp: 160K Energy source: solar, 40K

ENCELADUS Surface temp: 94K Energy source: tidal

Vol.%

Rock

Water/ice

C, H, N, O, S?

C, H, N, O

H, O, S

Ceres and Other Ocean Worlds

Global, Homogenous Composition: Ammoniated clays, serpentine, carbonates De Sanctis et al. (2015), Ammannito et al. (2016)

Points to global episode of hydrothermal alteration

Sodium carbonates Ammonium Salts De Sanctis et al. (2016)

Lake Searles Enceladus

Ceres’ surface shows mineralogy found only on Earth and Enceladus, so far

Ceres’ Surface Displays Material Formed at Depth

Organics!

Geology is Driven by Brines • Sodium bicarbonate and

ammonium salts found in bright deposits at Occator and other places (De Sanctis et al. 2016)

• The emplacement of Ahuna Mons implies the presence of brines at depth (Ruesch et al. 2016)

• Activity is recent – 10s My • Occurrence of salts in many

settings suggest near-surface abundance (Stein et al. 2017)

• Brine mixture eutectic is about 245K, possibly lower if ammonia remains

Ceres is Physically and Chemically Differentiated

Crater morphologies indicate ice content <40% (Bland et al. 2016)

Rocky Mantle ~2.4 g/cm3

Gravity suggests partially differentiated interior (Park et al. 2016)

Topography is explained by strong shell (<45 km) over soft, muddy interior Ermakov et al. (submitted), Fu et al. (submitted)

Ice/clathrates/ Salts/Silicates

~1.3 g/cm3

Ceres has a Muddy Interior Below a Frozen Ocean

• Ceres’ ocean has been slowly freezing due to the insulating role of salts and other hydrates – Brines accumulate between frozen ocean and rocky

core, in a mud • The icy shell was partially removed by impact-

accelerated sublimation and mixed with salts, clathrates, clays

• Clays offer a medium amenable to biochemical reactions

• Brines may be subject to radiolysis due to U and K decay creation of local redox gradients (Bouquet et al. 2017; Castillo-Rogez et al. 2017)

Muddy/Salty Ocean

Core with large rock particles

Ice

Rocky Core

Mud

Frozen Ocean

Oceanic material readily accessible below the regolith

Ceres is an Ocean World, Habitability TBD • Ceres shares same chemistry as Europa and Enceladus • Geology indicates recent (10s My) endogenic activity • Abundance of hydrated material led to slow freezing and

preservation of liquid until present • Ceres’s ocean is muddy – good for biochemistry • Surface exhibits oceanic material at shallow depth

KEY OPEN QUESTIONS: o Thickness and extent of mud layer? o Conditions of past and current liquid

environments? o Past and present extent of geochemical

gradients? o Origin of organics observed on the surface?

An in situ mission is the natural next step in the exploration of Ceres, the ocean world closest to Earth