Physical Mechanism of Comet Outbursts: An Experimental Result

Abstract

During experimental impacts into regoliths in vacuo at NASA Ames Research Center, an explosive eruption of gas-charged powder was observed and photographed. Analysis gives four main results applicable to comets: (1) Loose regoliths can retain interstitial gas pressure, supporting recent comet mantle theories. (2) The observed eruption could have produced a dust coma around a comet nucleus larger than a few kilometers. (3) The eruption involved discrete jets emitted by one primary vent (site of the initial disturbance) rather than a uniform spray from the vent or uniform dust/gas flow from the entire regolith surface. This behavior may clarify the discrete jets from P/Halley and other nuclei, but it is not well described in recent mathematical models of comet "mantles." (4) As discussed earlier by the author and colleagues, the mechanism for sporadic eruptions on comets probably involves gas pressure buildup in the porespace of regoliths having a slight effective tensile strength; outbursts occur spontaneously when pressure exceeds tensile strength. This model explains the recharge mechanism required by intermittent outbursts on P/Schwassmann-Wachmann 1, 2060 Chiron, and other bodies—a mechanism not well described by some mathematical models. This work is generally consistent with a 1990 comet regolith model by H. Rickman, J. Fernandez, and B. Gustafson ( Astron. Astrophys. 237, 524-535) and supports 1992 observations by A.D. Storrs, A. L. Cochran, and E. S. Barker ( Icarus 98, 163-178), who interpret their data as requiring "a porous … mantle of nonnegligible tensile strength." Such experiments may lead to improved theoretical modeling of comet mantle behavior.