  The ubiquity of information transmission via molecular communication between
cells is comprehensively documented on Earth; this phenomenon might even have
played a vital role in the origin(s) and early evolution of life. Motivated by
these considerations, a simple model for molecular communication entailing the
diffusion of signaling molecules from transmitter to receiver is elucidated.
The channel capacity $C$ (maximal rate of information transmission) and an
optimistic heuristic estimate of the actual information transmission rate
$\mathcal{I}$ are derived for this communication system; the two quantities,
especially the latter, are demonstrated to be broadly consistent with
laboratory experiments and more sophisticated theoretical models. The channel
capacity exhibits a potentially weak dependence on environmental parameters,
whereas the actual information transmission rate may scale with the
intercellular distance $d$ as $\mathcal{I} \propto d^{-4}$ and could vary
substantially across settings. These two variables are roughly calculated for
diverse astrobiological environments, ranging from Earth's upper oceans ($C
\sim 3.1 \times 10^3$ bits/s; $\mathcal{I} \sim 4.7 \times 10^{-2}$ bits/s) and
deep sea hydrothermal vents ($C \sim 4.2 \times 10^3$ bits/s; $\mathcal{I} \sim
1.2 \times 10^{-1}$ bits/s) to the hydrocarbon lakes and seas of Titan ($C \sim
3.8 \times 10^3$ bits/s; $\mathcal{I} \sim 2.6 \times 10^{-1}$ bits/s).
