SMALL COMETS' IMPLICATIONS FOR INTERPLANETARY LYMAN a

If small comets are as numerous in the inner solar system as Prank et al. [1986a] have recently proposed, large and easily detectable amounts of hydrogen would be generated in interplanetary space. Inhibition of water vapor production by a factor of 3 x 10 -9 by very thick dust mantles would be necessary to prevent excitation of a detectable interplanetary Lyman a glow. Other optical problems associated with the cometesimals are also discussed. Prank et al. [1986a], henceforth FSC, have proposed that small comets 12 meters in diameter, whose' density is 0.1 gm cm -3, impact he earth at a rate of about 20 per minute. This rate requires that the average distance between these cometesimals at 1 AU, before gravitational focussing by the earth, be about 5000 km and their spatial density be about 3 x 10 -26 cometscm -3 [Prank, et al., 1986b]. The "standard" rate for production of water molecules from comets at 1 AU is between 1017 and 1018 cm-2s -1 or 3 x 1024 molecules every second from each small comet. Dissociation of the water would result in the production of hydrogen atoms at a rate of 0.14 cm -3 s -1 from this family of small comets at 1 AU, if the water production rate is 5 x 1017 cm -2 s -1. If the lifetime of these atoms before charge exchange with the solar wind ions is 2 x 106 seconds, their density near 1 AU would be about 3 x 105 per cm 3. FSC, recognizing the problem of loading the solar wind with so many pick-up ions, proposed that the rate of gas production from the small comets must be kept low by sufficiently thick dust mantles on their exteriors. Reducing the production rate by orders of magnitude, to a level between 3 x 101ø and 3 x 10 TM cm -2 s -1, would avoid the difficulty with excess generation of pick-up ions [Frank, et al., 1986b]. It would not be sufficient to escape another difficulty the excitation of Lyman a radiation by the cometary hydrogen. If gas generation is assumed to vary with heliocentric distances a r -2'6 [Meier and Keller, 1985], a column perpendicular to the solar direction at 1 AU would contain 4.5 x 1018 atoms cm -2 and have an optical depth of 7 x 105 for Lyman a, a fact already pointed out by Chubb [1986]. Interplanetary hydrogen is not optically thick in the inner solar system [Purcell and Tousey, 1960; Meier and Prinz, 1970]. The interplanetary Lyman a glow at 1 AU is about 400 R at solar minimum and 1.2 kR at solar maximum. If the specific rate at which solar Lyman c• radiation is scattered by hydrogen atoms at 1 AU is taken as 1 x 10 -3 s -1 at solar minimum and 3 x 10 -3 s -1 at solar maximum [Hinteregger, 1981; Mount, et al., 1980], and the water production rate from small comets is 4 x 101ø molecules cm -2 s -1 (10 -7 times normal), the entire interplanetary glow would be ascribable to small comets. This interplanetary glow would be confined to the inner solar system. Observations performed Copyright 1987 by the American Geophysical Union. Paper number 6L6485. 0094-8276/87/006L-6485503.00 by ultraviolet instruments on Pioneer 10 and Voyagers 1 and 2 have demonstrated that this is not the case. The gas production rate set by FSC to prevent mass loading of the solar wind would result in Lyman a emission rates between 250 and 750 R (solar minimum and solar maximum) for the lowest part of its range (3 x 10 •ø cm -2 s-1). A contribution to the interplanetary Lyman a radiation at 1 AU of as much as 25 R by the hydrogen associated with small comets would have been detected as a signal rapidly decreasing with heliocentric distance by the ultraviolet spectrometers on Voyager 1 and 2. The FSC comets could emit water molecules at a rate no larger than 1.3 x 109 cm -2 s -• (3 x 10 -9 times the normal value for comets) if their contribution to interplanetary Lyman a is not to exceed 25 R at 1 AU, when the specific excitation rate for Lyman a is 2 x 10 -3 s -•. This limit is 0.04 times as large as the smaller of the limits set by FSC to avoid mass loading of the solar wind. FSC have invoked a model for cometary mantles developed by Fanale and Salvail [1984] to demonstrate that gas production rates can be kept as low as 3 x 1011 cm -2 s -1 at 1 AU, if the cometesimals have dust mantles whose thickness is 1 cm or less. As Morris [1986] has already pointed out, there is a serious problem with such comets retaining a mantle. Apart from this issue, the Fanale and Salvail model in fact does predict that the gas production rate will be between 3 x 101ø and 3 x 1011 molecules cm -2 s-• if the mantle thickness is between 15 and 1.5 cm. To drop the production rate another factor of 25 to meet the Lyman a constraint, the mantle would need to be 3.75 meters thickand the cometesimal mostly mantleaccording to the model of Fanale