Geoscience Reference
In-Depth Information
expected to be between
∼
5 and 20 kV/m, compared to
∼
3,000 kV/m on
Earth.
To have a rough idea of the brightness of a Mars lightning, we consider
an electric field,
E
, of 5 kV/m, the energy density is
10
−
4
J
/
m
3
,
u
=(1
/
2)
ε
0
E
2
=1
.
11
×
(11)
where
ε
0
is the vacuum dielectric permittivity.
If the dust cloud is assumed to be 1,000 m high, and to have a projected
surface area of 1 km
2
, then the total cloud volume is 10
9
cubic meters, and
the total energy due to charge separation within the cloud (assuming that
E
is uniform throughout) must be 1
.
11
10
5
J. If the camera recording
the flash were 200 km away from the cloud, and if 1/10 of the total energy
in the discharge produces light, then 1
.
11
×
×
10
4
J of light would be spread
over a sphere of area 5
10
11
m
2
. If the discharge takes 1 ms to occur, the
intensity at the camera is approximately 2
.
2
×
10
−
5
W
/
m
2
. Considering
that the Mars dust clouds can cover the whole planet surface, our estimate
intensity has to be considered a lower limit.
×
5.2.
Observation of Jupiter lightning
The possible occurrence of lightning in the Jupiter atmosphere was firstly
predicted by Bar-Nun.
2
The Voyagers 1 and 2 spacecrafts performed the
first observations of lightning on Jupiter in 1979. Later, even the Galileo
spacecraft has monitored the electric activity in the Jupiter atmosphere. A
typical Jovian storm is about 1,500 km in diameter and produces about
20 flashes per minute. The heights of the flashes are between 2 and
5 bar atmospheric pressure layer, in the region where the H
2
O clouds are
located. This suggests that the lightning generation mechanism is anal-
ogous to the terrestrial one (convective electrification of the clouds). In
the visible band, the flash intensity ranges from 4
.
3
10
8
×
(for those hav-
10
9
J for the more energetic ones.
9
The total
power is larger with respect to the optical one for a factor between 10
2
and 10
3
.
3
In average, in the above-mentioned optical range 0.01 flashes
per km
2
per year occur. Assuming a typical duration of 35 ms, as for
the terrestrial lightning,
11
ing mean energy) to 6
.
6
×
10
10
the optical power span from 1
.
2
×
to
10
11
W and, if observed from orbit, the corresponding apparent mag-
nitudes are negative till a distance of 10
5
km from the top of the Jupiter
clouds.
2
×
