On the possibility of sprites on other planets

Yoav Yair and Roy Yaniv

Lightning Activity in our solar system

Earth, Jupiter, Saturn - definite Venus – probable, debatable Uranus, Neptune – probable, partly confirmed Mars, Titan - theoretically possible Moon, Mercury, Pluto - impossible

If there are planetary lightning flashes, should there also be planetary TLEs?

2 2

1 1s

s s

E QZ Z Z Z

Computing conventional breakdown thresholds for various atmospheric compositions Ek is the critical electric field, where ionization

exactly balances attachment.

Ionization –

Attachment –

E/p is the electric field normalized to pressure, and Ek (the critical electric field) been expressed in normalized form of E/p. To find Ek at other pressures, we multiplied by the pressure (in torr units).

exp/i

i

BA

P E P

2

exp/a

a

BEA

P P E P

Parameters (Sentman, 2004)

Determination of Electrical Fields we assumed 3 different Electrical charges that a

Lightning can produce. For Earth and Venus we assumed 100 Coulomb,

500 Coulomb and 1000 Coulomb values produced by Lightning discharge.

For the Gas Giants we assumed 100 Coulomb, 1000 Coulomb and 10000 Coulomb values produced by Lightning discharge.

Verification: Earth Electrical Fields (10km cloud)

Calculations for Venus Clouds heights: 48, 55, 70 km

Ionospheric height is ~ 300 km

Venus Electrical Fields (48km

cloud)

Venus Electrical Fields (55km

cloud)

Venus Electrical Fields (70km

cloud)

Titan Electrical Fields

The Clouds of Jupiter Permanent global coverage organized

in belts and zones Lower cloud - probably water - 5 bar

pressure level, convective, mixed-phase

Middle cloud - NH4SH at the 2 bar pressure level, stratiform

Upper cloud - Ammonia Ice, Cirrus-like

•Ionospheric height is 1600 km above the 1-bar levelClouds: H2O 2 bar (-20km), 3 bar (-33 km), 4 bar (-44km).

Jupiter Electrical Fields (-20km cloud)

Jupiter Electrical Fields (-33km cloud)

Jupiter Electrical Fields (-44km cloud)

Saturn’s cloud layers (Atreya, 1986) Weaker convection, water

clouds deeper and warmer: probably no ice

H2O 4 bar (-90km), 6 bar (-120 km), 8 bar (-150km).

Saturn Electrical Fields (-90km cloud)

Saturn Electrical Fields (-120km cloud)

Saturn Electrical Fields (-150km cloud)

Summary The huge surface atmospheric pressure on Venus (~9 MPa)

means that no cloud-to-ground discharge from the high-level sulfuric acid clouds (Zs ~ 50 to 70 km) are possible.

Sprites can be produced by IC discharges (van der Velde et al., 2006), so even if all lightning activity in Venus is entirely intra-cloud discharges, sprites and Elves are still possible.

Venusian IC discharges would be slow to build-up and would exhibit different characteristics compared to their terrestrial analogues (Gurnett et al., 2001).

Potential emission bands from CO2 between 290 and 500 nm, with prominent lines near 288, 410 and 427 nm (Smyth, 1931). Also OI (777.7 nm).

In Jupiter and Saturn, the amount of charge required to initiate sprites is of the order of 104 C. Although these may seem highly unrealistic values, it has already been shown that lightning in these planets are far more energetic than on earth.