Daily Archives: 28. August 2020

Jupiter’s ocean moons raise tides on each other

Credit: NASA [click on image to enlarge]

The effects of relative proximity between these large moons seem to have been underrated. Not forgetting that Jupiter does have a big effect on Io, the closest Galilean moon to it.
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Jupiter’s “ocean world” moons may have strong gravitational effects on each other, raising big tides in each others’ subsurface seas, a new study suggests [Space.com reporting].

Surprisingly, these moon-moon tidal forces might generate more heat in the satellites’ oceans than the gravitational tugs of giant Jupiter, study team members found.

“That’s kind of interesting, because Jupiter is the biggest mass in that system, so its tidal forces are much bigger than one moon on another,” lead author Hamish Hay, who performed the work while at the University of Arizona’s Lunar and Planetary Laboratory, said in a statement.

Hay and his colleagues modeled the gravitational interactions among Jupiter’s four large Galilean moons — Io, Europa, Callisto and Ganymede.

The latter three are thought to harbor huge oceans of liquid water beneath their icy shells, whereas powerfully volcanic Io might have a subsurface sea of molten rock.

The researchers determined that the Galilean moons have an outsized influence on each other thanks to “tidal resonance” — basically, a reinforcing sync-up of a gravitational tug and the natural rocking of the satellites’ oceans.

The moons are more tidally resonant with each other than with Jupiter, which explains why the giant planet’s powerful pull doesn’t translate into bigger tidal effects.

As an example: Hay and his team calculated that Jupiter’s tug could generate a tidal wave in Europa’s buried ocean if that sea were about 660 feet (200 meters) deep.

Little Io, by contrast, could get a strong wave going in a Europan ocean 50 miles (80 kilometers) deep.

Full article here.

Related Talkshop post: Why Phi? – the resonance of Jupiter’s Galilean moons

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August 28, 2020 at 12:54PM

Amid crying over low Arctic ice, W Hudson Bay polar bears leave ice as late as 2009

Reposted from Dr. Susan Crockford’s Polar Bear Science

Posted on August 27, 2020 |

Amid crying over low Arctic ice, W Hudson Bay polar bears leave ice as late as 2009

This year, the last collared Western Hudson Bay polar bear to leave the ice left as late, or later, than the last collared bear did in 2009 (which was an unusually late breakup year) and so far, all bears spotted have been in good physical condition despite inhabiting one of the most southern regions of the Arctic. All the while, sea ice experts have been hand-wringing about low Arctic sea ice –– in general and as polar bear habitat.

A female with two yearling cubs on the shore of Wakusp National Park, Western Hudson Bay on 24 August 2020. Taken via livecam from almost a mile away.

By the third week of August this year, there was only a small patch of ice evident off Churchill in Western Hudson Bay, see below:

However, at least one bear with a satellite tracking collar stayed on that bit of ice until about 21 August, see tracking map below provided by Andrew Derocher, University of Alberta:

Said Derocher (22 August 2020) about the late date breakup and delays coming ashore: “One good year doesn’t help the population much but it’s better than a bad year.” Except he said something similar last year after admitting conditions had been good and like the 1980s, which he now seems to have forgotten. As of this summer, WH polar bears have now had six good years by any unbiased measure of the concept: ‘one good year’, year after year.

2020 vs. 2009

The last time that polar bears in Western Hudson Bay came off the ice so late in the summer was in 2009. Breakup of sea ice on Western Hudson Bay was unusually late and bears accordingly came ashore much later than the previous two decades – but even still, as far as I can tell from the published data (Cherry et al. 2013; Castro de la Guardia et al. 2017), the last collared bear came off the ice in 2009 on about August 19 or 20 (Day 231 or 232). Derocher announced his last bear came ashore on or about 21 August in 2020.

Condition of bears

Below is a video taken from a distance by one of the livecams on the shore of Wakusp National Park southeast of Churchill in Western Hudson Bay on 24 August 2020. This female with two cubs in good condition (the same group as in the still shot above) got spooked by something – probably a male bear approaching – and took off at a run across the tundra. But watch which of the three bears was first to notice something was up.

On 26 August, along the same stretch of shoreline, a solitary fat bear wandered by:

Polar Bear Alert Program Weekly Reports

Oddly, there has still been no first report of the season from the polar bear alert program. On Tuesday (25 August), I phoned Manitoba Conservation and the person I talked to said conservation officers had only just been sent out to Churchill this week. He didn’t say if this was a Covid-19 delay or whether there had been no need until now. However, it does suggest we won’t see a report until next week with some kind of assessment of how the bears are doing.

Arctic sea ice

Just to balance the above, it is certainly true that sea ice cover over the Arctic is near its yearly low. However, there is still quite a lot of ice left at 27 August 2020:

Keep in mind when you hear all the ‘sky is falling’ rhetoric that sea ice returns in the fall, as it did even after the lowest and second-lowest summer ice extents since 1979 (in 2012 and 2007/2016, respectively), see ice cover below at 15 November 2016, two months after a low at 15 September.

During this time of ice retreat, most bears endure a 4-5 month fast whether they are onshore or spend the summer on the remaining ice. But that’s OK, because all polar bears can survive and even thrive with a summer fast of 4-5 months as long as they get enough to eat during the spring (Crockford 2017, 2019).

References

Castro de la Guardia, L., Myers, P.G., Derocher, A.E., Lunn, N.J., Terwisscha van Scheltinga, A.D. 2017. Sea ice cycle in western Hudson Bay, Canada, from a polar bear perspective. Marine Ecology Progress Series 564: 225–233. http://www.int-res.com/abstracts/meps/v564/p225-233/

Cherry, S.G., Derocher, A.E., Thiemann, G.W., Lunn, N.J. 2013. Migration phenology and seasonal fidelity of an Arctic marine predator in relation to sea ice dynamics. Journal of Animal Ecology 82:912-921. http://onlinelibrary.wiley.com/doi/10.1111/1365-2656.12050/abstract

Crockford, S.J. 2017. Testing the hypothesis that routine sea ice coverage of 3-5 mkm2 results in a greater than 30% decline in population size of polar bears (Ursus maritimus). PeerJ Preprints 19 January 2017. Doi: 10.7287/peerj.preprints.2737v1 Open access. https://peerj.com/preprints/2737/

Crockford, S.J. 2019. The Polar Bear Catastrophe That Never Happened. Global Warming Policy Foundation, London. Available in paperback and ebook formats.

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August 28, 2020 at 12:50PM